Merge "Add LIFO for connection pooling"

doc/build/changelog/migration_13.rst

@@ -313,6 +313,34 @@ well as for casting decimal bind values for MySQL.
 
 :ticket:`3981`
 
+.. _change_pr467:
+
+New last-in-first-out strategy for QueuePool
+---------------------------------------------
+
+The connection pool usually used by :func:`.create_engine` is known
+as :class:`.QueuePool`.  This pool uses an object equivalent to Python's
+built-in ``Queue`` class in order to store database connections waiting
+to be used.   The ``Queue`` features first-in-first-out behavior, which is
+intended to provide a round-robin use of the database connections that are
+persistently in the pool.   However, a potential downside of this is that
+when the utilization of the pool is low, the re-use of each connection in series
+means that a server-side timeout strategy that attempts to reduce unused
+connections is prevented from shutting down these connections.   To suit
+this use case, a new flag :paramref:`.create_engine.pool_use_lifo` is added
+which reverses the ``.get()`` method of the ``Queue`` to pull the connection
+from the beginning of the queue instead of the end, essentially turning the
+"queue" into a "stack" (adding a whole new pool called ``StackPool`` was
+considered, however this was too much verbosity).
+
+.. seealso::
+
+    :ref:`pool_use_lifo`
+
+
+
+
+
 Key Behavioral Changes - Core
 =============================
 

doc/build/changelog/unreleased_13/pr467.rst

@@ -0,0 +1,13 @@
+.. change::
+   :tags: feature, engine
+
+   Added new "lifo" mode to :class:`.QueuePool`, typically enabled by setting
+   the flag :paramref:`.create_engine.pool_use_lifo` to True.   "lifo" mode
+   means the same connection just checked in will be the first to be checked
+   out again, allowing excess connections to be cleaned up from the server
+   side during periods of the pool being only partially utilized.  Pull request
+   courtesy Taem Park.
+
+   .. seealso::
+
+      :ref:`change_pr467`

doc/build/core/pooling.rst

@@ -396,6 +396,41 @@ a DBAPI connection might be invalidated include:
 All invalidations which occur will invoke the :meth:`.PoolEvents.invalidate`
 event.
 
+.. _pool_use_lifo:
+
+Using FIFO vs. LIFO
+-------------------
+
+The :class:`.QueuePool` class features a flag called
+:paramref:`.QueuePool.use_lifo`, which can also be accessed from
+:func:`.create_engine` via the flag :paramref:`.create_engine.pool_use_lifo`.
+Setting this flag to ``True`` causes the pool's "queue" behavior to instead be
+that of a "stack", e.g. the last connection to be returned to the pool is the
+first one to be used on the next request. In contrast to the pool's long-
+standing behavior of first-in-first-out, which produces a round-robin effect of
+using each connection in the pool in series, lifo mode allows excess
+connections to remain idle in the pool, allowing server-side timeout schemes to
+close these connections out.   The difference between FIFO and LIFO is
+basically whether or not its desirable for the pool to keep a full set of
+connections ready to go even during idle periods::
+
+    engine = create_engine(
+        "postgreql://", pool_use_lifo=True, pool_pre_ping=True)
+
+Above, we also make use of the :paramref:`.create_engine.pool_pre_ping` flag
+so that connections which are closed from the server side are gracefully
+handled by the connection pool and replaced with a new connection.
+
+Note that the flag only applies to :class:`.QueuePool` use.
+
+.. versionadded:: 1.3
+
+.. seealso::
+
+    :ref:`pool_disconnects`
+
+
+
 Using Connection Pools with Multiprocessing
 -------------------------------------------
 

lib/sqlalchemy/engine/__init__.py

@@ -399,6 +399,21 @@ def create_engine(*args, **kwargs):
         up on getting a connection from the pool. This is only used
         with :class:`~sqlalchemy.pool.QueuePool`.
 
+    :param pool_use_lifo=False: use LIFO (last-in-first-out) when retrieving
+        connections from :class:`.QueuePool` instead of FIFO
+        (first-in-first-out). Using LIFO, a server-side timeout scheme can
+        reduce the number of connections used during non- peak   periods of
+        use.   When planning for server-side timeouts, ensure that a recycle or
+        pre-ping strategy is in use to gracefully   handle stale connections.
+
+          .. versionadded:: 1.3
+
+          .. seealso::
+
+            :ref:`pool_use_lifo`
+
+            :ref:`pool_disconnects`
+
     :param plugins: string list of plugin names to load.  See
         :class:`.CreateEnginePlugin` for background.
 

lib/sqlalchemy/engine/strategies.py

@@ -123,7 +123,8 @@ class DefaultEngineStrategy(EngineStrategy):
                          'events': 'pool_events',
                          'use_threadlocal': 'pool_threadlocal',
                          'reset_on_return': 'pool_reset_on_return',
-                         'pre_ping': 'pool_pre_ping'}
+                         'pre_ping': 'pool_pre_ping',
+                         'use_lifo': 'pool_use_lifo'}
             for k in util.get_cls_kwargs(poolclass):
                 tk = translate.get(k, k)
                 if tk in kwargs:

lib/sqlalchemy/pool/impl.py

@@ -30,7 +30,7 @@ class QueuePool(Pool):
 
     """
 
-    def __init__(self, creator, pool_size=5, max_overflow=10, timeout=30,
+    def __init__(self, creator, pool_size=5, max_overflow=10, timeout=30, use_lifo=False,
                  **kw):
         r"""
         Construct a QueuePool.
@@ -63,6 +63,21 @@ class QueuePool(Pool):
         :param timeout: The number of seconds to wait before giving up
           on returning a connection. Defaults to 30.
 
+        :param use_lifo: use LIFO (last-in-first-out) when retrieving
+          connections instead of FIFO (first-in-first-out). Using LIFO, a
+          server-side timeout scheme can reduce the number of connections used
+          during non-peak periods of use.   When planning for server-side
+          timeouts, ensure that a recycle or pre-ping strategy is in use to
+          gracefully handle stale connections.
+
+          .. versionadded:: 1.3
+
+          .. seealso::
+
+            :ref:`pool_use_lifo`
+
+            :ref:`pool_disconnects`
+
         :param \**kw: Other keyword arguments including
           :paramref:`.Pool.recycle`, :paramref:`.Pool.echo`,
           :paramref:`.Pool.reset_on_return` and others are passed to the
@@ -70,7 +85,7 @@ class QueuePool(Pool):
 
         """
         Pool.__init__(self, creator, **kw)
-        self._pool = sqla_queue.Queue(pool_size)
+        self._pool = sqla_queue.Queue(pool_size, use_lifo=use_lifo)
         self._overflow = 0 - pool_size
         self._max_overflow = max_overflow
         self._timeout = timeout

lib/sqlalchemy/util/queue.py

@@ -39,10 +39,12 @@ class Full(Exception):
 
 
 class Queue:
-    def __init__(self, maxsize=0):
+    def __init__(self, maxsize=0, use_lifo=False):
         """Initialize a queue object with a given maximum size.
 
         If `maxsize` is <= 0, the queue size is infinite.
+
+        If `use_lifo` is True, this Queue acts like a Stack (LIFO).
         """
 
         self._init(maxsize)
@@ -57,6 +59,8 @@ class Queue:
         # Notify not_full whenever an item is removed from the queue;
         # a thread waiting to put is notified then.
         self.not_full = threading.Condition(self.mutex)
+        # If this queue uses LIFO or FIFO
+        self.use_lifo = use_lifo
 
     def qsize(self):
         """Return the approximate size of the queue (not reliable!)."""
@@ -196,4 +200,9 @@ class Queue:
 
     # Get an item from the queue
     def _get(self):
-        return self.queue.popleft()
+        if self.use_lifo:
+            # LIFO
+            return self.queue.pop()
+        else:
+            # FIFO
+            return self.queue.popleft()

test/engine/test_pool.py

@@ -1983,6 +1983,83 @@ class QueuePoolTest(PoolTestBase):
             rec.checkin
         )
 
+    def test_lifo(self):
+        c1, c2, c3 = Mock(), Mock(), Mock()
+        connections = [c1, c2, c3]
+
+        def creator():
+            return connections.pop(0)
+
+        p = pool.QueuePool(creator, use_lifo=True)
+
+        pc1 = p.connect()
+        pc2 = p.connect()
+        pc3 = p.connect()
+
+        pc1.close()
+        pc2.close()
+        pc3.close()
+
+        for i in range(5):
+            pc1 = p.connect()
+            is_(pc1.connection, c3)
+            pc1.close()
+
+            pc1 = p.connect()
+            is_(pc1.connection, c3)
+
+            pc2 = p.connect()
+            is_(pc2.connection, c2)
+            pc2.close()
+
+            pc3 = p.connect()
+            is_(pc3.connection, c2)
+
+            pc2 = p.connect()
+            is_(pc2.connection, c1)
+
+            pc2.close()
+            pc3.close()
+            pc1.close()
+
+    def test_fifo(self):
+        c1, c2, c3 = Mock(), Mock(), Mock()
+        connections = [c1, c2, c3]
+
+        def creator():
+            return connections.pop(0)
+
+        p = pool.QueuePool(creator)
+
+        pc1 = p.connect()
+        pc2 = p.connect()
+        pc3 = p.connect()
+
+        pc1.close()
+        pc2.close()
+        pc3.close()
+
+        pc1 = p.connect()
+        is_(pc1.connection, c1)
+        pc1.close()
+
+        pc1 = p.connect()
+        is_(pc1.connection, c2)
+
+        pc2 = p.connect()
+        is_(pc2.connection, c3)
+        pc2.close()
+
+        pc3 = p.connect()
+        is_(pc3.connection, c1)
+
+        pc2 = p.connect()
+        is_(pc2.connection, c3)
+
+        pc2.close()
+        pc3.close()
+        pc1.close()
+
 
 class ResetOnReturnTest(PoolTestBase):
     def _fixture(self, **kw):