Add LIFO option for getting pool resources (#162)

bb8/src/api.rs

@@ -95,11 +95,31 @@ pub struct Builder<M: ManageConnection> {
     pub(crate) error_sink: Box<dyn ErrorSink<M::Error>>,
     /// The time interval used to wake up and reap connections.
     pub(crate) reaper_rate: Duration,
+    /// Queue strategy (FIFO or LIFO)
+    pub(crate) queue_strategy: QueueStrategy,
     /// User-supplied trait object responsible for initializing connections
     pub(crate) connection_customizer: Option<Box<dyn CustomizeConnection<M::Connection, M::Error>>>,
     _p: PhantomData<M>,
 }
 
+#[derive(Debug)]
+pub enum QueueStrategy {
+    /// First in first out
+    /// This strategy behaves like a queue
+    /// It will evenly spread load on all existing connections, resetting their idle timeouts, maintaining the pool size
+    Fifo,
+    /// Last in first out
+    /// This behaves like a stack
+    /// It will use the most recently used connection and help to keep the total pool size small by evicting idle connections
+    Lifo,
+}
+
+impl Default for QueueStrategy {
+    fn default() -> Self {
+        QueueStrategy::Fifo
+    }
+}
+
 impl<M: ManageConnection> Default for Builder<M> {
     fn default() -> Self {
         Builder {
@@ -112,6 +132,7 @@ impl<M: ManageConnection> Default for Builder<M> {
             retry_connection: true,
             error_sink: Box::new(NopErrorSink),
             reaper_rate: Duration::from_secs(30),
+            queue_strategy: QueueStrategy::default(),
             connection_customizer: None,
             _p: PhantomData,
         }
@@ -260,6 +281,15 @@ impl<M: ManageConnection> Builder<M> {
         self
     }
 
+    /// Sets the queue strategy to be used by the pool
+    ///
+    /// Defaults to `Fifo`.
+    #[must_use]
+    pub fn queue_strategy(mut self, queue_strategy: QueueStrategy) -> Self {
+        self.queue_strategy = queue_strategy;
+        self
+    }
+
     /// Set the connection customizer to customize newly checked out connections
     #[must_use]
     pub fn connection_customizer(

bb8/src/internals.rs

@@ -2,7 +2,7 @@ use std::cmp::min;
 use std::sync::Arc;
 use std::time::Instant;
 
-use crate::lock::Mutex;
+use crate::{api::QueueStrategy, lock::Mutex};
 use futures_channel::oneshot;
 
 use crate::api::{Builder, ManageConnection};
@@ -42,6 +42,7 @@ where
     conns: VecDeque<IdleConn<M::Connection>>,
     num_conns: u32,
     pending_conns: u32,
+    queue_strategy: QueueStrategy,
 }
 
 impl<M> PoolInternals<M>
@@ -80,8 +81,11 @@ where
         }
 
         // Queue it in the idle queue
-        self.conns
-            .push_back(IdleConn::from(guard.conn.take().unwrap()));
+        let conn = IdleConn::from(guard.conn.take().unwrap());
+        match self.queue_strategy {
+            QueueStrategy::Fifo => self.conns.push_back(conn),
+            QueueStrategy::Lifo => self.conns.push_front(conn),
+        }
     }
 
     pub(crate) fn connect_failed(&mut self, _: Approval) {
@@ -163,6 +167,7 @@ where
             conns: VecDeque::new(),
             num_conns: 0,
             pending_conns: 0,
+            queue_strategy: QueueStrategy::Lifo,
         }
     }
 }