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file.promises.out.html
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file.promises.out.html
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File: promises.out
— Concurrent Ruby
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<div id="content"><div id='filecontents'><h1>Basics</h1>
<h2>Factory methods</h2>
<p>Future and Event are created indirectly with constructor methods in
FactoryMethods. They are not designed for inheritance but rather for
composition.</p>
<pre class="code ruby"><code class="ruby"><span class='const'><span class='object_link'><a href="Concurrent.html" title="Concurrent (module)">Concurrent</a></span></span><span class='op'>::</span><span class='const'><span class='object_link'><a href="Concurrent/Promises.html" title="Concurrent::Promises (module)">Promises</a></span></span><span class='op'>::</span><span class='const'><span class='object_link'><a href="Concurrent/Promises/FactoryMethods.html" title="Concurrent::Promises::FactoryMethods (module)">FactoryMethods</a></span></span><span class='period'>.</span><span class='id identifier rubyid_instance_methods'>instance_methods</span><span class='lparen'>(</span><span class='kw'>false</span><span class='rparen'>)</span><span class='period'>.</span><span class='id identifier rubyid_sort'>sort</span>
<span class='comment'># => [:any,
</span><span class='comment'># :any_event,
</span><span class='comment'># :any_event_on,
</span><span class='comment'># :any_fulfilled_future,
</span><span class='comment'># :any_fulfilled_future_on,
</span><span class='comment'># :any_resolved_future,
</span><span class='comment'># :any_resolved_future_on,
</span><span class='comment'># :delay,
</span><span class='comment'># :delay_on,
</span><span class='comment'># :fulfilled_future,
</span><span class='comment'># :future,
</span><span class='comment'># :future_on,
</span><span class='comment'># :make_future,
</span><span class='comment'># :rejected_future,
</span><span class='comment'># :resolvable_event,
</span><span class='comment'># :resolvable_event_on,
</span><span class='comment'># :resolvable_future,
</span><span class='comment'># :resolvable_future_on,
</span><span class='comment'># :resolved_event,
</span><span class='comment'># :resolved_future,
</span><span class='comment'># :schedule,
</span><span class='comment'># :schedule_on,
</span><span class='comment'># :zip,
</span><span class='comment'># :zip_events,
</span><span class='comment'># :zip_events_on,
</span><span class='comment'># :zip_futures,
</span><span class='comment'># :zip_futures_on,
</span><span class='comment'># :zip_futures_over,
</span><span class='comment'># :zip_futures_over_on]
</span></code></pre>
<p>The module can be included or extended where needed.</p>
<pre class="code ruby"><code class="ruby"><span class='const'>Class</span><span class='period'>.</span><span class='id identifier rubyid_new'>new</span> <span class='kw'>do</span>
<span class='id identifier rubyid_include'>include</span> <span class='const'><span class='object_link'><a href="Concurrent.html" title="Concurrent (module)">Concurrent</a></span></span><span class='op'>::</span><span class='const'><span class='object_link'><a href="Concurrent/Promises.html" title="Concurrent::Promises (module)">Promises</a></span></span><span class='op'>::</span><span class='const'><span class='object_link'><a href="Concurrent/Promises/FactoryMethods.html" title="Concurrent::Promises::FactoryMethods (module)">FactoryMethods</a></span></span>
<span class='kw'>def</span> <span class='id identifier rubyid_a_method'>a_method</span>
<span class='id identifier rubyid_resolvable_event'>resolvable_event</span>
<span class='kw'>end</span>
<span class='kw'>end</span><span class='period'>.</span><span class='id identifier rubyid_new'>new</span><span class='period'>.</span><span class='id identifier rubyid_a_method'>a_method</span>
<span class='comment'># => #<Concurrent::Promises::ResolvableEvent:0x000002 pending>
</span>
<span class='id identifier rubyid_mod'>mod</span> <span class='op'>=</span> <span class='const'>Module</span><span class='period'>.</span><span class='id identifier rubyid_new'>new</span> <span class='kw'>do</span>
<span class='id identifier rubyid_extend'>extend</span> <span class='const'><span class='object_link'><a href="Concurrent.html" title="Concurrent (module)">Concurrent</a></span></span><span class='op'>::</span><span class='const'><span class='object_link'><a href="Concurrent/Promises.html" title="Concurrent::Promises (module)">Promises</a></span></span><span class='op'>::</span><span class='const'><span class='object_link'><a href="Concurrent/Promises/FactoryMethods.html" title="Concurrent::Promises::FactoryMethods (module)">FactoryMethods</a></span></span>
<span class='kw'>end</span>
<span class='id identifier rubyid_mod'>mod</span><span class='period'>.</span><span class='id identifier rubyid_resolvable_event'>resolvable_event</span>
<span class='comment'># => #<Concurrent::Promises::ResolvableEvent:0x000003 pending>
</span></code></pre>
<p>The default executor can be changed by overriding <code>default_executor</code> method
inherited from <code>Concurrent::Promises::FactoryMethods</code>.</p>
<pre class="code ruby"><code class="ruby"><span class='id identifier rubyid_mod'>mod</span> <span class='op'>=</span> <span class='const'>Module</span><span class='period'>.</span><span class='id identifier rubyid_new'>new</span> <span class='kw'>do</span>
<span class='id identifier rubyid_extend'>extend</span> <span class='const'><span class='object_link'><a href="Concurrent.html" title="Concurrent (module)">Concurrent</a></span></span><span class='op'>::</span><span class='const'><span class='object_link'><a href="Concurrent/Promises.html" title="Concurrent::Promises (module)">Promises</a></span></span><span class='op'>::</span><span class='const'><span class='object_link'><a href="Concurrent/Promises/FactoryMethods.html" title="Concurrent::Promises::FactoryMethods (module)">FactoryMethods</a></span></span>
<span class='kw'>def</span> <span class='kw'>self</span><span class='period'>.</span><span class='id identifier rubyid_default_executor'>default_executor</span>
<span class='symbol'>:fast</span>
<span class='kw'>end</span>
<span class='kw'>end</span>
<span class='id identifier rubyid_mod'>mod</span><span class='period'>.</span><span class='id identifier rubyid_future'>future</span> <span class='lbrace'>{</span> <span class='int'>1</span> <span class='rbrace'>}</span><span class='period'>.</span><span class='id identifier rubyid_default_executor'>default_executor</span> <span class='comment'># => :fast
</span><span class='const'><span class='object_link'><a href="Concurrent.html" title="Concurrent (module)">Concurrent</a></span></span><span class='op'>::</span><span class='const'><span class='object_link'><a href="Concurrent/Promises.html" title="Concurrent::Promises (module)">Promises</a></span></span><span class='period'>.</span><span class='id identifier rubyid_future'><span class='object_link'><a href="Concurrent/Promises/FactoryMethods.html#future-instance_method" title="Concurrent::Promises::FactoryMethods#future (method)">future</a></span></span> <span class='lbrace'>{</span> <span class='int'>1</span> <span class='rbrace'>}</span><span class='period'>.</span><span class='id identifier rubyid_default_executor'>default_executor</span>
<span class='comment'># => :io
</span></code></pre>
<p>The module is already extended into <span class='object_link'><a href="Concurrent/Promises.html" title="Concurrent::Promises (module)">Concurrent::Promises</a></span> for convenience.</p>
<pre class="code ruby"><code class="ruby"><span class='const'><span class='object_link'><a href="Concurrent.html" title="Concurrent (module)">Concurrent</a></span></span><span class='op'>::</span><span class='const'><span class='object_link'><a href="Concurrent/Promises.html" title="Concurrent::Promises (module)">Promises</a></span></span><span class='period'>.</span><span class='id identifier rubyid_resolvable_event'><span class='object_link'><a href="Concurrent/Promises/FactoryMethods.html#resolvable_event-instance_method" title="Concurrent::Promises::FactoryMethods#resolvable_event (method)">resolvable_event</a></span></span>
<span class='comment'># => #<Concurrent::Promises::ResolvableEvent:0x000004 pending>
</span></code></pre>
<h2>Asynchronous task</h2>
<p>The most basic use-case of the framework is asynchronous processing. A task can
be processed asynchronously by using a <code>future</code> factory method. The block will
be executed on an internal thread pool.</p>
<p>Arguments of <code>future</code> are passed to the block and evaluation starts immediately.</p>
<pre class="code ruby"><code class="ruby"><span class='id identifier rubyid_future'>future</span> <span class='op'>=</span> <span class='const'><span class='object_link'><a href="Concurrent.html" title="Concurrent (module)">Concurrent</a></span></span><span class='op'>::</span><span class='const'><span class='object_link'><a href="Concurrent/Promises.html" title="Concurrent::Promises (module)">Promises</a></span></span><span class='period'>.</span><span class='id identifier rubyid_future'><span class='object_link'><a href="Concurrent/Promises/FactoryMethods.html#future-instance_method" title="Concurrent::Promises::FactoryMethods#future (method)">future</a></span></span><span class='lparen'>(</span><span class='float'>0.1</span><span class='rparen'>)</span> <span class='kw'>do</span> <span class='op'>|</span><span class='id identifier rubyid_duration'>duration</span><span class='op'>|</span>
<span class='id identifier rubyid_sleep'>sleep</span> <span class='id identifier rubyid_duration'>duration</span>
<span class='symbol'>:result</span>
<span class='kw'>end</span>
<span class='comment'># => #<Concurrent::Promises::Future:0x000005 pending>
</span></code></pre>
<p>Asks if the future is resolved, here it will be still in the middle of the
sleep call.</p>
<pre class="code ruby"><code class="ruby"><span class='id identifier rubyid_future'>future</span><span class='period'>.</span><span class='id identifier rubyid_resolved?'>resolved?</span> <span class='comment'># => false
</span></code></pre>
<p>Retrieving the value will block until the future is <strong>resolved</strong>.</p>
<pre class="code ruby"><code class="ruby"><span class='id identifier rubyid_future'>future</span><span class='period'>.</span><span class='id identifier rubyid_value'>value</span> <span class='comment'># => :result
</span><span class='id identifier rubyid_future'>future</span><span class='period'>.</span><span class='id identifier rubyid_resolved?'>resolved?</span> <span class='comment'># => true
</span></code></pre>
<p>If the task fails, we talk about the future being <strong>rejected</strong>.</p>
<pre class="code ruby"><code class="ruby"><span class='id identifier rubyid_future'>future</span> <span class='op'>=</span> <span class='const'><span class='object_link'><a href="Concurrent.html" title="Concurrent (module)">Concurrent</a></span></span><span class='op'>::</span><span class='const'><span class='object_link'><a href="Concurrent/Promises.html" title="Concurrent::Promises (module)">Promises</a></span></span><span class='period'>.</span><span class='id identifier rubyid_future'><span class='object_link'><a href="Concurrent/Promises/FactoryMethods.html#future-instance_method" title="Concurrent::Promises::FactoryMethods#future (method)">future</a></span></span> <span class='lbrace'>{</span> <span class='id identifier rubyid_sleep'>sleep</span> <span class='float'>0.1</span><span class='semicolon'>;</span> <span class='id identifier rubyid_raise'>raise</span> <span class='tstring'><span class='tstring_beg'>'</span><span class='tstring_content'>Boom</span><span class='tstring_end'>'</span></span> <span class='rbrace'>}</span>
<span class='comment'># => #<Concurrent::Promises::Future:0x000006 pending>
</span></code></pre>
<p>There is no result, the future was rejected with a reason.</p>
<pre class="code ruby"><code class="ruby"><span class='id identifier rubyid_future'>future</span><span class='period'>.</span><span class='id identifier rubyid_value'>value</span> <span class='comment'># => nil
</span><span class='id identifier rubyid_future'>future</span><span class='period'>.</span><span class='id identifier rubyid_reason'>reason</span> <span class='comment'># => #<RuntimeError: Boom>
</span></code></pre>
<p>It can be forced to raise the reason for rejection when retrieving the value.</p>
<pre class="code ruby"><code class="ruby"><span class='kw'>begin</span>
<span class='id identifier rubyid_future'>future</span><span class='period'>.</span><span class='id identifier rubyid_value!'>value!</span>
<span class='kw'>rescue</span> <span class='op'>=></span> <span class='id identifier rubyid_e'>e</span>
<span class='id identifier rubyid_e'>e</span>
<span class='kw'>end</span> <span class='comment'># => #<RuntimeError: Boom>
</span></code></pre>
<p>Which is the same as <code>future.value! rescue $!</code> which will be used hereafter.</p>
<p>Or it can be used directly as argument for raise, since it implements exception
method.</p>
<pre class="code ruby"><code class="ruby"><span class='id identifier rubyid_raise'>raise</span> <span class='id identifier rubyid_future'>future</span> <span class='kw'>rescue</span> <span class='gvar'>$!</span> <span class='comment'># => #<RuntimeError: Boom>
</span></code></pre>
<h2>States</h2>
<p>Let's define an inspection helper for methods.</p>
<pre class="code ruby"><code class="ruby"><span class='kw'>def</span> <span class='id identifier rubyid_inspect_methods'>inspect_methods</span><span class='lparen'>(</span><span class='op'>*</span><span class='id identifier rubyid_methods'>methods</span><span class='comma'>,</span> <span class='label'>of:</span><span class='rparen'>)</span>
<span class='id identifier rubyid_methods'>methods</span><span class='period'>.</span><span class='id identifier rubyid_reduce'>reduce</span><span class='lparen'>(</span><span class='lbrace'>{</span><span class='rbrace'>}</span><span class='rparen'>)</span> <span class='lbrace'>{</span> <span class='op'>|</span><span class='id identifier rubyid_h'>h</span><span class='comma'>,</span> <span class='id identifier rubyid_m'>m</span><span class='op'>|</span> <span class='id identifier rubyid_h'>h</span><span class='period'>.</span><span class='id identifier rubyid_update'>update</span> <span class='id identifier rubyid_m'>m</span> <span class='op'>=></span> <span class='id identifier rubyid_of'>of</span><span class='period'>.</span><span class='id identifier rubyid_send'>send</span><span class='lparen'>(</span><span class='id identifier rubyid_m'>m</span><span class='rparen'>)</span> <span class='rbrace'>}</span>
<span class='kw'>end</span>
</code></pre>
<p>Event has a <code>pending</code> and a <code>resolved</code> state. </p>
<pre class="code ruby"><code class="ruby"><span class='id identifier rubyid_event'>event</span> <span class='op'>=</span> <span class='const'><span class='object_link'><a href="Concurrent.html" title="Concurrent (module)">Concurrent</a></span></span><span class='op'>::</span><span class='const'><span class='object_link'><a href="Concurrent/Promises.html" title="Concurrent::Promises (module)">Promises</a></span></span><span class='period'>.</span><span class='id identifier rubyid_resolvable_event'><span class='object_link'><a href="Concurrent/Promises/FactoryMethods.html#resolvable_event-instance_method" title="Concurrent::Promises::FactoryMethods#resolvable_event (method)">resolvable_event</a></span></span>
<span class='id identifier rubyid_inspect_methods'>inspect_methods</span><span class='lparen'>(</span><span class='symbol'>:state</span><span class='comma'>,</span> <span class='symbol'>:pending?</span><span class='comma'>,</span> <span class='symbol'>:resolved?</span><span class='comma'>,</span> <span class='label'>of:</span> <span class='id identifier rubyid_event'>event</span><span class='rparen'>)</span>
<span class='comment'># => {:state=>:pending, :pending?=>true, :resolved?=>false}
</span>
<span class='id identifier rubyid_event'>event</span><span class='period'>.</span><span class='id identifier rubyid_resolve'>resolve</span>
<span class='id identifier rubyid_inspect_methods'>inspect_methods</span><span class='lparen'>(</span><span class='symbol'>:state</span><span class='comma'>,</span> <span class='symbol'>:pending?</span><span class='comma'>,</span> <span class='symbol'>:resolved?</span><span class='comma'>,</span> <span class='label'>of:</span> <span class='id identifier rubyid_event'>event</span><span class='rparen'>)</span>
<span class='comment'># => {:state=>:resolved, :pending?=>false, :resolved?=>true}
</span></code></pre>
<p>Future's <code>resolved</code> state is further specified to be <code>fulfilled</code> or <code>rejected</code>.</p>
<pre class="code ruby"><code class="ruby"><span class='id identifier rubyid_future'>future</span> <span class='op'>=</span> <span class='const'><span class='object_link'><a href="Concurrent.html" title="Concurrent (module)">Concurrent</a></span></span><span class='op'>::</span><span class='const'><span class='object_link'><a href="Concurrent/Promises.html" title="Concurrent::Promises (module)">Promises</a></span></span><span class='period'>.</span><span class='id identifier rubyid_resolvable_future'><span class='object_link'><a href="Concurrent/Promises/FactoryMethods.html#resolvable_future-instance_method" title="Concurrent::Promises::FactoryMethods#resolvable_future (method)">resolvable_future</a></span></span>
<span class='id identifier rubyid_inspect_methods'>inspect_methods</span><span class='lparen'>(</span><span class='symbol'>:state</span><span class='comma'>,</span> <span class='symbol'>:pending?</span><span class='comma'>,</span> <span class='symbol'>:resolved?</span><span class='comma'>,</span> <span class='symbol'>:fulfilled?</span><span class='comma'>,</span> <span class='symbol'>:rejected?</span><span class='comma'>,</span>
<span class='label'>of:</span> <span class='id identifier rubyid_future'>future</span><span class='rparen'>)</span>
<span class='comment'># => {:state=>:pending,
</span><span class='comment'># :pending?=>true,
</span><span class='comment'># :resolved?=>false,
</span><span class='comment'># :fulfilled?=>false,
</span><span class='comment'># :rejected?=>false}
</span>
<span class='id identifier rubyid_future'>future</span><span class='period'>.</span><span class='id identifier rubyid_fulfill'>fulfill</span> <span class='symbol'>:value</span>
<span class='id identifier rubyid_inspect_methods'>inspect_methods</span><span class='lparen'>(</span><span class='symbol'>:state</span><span class='comma'>,</span> <span class='symbol'>:pending?</span><span class='comma'>,</span> <span class='symbol'>:resolved?</span><span class='comma'>,</span> <span class='symbol'>:fulfilled?</span><span class='comma'>,</span> <span class='symbol'>:rejected?</span><span class='comma'>,</span>
<span class='symbol'>:result</span><span class='comma'>,</span> <span class='symbol'>:value</span><span class='comma'>,</span> <span class='symbol'>:reason</span><span class='comma'>,</span> <span class='label'>of:</span> <span class='id identifier rubyid_future'>future</span><span class='rparen'>)</span>
<span class='comment'># => {:state=>:fulfilled,
</span><span class='comment'># :pending?=>false,
</span><span class='comment'># :resolved?=>true,
</span><span class='comment'># :fulfilled?=>true,
</span><span class='comment'># :rejected?=>false,
</span><span class='comment'># :result=>[true, :value, nil],
</span><span class='comment'># :value=>:value,
</span><span class='comment'># :reason=>nil}
</span>
<span class='id identifier rubyid_future'>future</span> <span class='op'>=</span> <span class='const'><span class='object_link'><a href="Concurrent.html" title="Concurrent (module)">Concurrent</a></span></span><span class='op'>::</span><span class='const'><span class='object_link'><a href="Concurrent/Promises.html" title="Concurrent::Promises (module)">Promises</a></span></span><span class='period'>.</span><span class='id identifier rubyid_rejected_future'><span class='object_link'><a href="Concurrent/Promises/FactoryMethods.html#rejected_future-instance_method" title="Concurrent::Promises::FactoryMethods#rejected_future (method)">rejected_future</a></span></span> <span class='const'>StandardError</span><span class='period'>.</span><span class='id identifier rubyid_new'>new</span>
<span class='id identifier rubyid_inspect_methods'>inspect_methods</span><span class='lparen'>(</span><span class='symbol'>:state</span><span class='comma'>,</span> <span class='symbol'>:pending?</span><span class='comma'>,</span> <span class='symbol'>:resolved?</span><span class='comma'>,</span> <span class='symbol'>:fulfilled?</span><span class='comma'>,</span> <span class='symbol'>:rejected?</span><span class='comma'>,</span>
<span class='symbol'>:result</span><span class='comma'>,</span> <span class='symbol'>:value</span><span class='comma'>,</span> <span class='symbol'>:reason</span><span class='comma'>,</span> <span class='label'>of:</span> <span class='id identifier rubyid_future'>future</span><span class='rparen'>)</span>
<span class='comment'># => {:state=>:rejected,
</span><span class='comment'># :pending?=>false,
</span><span class='comment'># :resolved?=>true,
</span><span class='comment'># :fulfilled?=>false,
</span><span class='comment'># :rejected?=>true,
</span><span class='comment'># :result=>[false, nil, #<StandardError: StandardError>],
</span><span class='comment'># :value=>nil,
</span><span class='comment'># :reason=>#<StandardError: StandardError>}
</span></code></pre>
<h2>Direct creation of resolved futures</h2>
<p>When an existing value has to be wrapped in a future it does not have to go
through evaluation as follows.</p>
<pre class="code ruby"><code class="ruby"><span class='const'><span class='object_link'><a href="Concurrent.html" title="Concurrent (module)">Concurrent</a></span></span><span class='op'>::</span><span class='const'><span class='object_link'><a href="Concurrent/Promises.html" title="Concurrent::Promises (module)">Promises</a></span></span><span class='period'>.</span><span class='id identifier rubyid_future'><span class='object_link'><a href="Concurrent/Promises/FactoryMethods.html#future-instance_method" title="Concurrent::Promises::FactoryMethods#future (method)">future</a></span></span> <span class='lbrace'>{</span> <span class='id identifier rubyid_sleep'>sleep</span> <span class='float'>0.01</span><span class='semicolon'>;</span> <span class='symbol'>:value</span> <span class='rbrace'>}</span>
<span class='comment'># => #<Concurrent::Promises::Future:0x000007 pending>
</span></code></pre>
<p>Instead, it can be created directly as already-resolved:</p>
<pre class="code ruby"><code class="ruby"><span class='const'><span class='object_link'><a href="Concurrent.html" title="Concurrent (module)">Concurrent</a></span></span><span class='op'>::</span><span class='const'><span class='object_link'><a href="Concurrent/Promises.html" title="Concurrent::Promises (module)">Promises</a></span></span><span class='period'>.</span><span class='id identifier rubyid_fulfilled_future'><span class='object_link'><a href="Concurrent/Promises/FactoryMethods.html#fulfilled_future-instance_method" title="Concurrent::Promises::FactoryMethods#fulfilled_future (method)">fulfilled_future</a></span></span><span class='lparen'>(</span><span class='symbol'>:value</span><span class='rparen'>)</span>
<span class='comment'># => #<Concurrent::Promises::Future:0x000008 fulfilled>
</span><span class='const'><span class='object_link'><a href="Concurrent.html" title="Concurrent (module)">Concurrent</a></span></span><span class='op'>::</span><span class='const'><span class='object_link'><a href="Concurrent/Promises.html" title="Concurrent::Promises (module)">Promises</a></span></span><span class='period'>.</span><span class='id identifier rubyid_rejected_future'><span class='object_link'><a href="Concurrent/Promises/FactoryMethods.html#rejected_future-instance_method" title="Concurrent::Promises::FactoryMethods#rejected_future (method)">rejected_future</a></span></span><span class='lparen'>(</span><span class='const'>StandardError</span><span class='period'>.</span><span class='id identifier rubyid_new'>new</span><span class='lparen'>(</span><span class='tstring'><span class='tstring_beg'>'</span><span class='tstring_content'>Ups</span><span class='tstring_end'>'</span></span><span class='rparen'>)</span><span class='rparen'>)</span>
<span class='comment'># => #<Concurrent::Promises::Future:0x000009 rejected>
</span><span class='const'><span class='object_link'><a href="Concurrent.html" title="Concurrent (module)">Concurrent</a></span></span><span class='op'>::</span><span class='const'><span class='object_link'><a href="Concurrent/Promises.html" title="Concurrent::Promises (module)">Promises</a></span></span><span class='period'>.</span><span class='id identifier rubyid_resolved_future'><span class='object_link'><a href="Concurrent/Promises/FactoryMethods.html#resolved_future-instance_method" title="Concurrent::Promises::FactoryMethods#resolved_future (method)">resolved_future</a></span></span><span class='lparen'>(</span><span class='kw'>true</span><span class='comma'>,</span> <span class='symbol'>:value</span><span class='comma'>,</span> <span class='kw'>nil</span><span class='rparen'>)</span>
<span class='comment'># => #<Concurrent::Promises::Future:0x00000a fulfilled>
</span><span class='const'><span class='object_link'><a href="Concurrent.html" title="Concurrent (module)">Concurrent</a></span></span><span class='op'>::</span><span class='const'><span class='object_link'><a href="Concurrent/Promises.html" title="Concurrent::Promises (module)">Promises</a></span></span><span class='period'>.</span><span class='id identifier rubyid_resolved_future'><span class='object_link'><a href="Concurrent/Promises/FactoryMethods.html#resolved_future-instance_method" title="Concurrent::Promises::FactoryMethods#resolved_future (method)">resolved_future</a></span></span><span class='lparen'>(</span><span class='kw'>false</span><span class='comma'>,</span> <span class='kw'>nil</span><span class='comma'>,</span> <span class='const'>StandardError</span><span class='period'>.</span><span class='id identifier rubyid_new'>new</span><span class='lparen'>(</span><span class='tstring'><span class='tstring_beg'>'</span><span class='tstring_content'>Ups</span><span class='tstring_end'>'</span></span><span class='rparen'>)</span><span class='rparen'>)</span>
<span class='comment'># => #<Concurrent::Promises::Future:0x00000b rejected>
</span></code></pre>
<h2>Chaining</h2>
<p>A big advantage of promises is the ability to chain tasks together without blocking
the current thread.</p>
<pre class="code ruby"><code class="ruby"><span class='const'><span class='object_link'><a href="Concurrent.html" title="Concurrent (module)">Concurrent</a></span></span><span class='op'>::</span><span class='const'><span class='object_link'><a href="Concurrent/Promises.html" title="Concurrent::Promises (module)">Promises</a></span></span><span class='period'>.</span>
<span class='id identifier rubyid_future'>future</span><span class='lparen'>(</span><span class='int'>2</span><span class='rparen'>)</span> <span class='lbrace'>{</span> <span class='op'>|</span><span class='id identifier rubyid_v'>v</span><span class='op'>|</span> <span class='id identifier rubyid_v'>v</span><span class='period'>.</span><span class='id identifier rubyid_succ'>succ</span> <span class='rbrace'>}</span><span class='period'>.</span>
<span class='id identifier rubyid_then'>then</span><span class='lparen'>(</span><span class='op'>&</span><span class='symbol'>:succ</span><span class='rparen'>)</span><span class='period'>.</span>
<span class='id identifier rubyid_value!'>value!</span> <span class='comment'># => 4
</span></code></pre>
<p>As <code>future</code> factory method takes an argument, so does the <code>then</code> method. Any
supplied arguments are passed to the block, and the library ensures that they
are visible to the block.</p>
<pre class="code ruby"><code class="ruby"><span class='const'><span class='object_link'><a href="Concurrent.html" title="Concurrent (module)">Concurrent</a></span></span><span class='op'>::</span><span class='const'><span class='object_link'><a href="Concurrent/Promises.html" title="Concurrent::Promises (module)">Promises</a></span></span><span class='period'>.</span>
<span class='id identifier rubyid_future'>future</span><span class='lparen'>(</span><span class='tstring'><span class='tstring_beg'>'</span><span class='tstring_content'>3</span><span class='tstring_end'>'</span></span><span class='rparen'>)</span> <span class='lbrace'>{</span> <span class='op'>|</span><span class='id identifier rubyid_s'>s</span><span class='op'>|</span> <span class='id identifier rubyid_s'>s</span><span class='period'>.</span><span class='id identifier rubyid_to_i'>to_i</span> <span class='rbrace'>}</span><span class='period'>.</span>
<span class='id identifier rubyid_then'>then</span><span class='lparen'>(</span><span class='int'>2</span><span class='rparen'>)</span> <span class='lbrace'>{</span> <span class='op'>|</span><span class='id identifier rubyid_v'>v</span><span class='comma'>,</span> <span class='id identifier rubyid_arg'>arg</span><span class='op'>|</span> <span class='id identifier rubyid_v'>v</span> <span class='op'>+</span> <span class='id identifier rubyid_arg'>arg</span> <span class='rbrace'>}</span><span class='period'>.</span>
<span class='id identifier rubyid_value'>value</span> <span class='comment'># => 5
</span><span class='const'><span class='object_link'><a href="Concurrent.html" title="Concurrent (module)">Concurrent</a></span></span><span class='op'>::</span><span class='const'><span class='object_link'><a href="Concurrent/Promises.html" title="Concurrent::Promises (module)">Promises</a></span></span><span class='period'>.</span>
<span class='id identifier rubyid_fulfilled_future'>fulfilled_future</span><span class='lparen'>(</span><span class='tstring'><span class='tstring_beg'>'</span><span class='tstring_content'>3</span><span class='tstring_end'>'</span></span><span class='rparen'>)</span><span class='period'>.</span>
<span class='id identifier rubyid_then'>then</span><span class='lparen'>(</span><span class='op'>&</span><span class='symbol'>:to_i</span><span class='rparen'>)</span><span class='period'>.</span>
<span class='id identifier rubyid_then'>then</span><span class='lparen'>(</span><span class='int'>2</span><span class='comma'>,</span> <span class='op'>&</span><span class='symbol'>:+</span><span class='rparen'>)</span><span class='period'>.</span>
<span class='id identifier rubyid_value'>value</span> <span class='comment'># => 5
</span><span class='const'><span class='object_link'><a href="Concurrent.html" title="Concurrent (module)">Concurrent</a></span></span><span class='op'>::</span><span class='const'><span class='object_link'><a href="Concurrent/Promises.html" title="Concurrent::Promises (module)">Promises</a></span></span><span class='period'>.</span>
<span class='id identifier rubyid_fulfilled_future'>fulfilled_future</span><span class='lparen'>(</span><span class='int'>1</span><span class='rparen'>)</span><span class='period'>.</span>
<span class='id identifier rubyid_chain'>chain</span><span class='lparen'>(</span><span class='int'>2</span><span class='rparen'>)</span> <span class='lbrace'>{</span> <span class='op'>|</span><span class='id identifier rubyid_fulfilled'>fulfilled</span><span class='comma'>,</span> <span class='id identifier rubyid_value'>value</span><span class='comma'>,</span> <span class='id identifier rubyid_reason'>reason</span><span class='comma'>,</span> <span class='id identifier rubyid_arg'>arg</span><span class='op'>|</span> <span class='id identifier rubyid_value'>value</span> <span class='op'>+</span> <span class='id identifier rubyid_arg'>arg</span> <span class='rbrace'>}</span><span class='period'>.</span>
<span class='id identifier rubyid_value'>value</span> <span class='comment'># => 3
</span></code></pre>
<p>Passing the arguments in (similarly as for a thread <code>Thread.new(arg) { |arg|
do_stuff arg }</code>) is <strong>required</strong>. Both of the following bad examples may break:</p>
<pre class="code ruby"><code class="ruby"><span class='id identifier rubyid_arg'>arg</span> <span class='op'>=</span> <span class='int'>1</span> <span class='comment'># => 1
</span><span class='const'>Thread</span><span class='period'>.</span><span class='id identifier rubyid_new'>new</span> <span class='lbrace'>{</span> <span class='id identifier rubyid_do_stuff'>do_stuff</span> <span class='id identifier rubyid_arg'>arg</span> <span class='rbrace'>}</span>
<span class='comment'># => #<Thread:0x00000c@promises.in.md:203 run>
</span><span class='const'><span class='object_link'><a href="Concurrent.html" title="Concurrent (module)">Concurrent</a></span></span><span class='op'>::</span><span class='const'><span class='object_link'><a href="Concurrent/Promises.html" title="Concurrent::Promises (module)">Promises</a></span></span><span class='period'>.</span><span class='id identifier rubyid_future'><span class='object_link'><a href="Concurrent/Promises/FactoryMethods.html#future-instance_method" title="Concurrent::Promises::FactoryMethods#future (method)">future</a></span></span> <span class='lbrace'>{</span> <span class='id identifier rubyid_do_stuff'>do_stuff</span> <span class='id identifier rubyid_arg'>arg</span> <span class='rbrace'>}</span>
<span class='comment'># => #<Concurrent::Promises::Future:0x00000d pending>
</span></code></pre>
<p>Correct:</p>
<pre class="code ruby"><code class="ruby"><span class='id identifier rubyid_arg'>arg</span> <span class='op'>=</span> <span class='int'>1</span> <span class='comment'># => 1
</span><span class='const'>Thread</span><span class='period'>.</span><span class='id identifier rubyid_new'>new</span><span class='lparen'>(</span><span class='id identifier rubyid_arg'>arg</span><span class='rparen'>)</span> <span class='lbrace'>{</span> <span class='op'>|</span><span class='id identifier rubyid_arg'>arg</span><span class='op'>|</span> <span class='id identifier rubyid_do_stuff'>do_stuff</span> <span class='id identifier rubyid_arg'>arg</span> <span class='rbrace'>}</span>
<span class='comment'># => #<Thread:0x00000e@promises.in.md:211 run>
</span><span class='const'><span class='object_link'><a href="Concurrent.html" title="Concurrent (module)">Concurrent</a></span></span><span class='op'>::</span><span class='const'><span class='object_link'><a href="Concurrent/Promises.html" title="Concurrent::Promises (module)">Promises</a></span></span><span class='period'>.</span><span class='id identifier rubyid_future'><span class='object_link'><a href="Concurrent/Promises/FactoryMethods.html#future-instance_method" title="Concurrent::Promises::FactoryMethods#future (method)">future</a></span></span><span class='lparen'>(</span><span class='id identifier rubyid_arg'>arg</span><span class='rparen'>)</span> <span class='lbrace'>{</span> <span class='op'>|</span><span class='id identifier rubyid_arg'>arg</span><span class='op'>|</span> <span class='id identifier rubyid_do_stuff'>do_stuff</span> <span class='id identifier rubyid_arg'>arg</span> <span class='rbrace'>}</span>
<span class='comment'># => #<Concurrent::Promises::Future:0x00000f pending>
</span></code></pre>
<h2>Branching, and zipping</h2>
<p>Besides chaining it can also be branched.</p>
<pre class="code ruby"><code class="ruby"><span class='id identifier rubyid_head'>head</span> <span class='op'>=</span> <span class='const'><span class='object_link'><a href="Concurrent.html" title="Concurrent (module)">Concurrent</a></span></span><span class='op'>::</span><span class='const'><span class='object_link'><a href="Concurrent/Promises.html" title="Concurrent::Promises (module)">Promises</a></span></span><span class='period'>.</span><span class='id identifier rubyid_fulfilled_future'><span class='object_link'><a href="Concurrent/Promises/FactoryMethods.html#fulfilled_future-instance_method" title="Concurrent::Promises::FactoryMethods#fulfilled_future (method)">fulfilled_future</a></span></span> <span class='op'>-</span><span class='int'>1</span>
<span class='id identifier rubyid_branch1'>branch1</span> <span class='op'>=</span> <span class='id identifier rubyid_head'>head</span><span class='period'>.</span><span class='id identifier rubyid_then'>then</span><span class='lparen'>(</span><span class='op'>&</span><span class='symbol'>:abs</span><span class='rparen'>)</span>
<span class='id identifier rubyid_branch2'>branch2</span> <span class='op'>=</span> <span class='id identifier rubyid_head'>head</span><span class='period'>.</span><span class='id identifier rubyid_then'>then</span><span class='lparen'>(</span><span class='op'>&</span><span class='symbol'>:succ</span><span class='rparen'>)</span><span class='period'>.</span><span class='id identifier rubyid_then'>then</span><span class='lparen'>(</span><span class='op'>&</span><span class='symbol'>:succ</span><span class='rparen'>)</span>
<span class='id identifier rubyid_branch1'>branch1</span><span class='period'>.</span><span class='id identifier rubyid_value!'>value!</span> <span class='comment'># => 1
</span><span class='id identifier rubyid_branch2'>branch2</span><span class='period'>.</span><span class='id identifier rubyid_value!'>value!</span> <span class='comment'># => 1
</span></code></pre>
<p>It can be combined back to one future by zipping (<code>zip</code>, <code>&</code>).</p>
<pre class="code ruby"><code class="ruby"><span class='id identifier rubyid_branch1'>branch1</span><span class='period'>.</span><span class='id identifier rubyid_zip'>zip</span><span class='lparen'>(</span><span class='id identifier rubyid_branch2'>branch2</span><span class='rparen'>)</span><span class='period'>.</span><span class='id identifier rubyid_value!'>value!</span> <span class='comment'># => [1, 1]
</span><span class='lparen'>(</span><span class='id identifier rubyid_branch1'>branch1</span> <span class='op'>&</span> <span class='id identifier rubyid_branch2'>branch2</span><span class='rparen'>)</span><span class='period'>.</span>
<span class='id identifier rubyid_then'>then</span> <span class='lbrace'>{</span> <span class='op'>|</span><span class='id identifier rubyid_a'>a</span><span class='comma'>,</span> <span class='id identifier rubyid_b'>b</span><span class='op'>|</span> <span class='id identifier rubyid_a'>a</span> <span class='op'>+</span> <span class='id identifier rubyid_b'>b</span> <span class='rbrace'>}</span><span class='period'>.</span>
<span class='id identifier rubyid_value!'>value!</span> <span class='comment'># => 2
</span><span class='lparen'>(</span><span class='id identifier rubyid_branch1'>branch1</span> <span class='op'>&</span> <span class='id identifier rubyid_branch2'>branch2</span><span class='rparen'>)</span><span class='period'>.</span>
<span class='id identifier rubyid_then'>then</span><span class='lparen'>(</span><span class='op'>&</span><span class='symbol'>:+</span><span class='rparen'>)</span><span class='period'>.</span>
<span class='id identifier rubyid_value!'>value!</span> <span class='comment'># => 2
</span><span class='const'><span class='object_link'><a href="Concurrent.html" title="Concurrent (module)">Concurrent</a></span></span><span class='op'>::</span><span class='const'><span class='object_link'><a href="Concurrent/Promises.html" title="Concurrent::Promises (module)">Promises</a></span></span><span class='period'>.</span>
<span class='id identifier rubyid_zip'>zip</span><span class='lparen'>(</span><span class='id identifier rubyid_branch1'>branch1</span><span class='comma'>,</span> <span class='id identifier rubyid_branch2'>branch2</span><span class='comma'>,</span> <span class='id identifier rubyid_branch1'>branch1</span><span class='rparen'>)</span><span class='period'>.</span>
<span class='id identifier rubyid_then'>then</span> <span class='lbrace'>{</span> <span class='op'>|</span><span class='op'>*</span><span class='id identifier rubyid_values'>values</span><span class='op'>|</span> <span class='id identifier rubyid_values'>values</span><span class='period'>.</span><span class='id identifier rubyid_reduce'>reduce</span><span class='lparen'>(</span><span class='op'>&</span><span class='symbol'>:+</span><span class='rparen'>)</span> <span class='rbrace'>}</span><span class='period'>.</span>
<span class='id identifier rubyid_value!'>value!</span> <span class='comment'># => 3
</span></code></pre>
<p>Instead of zipping only the first one can be taken, if needed.</p>
<pre class="code ruby"><code class="ruby"><span class='const'><span class='object_link'><a href="Concurrent.html" title="Concurrent (module)">Concurrent</a></span></span><span class='op'>::</span><span class='const'><span class='object_link'><a href="Concurrent/Promises.html" title="Concurrent::Promises (module)">Promises</a></span></span><span class='period'>.</span><span class='id identifier rubyid_any'>any</span><span class='lparen'>(</span><span class='id identifier rubyid_branch1'>branch1</span><span class='comma'>,</span> <span class='id identifier rubyid_branch2'>branch2</span><span class='rparen'>)</span><span class='period'>.</span><span class='id identifier rubyid_value!'><span class='object_link'><a href="Concurrent/Promises/Future.html#value!-instance_method" title="Concurrent::Promises::Future#value! (method)">value!</a></span></span>
<span class='comment'># => 1
</span><span class='lparen'>(</span><span class='id identifier rubyid_branch1'>branch1</span> <span class='op'>|</span> <span class='id identifier rubyid_branch2'>branch2</span><span class='rparen'>)</span><span class='period'>.</span><span class='id identifier rubyid_value!'>value!</span> <span class='comment'># => 1
</span></code></pre>
<h2>Blocking methods</h2>
<p>In these examples we have used blocking methods like <code>value</code> extensively for
their convenience, however in practice is better to avoid them and continue
chaining.</p>
<p>If they need to be used (e.g. when integrating with threads), <code>value!</code> is a
better option over <code>value</code> when rejections are not dealt with differently.
Otherwise the rejections are not handled and probably silently forgotten.</p>
<h2>Error handling</h2>
<p>When a task in the chain fails, the rejection propagates down the
chain without executing the tasks created with <code>then</code>.</p>
<pre class="code ruby"><code class="ruby"><span class='const'><span class='object_link'><a href="Concurrent.html" title="Concurrent (module)">Concurrent</a></span></span><span class='op'>::</span><span class='const'><span class='object_link'><a href="Concurrent/Promises.html" title="Concurrent::Promises (module)">Promises</a></span></span><span class='period'>.</span>
<span class='id identifier rubyid_fulfilled_future'>fulfilled_future</span><span class='lparen'>(</span><span class='const'>Object</span><span class='period'>.</span><span class='id identifier rubyid_new'>new</span><span class='rparen'>)</span><span class='period'>.</span>
<span class='id identifier rubyid_then'>then</span><span class='lparen'>(</span><span class='op'>&</span><span class='symbol'>:succ</span><span class='rparen'>)</span><span class='period'>.</span>
<span class='id identifier rubyid_then'>then</span><span class='lparen'>(</span><span class='op'>&</span><span class='symbol'>:succ</span><span class='rparen'>)</span><span class='period'>.</span>
<span class='id identifier rubyid_result'>result</span>
<span class='comment'># => [false,
</span><span class='comment'># nil,
</span><span class='comment'># #<NoMethodError: undefined method `succ' for #<Object:0x000010>>]
</span></code></pre>
<p>As <code>then</code> chained tasks execute only on fulfilled futures, there is a <code>rescue</code>
method which chains a task which is executed only when the future is rejected.
It can be used to recover from rejection.</p>
<p>Using rescue to fulfill to 0 instead of the error.</p>
<pre class="code ruby"><code class="ruby"><span class='const'><span class='object_link'><a href="Concurrent.html" title="Concurrent (module)">Concurrent</a></span></span><span class='op'>::</span><span class='const'><span class='object_link'><a href="Concurrent/Promises.html" title="Concurrent::Promises (module)">Promises</a></span></span><span class='period'>.</span>
<span class='id identifier rubyid_fulfilled_future'>fulfilled_future</span><span class='lparen'>(</span><span class='const'>Object</span><span class='period'>.</span><span class='id identifier rubyid_new'>new</span><span class='rparen'>)</span><span class='period'>.</span>
<span class='id identifier rubyid_then'>then</span><span class='lparen'>(</span><span class='op'>&</span><span class='symbol'>:succ</span><span class='rparen'>)</span><span class='period'>.</span>
<span class='id identifier rubyid_then'>then</span><span class='lparen'>(</span><span class='op'>&</span><span class='symbol'>:succ</span><span class='rparen'>)</span><span class='period'>.</span>
<span class='id identifier rubyid_rescue'>rescue</span> <span class='lbrace'>{</span> <span class='op'>|</span><span class='id identifier rubyid_err'>err</span><span class='op'>|</span> <span class='int'>0</span> <span class='rbrace'>}</span><span class='period'>.</span>
<span class='id identifier rubyid_result'>result</span> <span class='comment'># => [true, 0, nil]
</span></code></pre>
<p>Rescue not executed when there is no rejection.</p>
<pre class="code ruby"><code class="ruby"><span class='const'><span class='object_link'><a href="Concurrent.html" title="Concurrent (module)">Concurrent</a></span></span><span class='op'>::</span><span class='const'><span class='object_link'><a href="Concurrent/Promises.html" title="Concurrent::Promises (module)">Promises</a></span></span><span class='period'>.</span>
<span class='id identifier rubyid_fulfilled_future'>fulfilled_future</span><span class='lparen'>(</span><span class='int'>1</span><span class='rparen'>)</span><span class='period'>.</span>
<span class='id identifier rubyid_then'>then</span><span class='lparen'>(</span><span class='op'>&</span><span class='symbol'>:succ</span><span class='rparen'>)</span><span class='period'>.</span>
<span class='id identifier rubyid_then'>then</span><span class='lparen'>(</span><span class='op'>&</span><span class='symbol'>:succ</span><span class='rparen'>)</span><span class='period'>.</span>
<span class='id identifier rubyid_rescue'>rescue</span> <span class='lbrace'>{</span> <span class='op'>|</span><span class='id identifier rubyid_e'>e</span><span class='op'>|</span> <span class='int'>0</span> <span class='rbrace'>}</span><span class='period'>.</span>
<span class='id identifier rubyid_result'>result</span> <span class='comment'># => [true, 3, nil]
</span></code></pre>
<p>Tasks added with <code>chain</code> are always evaluated.</p>
<pre class="code ruby"><code class="ruby"><span class='const'><span class='object_link'><a href="Concurrent.html" title="Concurrent (module)">Concurrent</a></span></span><span class='op'>::</span><span class='const'><span class='object_link'><a href="Concurrent/Promises.html" title="Concurrent::Promises (module)">Promises</a></span></span><span class='period'>.</span>
<span class='id identifier rubyid_fulfilled_future'>fulfilled_future</span><span class='lparen'>(</span><span class='int'>1</span><span class='rparen'>)</span><span class='period'>.</span>
<span class='id identifier rubyid_chain'>chain</span> <span class='lbrace'>{</span> <span class='op'>|</span><span class='id identifier rubyid_fulfilled'>fulfilled</span><span class='comma'>,</span> <span class='id identifier rubyid_value'>value</span><span class='comma'>,</span> <span class='id identifier rubyid_reason'>reason</span><span class='op'>|</span> <span class='id identifier rubyid_fulfilled'>fulfilled</span> <span class='op'>?</span> <span class='id identifier rubyid_value'>value</span> <span class='op'>:</span> <span class='id identifier rubyid_reason'>reason</span> <span class='rbrace'>}</span><span class='period'>.</span>
<span class='id identifier rubyid_value!'>value!</span> <span class='comment'># => 1
</span><span class='const'><span class='object_link'><a href="Concurrent.html" title="Concurrent (module)">Concurrent</a></span></span><span class='op'>::</span><span class='const'><span class='object_link'><a href="Concurrent/Promises.html" title="Concurrent::Promises (module)">Promises</a></span></span><span class='period'>.</span>
<span class='id identifier rubyid_rejected_future'>rejected_future</span><span class='lparen'>(</span><span class='const'>StandardError</span><span class='period'>.</span><span class='id identifier rubyid_new'>new</span><span class='lparen'>(</span><span class='tstring'><span class='tstring_beg'>'</span><span class='tstring_content'>Ups</span><span class='tstring_end'>'</span></span><span class='rparen'>)</span><span class='rparen'>)</span><span class='period'>.</span>
<span class='id identifier rubyid_chain'>chain</span> <span class='lbrace'>{</span> <span class='op'>|</span><span class='id identifier rubyid_fulfilled'>fulfilled</span><span class='comma'>,</span> <span class='id identifier rubyid_value'>value</span><span class='comma'>,</span> <span class='id identifier rubyid_reason'>reason</span><span class='op'>|</span> <span class='id identifier rubyid_fulfilled'>fulfilled</span> <span class='op'>?</span> <span class='id identifier rubyid_value'>value</span> <span class='op'>:</span> <span class='id identifier rubyid_reason'>reason</span> <span class='rbrace'>}</span><span class='period'>.</span>
<span class='id identifier rubyid_value!'>value!</span> <span class='comment'># => #<StandardError: Ups>
</span></code></pre>
<p>Zip is rejected if any of the zipped futures is.</p>
<pre class="code ruby"><code class="ruby"><span class='id identifier rubyid_rejected_zip'>rejected_zip</span> <span class='op'>=</span> <span class='const'><span class='object_link'><a href="Concurrent.html" title="Concurrent (module)">Concurrent</a></span></span><span class='op'>::</span><span class='const'><span class='object_link'><a href="Concurrent/Promises.html" title="Concurrent::Promises (module)">Promises</a></span></span><span class='period'>.</span><span class='id identifier rubyid_zip'>zip</span><span class='lparen'>(</span>
<span class='const'><span class='object_link'><a href="Concurrent.html" title="Concurrent (module)">Concurrent</a></span></span><span class='op'>::</span><span class='const'><span class='object_link'><a href="Concurrent/Promises.html" title="Concurrent::Promises (module)">Promises</a></span></span><span class='period'>.</span><span class='id identifier rubyid_fulfilled_future'><span class='object_link'><a href="Concurrent/Promises/FactoryMethods.html#fulfilled_future-instance_method" title="Concurrent::Promises::FactoryMethods#fulfilled_future (method)">fulfilled_future</a></span></span><span class='lparen'>(</span><span class='int'>1</span><span class='rparen'>)</span><span class='comma'>,</span>
<span class='const'><span class='object_link'><a href="Concurrent.html" title="Concurrent (module)">Concurrent</a></span></span><span class='op'>::</span><span class='const'><span class='object_link'><a href="Concurrent/Promises.html" title="Concurrent::Promises (module)">Promises</a></span></span><span class='period'>.</span><span class='id identifier rubyid_rejected_future'><span class='object_link'><a href="Concurrent/Promises/FactoryMethods.html#rejected_future-instance_method" title="Concurrent::Promises::FactoryMethods#rejected_future (method)">rejected_future</a></span></span><span class='lparen'>(</span><span class='const'>StandardError</span><span class='period'>.</span><span class='id identifier rubyid_new'>new</span><span class='lparen'>(</span><span class='tstring'><span class='tstring_beg'>'</span><span class='tstring_content'>Ups</span><span class='tstring_end'>'</span></span><span class='rparen'>)</span><span class='rparen'>)</span><span class='rparen'>)</span>
<span class='comment'># => #<Concurrent::Promises::Future:0x000011 rejected>
</span><span class='id identifier rubyid_rejected_zip'>rejected_zip</span><span class='period'>.</span><span class='id identifier rubyid_result'>result</span>
<span class='comment'># => [false, [1, nil], [nil, #<StandardError: Ups>]]
</span><span class='id identifier rubyid_rejected_zip'>rejected_zip</span><span class='period'>.</span>
<span class='id identifier rubyid_rescue'>rescue</span> <span class='lbrace'>{</span> <span class='op'>|</span><span class='id identifier rubyid_reason1'>reason1</span><span class='comma'>,</span> <span class='id identifier rubyid_reason2'>reason2</span><span class='op'>|</span> <span class='lparen'>(</span><span class='id identifier rubyid_reason1'>reason1</span> <span class='op'>||</span> <span class='id identifier rubyid_reason2'>reason2</span><span class='rparen'>)</span><span class='period'>.</span><span class='id identifier rubyid_message'>message</span> <span class='rbrace'>}</span><span class='period'>.</span>
<span class='id identifier rubyid_value'>value</span> <span class='comment'># => "Ups"
</span></code></pre>
<h2>Delayed futures</h2>
<p>Delayed futures will not evaluate until asked by <code>touch</code> or other method
requiring resolution. </p>
<pre class="code ruby"><code class="ruby"><span class='id identifier rubyid_future'>future</span> <span class='op'>=</span> <span class='const'><span class='object_link'><a href="Concurrent.html" title="Concurrent (module)">Concurrent</a></span></span><span class='op'>::</span><span class='const'><span class='object_link'><a href="Concurrent/Promises.html" title="Concurrent::Promises (module)">Promises</a></span></span><span class='period'>.</span><span class='id identifier rubyid_delay'><span class='object_link'><a href="Concurrent/Promises/FactoryMethods.html#delay-instance_method" title="Concurrent::Promises::FactoryMethods#delay (method)">delay</a></span></span> <span class='lbrace'>{</span> <span class='id identifier rubyid_sleep'>sleep</span> <span class='float'>0.1</span><span class='semicolon'>;</span> <span class='tstring'><span class='tstring_beg'>'</span><span class='tstring_content'>lazy</span><span class='tstring_end'>'</span></span> <span class='rbrace'>}</span>
<span class='comment'># => #<Concurrent::Promises::Future:0x000012 pending>
</span><span class='id identifier rubyid_sleep'>sleep</span> <span class='float'>0.1</span>
<span class='id identifier rubyid_future'>future</span><span class='period'>.</span><span class='id identifier rubyid_resolved?'>resolved?</span> <span class='comment'># => false
</span><span class='id identifier rubyid_future'>future</span><span class='period'>.</span><span class='id identifier rubyid_touch'>touch</span>
<span class='comment'># => #<Concurrent::Promises::Future:0x000012 pending>
</span><span class='id identifier rubyid_sleep'>sleep</span> <span class='float'>0.2</span>
<span class='id identifier rubyid_future'>future</span><span class='period'>.</span><span class='id identifier rubyid_resolved?'>resolved?</span> <span class='comment'># => true
</span></code></pre>
<p>All blocking methods like <code>wait</code>, <code>value</code> call <code>touch</code> and trigger evaluation.</p>
<pre class="code ruby"><code class="ruby"><span class='const'><span class='object_link'><a href="Concurrent.html" title="Concurrent (module)">Concurrent</a></span></span><span class='op'>::</span><span class='const'><span class='object_link'><a href="Concurrent/Promises.html" title="Concurrent::Promises (module)">Promises</a></span></span><span class='period'>.</span><span class='id identifier rubyid_delay'><span class='object_link'><a href="Concurrent/Promises/FactoryMethods.html#delay-instance_method" title="Concurrent::Promises::FactoryMethods#delay (method)">delay</a></span></span> <span class='lbrace'>{</span> <span class='symbol'>:value</span> <span class='rbrace'>}</span><span class='period'>.</span><span class='id identifier rubyid_value'>value</span>
<span class='comment'># => :value
</span></code></pre>
<p>It propagates up through the chain, allowing whole or partial lazy chains.</p>
<pre class="code ruby"><code class="ruby"><span class='id identifier rubyid_head'>head</span> <span class='op'>=</span> <span class='const'><span class='object_link'><a href="Concurrent.html" title="Concurrent (module)">Concurrent</a></span></span><span class='op'>::</span><span class='const'><span class='object_link'><a href="Concurrent/Promises.html" title="Concurrent::Promises (module)">Promises</a></span></span><span class='period'>.</span><span class='id identifier rubyid_delay'><span class='object_link'><a href="Concurrent/Promises/FactoryMethods.html#delay-instance_method" title="Concurrent::Promises::FactoryMethods#delay (method)">delay</a></span></span> <span class='lbrace'>{</span> <span class='int'>1</span> <span class='rbrace'>}</span>
<span class='id identifier rubyid_branch1'>branch1</span> <span class='op'>=</span> <span class='id identifier rubyid_head'>head</span><span class='period'>.</span><span class='id identifier rubyid_then'>then</span><span class='lparen'>(</span><span class='op'>&</span><span class='symbol'>:succ</span><span class='rparen'>)</span>
<span class='id identifier rubyid_branch2'>branch2</span> <span class='op'>=</span> <span class='id identifier rubyid_head'>head</span><span class='period'>.</span><span class='id identifier rubyid_delay'>delay</span><span class='period'>.</span><span class='id identifier rubyid_then'>then</span><span class='lparen'>(</span><span class='op'>&</span><span class='symbol'>:succ</span><span class='rparen'>)</span>
<span class='id identifier rubyid_join'>join</span> <span class='op'>=</span> <span class='id identifier rubyid_branch1'>branch1</span> <span class='op'>&</span> <span class='id identifier rubyid_branch2'>branch2</span>
<span class='id identifier rubyid_sleep'>sleep</span> <span class='float'>0.1</span>
</code></pre>
<p>Nothing resolves.</p>
<pre class="code ruby"><code class="ruby"><span class='lbracket'>[</span><span class='id identifier rubyid_head'>head</span><span class='comma'>,</span> <span class='id identifier rubyid_branch1'>branch1</span><span class='comma'>,</span> <span class='id identifier rubyid_branch2'>branch2</span><span class='comma'>,</span> <span class='id identifier rubyid_join'>join</span><span class='rbracket'>]</span><span class='period'>.</span><span class='id identifier rubyid_map'>map</span><span class='lparen'>(</span><span class='op'>&</span><span class='symbol'>:resolved?</span><span class='rparen'>)</span>
<span class='comment'># => [false, false, false, false]
</span></code></pre>
<p>Force <code>branch1</code> evaluation.</p>
<pre class="code ruby"><code class="ruby"><span class='id identifier rubyid_branch1'>branch1</span><span class='period'>.</span><span class='id identifier rubyid_value'>value</span> <span class='comment'># => 2
</span><span class='id identifier rubyid_sleep'>sleep</span> <span class='float'>0.1</span>
<span class='lbracket'>[</span><span class='id identifier rubyid_head'>head</span><span class='comma'>,</span> <span class='id identifier rubyid_branch1'>branch1</span><span class='comma'>,</span> <span class='id identifier rubyid_branch2'>branch2</span><span class='comma'>,</span> <span class='id identifier rubyid_join'>join</span><span class='rbracket'>]</span><span class='period'>.</span><span class='id identifier rubyid_map'>map</span><span class='lparen'>(</span><span class='op'>&</span><span class='symbol'>:resolved?</span><span class='rparen'>)</span>
<span class='comment'># => [true, true, false, false]
</span></code></pre>
<p>Force evaluation of both by calling <code>value</code> on <code>join</code>.</p>
<pre class="code ruby"><code class="ruby"><span class='id identifier rubyid_join'>join</span><span class='period'>.</span><span class='id identifier rubyid_value'>value</span> <span class='comment'># => [2, 2]
</span><span class='lbracket'>[</span><span class='id identifier rubyid_head'>head</span><span class='comma'>,</span> <span class='id identifier rubyid_branch1'>branch1</span><span class='comma'>,</span> <span class='id identifier rubyid_branch2'>branch2</span><span class='comma'>,</span> <span class='id identifier rubyid_join'>join</span><span class='rbracket'>]</span><span class='period'>.</span><span class='id identifier rubyid_map'>map</span><span class='lparen'>(</span><span class='op'>&</span><span class='symbol'>:resolved?</span><span class='rparen'>)</span>
<span class='comment'># => [true, true, true, true]
</span></code></pre>
<h2>Flatting</h2>
<p>Sometimes it is needed to wait for an inner future. An apparent solution is to wait
inside the future <code>Concurrent::Promises.future { Concurrent::Promises.future { 1+1 }.value }.value</code>.
However, as mentioned before, <code>value</code> calls should be <strong>avoided</strong> to avoid
blocking threads. Therefore there is a <code>#flat</code> method which is a correct solution
in this situation and does not block any thread.</p>
<pre class="code ruby"><code class="ruby"><span class='const'><span class='object_link'><a href="Concurrent.html" title="Concurrent (module)">Concurrent</a></span></span><span class='op'>::</span><span class='const'><span class='object_link'><a href="Concurrent/Promises.html" title="Concurrent::Promises (module)">Promises</a></span></span><span class='period'>.</span><span class='id identifier rubyid_future'><span class='object_link'><a href="Concurrent/Promises/FactoryMethods.html#future-instance_method" title="Concurrent::Promises::FactoryMethods#future (method)">future</a></span></span> <span class='lbrace'>{</span> <span class='const'><span class='object_link'><a href="Concurrent.html" title="Concurrent (module)">Concurrent</a></span></span><span class='op'>::</span><span class='const'><span class='object_link'><a href="Concurrent/Promises.html" title="Concurrent::Promises (module)">Promises</a></span></span><span class='period'>.</span><span class='id identifier rubyid_future'><span class='object_link'><a href="Concurrent/Promises/FactoryMethods.html#future-instance_method" title="Concurrent::Promises::FactoryMethods#future (method)">future</a></span></span> <span class='lbrace'>{</span> <span class='int'>1</span><span class='op'>+</span><span class='int'>1</span> <span class='rbrace'>}</span> <span class='rbrace'>}</span><span class='period'>.</span><span class='id identifier rubyid_flat'>flat</span><span class='period'>.</span><span class='id identifier rubyid_value!'>value!</span>
<span class='comment'># => 2
</span></code></pre>
<p>A more complicated example.</p>
<pre class="code ruby"><code class="ruby"><span class='const'><span class='object_link'><a href="Concurrent.html" title="Concurrent (module)">Concurrent</a></span></span><span class='op'>::</span><span class='const'><span class='object_link'><a href="Concurrent/Promises.html" title="Concurrent::Promises (module)">Promises</a></span></span><span class='period'>.</span>
<span class='id identifier rubyid_future'>future</span> <span class='lbrace'>{</span> <span class='const'><span class='object_link'><a href="Concurrent.html" title="Concurrent (module)">Concurrent</a></span></span><span class='op'>::</span><span class='const'><span class='object_link'><a href="Concurrent/Promises.html" title="Concurrent::Promises (module)">Promises</a></span></span><span class='period'>.</span><span class='id identifier rubyid_future'><span class='object_link'><a href="Concurrent/Promises/FactoryMethods.html#future-instance_method" title="Concurrent::Promises::FactoryMethods#future (method)">future</a></span></span> <span class='lbrace'>{</span> <span class='const'><span class='object_link'><a href="Concurrent.html" title="Concurrent (module)">Concurrent</a></span></span><span class='op'>::</span><span class='const'><span class='object_link'><a href="Concurrent/Promises.html" title="Concurrent::Promises (module)">Promises</a></span></span><span class='period'>.</span><span class='id identifier rubyid_future'><span class='object_link'><a href="Concurrent/Promises/FactoryMethods.html#future-instance_method" title="Concurrent::Promises::FactoryMethods#future (method)">future</a></span></span> <span class='lbrace'>{</span> <span class='int'>1</span> <span class='op'>+</span> <span class='int'>1</span> <span class='rbrace'>}</span> <span class='rbrace'>}</span> <span class='rbrace'>}</span><span class='period'>.</span>
<span class='id identifier rubyid_flat'>flat</span><span class='lparen'>(</span><span class='int'>1</span><span class='rparen'>)</span><span class='period'>.</span>
<span class='id identifier rubyid_then'>then</span> <span class='lbrace'>{</span> <span class='op'>|</span><span class='id identifier rubyid_future'>future</span><span class='op'>|</span> <span class='id identifier rubyid_future'>future</span><span class='period'>.</span><span class='id identifier rubyid_then'>then</span><span class='lparen'>(</span><span class='op'>&</span><span class='symbol'>:succ</span><span class='rparen'>)</span> <span class='rbrace'>}</span><span class='period'>.</span>
<span class='id identifier rubyid_flat'>flat</span><span class='lparen'>(</span><span class='int'>1</span><span class='rparen'>)</span><span class='period'>.</span>
<span class='id identifier rubyid_value!'>value!</span> <span class='comment'># => 3
</span></code></pre>
<h2>Scheduling</h2>
<p>Tasks can be planned to be executed with a time delay.</p>
<p>Schedule task to be executed in 0.1 seconds.</p>
<pre class="code ruby"><code class="ruby"><span class='id identifier rubyid_scheduled'>scheduled</span> <span class='op'>=</span> <span class='const'><span class='object_link'><a href="Concurrent.html" title="Concurrent (module)">Concurrent</a></span></span><span class='op'>::</span><span class='const'><span class='object_link'><a href="Concurrent/Promises.html" title="Concurrent::Promises (module)">Promises</a></span></span><span class='period'>.</span><span class='id identifier rubyid_schedule'><span class='object_link'><a href="Concurrent/Promises/FactoryMethods.html#schedule-instance_method" title="Concurrent::Promises::FactoryMethods#schedule (method)">schedule</a></span></span><span class='lparen'>(</span><span class='float'>0.1</span><span class='rparen'>)</span> <span class='lbrace'>{</span> <span class='int'>1</span> <span class='rbrace'>}</span>
<span class='comment'># => #<Concurrent::Promises::Future:0x000013 pending>
</span><span class='id identifier rubyid_scheduled'>scheduled</span><span class='period'>.</span><span class='id identifier rubyid_resolved?'>resolved?</span> <span class='comment'># => false
</span></code></pre>
<p>Value will become available after 0.1 seconds. </p>
<pre class="code ruby"><code class="ruby"><span class='id identifier rubyid_scheduled'>scheduled</span><span class='period'>.</span><span class='id identifier rubyid_value'>value</span> <span class='comment'># => 1
</span></code></pre>
<p>It can be used in the chain as well, where the delay is counted from the moment
its parent resolves. Therefore, the following future will be resolved in 0.2 seconds.</p>
<pre class="code ruby"><code class="ruby"><span class='id identifier rubyid_future'>future</span> <span class='op'>=</span> <span class='const'><span class='object_link'><a href="Concurrent.html" title="Concurrent (module)">Concurrent</a></span></span><span class='op'>::</span><span class='const'><span class='object_link'><a href="Concurrent/Promises.html" title="Concurrent::Promises (module)">Promises</a></span></span><span class='period'>.</span>
<span class='id identifier rubyid_future'>future</span> <span class='lbrace'>{</span> <span class='id identifier rubyid_sleep'>sleep</span> <span class='float'>0.1</span><span class='semicolon'>;</span> <span class='symbol'>:result</span> <span class='rbrace'>}</span><span class='period'>.</span>
<span class='id identifier rubyid_schedule'>schedule</span><span class='lparen'>(</span><span class='float'>0.1</span><span class='rparen'>)</span><span class='period'>.</span>
<span class='id identifier rubyid_then'>then</span><span class='lparen'>(</span><span class='op'>&</span><span class='symbol'>:to_s</span><span class='rparen'>)</span><span class='period'>.</span>
<span class='id identifier rubyid_value!'>value!</span> <span class='comment'># => "result"
</span></code></pre>
<p>Time can be used as well.</p>
<pre class="code ruby"><code class="ruby"><span class='const'><span class='object_link'><a href="Concurrent.html" title="Concurrent (module)">Concurrent</a></span></span><span class='op'>::</span><span class='const'><span class='object_link'><a href="Concurrent/Promises.html" title="Concurrent::Promises (module)">Promises</a></span></span><span class='period'>.</span><span class='id identifier rubyid_schedule'><span class='object_link'><a href="Concurrent/Promises/FactoryMethods.html#schedule-instance_method" title="Concurrent::Promises::FactoryMethods#schedule (method)">schedule</a></span></span><span class='lparen'>(</span><span class='const'>Time</span><span class='period'>.</span><span class='id identifier rubyid_now'>now</span> <span class='op'>+</span> <span class='int'>10</span><span class='rparen'>)</span> <span class='lbrace'>{</span> <span class='symbol'>:val</span> <span class='rbrace'>}</span>
<span class='comment'># => #<Concurrent::Promises::Future:0x000014 pending>
</span></code></pre>
<h2>Resolvable Future and Event:</h2>
<p>Sometimes it is required to resolve a future externally, in these cases
<code>resolvable_future</code> and <code>resolvable_event</code> factory methods can be used. See
<span class='object_link'><a href="Concurrent/Promises/ResolvableFuture.html" title="Concurrent::Promises::ResolvableFuture (class)">Concurrent::Promises::ResolvableFuture</a></span> and
<span class='object_link'><a href="Concurrent/Promises/ResolvableEvent.html" title="Concurrent::Promises::ResolvableEvent (class)">Concurrent::Promises::ResolvableEvent</a></span>.</p>
<pre class="code ruby"><code class="ruby"><span class='id identifier rubyid_future'>future</span> <span class='op'>=</span> <span class='const'><span class='object_link'><a href="Concurrent.html" title="Concurrent (module)">Concurrent</a></span></span><span class='op'>::</span><span class='const'><span class='object_link'><a href="Concurrent/Promises.html" title="Concurrent::Promises (module)">Promises</a></span></span><span class='period'>.</span><span class='id identifier rubyid_resolvable_future'><span class='object_link'><a href="Concurrent/Promises/FactoryMethods.html#resolvable_future-instance_method" title="Concurrent::Promises::FactoryMethods#resolvable_future (method)">resolvable_future</a></span></span>
<span class='comment'># => #<Concurrent::Promises::ResolvableFuture:0x000015 pending>
</span></code></pre>
<p>The thread will be blocked until the future is resolved</p>
<pre class="code ruby"><code class="ruby"><span class='id identifier rubyid_thread'>thread</span> <span class='op'>=</span> <span class='const'>Thread</span><span class='period'>.</span><span class='id identifier rubyid_new'>new</span> <span class='lbrace'>{</span> <span class='id identifier rubyid_future'>future</span><span class='period'>.</span><span class='id identifier rubyid_value'>value</span> <span class='rbrace'>}</span>
<span class='id identifier rubyid_future'>future</span><span class='period'>.</span><span class='id identifier rubyid_fulfill'>fulfill</span> <span class='int'>1</span>
<span class='comment'># => #<Concurrent::Promises::ResolvableFuture:0x000015 fulfilled>
</span><span class='id identifier rubyid_thread'>thread</span><span class='period'>.</span><span class='id identifier rubyid_value'>value</span> <span class='comment'># => 1
</span></code></pre>
<p>A future can be resolved only once.</p>
<pre class="code ruby"><code class="ruby"><span class='id identifier rubyid_future'>future</span><span class='period'>.</span><span class='id identifier rubyid_fulfill'>fulfill</span> <span class='int'>1</span> <span class='kw'>rescue</span> <span class='gvar'>$!</span>
<span class='comment'># => #<Concurrent::MultipleAssignmentError: Future can be resolved only once. It's [true, 1, nil], trying to set [true, 1, nil]. {:current_result=>[true, 1, nil], :new_result=>[true, 1, nil]}>
</span><span class='id identifier rubyid_future'>future</span><span class='period'>.</span><span class='id identifier rubyid_fulfill'>fulfill</span> <span class='int'>2</span><span class='comma'>,</span> <span class='kw'>false</span> <span class='comment'># => false
</span></code></pre>
<h2>How are promises executed?</h2>
<p>Promises use global pools to execute the tasks. Therefore each task may run on
different threads which implies that users have to be careful not to depend on
Thread-local variables (or they have to be set at the beginning of the task and
cleaned up at the end of the task).</p>
<p>Since the tasks are running on may different threads of the thread pool, it's
better to follow following rules:</p>
<ul>
<li> Use only data passed via arguments or values of parent futures, to
have better control over what are futures accessing.</li>
<li> The data passed in and out of futures is easier to deal with if it is
immutable or at least treated as such.</li>
<li> Any mutable and mutated object accessed by more than one thread or future
must be thread-safe, see <span class='object_link'><a href="Concurrent/Array.html" title="Concurrent::Array (class)">Concurrent::Array</a></span>, <span class='object_link'><a href="Concurrent/Hash.html" title="Concurrent::Hash (class)">Concurrent::Hash</a></span>, and
<span class='object_link'><a href="Concurrent/Map.html" title="Concurrent::Map (class)">Concurrent::Map</a></span>. (The value of a future may be consumed by many futures.)</li>
<li> Futures can access outside objects, but they have to be thread-safe.</li>
</ul>
<blockquote>
<p><em>TODO: This part to be extended</em></p>
</blockquote>
<h1>Advanced</h1>
<h2>Callbacks</h2>
<pre class="code ruby"><code class="ruby"><span class='id identifier rubyid_queue'>queue</span> <span class='op'>=</span> <span class='const'>Queue</span><span class='period'>.</span><span class='id identifier rubyid_new'>new</span> <span class='comment'># => #<Thread::Queue:0x000016>
</span><span class='id identifier rubyid_future'>future</span> <span class='op'>=</span> <span class='const'><span class='object_link'><a href="Concurrent.html" title="Concurrent (module)">Concurrent</a></span></span><span class='op'>::</span><span class='const'><span class='object_link'><a href="Concurrent/Promises.html" title="Concurrent::Promises (module)">Promises</a></span></span><span class='period'>.</span><span class='id identifier rubyid_delay'><span class='object_link'><a href="Concurrent/Promises/FactoryMethods.html#delay-instance_method" title="Concurrent::Promises::FactoryMethods#delay (method)">delay</a></span></span> <span class='lbrace'>{</span> <span class='int'>1</span> <span class='op'>+</span> <span class='int'>1</span> <span class='rbrace'>}</span>
<span class='comment'># => #<Concurrent::Promises::Future:0x000017 pending>
</span>
<span class='id identifier rubyid_future'>future</span><span class='period'>.</span><span class='id identifier rubyid_on_fulfillment'>on_fulfillment</span> <span class='lbrace'>{</span> <span class='id identifier rubyid_queue'>queue</span> <span class='op'><<</span> <span class='int'>1</span> <span class='rbrace'>}</span> <span class='comment'># evaluated asynchronously
</span><span class='id identifier rubyid_future'>future</span><span class='period'>.</span><span class='id identifier rubyid_on_fulfillment!'>on_fulfillment!</span> <span class='lbrace'>{</span> <span class='id identifier rubyid_queue'>queue</span> <span class='op'><<</span> <span class='int'>2</span> <span class='rbrace'>}</span> <span class='comment'># evaluated on resolving thread
</span>
<span class='id identifier rubyid_queue'>queue</span><span class='period'>.</span><span class='id identifier rubyid_empty?'>empty?</span> <span class='comment'># => true
</span><span class='id identifier rubyid_future'>future</span><span class='period'>.</span><span class='id identifier rubyid_value'>value</span> <span class='comment'># => 2
</span><span class='id identifier rubyid_queue'>queue</span><span class='period'>.</span><span class='id identifier rubyid_pop'>pop</span> <span class='comment'># => 2
</span><span class='id identifier rubyid_queue'>queue</span><span class='period'>.</span><span class='id identifier rubyid_pop'>pop</span> <span class='comment'># => 1
</span></code></pre>
<h2>Using executors</h2>
<p>Factory methods, chain, and callback methods all have other versions of them
which takes an executor argument.</p>
<p>It takes an instance of an executor, or a symbol which is a shortcut for the
two global pools in concurrent-ruby. <code>:fast</code> for short and non-blocking tasks
and <code>:io</code> for long-running and blocking tasks.</p>
<pre class="code ruby"><code class="ruby"><span class='const'><span class='object_link'><a href="Concurrent.html" title="Concurrent (module)">Concurrent</a></span></span><span class='op'>::</span><span class='const'><span class='object_link'><a href="Concurrent/Promises.html" title="Concurrent::Promises (module)">Promises</a></span></span><span class='period'>.</span><span class='id identifier rubyid_future_on'><span class='object_link'><a href="Concurrent/Promises/FactoryMethods.html#future_on-instance_method" title="Concurrent::Promises::FactoryMethods#future_on (method)">future_on</a></span></span><span class='lparen'>(</span><span class='symbol'>:fast</span><span class='rparen'>)</span> <span class='lbrace'>{</span> <span class='int'>2</span> <span class='rbrace'>}</span><span class='period'>.</span>
<span class='id identifier rubyid_then_on'>then_on</span><span class='lparen'>(</span><span class='symbol'>:io</span><span class='rparen'>)</span> <span class='lbrace'>{</span> <span class='const'>File</span><span class='period'>.</span><span class='id identifier rubyid_read'>read</span> <span class='kw'>__FILE__</span> <span class='rbrace'>}</span><span class='period'>.</span>
<span class='id identifier rubyid_value'>value</span><span class='period'>.</span><span class='id identifier rubyid_size'>size</span> <span class='comment'># => 27115
</span></code></pre>
<h2>Run (simulated process)</h2>
<p>Similar to flatting is running. When <code>run</code> is called on a future it will flat
indefinitely as long the future fulfils into a <code>Future</code> value. It can be used
to simulate a thread-like processing without actually occupying the thread.</p>
<pre class="code ruby"><code class="ruby"><span class='id identifier rubyid_count'>count</span> <span class='op'>=</span> <span class='id identifier rubyid_lambda'>lambda</span> <span class='kw'>do</span> <span class='op'>|</span><span class='id identifier rubyid_v'>v</span><span class='op'>|</span>
<span class='id identifier rubyid_v'>v</span> <span class='op'>+=</span> <span class='int'>1</span>
<span class='id identifier rubyid_v'>v</span> <span class='op'><</span> <span class='int'>5</span> <span class='op'>?</span> <span class='const'><span class='object_link'><a href="Concurrent.html" title="Concurrent (module)">Concurrent</a></span></span><span class='op'>::</span><span class='const'><span class='object_link'><a href="Concurrent/Promises.html" title="Concurrent::Promises (module)">Promises</a></span></span><span class='period'>.</span><span class='id identifier rubyid_future_on'><span class='object_link'><a href="Concurrent/Promises/FactoryMethods.html#future_on-instance_method" title="Concurrent::Promises::FactoryMethods#future_on (method)">future_on</a></span></span><span class='lparen'>(</span><span class='symbol'>:fast</span><span class='comma'>,</span> <span class='id identifier rubyid_v'>v</span><span class='comma'>,</span> <span class='op'>&</span><span class='id identifier rubyid_count'>count</span><span class='rparen'>)</span> <span class='op'>:</span> <span class='id identifier rubyid_v'>v</span>
<span class='kw'>end</span>
<span class='comment'># => #<Proc:0x000018@promises.in.md:520 (lambda)>
</span><span class='int'>400</span><span class='period'>.</span><span class='id identifier rubyid_times'>times</span><span class='period'>.</span>
<span class='id identifier rubyid_map'>map</span> <span class='lbrace'>{</span> <span class='const'><span class='object_link'><a href="Concurrent.html" title="Concurrent (module)">Concurrent</a></span></span><span class='op'>::</span><span class='const'><span class='object_link'><a href="Concurrent/Promises.html" title="Concurrent::Promises (module)">Promises</a></span></span><span class='period'>.</span><span class='id identifier rubyid_future_on'><span class='object_link'><a href="Concurrent/Promises/FactoryMethods.html#future_on-instance_method" title="Concurrent::Promises::FactoryMethods#future_on (method)">future_on</a></span></span><span class='lparen'>(</span><span class='symbol'>:fast</span><span class='comma'>,</span> <span class='int'>0</span><span class='comma'>,</span> <span class='op'>&</span><span class='id identifier rubyid_count'>count</span><span class='rparen'>)</span><span class='period'>.</span><span class='id identifier rubyid_run'><span class='object_link'><a href="Concurrent/Promises/Future.html#run-instance_method" title="Concurrent::Promises::Future#run (method)">run</a></span></span><span class='period'>.</span><span class='id identifier rubyid_value!'><span class='object_link'><a href="Concurrent/Promises/Future.html#value!-instance_method" title="Concurrent::Promises::Future#value! (method)">value!</a></span></span> <span class='rbrace'>}</span><span class='period'>.</span>
<span class='id identifier rubyid_all?'>all?</span> <span class='lbrace'>{</span> <span class='op'>|</span><span class='id identifier rubyid_v'>v</span><span class='op'>|</span> <span class='id identifier rubyid_v'>v</span> <span class='op'>==</span> <span class='int'>5</span> <span class='rbrace'>}</span> <span class='comment'># => true
</span></code></pre>
<p>Therefore the above example finished fine on the the <code>:fast</code> thread pool even
though it has much fewer threads than are simulated in the simulated process.</p>
<h1>Interoperability</h1>
<h2>Actors</h2>
<p>Create an actor which takes received numbers and returns the number squared. </p>
<pre class="code ruby"><code class="ruby"><span class='id identifier rubyid_actor'>actor</span> <span class='op'>=</span> <span class='const'><span class='object_link'><a href="Concurrent.html" title="Concurrent (module)">Concurrent</a></span></span><span class='op'>::</span><span class='const'><span class='object_link'><a href="Concurrent/Actor.html" title="Concurrent::Actor (module)">Actor</a></span></span><span class='op'>::</span><span class='const'><span class='object_link'><a href="Concurrent/Actor/Utils.html" title="Concurrent::Actor::Utils (module)">Utils</a></span></span><span class='op'>::</span><span class='const'><span class='object_link'><a href="Concurrent/Actor/Utils/AdHoc.html" title="Concurrent::Actor::Utils::AdHoc (class)">AdHoc</a></span></span><span class='period'>.</span><span class='id identifier rubyid_spawn'><span class='object_link'><a href="Concurrent/Actor/AbstractContext.html#spawn-class_method" title="Concurrent::Actor::AbstractContext.spawn (method)">spawn</a></span></span> <span class='symbol'>:square</span> <span class='kw'>do</span>
<span class='tlambda'>-></span> <span class='id identifier rubyid_v'>v</span> <span class='tlambeg'>{</span> <span class='id identifier rubyid_v'>v</span> <span class='op'>**</span> <span class='int'>2</span> <span class='rbrace'>}</span>
<span class='kw'>end</span>
<span class='comment'># => #<Concurrent::Actor::Reference:0x000019 /square (Concurrent::Actor::Utils::AdHoc)>
</span></code></pre>
<p>Send result of <code>1+1</code> to the actor, and add 2 to the result sent back from the
actor.</p>
<pre class="code ruby"><code class="ruby"><span class='const'><span class='object_link'><a href="Concurrent.html" title="Concurrent (module)">Concurrent</a></span></span><span class='op'>::</span><span class='const'><span class='object_link'><a href="Concurrent/Promises.html" title="Concurrent::Promises (module)">Promises</a></span></span><span class='period'>.</span>
<span class='id identifier rubyid_future'>future</span> <span class='lbrace'>{</span> <span class='int'>1</span> <span class='op'>+</span> <span class='int'>1</span> <span class='rbrace'>}</span><span class='period'>.</span>
<span class='id identifier rubyid_then_ask'>then_ask</span><span class='lparen'>(</span><span class='id identifier rubyid_actor'>actor</span><span class='rparen'>)</span><span class='period'>.</span>
<span class='id identifier rubyid_then'>then</span> <span class='lbrace'>{</span> <span class='op'>|</span><span class='id identifier rubyid_v'>v</span><span class='op'>|</span> <span class='id identifier rubyid_v'>v</span> <span class='op'>+</span> <span class='int'>2</span> <span class='rbrace'>}</span><span class='period'>.</span>
<span class='id identifier rubyid_value!'>value!</span> <span class='comment'># => 6
</span></code></pre>
<p>So <code>(1 + 1)**2 + 2 = 6</code>.</p>
<p>The <code>ask</code> method returns future.</p>
<pre class="code ruby"><code class="ruby"><span class='id identifier rubyid_actor'>actor</span><span class='period'>.</span><span class='id identifier rubyid_ask'>ask</span><span class='lparen'>(</span><span class='int'>2</span><span class='rparen'>)</span><span class='period'>.</span><span class='id identifier rubyid_then'>then</span><span class='lparen'>(</span><span class='op'>&</span><span class='symbol'>:succ</span><span class='rparen'>)</span><span class='period'>.</span><span class='id identifier rubyid_value!'>value!</span> <span class='comment'># => 5
</span></code></pre>
<h2>Channel</h2>
<p>There is an implementation of channel as well. Let's start by creating a
channel with a capacity of 2 messages.</p>
<pre class="code ruby"><code class="ruby"><span class='id identifier rubyid_ch1'>ch1</span> <span class='op'>=</span> <span class='const'><span class='object_link'><a href="Concurrent.html" title="Concurrent (module)">Concurrent</a></span></span><span class='op'>::</span><span class='const'><span class='object_link'><a href="Concurrent/Promises.html" title="Concurrent::Promises (module)">Promises</a></span></span><span class='op'>::</span><span class='const'><span class='object_link'><a href="Concurrent/Promises/Channel.html" title="Concurrent::Promises::Channel (class)">Channel</a></span></span><span class='period'>.</span><span class='id identifier rubyid_new'><span class='object_link'><a href="Concurrent/Synchronization/Object.html#new-class_method" title="Concurrent::Synchronization::Object.new (method)">new</a></span></span> <span class='int'>2</span>
<span class='comment'># => #<Concurrent::Promises::Channel:0x00001a size:2>
</span></code></pre>
<p>We push 3 messages, it can be observed that the last future representing the
push is not fulfilled since the capacity prevents it. When the work which fills
the channel depends on the futures created by push it can be used to create
backpressure – the filling work is delayed until the channel has space for
more messages.</p>
<pre class="code ruby"><code class="ruby"><span class='id identifier rubyid_pushes'>pushes</span> <span class='op'>=</span> <span class='int'>3</span><span class='period'>.</span><span class='id identifier rubyid_times'>times</span><span class='period'>.</span><span class='id identifier rubyid_map'>map</span> <span class='lbrace'>{</span> <span class='op'>|</span><span class='id identifier rubyid_i'>i</span><span class='op'>|</span> <span class='id identifier rubyid_ch1'>ch1</span><span class='period'>.</span><span class='id identifier rubyid_push'>push</span> <span class='id identifier rubyid_i'>i</span> <span class='rbrace'>}</span>
<span class='comment'># => [#<Concurrent::Promises::Future:0x00001b fulfilled>,
</span><span class='comment'># #<Concurrent::Promises::Future:0x00001c fulfilled>,
</span><span class='comment'># #<Concurrent::Promises::Future:0x00001d pending>]
</span><span class='id identifier rubyid_ch1'>ch1</span><span class='period'>.</span><span class='id identifier rubyid_pop'>pop</span><span class='period'>.</span><span class='id identifier rubyid_value!'>value!</span> <span class='comment'># => 0
</span><span class='id identifier rubyid_pushes'>pushes</span>
<span class='comment'># => [#<Concurrent::Promises::Future:0x00001b fulfilled>,
</span><span class='comment'># #<Concurrent::Promises::Future:0x00001c fulfilled>,
</span><span class='comment'># #<Concurrent::Promises::Future:0x00001d fulfilled>]
</span></code></pre>
<p>A selection over channels can be created with the <code>.select_channel</code> factory method. It
will be fulfilled with a first message available in any of the channels. It
returns a pair to be able to find out which channel had the message available.</p>
<pre class="code ruby"><code class="ruby"><span class='id identifier rubyid_ch2'>ch2</span> <span class='op'>=</span> <span class='const'><span class='object_link'><a href="Concurrent.html" title="Concurrent (module)">Concurrent</a></span></span><span class='op'>::</span><span class='const'><span class='object_link'><a href="Concurrent/Promises.html" title="Concurrent::Promises (module)">Promises</a></span></span><span class='op'>::</span><span class='const'><span class='object_link'><a href="Concurrent/Promises/Channel.html" title="Concurrent::Promises::Channel (class)">Channel</a></span></span><span class='period'>.</span><span class='id identifier rubyid_new'><span class='object_link'><a href="Concurrent/Synchronization/Object.html#new-class_method" title="Concurrent::Synchronization::Object.new (method)">new</a></span></span> <span class='int'>2</span>
<span class='comment'># => #<Concurrent::Promises::Channel:0x00001e size:2>
</span><span class='id identifier rubyid_result'>result</span> <span class='op'>=</span> <span class='const'><span class='object_link'><a href="Concurrent.html" title="Concurrent (module)">Concurrent</a></span></span><span class='op'>::</span><span class='const'><span class='object_link'><a href="Concurrent/Promises.html" title="Concurrent::Promises (module)">Promises</a></span></span><span class='period'>.</span><span class='id identifier rubyid_select_channel'><span class='object_link'><a href="Concurrent/Promises/FactoryMethods/NewChannelIntegration.html#select_channel-instance_method" title="Concurrent::Promises::FactoryMethods::NewChannelIntegration#select_channel (method)">select_channel</a></span></span><span class='lparen'>(</span><span class='id identifier rubyid_ch1'>ch1</span><span class='comma'>,</span> <span class='id identifier rubyid_ch2'>ch2</span><span class='rparen'>)</span>
<span class='comment'># => #<Concurrent::Promises::ResolvableFuture:0x00001f fulfilled>
</span><span class='id identifier rubyid_result'>result</span><span class='period'>.</span><span class='id identifier rubyid_value!'>value!</span>
<span class='comment'># => [#<Concurrent::Promises::Channel:0x00001a size:2>, 1]
</span>
<span class='const'><span class='object_link'><a href="Concurrent.html" title="Concurrent (module)">Concurrent</a></span></span><span class='op'>::</span><span class='const'><span class='object_link'><a href="Concurrent/Promises.html" title="Concurrent::Promises (module)">Promises</a></span></span><span class='period'>.</span><span class='id identifier rubyid_future'><span class='object_link'><a href="Concurrent/Promises/FactoryMethods.html#future-instance_method" title="Concurrent::Promises::FactoryMethods#future (method)">future</a></span></span> <span class='lbrace'>{</span> <span class='int'>1</span><span class='op'>+</span><span class='int'>1</span> <span class='rbrace'>}</span><span class='period'>.</span><span class='id identifier rubyid_then_push_channel'>then_push_channel</span><span class='lparen'>(</span><span class='id identifier rubyid_ch1'>ch1</span><span class='rparen'>)</span>
<span class='comment'># => #<Concurrent::Promises::Future:0x000020 pending>
</span><span class='id identifier rubyid_result'>result</span> <span class='op'>=</span> <span class='lparen'>(</span>
<span class='const'><span class='object_link'><a href="Concurrent.html" title="Concurrent (module)">Concurrent</a></span></span><span class='op'>::</span><span class='const'><span class='object_link'><a href="Concurrent/Promises.html" title="Concurrent::Promises (module)">Promises</a></span></span><span class='period'>.</span><span class='id identifier rubyid_fulfilled_future'><span class='object_link'><a href="Concurrent/Promises/FactoryMethods.html#fulfilled_future-instance_method" title="Concurrent::Promises::FactoryMethods#fulfilled_future (method)">fulfilled_future</a></span></span><span class='lparen'>(</span><span class='tstring'><span class='tstring_beg'>'</span><span class='tstring_content'>%02d</span><span class='tstring_end'>'</span></span><span class='rparen'>)</span> <span class='op'>&</span>
<span class='const'><span class='object_link'><a href="Concurrent.html" title="Concurrent (module)">Concurrent</a></span></span><span class='op'>::</span><span class='const'><span class='object_link'><a href="Concurrent/Promises.html" title="Concurrent::Promises (module)">Promises</a></span></span><span class='period'>.</span><span class='id identifier rubyid_select_channel'><span class='object_link'><a href="Concurrent/Promises/FactoryMethods/NewChannelIntegration.html#select_channel-instance_method" title="Concurrent::Promises::FactoryMethods::NewChannelIntegration#select_channel (method)">select_channel</a></span></span><span class='lparen'>(</span><span class='id identifier rubyid_ch1'>ch1</span><span class='comma'>,</span> <span class='id identifier rubyid_ch2'>ch2</span><span class='rparen'>)</span><span class='rparen'>)</span><span class='period'>.</span>
<span class='id identifier rubyid_then'>then</span> <span class='lbrace'>{</span> <span class='op'>|</span><span class='id identifier rubyid_format'>format</span><span class='comma'>,</span> <span class='lparen'>(</span><span class='id identifier rubyid_channel'>channel</span><span class='comma'>,</span> <span class='id identifier rubyid_value'>value</span><span class='rparen'>)</span><span class='op'>|</span> <span class='id identifier rubyid_format'>format</span> <span class='id identifier rubyid_format'>format</span><span class='comma'>,</span> <span class='id identifier rubyid_value'>value</span> <span class='rbrace'>}</span>
<span class='comment'># => #<Concurrent::Promises::Future:0x000021 pending>
</span><span class='id identifier rubyid_result'>result</span><span class='period'>.</span><span class='id identifier rubyid_value!'>value!</span> <span class='comment'># => "02"
</span></code></pre>
<h2>ProcessingActor</h2>
<p>There is also a new implementation of actors based on the Channel and the
ability of promises to simulate processes. The actor runs as a process but also
does not occupy a thread per actor as the previously-described Concurrent::Actor
implementation. This implementation is close to Erlang actors, therefore OTP
can be ported for this actors (and it's planned).</p>
<p>The simplest actor is one which just computes without even receiving a
message.</p>
<pre class="code ruby"><code class="ruby"><span class='id identifier rubyid_actor'>actor</span> <span class='op'>=</span> <span class='const'><span class='object_link'><a href="Concurrent.html" title="Concurrent (module)">Concurrent</a></span></span><span class='op'>::</span><span class='const'><span class='object_link'><a href="Concurrent/ProcessingActor.html" title="Concurrent::ProcessingActor (class)">ProcessingActor</a></span></span><span class='period'>.</span><span class='id identifier rubyid_act'><span class='object_link'><a href="Concurrent/ProcessingActor.html#act-class_method" title="Concurrent::ProcessingActor.act (method)">act</a></span></span><span class='lparen'>(</span><span class='id identifier rubyid_an_argument'>an_argument</span> <span class='op'>=</span> <span class='int'>2</span><span class='rparen'>)</span> <span class='kw'>do</span> <span class='op'>|</span><span class='id identifier rubyid_actor'>actor</span><span class='comma'>,</span> <span class='id identifier rubyid_number'>number</span><span class='op'>|</span>
<span class='id identifier rubyid_number'>number</span> <span class='op'>**</span> <span class='int'>3</span>
<span class='kw'>end</span>
<span class='comment'># => #<Concurrent::ProcessingActor:0x000022 ... termination:pending>
</span><span class='id identifier rubyid_actor'>actor</span><span class='period'>.</span><span class='id identifier rubyid_termination'>termination</span><span class='period'>.</span><span class='id identifier rubyid_value!'>value!</span> <span class='comment'># => 8
</span></code></pre>
<p>Let's receive some messages though.</p>
<pre class="code ruby"><code class="ruby"><span class='id identifier rubyid_add_2_messages'>add_2_messages</span> <span class='op'>=</span> <span class='const'><span class='object_link'><a href="Concurrent.html" title="Concurrent (module)">Concurrent</a></span></span><span class='op'>::</span><span class='const'><span class='object_link'><a href="Concurrent/ProcessingActor.html" title="Concurrent::ProcessingActor (class)">ProcessingActor</a></span></span><span class='period'>.</span><span class='id identifier rubyid_act'><span class='object_link'><a href="Concurrent/ProcessingActor.html#act-class_method" title="Concurrent::ProcessingActor.act (method)">act</a></span></span> <span class='kw'>do</span> <span class='op'>|</span><span class='id identifier rubyid_actor'>actor</span><span class='op'>|</span>
<span class='comment'># Receive two messages then terminate normally with the sum.
</span> <span class='lparen'>(</span><span class='id identifier rubyid_actor'>actor</span><span class='period'>.</span><span class='id identifier rubyid_receive'>receive</span> <span class='op'>&</span> <span class='id identifier rubyid_actor'>actor</span><span class='period'>.</span><span class='id identifier rubyid_receive'>receive</span><span class='rparen'>)</span><span class='period'>.</span><span class='id identifier rubyid_then'>then</span> <span class='kw'>do</span> <span class='op'>|</span><span class='id identifier rubyid_a'>a</span><span class='comma'>,</span> <span class='id identifier rubyid_b'>b</span><span class='op'>|</span>
<span class='id identifier rubyid_a'>a</span> <span class='op'>+</span> <span class='id identifier rubyid_b'>b</span>
<span class='kw'>end</span>
<span class='kw'>end</span>
<span class='id identifier rubyid_add_2_messages'>add_2_messages</span><span class='period'>.</span><span class='id identifier rubyid_tell'>tell</span> <span class='int'>1</span>
<span class='comment'># => #<Concurrent::Promises::Future:0x000025 pending>
</span><span class='id identifier rubyid_add_2_messages'>add_2_messages</span><span class='period'>.</span><span class='id identifier rubyid_termination'>termination</span><span class='period'>.</span><span class='id identifier rubyid_resolved?'>resolved?</span> <span class='comment'># => false
</span><span class='id identifier rubyid_add_2_messages'>add_2_messages</span><span class='period'>.</span><span class='id identifier rubyid_tell'>tell</span> <span class='int'>3</span>
<span class='comment'># => #<Concurrent::Promises::Future:0x000026 pending>
</span><span class='id identifier rubyid_add_2_messages'>add_2_messages</span><span class='period'>.</span><span class='id identifier rubyid_termination'>termination</span><span class='period'>.</span><span class='id identifier rubyid_value!'>value!</span> <span class='comment'># => 4
</span></code></pre>
<p>Actors can also be used to apply backpressure to a producer. Let's start by
defining an actor which a mailbox of size 2.</p>
<pre class="code ruby"><code class="ruby"><span class='id identifier rubyid_slow_counter'>slow_counter</span> <span class='op'>=</span> <span class='tlambda'>-></span> <span class='lparen'>(</span><span class='id identifier rubyid_actor'>actor</span><span class='comma'>,</span> <span class='id identifier rubyid_count'>count</span><span class='rparen'>)</span> <span class='kw'>do</span>
<span class='id identifier rubyid_actor'>actor</span><span class='period'>.</span><span class='id identifier rubyid_receive'>receive</span><span class='period'>.</span><span class='id identifier rubyid_then'>then</span> <span class='kw'>do</span> <span class='op'>|</span><span class='id identifier rubyid_command'>command</span><span class='comma'>,</span> <span class='id identifier rubyid_number'>number</span><span class='op'>|</span>
<span class='id identifier rubyid_sleep'>sleep</span> <span class='float'>0.1</span>
<span class='kw'>case</span> <span class='id identifier rubyid_command'>command</span>
<span class='kw'>when</span> <span class='symbol'>:add</span>
<span class='id identifier rubyid_slow_counter'>slow_counter</span><span class='period'>.</span><span class='id identifier rubyid_call'>call</span> <span class='id identifier rubyid_actor'>actor</span><span class='comma'>,</span> <span class='id identifier rubyid_count'>count</span> <span class='op'>+</span> <span class='id identifier rubyid_number'>number</span>
<span class='kw'>when</span> <span class='symbol'>:done</span>
<span class='comment'># terminate
</span> <span class='id identifier rubyid_count'>count</span>
<span class='kw'>end</span>
<span class='kw'>end</span>
<span class='kw'>end</span>
<span class='id identifier rubyid_actor'>actor</span> <span class='op'>=</span> <span class='const'><span class='object_link'><a href="Concurrent.html" title="Concurrent (module)">Concurrent</a></span></span><span class='op'>::</span><span class='const'><span class='object_link'><a href="Concurrent/ProcessingActor.html" title="Concurrent::ProcessingActor (class)">ProcessingActor</a></span></span><span class='period'>.</span><span class='id identifier rubyid_act_listening'><span class='object_link'><a href="Concurrent/ProcessingActor.html#act_listening-class_method" title="Concurrent::ProcessingActor.act_listening (method)">act_listening</a></span></span><span class='lparen'>(</span>
<span class='const'><span class='object_link'><a href="Concurrent.html" title="Concurrent (module)">Concurrent</a></span></span><span class='op'>::</span><span class='const'><span class='object_link'><a href="Concurrent/Promises.html" title="Concurrent::Promises (module)">Promises</a></span></span><span class='op'>::</span><span class='const'><span class='object_link'><a href="Concurrent/Promises/Channel.html" title="Concurrent::Promises::Channel (class)">Channel</a></span></span><span class='period'>.</span><span class='id identifier rubyid_new'><span class='object_link'><a href="Concurrent/Synchronization/Object.html#new-class_method" title="Concurrent::Synchronization::Object.new (method)">new</a></span></span><span class='lparen'>(</span><span class='int'>2</span><span class='rparen'>)</span><span class='comma'>,</span>
<span class='int'>0</span><span class='comma'>,</span>
<span class='op'>&</span><span class='id identifier rubyid_slow_counter'>slow_counter</span><span class='rparen'>)</span>
<span class='comment'># => #<Concurrent::ProcessingActor:0x000027 ... termination:pending>
</span></code></pre>
<p>Now we can create a producer which will push messages only when there is a
space available in the mailbox. We use promises to free a thread during waiting
on a free space in the mailbox.</p>
<pre class="code ruby"><code class="ruby"><span class='id identifier rubyid_produce'>produce</span> <span class='op'>=</span> <span class='tlambda'>-></span> <span class='id identifier rubyid_receiver'>receiver</span><span class='comma'>,</span> <span class='id identifier rubyid_i'>i</span> <span class='kw'>do</span>
<span class='kw'>if</span> <span class='id identifier rubyid_i'>i</span> <span class='op'><</span> <span class='int'>10</span>
<span class='id identifier rubyid_receiver'>receiver</span><span class='period'>.</span>
<span class='comment'># send a message to the actor, resolves only after the message is
</span> <span class='comment'># accepted by the actor's mailbox
</span> <span class='id identifier rubyid_tell'>tell</span><span class='lparen'>(</span><span class='lbracket'>[</span><span class='symbol'>:add</span><span class='comma'>,</span> <span class='id identifier rubyid_i'>i</span><span class='rbracket'>]</span><span class='rparen'>)</span><span class='period'>.</span>
<span class='comment'># send incremented message when the above message is accepted
</span> <span class='id identifier rubyid_then'>then</span><span class='lparen'>(</span><span class='id identifier rubyid_i'>i</span><span class='op'>+</span><span class='int'>1</span><span class='comma'>,</span> <span class='op'>&</span><span class='id identifier rubyid_produce'>produce</span><span class='rparen'>)</span>
<span class='kw'>else</span>
<span class='id identifier rubyid_receiver'>receiver</span><span class='period'>.</span><span class='id identifier rubyid_tell'>tell</span><span class='lparen'>(</span><span class='symbol'>:done</span><span class='rparen'>)</span>
<span class='comment'># do not continue
</span> <span class='kw'>end</span>
<span class='kw'>end</span>
<span class='const'><span class='object_link'><a href="Concurrent.html" title="Concurrent (module)">Concurrent</a></span></span><span class='op'>::</span><span class='const'><span class='object_link'><a href="Concurrent/Promises.html" title="Concurrent::Promises (module)">Promises</a></span></span><span class='period'>.</span><span class='id identifier rubyid_future'><span class='object_link'><a href="Concurrent/Promises/FactoryMethods.html#future-instance_method" title="Concurrent::Promises::FactoryMethods#future (method)">future</a></span></span><span class='lparen'>(</span><span class='id identifier rubyid_actor'>actor</span><span class='comma'>,</span> <span class='int'>0</span><span class='comma'>,</span> <span class='op'>&</span><span class='id identifier rubyid_produce'>produce</span><span class='rparen'>)</span><span class='period'>.</span><span class='id identifier rubyid_run'><span class='object_link'><a href="Concurrent/Promises/Future.html#run-instance_method" title="Concurrent::Promises::Future#run (method)">run</a></span></span><span class='period'>.</span><span class='id identifier rubyid_wait!'><span class='object_link'><a href="Concurrent/Promises/Future.html#wait!-instance_method" title="Concurrent::Promises::Future#wait! (method)">wait!</a></span></span>
<span class='comment'># => #<Concurrent::Promises::Future:0x00002a fulfilled>
</span>
<span class='id identifier rubyid_actor'>actor</span><span class='period'>.</span><span class='id identifier rubyid_termination'>termination</span><span class='period'>.</span><span class='id identifier rubyid_value!'>value!</span> <span class='comment'># => 45
</span></code></pre>
<h1>Use-cases</h1>
<h2>Simple background processing</h2>
<pre class="code ruby"><code class="ruby"><span class='const'><span class='object_link'><a href="Concurrent.html" title="Concurrent (module)">Concurrent</a></span></span><span class='op'>::</span><span class='const'><span class='object_link'><a href="Concurrent/Promises.html" title="Concurrent::Promises (module)">Promises</a></span></span><span class='period'>.</span><span class='id identifier rubyid_future'><span class='object_link'><a href="Concurrent/Promises/FactoryMethods.html#future-instance_method" title="Concurrent::Promises::FactoryMethods#future (method)">future</a></span></span> <span class='lbrace'>{</span> <span class='id identifier rubyid_do_stuff'>do_stuff</span> <span class='rbrace'>}</span>
<span class='comment'># => #<Concurrent::Promises::Future:0x00002b pending>
</span></code></pre>
<h2>Parallel background processing</h2>
<pre class="code ruby"><code class="ruby"><span class='id identifier rubyid_tasks'>tasks</span> <span class='op'>=</span> <span class='int'>4</span><span class='period'>.</span><span class='id identifier rubyid_times'>times</span><span class='period'>.</span><span class='id identifier rubyid_map'>map</span> <span class='lbrace'>{</span> <span class='op'>|</span><span class='id identifier rubyid_i'>i</span><span class='op'>|</span> <span class='const'><span class='object_link'><a href="Concurrent.html" title="Concurrent (module)">Concurrent</a></span></span><span class='op'>::</span><span class='const'><span class='object_link'><a href="Concurrent/Promises.html" title="Concurrent::Promises (module)">Promises</a></span></span><span class='period'>.</span><span class='id identifier rubyid_future'><span class='object_link'><a href="Concurrent/Promises/FactoryMethods.html#future-instance_method" title="Concurrent::Promises::FactoryMethods#future (method)">future</a></span></span><span class='lparen'>(</span><span class='id identifier rubyid_i'>i</span><span class='rparen'>)</span> <span class='lbrace'>{</span> <span class='op'>|</span><span class='id identifier rubyid_i'>i</span><span class='op'>|</span> <span class='id identifier rubyid_i'>i</span><span class='op'>*</span><span class='int'>2</span> <span class='rbrace'>}</span> <span class='rbrace'>}</span>
<span class='comment'># => [#<Concurrent::Promises::Future:0x00002c pending>,
</span><span class='comment'># #<Concurrent::Promises::Future:0x00002d pending>,
</span><span class='comment'># #<Concurrent::Promises::Future:0x00002e pending>,
</span><span class='comment'># #<Concurrent::Promises::Future:0x00002f pending>]
</span><span class='const'><span class='object_link'><a href="Concurrent.html" title="Concurrent (module)">Concurrent</a></span></span><span class='op'>::</span><span class='const'><span class='object_link'><a href="Concurrent/Promises.html" title="Concurrent::Promises (module)">Promises</a></span></span><span class='period'>.</span><span class='id identifier rubyid_zip'>zip</span><span class='lparen'>(</span><span class='op'>*</span><span class='id identifier rubyid_tasks'>tasks</span><span class='rparen'>)</span><span class='period'>.</span><span class='id identifier rubyid_value!'><span class='object_link'><a href="Concurrent/Promises/Future.html#value!-instance_method" title="Concurrent::Promises::Future#value! (method)">value!</a></span></span>
<span class='comment'># => [0, 2, 4, 6]
</span></code></pre>
<h2>Actor background processing</h2>
<p>Actors are mainly keep and isolate state, they should stay responsive not being
blocked by a longer running computations. It desirable to offload the work to
stateless promises.</p>
<p>Lets define an actor which will process jobs, while staying responsive, and
tracking the number of tasks being processed.</p>
<pre class="code ruby"><code class="ruby"><span class='kw'>class</span> <span class='const'>Computer</span> <span class='op'><</span> <span class='const'><span class='object_link'><a href="Concurrent.html" title="Concurrent (module)">Concurrent</a></span></span><span class='op'>::</span><span class='const'><span class='object_link'><a href="Concurrent/Actor.html" title="Concurrent::Actor (module)">Actor</a></span></span><span class='op'>::</span><span class='const'><span class='object_link'><a href="Concurrent/Actor/RestartingContext.html" title="Concurrent::Actor::RestartingContext (class)">RestartingContext</a></span></span>
<span class='kw'>def</span> <span class='id identifier rubyid_initialize'>initialize</span>
<span class='kw'>super</span><span class='lparen'>(</span><span class='rparen'>)</span>
<span class='ivar'>@jobs</span> <span class='op'>=</span> <span class='lbrace'>{</span><span class='rbrace'>}</span>
<span class='kw'>end</span>
<span class='kw'>def</span> <span class='id identifier rubyid_on_message'>on_message</span><span class='lparen'>(</span><span class='id identifier rubyid_msg'>msg</span><span class='rparen'>)</span>
<span class='id identifier rubyid_command'>command</span><span class='comma'>,</span> <span class='op'>*</span><span class='id identifier rubyid_args'>args</span> <span class='op'>=</span> <span class='id identifier rubyid_msg'>msg</span>
<span class='kw'>case</span> <span class='id identifier rubyid_command'>command</span>
<span class='comment'># new job to process
</span> <span class='kw'>when</span> <span class='symbol'>:run</span>
<span class='id identifier rubyid_job'>job</span> <span class='op'>=</span> <span class='id identifier rubyid_args'>args</span><span class='lbracket'>[</span><span class='int'>0</span><span class='rbracket'>]</span>
<span class='ivar'>@jobs</span><span class='lbracket'>[</span><span class='id identifier rubyid_job'>job</span><span class='rbracket'>]</span> <span class='op'>=</span> <span class='id identifier rubyid_envelope'>envelope</span><span class='period'>.</span><span class='id identifier rubyid_future'>future</span>
<span class='comment'># Process asynchronously and send message back when done.
</span> <span class='const'><span class='object_link'><a href="Concurrent.html" title="Concurrent (module)">Concurrent</a></span></span><span class='op'>::</span><span class='const'><span class='object_link'><a href="Concurrent/Promises.html" title="Concurrent::Promises (module)">Promises</a></span></span><span class='period'>.</span><span class='id identifier rubyid_future'><span class='object_link'><a href="Concurrent/Promises/FactoryMethods.html#future-instance_method" title="Concurrent::Promises::FactoryMethods#future (method)">future</a></span></span><span class='lparen'>(</span><span class='op'>&</span><span class='id identifier rubyid_job'>job</span><span class='rparen'>)</span><span class='period'>.</span><span class='id identifier rubyid_chain'><span class='object_link'><a href="Concurrent/Promises/AbstractEventFuture.html#chain-instance_method" title="Concurrent::Promises::AbstractEventFuture#chain (method)">chain</a></span></span><span class='lparen'>(</span><span class='id identifier rubyid_job'>job</span><span class='rparen'>)</span> <span class='kw'>do</span> <span class='op'>|</span><span class='id identifier rubyid_fulfilled'>fulfilled</span><span class='comma'>,</span> <span class='id identifier rubyid_value'>value</span><span class='comma'>,</span> <span class='id identifier rubyid_reason'>reason</span><span class='comma'>,</span> <span class='id identifier rubyid_job'>job</span><span class='op'>|</span>
<span class='kw'>self</span><span class='period'>.</span><span class='id identifier rubyid_tell'>tell</span> <span class='lbracket'>[</span><span class='symbol'>:done</span><span class='comma'>,</span> <span class='id identifier rubyid_job'>job</span><span class='comma'>,</span> <span class='id identifier rubyid_fulfilled'>fulfilled</span><span class='comma'>,</span> <span class='id identifier rubyid_value'>value</span><span class='comma'>,</span> <span class='id identifier rubyid_reason'>reason</span><span class='rbracket'>]</span>
<span class='kw'>end</span>
<span class='comment'># Do not make return value of this method to be answer of this message.
</span> <span class='comment'># We are answering later in :done by resolving the future kept in @jobs.
</span> <span class='const'><span class='object_link'><a href="Concurrent.html" title="Concurrent (module)">Concurrent</a></span></span><span class='op'>::</span><span class='const'><span class='object_link'><a href="Concurrent/Actor.html" title="Concurrent::Actor (module)">Actor</a></span></span><span class='op'>::</span><span class='const'><span class='object_link'><a href="Concurrent/Actor/Behaviour.html" title="Concurrent::Actor::Behaviour (module)">Behaviour</a></span></span><span class='op'>::</span><span class='const'><span class='object_link'><a href="Concurrent/Actor/Behaviour.html#MESSAGE_PROCESSED-constant" title="Concurrent::Actor::Behaviour::MESSAGE_PROCESSED (constant)">MESSAGE_PROCESSED</a></span></span>
<span class='kw'>when</span> <span class='symbol'>:done</span>
<span class='id identifier rubyid_job'>job</span><span class='comma'>,</span> <span class='id identifier rubyid_fulfilled'>fulfilled</span><span class='comma'>,</span> <span class='id identifier rubyid_value'>value</span><span class='comma'>,</span> <span class='id identifier rubyid_reason'>reason</span> <span class='op'>=</span> <span class='op'>*</span><span class='id identifier rubyid_args'>args</span>
<span class='id identifier rubyid_future'>future</span> <span class='op'>=</span> <span class='ivar'>@jobs</span><span class='period'>.</span><span class='id identifier rubyid_delete'>delete</span> <span class='id identifier rubyid_job'>job</span>
<span class='comment'># Answer the job's result.
</span> <span class='id identifier rubyid_future'>future</span><span class='period'>.</span><span class='id identifier rubyid_resolve'>resolve</span> <span class='id identifier rubyid_fulfilled'>fulfilled</span><span class='comma'>,</span> <span class='id identifier rubyid_value'>value</span><span class='comma'>,</span> <span class='id identifier rubyid_reason'>reason</span>
<span class='kw'>when</span> <span class='symbol'>:status</span>
<span class='lbrace'>{</span> <span class='label'>running_jobs:</span> <span class='ivar'>@jobs</span><span class='period'>.</span><span class='id identifier rubyid_size'>size</span> <span class='rbrace'>}</span>
<span class='kw'>else</span>
<span class='comment'># Continue to fail with unknown message.
</span> <span class='id identifier rubyid_pass'>pass</span>
<span class='kw'>end</span>
<span class='kw'>end</span>
<span class='kw'>end</span>
</code></pre>
<p>Create the computer actor and send it 3 jobs.</p>
<pre class="code ruby"><code class="ruby"><span class='id identifier rubyid_computer'>computer</span> <span class='op'>=</span> <span class='const'><span class='object_link'><a href="Concurrent.html" title="Concurrent (module)">Concurrent</a></span></span><span class='op'>::</span><span class='const'><span class='object_link'><a href="Concurrent/Actor.html" title="Concurrent::Actor (module)">Actor</a></span></span><span class='period'>.</span><span class='id identifier rubyid_spawn'><span class='object_link'><a href="Concurrent/Actor.html#spawn-class_method" title="Concurrent::Actor.spawn (method)">spawn</a></span></span> <span class='const'>Computer</span><span class='comma'>,</span> <span class='symbol'>:computer</span>
<span class='comment'># => #<Concurrent::Actor::Reference:0x000030 /computer (Computer)>
</span><span class='id identifier rubyid_results'>results</span> <span class='op'>=</span> <span class='int'>3</span><span class='period'>.</span><span class='id identifier rubyid_times'>times</span><span class='period'>.</span><span class='id identifier rubyid_map'>map</span> <span class='lbrace'>{</span> <span class='id identifier rubyid_computer'>computer</span><span class='period'>.</span><span class='id identifier rubyid_ask'>ask</span> <span class='lbracket'>[</span><span class='symbol'>:run</span><span class='comma'>,</span> <span class='tlambda'>-></span> <span class='tlambeg'>{</span> <span class='id identifier rubyid_sleep'>sleep</span> <span class='float'>0.1</span><span class='semicolon'>;</span> <span class='symbol'>:result</span> <span class='rbrace'>}</span><span class='rbracket'>]</span> <span class='rbrace'>}</span>
<span class='comment'># => [#<Concurrent::Promises::Future:0x000031 pending>,
</span><span class='comment'># #<Concurrent::Promises::Future:0x000032 pending>,
</span><span class='comment'># #<Concurrent::Promises::Future:0x000033 pending>]
</span><span class='id identifier rubyid_computer'>computer</span><span class='period'>.</span><span class='id identifier rubyid_ask'>ask</span><span class='lparen'>(</span><span class='symbol'>:status</span><span class='rparen'>)</span><span class='period'>.</span><span class='id identifier rubyid_value!'>value!</span> <span class='comment'># => {:running_jobs=>3}
</span><span class='id identifier rubyid_results'>results</span><span class='period'>.</span><span class='id identifier rubyid_map'>map</span><span class='lparen'>(</span><span class='op'>&</span><span class='symbol'>:value!</span><span class='rparen'>)</span> <span class='comment'># => [:result, :result, :result]
</span></code></pre>
<h2>Solving the Thread count limit by thread simulation</h2>
<p>Sometimes an application requires to process a lot of tasks concurrently. If
the number of concurrent tasks is high enough than it is not possible to create
a Thread for each of them. A partially satisfactory solution could be to use
Fibers, but that solution locks the application on MRI since other Ruby
implementations are using threads for each Fiber.</p>
<p>This library provides a <span class='object_link'><a href="Concurrent/Promises/Future.html#run-instance_method" title="Concurrent::Promises::Future#run (method)">Concurrent::Promises::Future#run</a></span> method on a future
to simulate threads without actually accepting one all the time. The run method
is similar to <span class='object_link'><a href="Concurrent/Promises/Future.html#flat-instance_method" title="Concurrent::Promises::Future#flat (method)">Concurrent::Promises::Future#flat</a></span> but it will keep flattening
until it's fulfilled with non future value, then the value is taken as a result
of the process simulated by <code>run</code>.</p>
<pre class="code ruby"><code class="ruby"><span class='id identifier rubyid_body'>body</span> <span class='op'>=</span> <span class='id identifier rubyid_lambda'>lambda</span> <span class='kw'>do</span> <span class='op'>|</span><span class='id identifier rubyid_v'>v</span><span class='op'>|</span>
<span class='comment'># Some computation step of the process
</span> <span class='id identifier rubyid_new_v'>new_v</span> <span class='op'>=</span> <span class='id identifier rubyid_v'>v</span> <span class='op'>+</span> <span class='int'>1</span>
<span class='comment'># Is the process finished?
</span> <span class='kw'>if</span> <span class='id identifier rubyid_new_v'>new_v</span> <span class='op'><</span> <span class='int'>5</span>
<span class='comment'># Continue computing with new value, does not have to be recursive.
</span> <span class='comment'># It just has to return a future.
</span> <span class='const'><span class='object_link'><a href="Concurrent.html" title="Concurrent (module)">Concurrent</a></span></span><span class='op'>::</span><span class='const'><span class='object_link'><a href="Concurrent/Promises.html" title="Concurrent::Promises (module)">Promises</a></span></span><span class='period'>.</span><span class='id identifier rubyid_future'><span class='object_link'><a href="Concurrent/Promises/FactoryMethods.html#future-instance_method" title="Concurrent::Promises::FactoryMethods#future (method)">future</a></span></span><span class='lparen'>(</span><span class='id identifier rubyid_new_v'>new_v</span><span class='comma'>,</span> <span class='op'>&</span><span class='id identifier rubyid_body'>body</span><span class='rparen'>)</span>
<span class='kw'>else</span>
<span class='comment'># The process is finished, fulfill the final value with `new_v`.
</span> <span class='id identifier rubyid_new_v'>new_v</span>
<span class='kw'>end</span>
<span class='kw'>end</span>
<span class='const'><span class='object_link'><a href="Concurrent.html" title="Concurrent (module)">Concurrent</a></span></span><span class='op'>::</span><span class='const'><span class='object_link'><a href="Concurrent/Promises.html" title="Concurrent::Promises (module)">Promises</a></span></span><span class='period'>.</span><span class='id identifier rubyid_future'><span class='object_link'><a href="Concurrent/Promises/FactoryMethods.html#future-instance_method" title="Concurrent::Promises::FactoryMethods#future (method)">future</a></span></span><span class='lparen'>(</span><span class='int'>0</span><span class='comma'>,</span> <span class='op'>&</span><span class='id identifier rubyid_body'>body</span><span class='rparen'>)</span><span class='period'>.</span><span class='id identifier rubyid_run'><span class='object_link'><a href="Concurrent/Promises/Future.html#run-instance_method" title="Concurrent::Promises::Future#run (method)">run</a></span></span><span class='period'>.</span><span class='id identifier rubyid_value!'><span class='object_link'><a href="Concurrent/Promises/Future.html#value!-instance_method" title="Concurrent::Promises::Future#value! (method)">value!</a></span></span> <span class='comment'># => 5
</span></code></pre>
<p>This solution works well an any Ruby implementation.</p>
<blockquote>
<p><em>TODO: More examples to be added.</em></p>
</blockquote>
<h2>Cancellation</h2>
<h3>Simple</h3>
<p>Lets have two processes which will count until cancelled.</p>
<pre class="code ruby"><code class="ruby"><span class='id identifier rubyid_source'>source</span><span class='comma'>,</span> <span class='id identifier rubyid_token'>token</span> <span class='op'>=</span> <span class='const'><span class='object_link'><a href="Concurrent.html" title="Concurrent (module)">Concurrent</a></span></span><span class='op'>::</span><span class='const'><span class='object_link'><a href="Concurrent/Cancellation.html" title="Concurrent::Cancellation (class)">Cancellation</a></span></span><span class='period'>.</span><span class='id identifier rubyid_create'><span class='object_link'><a href="Concurrent/Cancellation.html#create-class_method" title="Concurrent::Cancellation.create (method)">create</a></span></span>
<span class='comment'># => [#<Concurrent::Cancellation:0x000034 canceled:false>,
</span><span class='comment'># #<Concurrent::Cancellation::Token:0x000035 canceled:false>]
</span>
<span class='id identifier rubyid_count_until_cancelled'>count_until_cancelled</span> <span class='op'>=</span> <span class='tlambda'>-></span> <span class='id identifier rubyid_token'>token</span><span class='comma'>,</span> <span class='id identifier rubyid_count'>count</span> <span class='kw'>do</span>
<span class='kw'>if</span> <span class='id identifier rubyid_token'>token</span><span class='period'>.</span><span class='id identifier rubyid_canceled?'>canceled?</span>
<span class='id identifier rubyid_count'>count</span>
<span class='kw'>else</span>
<span class='const'><span class='object_link'><a href="Concurrent.html" title="Concurrent (module)">Concurrent</a></span></span><span class='op'>::</span><span class='const'><span class='object_link'><a href="Concurrent/Promises.html" title="Concurrent::Promises (module)">Promises</a></span></span><span class='period'>.</span><span class='id identifier rubyid_future'><span class='object_link'><a href="Concurrent/Promises/FactoryMethods.html#future-instance_method" title="Concurrent::Promises::FactoryMethods#future (method)">future</a></span></span> <span class='id identifier rubyid_token'>token</span><span class='comma'>,</span> <span class='id identifier rubyid_count'>count</span><span class='op'>+</span><span class='int'>1</span><span class='comma'>,</span> <span class='op'>&</span><span class='id identifier rubyid_count_until_cancelled'>count_until_cancelled</span>
<span class='kw'>end</span>
<span class='kw'>end</span>
<span class='id identifier rubyid_futures'>futures</span> <span class='op'>=</span> <span class='const'>Array</span><span class='period'>.</span><span class='id identifier rubyid_new'>new</span><span class='lparen'>(</span><span class='int'>2</span><span class='rparen'>)</span> <span class='kw'>do</span>
<span class='const'><span class='object_link'><a href="Concurrent.html" title="Concurrent (module)">Concurrent</a></span></span><span class='op'>::</span><span class='const'><span class='object_link'><a href="Concurrent/Promises.html" title="Concurrent::Promises (module)">Promises</a></span></span><span class='period'>.</span><span class='id identifier rubyid_future'><span class='object_link'><a href="Concurrent/Promises/FactoryMethods.html#future-instance_method" title="Concurrent::Promises::FactoryMethods#future (method)">future</a></span></span><span class='lparen'>(</span><span class='id identifier rubyid_token'>token</span><span class='comma'>,</span> <span class='int'>0</span><span class='comma'>,</span> <span class='op'>&</span><span class='id identifier rubyid_count_until_cancelled'>count_until_cancelled</span><span class='rparen'>)</span><span class='period'>.</span><span class='id identifier rubyid_run'><span class='object_link'><a href="Concurrent/Promises/Future.html#run-instance_method" title="Concurrent::Promises::Future#run (method)">run</a></span></span>
<span class='kw'>end</span>
<span class='comment'># => [#<Concurrent::Promises::Future:0x000036 pending>,
</span><span class='comment'># #<Concurrent::Promises::Future:0x000037 pending>]
</span>
<span class='id identifier rubyid_sleep'>sleep</span> <span class='float'>0.01</span>
<span class='id identifier rubyid_source'>source</span><span class='period'>.</span><span class='id identifier rubyid_cancel'>cancel</span> <span class='comment'># => true
</span><span class='id identifier rubyid_futures'>futures</span><span class='period'>.</span><span class='id identifier rubyid_map'>map</span><span class='lparen'>(</span><span class='op'>&</span><span class='symbol'>:value!</span><span class='rparen'>)</span> <span class='comment'># => [68, 71]
</span></code></pre>
<p>Cancellation can also be used as event or future to log or plan re-execution.</p>
<pre class="code ruby"><code class="ruby"><span class='id identifier rubyid_token'>token</span><span class='period'>.</span><span class='id identifier rubyid_to_event'>to_event</span><span class='period'>.</span><span class='id identifier rubyid_chain'>chain</span> <span class='kw'>do</span>
<span class='comment'># log cancellation
</span> <span class='comment'># plane re-execution
</span><span class='kw'>end</span>
</code></pre>
<h3>Parallel background processing with cancellation</h3>
<p>Each task tries to count to 1000 but there is a randomly failing test. The
tasks share a cancellation, when one of them fails it cancels the others.</p>
<pre class="code ruby"><code class="ruby"><span class='id identifier rubyid_source'>source</span><span class='comma'>,</span> <span class='id identifier rubyid_token'>token</span> <span class='op'>=</span> <span class='const'><span class='object_link'><a href="Concurrent.html" title="Concurrent (module)">Concurrent</a></span></span><span class='op'>::</span><span class='const'><span class='object_link'><a href="Concurrent/Cancellation.html" title="Concurrent::Cancellation (class)">Cancellation</a></span></span><span class='period'>.</span><span class='id identifier rubyid_create'><span class='object_link'><a href="Concurrent/Cancellation.html#create-class_method" title="Concurrent::Cancellation.create (method)">create</a></span></span>
<span class='comment'># => [#<Concurrent::Cancellation:0x000038 canceled:false>,
</span><span class='comment'># #<Concurrent::Cancellation::Token:0x000039 canceled:false>]
</span><span class='id identifier rubyid_tasks'>tasks</span> <span class='op'>=</span> <span class='int'>4</span><span class='period'>.</span><span class='id identifier rubyid_times'>times</span><span class='period'>.</span><span class='id identifier rubyid_map'>map</span> <span class='kw'>do</span> <span class='op'>|</span><span class='id identifier rubyid_i'>i</span><span class='op'>|</span>
<span class='const'><span class='object_link'><a href="Concurrent.html" title="Concurrent (module)">Concurrent</a></span></span><span class='op'>::</span><span class='const'><span class='object_link'><a href="Concurrent/Promises.html" title="Concurrent::Promises (module)">Promises</a></span></span><span class='period'>.</span><span class='id identifier rubyid_future'><span class='object_link'><a href="Concurrent/Promises/FactoryMethods.html#future-instance_method" title="Concurrent::Promises::FactoryMethods#future (method)">future</a></span></span><span class='lparen'>(</span><span class='id identifier rubyid_source'>source</span><span class='comma'>,</span> <span class='id identifier rubyid_token'>token</span><span class='comma'>,</span> <span class='id identifier rubyid_i'>i</span><span class='rparen'>)</span> <span class='kw'>do</span> <span class='op'>|</span><span class='id identifier rubyid_source'>source</span><span class='comma'>,</span> <span class='id identifier rubyid_token'>token</span><span class='comma'>,</span> <span class='id identifier rubyid_i'>i</span><span class='op'>|</span>
<span class='id identifier rubyid_count'>count</span> <span class='op'>=</span> <span class='int'>0</span>
<span class='int'>1000</span><span class='period'>.</span><span class='id identifier rubyid_times'>times</span> <span class='kw'>do</span>
<span class='kw'>break</span> <span class='id identifier rubyid_count'>count</span> <span class='op'>=</span> <span class='symbol'>:cancelled</span> <span class='kw'>if</span> <span class='id identifier rubyid_token'>token</span><span class='period'>.</span><span class='id identifier rubyid_canceled?'>canceled?</span>
<span class='id identifier rubyid_count'>count</span> <span class='op'>+=</span> <span class='int'>1</span>
<span class='id identifier rubyid_sleep'>sleep</span> <span class='float'>0.01</span>
<span class='kw'>if</span> <span class='id identifier rubyid_rand'>rand</span> <span class='op'>></span> <span class='float'>0.95</span>
<span class='id identifier rubyid_source'>source</span><span class='period'>.</span><span class='id identifier rubyid_cancel'>cancel</span>
<span class='id identifier rubyid_raise'>raise</span> <span class='tstring'><span class='tstring_beg'>'</span><span class='tstring_content'>random error</span><span class='tstring_end'>'</span></span>
<span class='kw'>end</span>
<span class='id identifier rubyid_count'>count</span>
<span class='kw'>end</span>
<span class='kw'>end</span>
<span class='kw'>end</span>
<span class='comment'># => [#<Concurrent::Promises::Future:0x00003a pending>,
</span><span class='comment'># #<Concurrent::Promises::Future:0x00003b pending>,
</span><span class='comment'># #<Concurrent::Promises::Future:0x00003c pending>,
</span><span class='comment'># #<Concurrent::Promises::Future:0x00003d pending>]
</span><span class='const'><span class='object_link'><a href="Concurrent.html" title="Concurrent (module)">Concurrent</a></span></span><span class='op'>::</span><span class='const'><span class='object_link'><a href="Concurrent/Promises.html" title="Concurrent::Promises (module)">Promises</a></span></span><span class='period'>.</span><span class='id identifier rubyid_zip'>zip</span><span class='lparen'>(</span><span class='op'>*</span><span class='id identifier rubyid_tasks'>tasks</span><span class='rparen'>)</span><span class='period'>.</span><span class='id identifier rubyid_result'><span class='object_link'><a href="Concurrent/Promises/Future.html#result-instance_method" title="Concurrent::Promises::Future#result (method)">result</a></span></span>
<span class='comment'># => [false,
</span><span class='comment'># [:cancelled, nil, :cancelled, :cancelled],
</span><span class='comment'># [nil, #<RuntimeError: random error>, nil, nil]]
</span></code></pre>
<p>Without the randomly failing part it produces following.</p>
<pre class="code ruby"><code class="ruby"><span class='id identifier rubyid_source'>source</span><span class='comma'>,</span> <span class='id identifier rubyid_token'>token</span> <span class='op'>=</span> <span class='const'><span class='object_link'><a href="Concurrent.html" title="Concurrent (module)">Concurrent</a></span></span><span class='op'>::</span><span class='const'><span class='object_link'><a href="Concurrent/Cancellation.html" title="Concurrent::Cancellation (class)">Cancellation</a></span></span><span class='period'>.</span><span class='id identifier rubyid_create'><span class='object_link'><a href="Concurrent/Cancellation.html#create-class_method" title="Concurrent::Cancellation.create (method)">create</a></span></span>
<span class='comment'># => [#<Concurrent::Cancellation:0x00003e canceled:false>,
</span><span class='comment'># #<Concurrent::Cancellation::Token:0x00003f canceled:false>]
</span><span class='id identifier rubyid_tasks'>tasks</span> <span class='op'>=</span> <span class='int'>4</span><span class='period'>.</span><span class='id identifier rubyid_times'>times</span><span class='period'>.</span><span class='id identifier rubyid_map'>map</span> <span class='kw'>do</span> <span class='op'>|</span><span class='id identifier rubyid_i'>i</span><span class='op'>|</span>
<span class='const'><span class='object_link'><a href="Concurrent.html" title="Concurrent (module)">Concurrent</a></span></span><span class='op'>::</span><span class='const'><span class='object_link'><a href="Concurrent/Promises.html" title="Concurrent::Promises (module)">Promises</a></span></span><span class='period'>.</span><span class='id identifier rubyid_future'><span class='object_link'><a href="Concurrent/Promises/FactoryMethods.html#future-instance_method" title="Concurrent::Promises::FactoryMethods#future (method)">future</a></span></span><span class='lparen'>(</span><span class='id identifier rubyid_source'>source</span><span class='comma'>,</span> <span class='id identifier rubyid_token'>token</span><span class='comma'>,</span> <span class='id identifier rubyid_i'>i</span><span class='rparen'>)</span> <span class='kw'>do</span> <span class='op'>|</span><span class='id identifier rubyid_source'>source</span><span class='comma'>,</span> <span class='id identifier rubyid_token'>token</span><span class='comma'>,</span> <span class='id identifier rubyid_i'>i</span><span class='op'>|</span>
<span class='id identifier rubyid_count'>count</span> <span class='op'>=</span> <span class='int'>0</span>
<span class='int'>1000</span><span class='period'>.</span><span class='id identifier rubyid_times'>times</span> <span class='kw'>do</span>
<span class='kw'>break</span> <span class='id identifier rubyid_count'>count</span> <span class='op'>=</span> <span class='symbol'>:cancelled</span> <span class='kw'>if</span> <span class='id identifier rubyid_token'>token</span><span class='period'>.</span><span class='id identifier rubyid_canceled?'>canceled?</span>
<span class='id identifier rubyid_count'>count</span> <span class='op'>+=</span> <span class='int'>1</span>
<span class='comment'># sleep 0.01
</span> <span class='comment'># if rand > 0.95
</span> <span class='comment'># source.cancel
</span> <span class='comment'># raise 'random error'
</span> <span class='comment'># end
</span> <span class='kw'>end</span>
<span class='id identifier rubyid_count'>count</span>
<span class='kw'>end</span>
<span class='kw'>end</span>
<span class='const'><span class='object_link'><a href="Concurrent.html" title="Concurrent (module)">Concurrent</a></span></span><span class='op'>::</span><span class='const'><span class='object_link'><a href="Concurrent/Promises.html" title="Concurrent::Promises (module)">Promises</a></span></span><span class='period'>.</span><span class='id identifier rubyid_zip'>zip</span><span class='lparen'>(</span><span class='op'>*</span><span class='id identifier rubyid_tasks'>tasks</span><span class='rparen'>)</span><span class='period'>.</span><span class='id identifier rubyid_result'><span class='object_link'><a href="Concurrent/Promises/Future.html#result-instance_method" title="Concurrent::Promises::Future#result (method)">result</a></span></span>
<span class='comment'># => [true, [1000, 1000, 1000, 1000], nil]
</span></code></pre>
<h2>Throttling concurrency</h2>
<p>By creating an actor managing the resource we can control how many threads is
accessing the resource. In this case one at the time.</p>