6
.. module:: sqlalchemy.pool
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A connection pool is a standard technique used to maintain
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long running connections in memory for efficient re-use,
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management for the total number of connections an application
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might use simultaneously.
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server-side web applications, a connection pool is the standard way to
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maintain a "pool" of active database connections in memory which are
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reused across requests.
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SQLAlchemy includes several connection pool implementations
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which integrate with the :class:`.Engine`. They can also be used
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directly for applications that want to add pooling to an otherwise
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Connection Pool Configuration
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-----------------------------
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The :class:`~.engine.Engine` returned by the
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:func:`~sqlalchemy.create_engine` function in most cases has a :class:`.QueuePool`
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integrated, pre-configured with reasonable pooling defaults. If
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you're reading this section only to learn how to enable pooling - congratulations!
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The most common :class:`.QueuePool` tuning parameters can be passed
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directly to :func:`~sqlalchemy.create_engine` as keyword arguments:
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``pool_size``, ``max_overflow``, ``pool_recycle`` and
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``pool_timeout``. For example::
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engine = create_engine('postgresql://me@localhost/mydb',
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pool_size=20, max_overflow=0)
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In the case of SQLite, the :class:`.SingletonThreadPool` or
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:class:`.NullPool` are selected by the dialect to provide
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greater compatibility with SQLite's threading and locking
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model, as well as to provide a reasonable default behavior
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to SQLite "memory" databases, which maintain their entire
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dataset within the scope of a single connection.
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All SQLAlchemy pool implementations have in common
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that none of them "pre create" connections - all implementations wait
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until first use before creating a connection. At that point, if
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no additional concurrent checkout requests for more connections
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are made, no additional connections are created. This is why it's perfectly
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fine for :func:`.create_engine` to default to using a :class:`.QueuePool`
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of size five without regard to whether or not the application really needs five connections
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queued up - the pool would only grow to that size if the application
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actually used five connections concurrently, in which case the usage of a
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small pool is an entirely appropriate default behavior.
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Switching Pool Implementations
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------------------------------
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The usual way to use a different kind of pool with :func:`.create_engine`
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is to use the ``poolclass`` argument. This argument accepts a class
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imported from the ``sqlalchemy.pool`` module, and handles the details
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of building the pool for you. Common options include specifying
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:class:`.QueuePool` with SQLite::
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from sqlalchemy.pool import QueuePool
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engine = create_engine('sqlite:///file.db', poolclass=QueuePool)
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Disabling pooling using :class:`.NullPool`::
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from sqlalchemy.pool import NullPool
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engine = create_engine(
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'postgresql+psycopg2://scott:tiger@localhost/test',
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Using a Custom Connection Function
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----------------------------------
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All :class:`.Pool` classes accept an argument ``creator`` which is
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a callable that creates a new connection. :func:`.create_engine`
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accepts this function to pass onto the pool via an argument of
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import sqlalchemy.pool as pool
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c = psycopg2.connect(username='ed', host='127.0.0.1', dbname='test')
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# do things with 'c' to set up
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engine = create_engine('postgresql+psycopg2://', creator=getconn)
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For most "initialize on connection" routines, it's more convenient
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to use the :class:`.PoolEvents` event hooks, so that the usual URL argument to
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:func:`.create_engine` is still usable. ``creator`` is there as
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a last resort for when a DBAPI has some form of ``connect``
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that is not at all supported by SQLAlchemy.
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------------------------
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To use a :class:`.Pool` by itself, the ``creator`` function is
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the only argument that's required and is passed first, followed
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by any additional options::
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import sqlalchemy.pool as pool
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c = psycopg2.connect(username='ed', host='127.0.0.1', dbname='test')
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mypool = pool.QueuePool(getconn, max_overflow=10, pool_size=5)
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DBAPI connections can then be procured from the pool using the :meth:`.Pool.connect`
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function. The return value of this method is a DBAPI connection that's contained
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within a transparent proxy::
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conn = mypool.connect()
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cursor = conn.cursor()
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cursor.execute("select foo")
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The purpose of the transparent proxy is to intercept the ``close()`` call,
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such that instead of the DBAPI connection being closed, it's returned to the
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# "close" the connection. Returns
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The proxy also returns its contained DBAPI connection to the pool
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when it is garbage collected,
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though it's not deterministic in Python that this occurs immediately (though
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it is typical with cPython).
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The ``close()`` step also performs the important step of calling the
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``rollback()`` method of the DBAPI connection. This is so that any
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existing transaction on the connection is removed, not only ensuring
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that no existing state remains on next usage, but also so that table
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and row locks are released as well as that any isolated data snapshots
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are removed. This behavior can be disabled using the ``reset_on_return``
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option of :class:`.Pool`.
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A particular pre-created :class:`.Pool` can be shared with one or more
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engines by passing it to the ``pool`` argument of :func:`.create_engine`::
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e = create_engine('postgresql://', pool=mypool)
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Connection pools support an event interface that allows hooks to execute
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upon first connect, upon each new connection, and upon checkout and
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checkin of connections. See :class:`.PoolEvents` for details.
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Dealing with Disconnects
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------------------------
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The connection pool has the ability to refresh individual connections as well as
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its entire set of connections, setting the previously pooled connections as
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"invalid". A common use case is allow the connection pool to gracefully recover
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when the database server has been restarted, and all previously established connections
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are no longer functional. There are two approaches to this.
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Disconnect Handling - Optimistic
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^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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The most common approach is to let SQLAlchemy handle disconnects as they
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occur, at which point the pool is refreshed. This assumes the :class:`.Pool`
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is used in conjunction with a :class:`.Engine`. The :class:`.Engine` has
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logic which can detect disconnection events and refresh the pool automatically.
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When the :class:`.Connection` attempts to use a DBAPI connection, and an
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exception is raised that corresponds to a "disconnect" event, the connection
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is invalidated. The :class:`.Connection` then calls the :meth:`.Pool.recreate`
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method, effectively invalidating all connections not currently checked out so
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that they are replaced with new ones upon next checkout::
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from sqlalchemy import create_engine, exc
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e = create_engine(...)
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# suppose the database has been restarted.
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c.execute("SELECT * FROM table")
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except exc.DBAPIError, e:
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# an exception is raised, Connection is invalidated.
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if e.connection_invalidated:
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print "Connection was invalidated!"
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# after the invalidate event, a new connection
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# starts with a new Pool
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c.execute("SELECT * FROM table")
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The above example illustrates that no special intervention is needed, the pool
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continues normally after a disconnection event is detected. However, an exception is
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raised. In a typical web application using an ORM Session, the above condition would
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correspond to a single request failing with a 500 error, then the web application
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continuing normally beyond that. Hence the approach is "optimistic" in that frequent
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database restarts are not anticipated.
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~~~~~~~~~~~~~~~~~~~~~~~
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An additional setting that can augment the "optimistic" approach is to set the
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pool recycle parameter. This parameter prevents the pool from using a particular
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connection that has passed a certain age, and is appropriate for database backends
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such as MySQL that automatically close connections that have been stale after a particular
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from sqlalchemy import create_engine
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e = create_engine("mysql://scott:tiger@localhost/test", pool_recycle=3600)
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Above, any DBAPI connection that has been open for more than one hour will be invalidated and replaced,
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upon next checkout. Note that the invalidation **only** occurs during checkout - not on
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any connections that are held in a checked out state. ``pool_recycle`` is a function
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of the :class:`.Pool` itself, independent of whether or not an :class:`.Engine` is in use.
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Disconnect Handling - Pessimistic
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^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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At the expense of some extra SQL emitted for each connection checked out from the pool,
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a "ping" operation established by a checkout event handler
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can detect an invalid connection before it's used::
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from sqlalchemy import exc
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from sqlalchemy import event
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from sqlalchemy.pool import Pool
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@event.listens_for(Pool, "checkout")
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def ping_connection(dbapi_connection, connection_record, connection_proxy):
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cursor = dbapi_connection.cursor()
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cursor.execute("SELECT 1")
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# optional - dispose the whole pool
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# instead of invalidating one at a time
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# connection_proxy._pool.dispose()
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# raise DisconnectionError - pool will try
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# connecting again up to three times before raising.
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raise exc.DisconnectionError()
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Above, the :class:`.Pool` object specifically catches :class:`~sqlalchemy.exc.DisconnectionError` and attempts
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to create a new DBAPI connection, up to three times, before giving up and then raising
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:class:`~sqlalchemy.exc.InvalidRequestError`, failing the connection. This recipe will ensure
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that a new :class:`.Connection` will succeed even if connections
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in the pool have gone stale, provided that the database server is actually running. The expense
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is that of an additional execution performed per checkout. When using the ORM :class:`.Session`,
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there is one connection checkout per transaction, so the expense is fairly low. The ping approach
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above also works with straight connection pool usage, that is, even if no :class:`.Engine` were
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The event handler can be tested using a script like the following, restarting the database
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server at the point at which the script pauses for input::
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from sqlalchemy import create_engine
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e = create_engine("mysql://scott:tiger@localhost/test", echo_pool=True)
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# pool size is now three.
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print "Restart the server"
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print c.execute("select 1").fetchall()
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.. _pool_connection_invalidation:
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The :class:`.Pool` provides "connection invalidation" services which allow
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both explicit invalidation of a connection as well as automatic invalidation
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in response to conditions that are determined to render a connection unusable.
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"Invalidation" means that a particular DBAPI connection is removed from the
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pool and discarded. The ``.close()`` method is called on this connection
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if it is not clear that the connection itself might not be closed, however
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if this method fails, the exception is logged but the operation still proceeds.
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When using a :class:`.Engine`, the :meth:`.Connection.invalidate` method is
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the usual entrypoint to explicit invalidation. Other conditions by which
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a DBAPI connection might be invalidated include:
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* a DBAPI exception such as :class:`.OperationalError`, raised when a
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method like ``connection.execute()`` is called, is detected as indicating
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a so-called "disconnect" condition. As the Python DBAPI provides no
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standard system for determining the nature of an exception, all SQLAlchemy
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dialects include a system called ``is_disconnect()`` which will examine
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the contents of an exception object, including the string message and
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any potential error codes included with it, in order to determine if this
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exception indicates that the connection is no longer usable. If this is the
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case, the :meth:`._ConnectionFairy.invalidate` method is called and the
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DBAPI connection is then discarded.
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* When the connection is returned to the pool, and
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calling the ``connection.rollback()`` or ``connection.commit()`` methods,
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as dictated by the pool's "reset on return" behavior, throws an exception.
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A final attempt at calling ``.close()`` on the connection will be made,
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and it is then discarded.
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* When a listener implementing :meth:`.PoolEvents.checkout` raises the
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:class:`~sqlalchemy.exc.DisconnectionError` exception, indicating that the connection
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won't be usable and a new connection attempt needs to be made.
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All invalidations which occur will invoke the :meth:`.PoolEvents.invalidate`
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API Documentation - Available Pool Implementations
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---------------------------------------------------
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.. autoclass:: sqlalchemy.pool.Pool
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.. automethod:: __init__
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.. automethod:: connect
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.. automethod:: dispose
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.. automethod:: recreate
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.. automethod:: unique_connection
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.. autoclass:: sqlalchemy.pool.QueuePool
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.. automethod:: __init__
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.. automethod:: connect
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.. automethod:: unique_connection
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.. autoclass:: SingletonThreadPool
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.. automethod:: __init__
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.. autoclass:: AssertionPool
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.. autoclass:: NullPool
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.. autoclass:: StaticPool
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.. autoclass:: _ConnectionFairy
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.. autoattribute:: _connection_record
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.. autoclass:: _ConnectionRecord
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Pooling Plain DB-API Connections
369
--------------------------------
371
Any :pep:`249` DB-API module can be "proxied" through the connection
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pool transparently. Usage of the DB-API is exactly as before, except
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the ``connect()`` method will consult the pool. Below we illustrate
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this with ``psycopg2``::
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import sqlalchemy.pool as pool
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import psycopg2 as psycopg
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psycopg = pool.manage(psycopg)
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# then connect normally
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connection = psycopg.connect(database='test', username='scott',
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This produces a :class:`_DBProxy` object which supports the same
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``connect()`` function as the original DB-API module. Upon
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connection, a connection proxy object is returned, which delegates its
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calls to a real DB-API connection object. This connection object is
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stored persistently within a connection pool (an instance of
390
:class:`.Pool`) that corresponds to the exact connection arguments sent
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to the ``connect()`` function.
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The connection proxy supports all of the methods on the original
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connection object, most of which are proxied via ``__getattr__()``.
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The ``close()`` method will return the connection to the pool, and the
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``cursor()`` method will return a proxied cursor object. Both the
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connection proxy and the cursor proxy will also return the underlying
398
connection to the pool after they have both been garbage collected,
399
which is detected via weakref callbacks (``__del__`` is not used).
401
Additionally, when connections are returned to the pool, a
402
``rollback()`` is issued on the connection unconditionally. This is
403
to release any locks still held by the connection that may have
404
resulted from normal activity.
406
By default, the ``connect()`` method will return the same connection
407
that is already checked out in the current thread. This allows a
408
particular connection to be used in a given thread without needing to
409
pass it around between functions. To disable this behavior, specify
410
``use_threadlocal=False`` to the ``manage()`` function.
412
.. autofunction:: sqlalchemy.pool.manage
414
.. autofunction:: sqlalchemy.pool.clear_managers
added
removed
doc/_sources/core/pooling.txt
