20-idiomatic-trading-strategy.Rmd

# Idiomatic trading strategy

## Objectives
- Following the OHLC footsteps
- Simplifying the Naive supervision tree
- Supporting multiple positions
- Retrofitting the "shutdown" functionality
- Updating the Strategy to handle rebuys
- Fetching active positions
- Tidying up

## Following the OHLC footsteps

In the last chapter, we looked into the idiomatic Elixir. We refactored our initial implementation of OHLC aggregation to maximise the amount of pure code by limiting the number of processes running per symbol. It was a nice standalone functionality to showcase the concept.

In this chapter, we will look into how could we refactor our naive strategy's code to achieve the same result. At this moment, the naive strategy uses multiple processes per symbol, including  `Trader` processes, dedicated `Leader` and `SymbolSupervisor`. We will update the `Trader` process to be responsible for multiple trades(we will call them "positions" from now on) on a single symbol. This way, we will have a single `Trader` process per symbol as well, as we will get rid of both the `Leader`(we will move rebuy/shutdown logic to our strategy - where it belongs) and the `SymbolSupervisor`: 

```{r, fig.align="center", out.width="100%", echo=FALSE}
knitr::include_graphics("images/chapter_20_01_current_hierarchy.jpg")
```

There are multiple benefits of simplifying the supervision hierarchy, and we will look at them closely as we refactor the code - let's get to it.

## Simplifying the Naive supervision tree

Starting from the top of the tree, the first module we need to update will be the `Naive.DynamicSymbolSupervisor` module.

### The `Naive.DynamicTraderSupervisor` module

The filename needs to be updated to `dynamic_trader_supervisor.ex` and the module name to `Naive.DynamicTraderSupervisor`.

Next, there's the `@registry` attribute that we will rename to `:naive_traders`.

Finally, update the alias to the `Naive.SymbolSupervisor` to the `Naive.Trader` and use it inside the `start_child/1` function:

```{r, engine = 'elixir', eval = FALSE}
  # /apps/naive/lib/naive/dynamic_trader_supervisor.ex
  alias Naive.Trader 
  ...
  defp start_child(args) do
    DynamicSupervisor.start_child(
      __MODULE__,
      {Trader, args}
    )
  end
```

### The `Naive` module

The `Naive` module heavily depends on the `Naive.DynamicSymbolSupervisor`(now called the

`Naive.DynamicTraderSupervisor`), we need to update all the references to it:

```{r, engine = 'elixir', eval = FALSE}
  # /apps/naive/lib/naive.ex
  alias Naive.DynamicTraderSupervisor
  ...
  |> DynamicTraderSupervisor.start_worker()
  ...
  |> DynamicTraderSupervisor.stop_worker()
  ...
  |> DynamicTraderSupervisor.shutdown_worker()
```

### The `Naive.Supervisor` module

The `Naive.Supervisor` supervises the `Naive.DynamicSymbolSupervisor`(now called the

`Naive.DynamicTraderSupervisor`) and the registry that stores the traders' PIDs - we need to update both:

```{r, engine = 'elixir', eval = FALSE}
  # /apps/naive/lib/naive/supervisor.ex
  alias Naive.DynamicTraderSupervisor # <= updated

  @registry :naive_traders # <= updated
  ...
    children = [
      {Registry, [keys: :unique, name: @registry]},
      {DynamicTradersSupervisor, []}, # <= updated
      {Task,
       fn ->
         DynamicTradersSupervisor.autostart_workers() # <= updated
       end}
    ]
```

### The `Naive.Trader` module

The final module to be updated will be the `Naive.Trader`. It needs to register process' PID inside the Registry:

```{r, engine = 'elixir', eval = FALSE}
  # /apps/naive/lib/naive/trader.ex
  @registry :naive_traders # <= added
  ...
  def start_link(%State{} = state) do
    symbol = String.upcase(state.symbol) # <= updated

    GenServer.start_link(
      __MODULE__,
      state,
      name: via_tuple(symbol)  # <= updated
    )
  end
  ...
  defp via_tuple(symbol) do
    {:via, Registry, {@registry, symbol}}
  end
```

That finishes the changes to the supervision tree - the `Naive.Leader` and the `Naive.SymbolSupervisor` aren't used anymore. At this moment, our codebase won't work as we need to retrofit the functionality that the `Naive.Leader` was offering into our `Naive.Trader` and `Naive.Strategy` modules, which we will focus on in the next section.

## Supporting multiple positions

The current `State` struct inside the `Naive.Trader` was geared toward a single trade cycle. As we now need to handle multiple positions inside a single `Trader` process, we will need to update this struct. We will start by moving the current `State` struct from the `Naive.Trader` into the `Naive.Strategy` and renaming it to `Position`:

```{r, engine = 'elixir', eval = FALSE}
# /apps/naive/lib/naive/strategy.ex
defmodule Naive.Strategy do
  ...
  defmodule Position do
    @enforce_keys [
	# keys copied from `Naive.Trader.State` struct
    ]
    defstruct [
	# keys copied from `Naive.Trader.State` struct
    ]
  end
```

This will break all the references to `Naive.Trader.State` inside the `Naive.Strategy`, which we need to update to `Position`(and remove the alias of `Naive.Trader.State`):

```{r, engine = 'elixir', eval = FALSE}
  # /apps/naive/lib/naive/strategy.ex
  ...
  def execute(%TradeEvent{} = trade_event, %Position{} = position do
    generate_decision(trade_event, position)
    |> execute_decision(position)
  end
  ...
  def generate_decision(
        %TradeEvent{
          ...
        },
        %Position{ # <= in all 8 clauses
        ...

  defp execute_decision(
    {.... # decision },
    %Position{ # <= updated
      ...
    } = position # <= updated
  ) do
    ...
    new_position = %{position | buy_order: order} # <= updated
    @leader.notify(:trader_state_updated, new_position) # <= updated
    {:ok, new_position} # <^= similar changes in all execute_decision
```

We will ignore the fact that we are still calling the `@leader` and still dealing with a single position(inside the strategy) - we will fix that in the following section. One step at a time ;)

As we are already changing strategy to work with positions, we will update all the logger messages:

```{r, engine = 'elixir', eval = FALSE}
  # /apps/naive/lib/naive/strategy.ex
  defp execute_decision(
         {:place_buy_order, price, quantity},
         ...
  ) do
       ) do
    @logger.info(
      "Position (#{symbol}/#{id}): " <>
        "Placing a BUY order @ #{price}, quantity: #{quantity}"
    ) # ^ updated message
  ...

    defp execute_decision(
         {:place_sell_order, sell_price},
         ...
       ) do
    @logger.info(
      "Position (#{symbol}/#{id}): The BUY order is now filled. " <>
        "Placing a SELL order @ #{sell_price}, quantity: #{quantity}"
    ) # ^ updated message
    ...

  defp execute_decision(
         :fetch_buy_order,
         ...
       ) do
    @logger.info("Position (#{symbol}/#{id}): The BUY order is now partially filled")
    ... # ^^^ updated message

  defp execute_decision(
         :exit,
         ...
       ) do
    @logger.info("Position (#{symbol}/#{id}): Trade cycle finished")
    ... # ^^^ updated message

  defp execute_decision(
         :fetch_sell_order,
         ...
       ) do
    @logger.info("Position (#{symbol}/#{id}): The SELL order is now partially filled")
     ... # ^^^ updated message

  defp execute_decision(
         :rebuy,
         ...
       ) do
    @logger.info("Position (#{symbol}/#{id}): Rebuy triggered")
     ... # ^^^ updated message
```

Our code is still far from working, but we are incrementally updating it to work with multiple positions.

### Initialization

At this moment, the `Naive.Trader` expects the `state` to be injected on start (using the `start_link/1` function). We were able to do that because the `Naive.Leader` was fetching the settings and building the fresh trader state.

First, let's update the `State` of the `Naive.Trader` - it will now hold the symbol's settings(previously held in the leader) and list of positions(list of the `Naive.Strategy.Position` structs):

```{r, engine = 'elixir', eval = FALSE}
  # /apps/naive/lib/naive/trader.ex
  defmodule State do
    @enforce_keys [:settings, :positions]
    defstruct [:settings, positions: []]
  end
```

Now we need to update the `start_link/1` and `init/1` functions as well as add the `handle_continue/2` callback to fetch settings and store them together with an initial position in the state:

```{r, engine = 'elixir', eval = FALSE}
  # /apps/naive/lib/naive/trader.ex
   alias Naive.Strategy
   ...

  def start_link(symbol) do # <= now expecting symbol
    symbol = String.upcase(symbol) # <= updated

    GenServer.start_link(
      __MODULE__,
      symbol,   # <= updated
      name: via_tuple(symbol)
    )
    ...

  def init(symbol) do # <= updated
    @logger.info("Initializing new trader for #{symbol}") # <= updated

    @pubsub_client.subscribe(
      Core.PubSub,
      "TRADE_EVENTS:#{symbol}"
    )

    {:ok, nil, {:continue, {:start_position, symbol}}} # <= updated
  end

  def handle_continue({:start_position, symbol}, _state) do
    settings = Strategy.fetch_symbol_settings(symbol)
    positions = [Strategy.generate_fresh_position(settings)]

    {:noreply, %State{settings: settings, positions: positions}}
  end # ^^^ new function/callback
```

As the `Naive.Trader` starts, it returns the `{:continue, ...}` tuple from the `init/1` function. This will cause the `handle_continue/2` callback to be called asynchronously. Inside it, we fetch settings and add a single fresh position to the list of positions - both stored in Trader's state.

Both functions inside the `handle_continue/2` callback previously were part of the `Naive.Leader` - we need to move them across to the `Naive.Strategy`:

```{r, engine = 'elixir', eval = FALSE}
  # /apps/naive/lib/naive/strategy.ex
  alias Naive.Schema.Settings
  ...
  @repo Application.compile_env(:naive, :repo)
  ...
  def fetch_symbol_settings(symbol) do
    exchange_info = @binance_client.get_exchange_info()
    db_settings = @repo.get_by!(Settings, symbol: symbol)

    merge_filters_into_settings(exchange_info, db_settings, symbol)
  end

  def merge_filters_into_settings(exchange_info, db_settings, symbol) do
    symbol_filters =
      exchange_info
      |> elem(1)
      |> Map.get(:symbols)
      |> Enum.find(&(&1["symbol"] == symbol))
      |> Map.get("filters")

    tick_size =
      symbol_filters
      |> Enum.find(&(&1["filterType"] == "PRICE_FILTER"))
      |> Map.get("tickSize")

    step_size =
      symbol_filters
      |> Enum.find(&(&1["filterType"] == "LOT_SIZE"))
      |> Map.get("stepSize")

    Map.merge(
      %{
        tick_size: tick_size,
        step_size: step_size
      },
      db_settings |> Map.from_struct()
    )
  end

  def generate_fresh_position(settings, id \\ :os.system_time(:millisecond)) do
    %{
      struct(Position, settings)
      | id: id,
        budget: D.div(settings.budget, settings.chunks),
        rebuy_notified: false
    }
  end
```

Inside the above code, we modified the `fetch_symbol_settings/1` function to fetch settings from binance and DB first and then progress with the "pure" part. This update allows us to test most of the logic easily without using mocks.

The `generate_fresh_position/2` was previously called `fresh_trader_state/1` inside the `Naive.Leader`. It had an `id` assigned inside the function based on the current system time. That made it a bit more difficult to test as we don't know what should we expect there as a value. By moving the `id` to the arguments and assigning the current time there, we are now able to test it by passing our dummy value.

We are now using `@repo` inside the `Naive.Strategy` so we need to add it to configuration files(including test configuration):

```{r, engine = 'elixir', eval = FALSE}
# /config/config.exs
config :naive,
  ...
  repo: Naive.Repo,
```

```{r, engine = 'elixir', eval = FALSE}
# /config/test.exs
config :naive,
  ...
  repo: Test.Naive.RepoMock,
```

### Parallelising the strategy

We can now move on to the strategy, but first, let's update the `Naive.Trader` to pass positions and settings separately:

```{r, engine = 'elixir', eval = FALSE}
  # /apps/naive/lib/naive/trader.ex
  def handle_info(%TradeEvent{} = trade_event, %State{} = state) do
    case Naive.Strategy.execute(trade_event, state.positions, state.settings) do # <= updated
      {:ok, updated_positions} -> # <= updated
        {:noreply, %{state | positions: updated_positions}} # <= updated

      :exit ->
        {:stop, :normal, state}
    end
```

We need all the `positions` to iterate through them, deciding and executing appropriate actions. The `settings` will be used inside the strategy later, but we will pass it on now to avoid going back and forward later.

Additionally, we updated the `case` match to expect a list of updated positions which we will assign to the `Trader`'s state.

Now we can modify the `Naive.Strategy` to handle multiple positions:

```{r, engine = 'elixir', eval = FALSE}
   # /apps/naive/lib/naive/strategy.ex
  def execute(%TradeEvent{} = trade_event, positions, settings) do
    generate_decisions(positions, [], trade_event, settings)
    |> Enum.map(fn {decision, position} ->
      Task.async(fn -> execute_decision(decision, position, settings) end)
    end)
    |> Task.await_many()
    |> then(&parse_results/1)
  end
```

\newpage

We need to write most of the functions used above, but we can already see the idea. We will map each of the decisions that we generate to async tasks that execute them. Next, we wait for all of them to finish and parse the results.

First, we are calling a new function `generate_decisions/4`, which is a recursive function on top of the existing `generate_decision/2`:

```{r, engine = 'elixir', eval = FALSE}
  # /apps/naive/lib/naive/strategy.ex
  def generate_decisions([], generated_results, _trade_event, _settings) do
    generated_results
  end

  def generate_decisions([position | rest] = positions, generated_results, trade_event, settings) do
    current_positions = positions ++ (generated_results |> Enum.map(&elem(&1, 0)))

    case generate_decision(trade_event, position, current_positions, settings) do
      decision ->
        generate_decisions(
          rest,
          [{decision, position} | generated_results],
          trade_event,
          settings
        )
    end
  end
```

At this moment, the `generate_decisions/4` can look like overengineered `Enum.map/2` function, but we are actually preparing the ground for the consequent updates later in this chapter(to get the rest of the functionality running).

It's important to note that we are now passing four arguments into the `generate_decision` function - we added `current_positions` and `settings` - those will be required in the further updates as it was mentioned above. At this moment though, we will update **all** the `generate_decision/2` clauses to include two additional arguments:

```{r, engine = 'elixir', eval = FALSE}
  # /apps/naive/lib/naive/strategy.ex
  def generate_decision(
        %TradeEvent{...},
        %Position{
          ...
        },
        _positions, # <= add this 8 times
       _settings    # <= add this 8 times
      ) do
```

\newpage

Now back to the main `execute/3` function where we are calling `execute_decision/3`, which we need to update as well(**all** clauses):

```{r, engine = 'elixir', eval = FALSE}
  # /apps/naive/lib/naive/strategy.ex
  defp execute_decision(
         {...},
         %Position{
           ...
         } = position,
         _settings  # <= added 7 times
       ) do
```

The final function that gets called from the `execute/3` function is `parse_results/1`, which will aggregate all the results into a single tuple:

```{r, engine = 'elixir', eval = FALSE}
  # /apps/naive/lib/naive/strategy.ex
  def parse_results([_ | _] = results) do
    results
    |> Enum.map(fn {:ok, new_position} -> new_position end)
    |> then(&{:ok, &1})
  end
```

At this moment, we should be able to run our code:

```{r, engine = 'bash', eval = FALSE}
$ iex -S mix
...
iex(1)> Naive.start_trading("XRPUSDT")
...
iex(2)> Streamer.start_streaming("XRPUSDT")
21:29:17.998 [info]  Starting streaming XRPUSDT trade events
...
21:29:21.037 [info]  Position (XRPUSDT/1651696014179): Placing a BUY order @ 0.64010000,
quantity: 31.00000000
21:29:21.037 [error] Task #PID<0.10293.0> started from #PID<0.480.0> terminating
** (stop) exited in: GenServer.call(:"Elixir.Naive.Leader-XRPUSDT"...
```

So we have a trader that start and places a buy order but then it tries to update the leader with it's new state - we can update the `execute_decision/3` function to drop the updates(in **all** of the clauses):

```{r, engine = 'elixir', eval = FALSE}
    # /apps/naive/lib/naive/strategy.ex
  defp execute_decision(
    ...
       ) do
    ...
    # convert the below:
    new_position = %{position | buy_order: order}
    @leader.notify(:trader_state_updated, new_position)
    {:ok, new_position}
    # to:
    {:ok, %{position | buy_order: order}}
  end
```

Apply similar changes to all the clauses of the `execute_decision/3` to get rid of the references to the `@leader` - remember to remove the module's attribute as well, as we won't need it anymore.

Important note - one of those references to the `@leader` will be the notification that rebuy was triggered:

```{r, engine = 'elixir', eval = FALSE}
    # /apps/naive/lib/naive/strategy.ex
    @leader.notify(:rebuy_triggered, new_position)
```

At this moment, remove that reference as well. We will get back to the rebuy functionality in the next section.

We can now rerun our code:

```{r, engine = 'bash', eval = FALSE}
$ iex -S mix
...
iex(1)> Streamer.start_streaming("ETHUSDT")
...
iex(2)> Naive.start_trading("ETHUSDT")
...
21:59:19.836 [info]  Position (ETHUSDT/1651697959836): Placing a BUY order @ 2945.31000000,
quantity: 0.06790000
 21:59:46.997 [info]  Position (ETHUSDT/1651697959836): The BUY order is now partially
 filled
 21:59:46.997 [info]  Position (ETHUSDT/1651697959836): The BUY order is now filled.
 Placing a SELL order @ 2947.66000000, quantity: 0.06790000
 22:00:21.631 [info]  Position (ETHUSDT/1651697959836): The SELL order is now partially
 filled
 22:00:21.734 [info]  Position (ETHUSDT/1651697959836): Trade cycle finished
22:00:21.737 [error] GenServer {:naive_traders, "ETHUSDT"} terminating
** (FunctionClauseError) no function clause matching in anonymous fn/1 in
Naive.Strategy.parse_results/1
    (naive 0.1.0) lib/naive/strategy.ex:56: anonymous fn(:exit) in
    Naive.Strategy.parse_results/1
```

We can see that our trader process can now go through the whole trade cycle, but it fails to start a new position after the first trade cycle finishes and returns `:exit`.

To fix this issue, we need to return `:finished` instead of `:exit` from the `generate_decision/3` clause responsible for matching end of the trade cycle:

```{r, engine = 'elixir', eval = FALSE}
  # /apps/naive/lib/naive/strategy.ex
  def generate_decision(
        %TradeEvent{},
        %Position{
          sell_order: %Binance.OrderResponse{
            status: "FILLED"
          }
        },
        _positions,
        _settings
      ) do
    :finished # <= updated
  end
```

This decision will end up inside the `execute_decision/3` where previously we were returning `:exit` atom, which was causing an error - let's move this clause to be the last clause and update its body to generate a fresh state instead of returning a dummy atom:

```{r, engine = 'elixir', eval = FALSE}
  # /apps/naive/lib/naive/strategy.ex
  defp execute_decision(
         :finished, # <= previously :exit; updated
         %Position{
           id: id,
           symbol: symbol
         },
         settings # <= now used
       ) do
    new_position = generate_fresh_position(settings) # <= added

    @logger.info("Position (#{symbol}/#{id}): Trade cycle finished")

    {:ok, new_position} # <= updated
  end
```

At this moment, our trader process should be able to run across multiple trade cycles one after another:

```
$ iex -S mix
...
iex(1)> Streamer.start_streaming("ETHUSDT")
...
iex(2)> Naive.start_trading("ETHUSDT")
...
22:46:46.568 [info]  Position (ETHUSDT/1651697959836): Trade cycle finished
22:46:46.577 [info]  Position (ETHUSDT/1651697959836): Placing a BUY order @ 2945.31000000,
  quantity: 0.06790000
```

This finishes direct changes related to making the trader/strategy work with multiple positions, but it lacks all the features that the `Naive.Leader` offered. We will now iterate on this code to bring that missing functionality.

## Retrofitting the "shutdown" functionality

Previously, the shutdown logic was scattered around in multiple places inside the `Naive.Leader`, for example, when the rebuy was triggered - making sure that new Trader processes won't get started in the "shutdown" state.

Now, we have an opportunity to make the shutdown functionality part of our strategy.

We will start by modifying the `DynamicTraderSupervisor` where we will update the `shutdown_worker/1` function to call the `Naive.Trader` instead of the `Naive.Leader`:

```{r, engine = 'elixir', eval = FALSE}
  # /apps/naive/lib/naive/dynamic_trader_supervisor.ex
  def shutdown_worker(symbol) when is_binary(symbol) do
    Logger.info("Shutdown of trading on #{symbol} initialized")
    {:ok, settings} = update_status(symbol, "shutdown")
    Trader.notify(:settings_updated, settings) # <= updated
    {:ok, settings}
  end
```

Now, the Trader will handle updating the settings, which we will add next, but before we do that, we should move the `update_status/2` function into the `Naive.Strategy` as it will be used from both the `DynamicTraderSupervisor` and the `Naive.Strategy`:

```{r, engine = 'elixir', eval = FALSE}
  # /apps/naive/lib/naive/strategy.ex
  def update_status(symbol, status) # <= updated to public
      when is_binary(symbol) and is_binary(status) do
    @repo.get_by(Settings, symbol: symbol) # <= updated to use @repo
    |> Ecto.Changeset.change(%{status: status})
    |> @repo.update() # <= updated to use @repo
  end
```

Now we need to update the `DynamicTraderSupervisor` module to call the `update_status/2` from the `Naive.Strategy` module:

```{r, engine = 'elixir', eval = FALSE}
  # /apps/naive/lib/naive/dynamic_trader_supervisor.ex
  alias Naive.Strategy
  ...

  def start_worker(symbol) do
    ...
    Strategy.update_status(symbol, "on") # <= updated
    ..

  def stop_worker(symbol) do
    ...
    Strategy.update_status(symbol, "off") # <= updated
    ...

  def shutdown_worker(symbol) when is_binary(symbol) do
    ...
    {:ok, settings} = Strategy.update_status(symbol, "shutdown") # <= updated
```

### Handling updated settings

We can now move on to the `Naive.Trader` module, where we need to add  a new `notify/2` interface function:

```{r, engine = 'elixir', eval = FALSE}
  # /apps/naive/lib/naive/trader.ex
  def notify(:settings_updated, settings) do
    call_trader(settings.symbol, {:update_settings, settings})
  end
  ...
  defp call_trader(symbol, data) do
    case Registry.lookup(@registry, symbol) do
      [{pid, _}] ->
        GenServer.call(
          pid,
          data
        )

      _ ->
        Logger.warning("Unable to locate trader process assigned to #{symbol}")
        {:error, :unable_to_locate_trader}
    end
  end
```

The `notify/2` function acts as a part of the public interface of the `Naive.Trader` module. It uses the `call_trader/2` helper function to abstract away looking up the `Trader` process from the `Registry` and making a `GenServer.call`. Besides the "looking up" part being an implementation detail that should be abstracted, we will also need to look up traders' PIDs to provide other functionalities in the upcoming sections.

As we are making a call to the trader process, we need to add a callback:

```{r, engine = 'elixir', eval = FALSE}
  # /apps/naive/lib/naive/trader.ex
  def handle_call(
        {:update_settings, new_settings},
        _,
        state
      ) do
    {:reply, :ok, %{state | settings: new_settings}}
  end
```

### Updating the `Naive.Strategy` to honour the "shutdown" state

We updated all of the modules to update the `settings` inside the `%State{}` of the `Trader` process. That's the first step, but now we need to modify our strategy to act appropriately.

The first step will be to update the `generate_decision/4` clause that handles the rebuy being triggered to take under consideration the `settings.status`:

```{r, engine = 'elixir', eval = FALSE}
  # /apps/naive/lib/naive/strategy.ex
  def generate_decision(
        %TradeEvent{
          price: current_price
        },
        %Position{
          buy_order: %Binance.OrderResponse{
            price: buy_price
          },
          rebuy_interval: rebuy_interval,
          rebuy_notified: false
        },
        _positions,
        settings # <= updated
      ) do
    if trigger_rebuy?(buy_price, current_price, rebuy_interval) &&
         settings.status != "shutdown" do # <= updated
      :rebuy
    else
      :skip
    end
  end
```

Another clause that we need to update is the one responsible for matching end of the trading cycle:

```{r, engine = 'elixir', eval = FALSE}
  # /apps/naive/lib/naive/strategy.ex
  def generate_decision(
        %TradeEvent{},
        %Position{
          sell_order: %Binance.OrderResponse{
            status: "FILLED"
          }
        },
        _positions,
        settings # <= updated
      ) do
    if settings.status != "shutdown" do # <= updated
      :finished
    else
      :exit # <= new decision
    end
  end
```

As we added a new `:exit` decision that we need to handle inside the `generate_decisions/4` - it needs to remove this decision from the list of generated decisions:

```{r, engine = 'elixir', eval = FALSE}
  # /apps/naive/lib/naive/strategy.ex
  def generate_decisions([position | rest] = positions, generated_results, trade_event, settings) do
    ...
    case generate_decision(trade_event, position, current_positions, settings) do
      :exit ->
        generate_decisions(rest, generated_results, trade_event, settings)

      decision -> ...
      ...
```

Inside the recursive function, we are skipping all the positions that ended up with the `:exit` decisions. This will slowly cause the list of positions to drain to an empty list, which will cause the `parse_results/1` function to fail(as it expects non-empty list). We will add a new first clause to match the empty list of positions and return the `:exit` atom:

```{r, engine = 'elixir', eval = FALSE}
  # /apps/naive/lib/naive/strategy.ex
  def parse_results([]) do # <= added clause
    :exit
  end

  def parse_results([_ | _] = results) do
    ...
  end
```

In the end, the `:exit` atom will cause the `Naive.Trader` module to stop the process.

The final step will be to update the `Naive.Trader` to log a message and update the status to `"off"` before exiting the process:

```{r, engine = 'elixir', eval = FALSE}
  # /apps/naive/lib/naive/trader.ex
  def handle_info(%TradeEvent{} = trade_event, %State{} = state) do
    ...
    case Naive.Strategy.execute(trade_event, state.positions, state.settings) do
      ...
      :exit ->
        {:ok, _settings} = Strategy.update_status(trade_event.symbol, "off")
        Logger.info("Trading for #{trade_event.symbol} stopped")
        {:stop, :normal, state}
```

We can test this by running the following:

```{r, engine = 'bash', eval = FALSE}
$ iex -S mix
...
iex(1)> Streamer.start_streaming("ETHUSDT")
...
iex(4)> Naive.start_trading("ETHUSDT")
...
iex(4)> Naive.shutdown_trading("ETHUSDT")
22:35:58.929 [info]  Shutdown of trading on ETHUSDT initialized
23:05:40.068 [info]  Position (ETHUSDT/1651788334058): The SELL order is now partially filled
23:05:40.123 [info]  Trading for ETHUSDT stopped
```

That finishes the shutdown functionality. As mentioned previously, one after another, positions will complete their trading cycles, and the whole process will exit at the end.

## Updating the Strategy to handle rebuys

 Previously, both the `Trader` and the `Leader` were involved in the rebuy functionality. As now we removed the `Leader`, it's an excellent opportunity to move as much as possible of that logic into our strategy.

We will start by updating the `generate_decision/4` clause responsible for matching the rebuy scenario. We will take into consideration the number of currently open positions(this check was previously done inside the `Naive.Leader`):

```{r, engine = 'elixir', eval = FALSE}
  # /apps/naive/lib/naive/strategy.ex
  def generate_decision(
        %TradeEvent{
          price: current_price
        },
        %Position{
          buy_order: %Binance.OrderResponse{
            price: buy_price
          },
          rebuy_interval: rebuy_interval,
          rebuy_notified: false
        },
        positions, # <= updated
        settings
      ) do
    if trigger_rebuy?(buy_price, current_price, rebuy_interval) &&
         settings.status != "shutdown" &&
         length(positions) < settings.chunks do # <= added
      :rebuy
    else
      :skip
    end
  end
```

Now we need to deal with the `:rebuy` decision(previously, we removed the logic notifying the `Naive.Leader` about the rebuy being triggered).

\newpage

In case of rebuy decision we need to add a new position to the positions list which can be done by modifying the `generate_decisions/4` function:

```{r, engine = 'elixir', eval = FALSE}
  # /apps/naive/lib/naive/strategy.ex
  def generate_decisions([position | rest] = positions, generated_results, trade_event, settings) do
    ...
    case generate_decision(trade_event, position, current_positions, settings) do
      :exit -> ...
      :rebuy ->
        generate_decisions(
          rest,
          [{:skip, %{position | rebuy_notified: true}}, {:rebuy, position}] ++ generated_results,
          trade_event,
          settings
        ) # ^^^^^ added
      decision -> ...
```

In the case of the `:rebuy` decision, we are updating the `rebuy_notified` of the position that triggered it, as well as adding another position to the list with the `:rebuy` decision(it's the same position that triggered rebuy but we will ignore it further down the line).

The final step will be to update the `execute_decision/3` clause that matches the `:rebuy` decision to

`generate_fresh_position/1`, log and return that newly created position:

```{r, engine = 'elixir', eval = FALSE}
  # /apps/naive/lib/naive/strategy.ex
  defp execute_decision(
         :rebuy,
         %Position{
           id: id,
           symbol: symbol
         }, # <= position removed
         settings # <= updated
       ) do
    new_position = generate_fresh_position(settings) # <= updated

    @logger.info("Position (#{symbol}/#{id}): Rebuy triggered. Starting new position") # <= updated

    {:ok, new_position} # <= updated
  end
```

We updated the whole function body as now it deals with initialising a new position instead of just flipping the `rebuy_triggered` flag inside the original position.

\newpage

We can now run the strategy to confirm that rebuy starts new positions:

```{r, engine = 'bash', eval = FALSE}
$ iex -S mix
...
iex(1)> Streamer.start_streaming("ETHUSDT")
...
iex(2)> Naive.start_trading("ETHUSDT")
...
18:00:29.872 [info]  Position (ETHUSDT/1651856406828): Rebuy triggered. Starting new position
18:00:29.880 [info]  Position (ETHUSDT/1651856429871): Placing a BUY order @ 13.39510000,
  quantity: 14.93000000
```

The above shows that a single buy position can trigger rebuy, starting a new position immediately placing another buy order.

At this moment the integration tests should already be passing, but first, we need to fix the `Naive.TraderTest` a bit to make the test code compile:

```{r, engine = 'elixir', eval = FALSE}
  # /apps/naive/test/naive/trader_test.exs
  defp dummy_trader_state() do
    %Naive.Strategy.Position{ # <= updated
```

That's just the bare minimum as this test won't run, but Elixir would not be able to find the `:id` attribute inside the `State` struct at the compilation time. We can now run the integration tests:

```{r, engine = 'bash', eval = FALSE}
$ MIX_ENV=integration mix test.integration
...
Finished in 7.2 seconds (0.00s async, 7.2s sync)
2 tests, 0 failures, 1 excluded
```

Yay! We reached the point where our strategy took over all the functionality that the `Naive.Leader` provided.

## Fetching active positions

Previously, we were able to figure out the number of currently open positions by looking at the supervision tree, but now there's just a single trader process with possibly multiple open positions.

To aid observability of the state of our trading on the symbols, we will add an interface that can be used to fetch the currently open positions of the trader process.

We will start with the interface itself. It will take a symbol to be able to find the trader responsible for it:

```{r, engine = 'elixir', eval = FALSE}
  # /apps/naive/lib/naive.ex
  alias Naive.Trader
  ...
  def get_positions(symbol) do
    symbol
    |> String.upcase()
    |> Trader.get_positions()
  end
```

Now the trader's interface function will forward the symbol to the `GenServer.call/2` to the actual process `Naive.Trader` process responsible for trading on that symbol:

```{r, engine = 'elixir', eval = FALSE}
  # /apps/naive/lib/naive/trader.ex
  def get_positions(symbol) do
    call_trader(symbol, {:get_positions, symbol})
  end
```

As we need to look up the PID of the trader process in the `Registry`, we can use the same `call_trader/2` helper as in the case of the `notify/2` function.

The message will get sent to the `Trader` process, where we need to add a callback that will return all the current positions:

```{r, engine = 'elixir', eval = FALSE}
  # /apps/naive/lib/naive/trader.ex
  def handle_call(
        {:get_positions, _symbol},
        _,
        state
      ) do
    {:reply, state.positions, state}
  end
```

We can now test fetching currently open positions by running:

```{r, engine = 'bash', eval = FALSE}
$ iex -S mix
...
iex(1)> Streamer.start_streaming("ETHUSDT")
...
iex(2)> Naive.start_trading("ETHUSDT")
...
iex(3)> Naive.get_positions("ETHUSDT")
[
  %Naive.Strategy.Position{
    ...
  },
  %Naive.Strategy.Position{
    ...
  },
  ...
]  
```

We can see that we now have a better overview of what's happening. Previously we needed to go to the database as the state was shared between multiple Trader processes. Now everything is in one place, which we could leverage to load the initial state for some frontend dashboards(subsequent positions' updates could be done by listening to the PubSub topic and pushing diffs to the browser via WebSocket).

## Tidying up

Let's tidy up the codebase start with removing the `/apps/naive/lib/naive/leader.ex`

and `/apps/naive/lib/naive/symbol_supervisor.ex` as we don't need them anymore.

This will cause problems with our mocks that we need to update in the test helper:

```{r, engine = 'elixir', eval = FALSE}
# /apps/naive/test/test_helper.exs
Mox.defmock(Test.Naive.LeaderMock, for: Naive.Leader) # <= remove
```

Our integration test will now run again and pass. Sadly that won't be the case for our unit tests. We will revisit the mocking and unit tests in the next chapter, where we will once again look into how we should structure our code to be more testable and "mockable".

## Final thoughts

In this chapter, we gathered parts of our strategy that were spread across multiple processes and put them into the `Naive.Strategy` module. Furthermore, we made efforts to separate parts of the strategy that are pure from side-effectfull code, which we pushed to the edge. It should be visible that this way, we can cover the vast amount of logic with simple and easy to understand tests that don't require mocking or setup. In the next chapter, we will look into how we could improve the testing of the parts "pushed to the edge"(side effects).

[Note] Please remember to run the `mix format` to keep things nice and tidy.

The source code for this chapter can be found on [GitHub](https://github.com/Cinderella-Man/hands-on-elixir-and-otp-cryptocurrency-trading-bot-source-code/tree/chapter_20)