Working with Engines and Connections
The establishment of a database connection is typically a somewhat expensive operation, and applications need a way to get at database connections repeatedly with minimal overhead. Particularly for server-side web applications, a connection pool is the standard way to maintain a “pool” of active database connections in memory which are reused across requests.
SQLAlchemy includes several connection pool implementations which integrate with the Engine. They can also be used directly for applications that want to add pooling to an otherwise plain DBAPI approach.
The Engine returned by the create_engine() function in most cases has a QueuePool integrated, pre-configured with reasonable pooling defaults. If you’re reading this section to simply enable pooling- congratulations! You’re already done.
The most common QueuePool tuning parameters can be passed directly to create_engine() as keyword arguments: pool_size, max_overflow, pool_recycle and pool_timeout. For example:
engine = create_engine('postgresql://me@localhost/mydb',
pool_size=20, max_overflow=0)In the case of SQLite, a SingletonThreadPool is provided instead, to provide compatibility with SQLite’s restricted threading model, as well as to provide a reasonable default behavior to SQLite “memory” databases, which maintain their entire dataset within the scope of a single connection.
All SQLAlchemy pool implementations have in common that none of them “pre create” connections - all implementations wait until first use before creating a connection. At that point, if no additional concurrent checkout requests for more connections are made, no additional connections are created. This is why it’s perfectly fine for create_engine() to default to using a QueuePool of size five without regard to whether or not the application really needs five connections queued up - the pool would only grow to that size if the application actually used five connections concurrently, in which case the usage of a small pool is an entirely appropriate default behavior.
The usual way to use a different kind of pool with create_engine() is to use the poolclass argument. This argument accepts a class imported from the sqlalchemy.pool module, and handles the details of building the pool for you. Common options include specifying QueuePool with SQLite:
from sqlalchemy.pool import QueuePool
engine = create_engine('sqlite:///file.db', poolclass=QueuePool)Disabling pooling using NullPool:
from sqlalchemy.pool import NullPool
engine = create_engine(
'postgresql+psycopg2://scott:tiger@localhost/test',
poolclass=NullPool)All Pool classes accept an argument creator which is a callable that creates a new connection. create_engine() accepts this function to pass onto the pool via an argument of the same name:
import sqlalchemy.pool as pool
import psycopg2
def getconn():
c = psycopg2.connect(username='ed', host='127.0.0.1', dbname='test')
# do things with 'c' to set up
return c
engine = create_engine('postgresql+psycopg2://', creator=getconn)For most “initialize on connection” routines, it’s more convenient to use a PoolListener, so that the usual URL argument to create_engine() is still usable. creator is there as a total last resort for when a DBAPI has some form of connect that is not at all supported by SQLAlchemy.
To use a Pool by itself, the creator function is the only argument that’s required and is passed first, followed by any additional options:
import sqlalchemy.pool as pool
import psycopg2
def getconn():
c = psycopg2.connect(username='ed', host='127.0.0.1', dbname='test')
return c
mypool = pool.QueuePool(getconn, max_overflow=10, pool_size=5)DBAPI connections can then be procured from the pool using the Pool.connect() function. The return value of this method is a DBAPI connection that’s contained within a transparent proxy:
# get a connection
conn = mypool.connect()
# use it
cursor = conn.cursor()
cursor.execute("select foo")The purpose of the transparent proxy is to intercept the close() call, such that instead of the DBAPI connection being closed, its returned to the pool:
# "close" the connection. Returns
# it to the pool.
conn.close()The proxy also returns its contained DBAPI connection to the pool when it is garbage collected, though it’s not deterministic in Python that this occurs immediately (though it is typical with cPython).
A particular pre-created Pool can be shared with one or more engines by passing it to the pool argument of create_engine():
e = create_engine('postgresql://', pool=mypool)Connection pools support an event interface that allows hooks to execute upon first connect, upon each new connection, and upon checkout and checkin of connections. See PoolListener for details.
Abstract base class for connection pools.
Construct a Pool.
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Return a DBAPI connection from the pool.
The connection is instrumented such that when its close() method is called, the connection will be returned to the pool.
Dispose of this pool.
This method leaves the possibility of checked-out connections remaining open, It is advised to not reuse the pool once dispose() is called, and to instead use a new pool constructed by the recreate() method.
Bases: sqlalchemy.pool.Pool
A Pool that imposes a limit on the number of open connections.
Construct a QueuePool.
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Bases: sqlalchemy.pool.Pool
A Pool that maintains one connection per thread.
Maintains one connection per each thread, never moving a connection to a thread other than the one which it was created in.
This is used for SQLite, which both does not handle multithreading by default, and also requires a singleton connection if a :memory: database is being used.
Options are the same as those of Pool, as well as:
| Parameters: | pool_size – The number of threads in which to maintain connections at once. Defaults to five. |
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Bases: sqlalchemy.pool.Pool
A Pool that allows at most one checked out connection at any given time.
This will raise an exception if more than one connection is checked out at a time. Useful for debugging code that is using more connections than desired.
Bases: sqlalchemy.pool.Pool
A Pool which does not pool connections.
Instead it literally opens and closes the underlying DB-API connection per each connection open/close.
Reconnect-related functions such as recycle and connection invalidation are not supported by this Pool implementation, since no connections are held persistently.
Bases: sqlalchemy.pool.Pool
A Pool of exactly one connection, used for all requests.
Reconnect-related functions such as recycle and connection invalidation (which is also used to support auto-reconnect) are not currently supported by this Pool implementation but may be implemented in a future release.
Any PEP 249 DB-API module can be “proxied” through the connection pool transparently. Usage of the DB-API is exactly as before, except the connect() method will consult the pool. Below we illustrate this with psycopg2:
import sqlalchemy.pool as pool
import psycopg2 as psycopg
psycopg = pool.manage(psycopg)
# then connect normally
connection = psycopg.connect(database='test', username='scott',
password='tiger')This produces a _DBProxy object which supports the same connect() function as the original DB-API module. Upon connection, a connection proxy object is returned, which delegates its calls to a real DB-API connection object. This connection object is stored persistently within a connection pool (an instance of Pool) that corresponds to the exact connection arguments sent to the connect() function.
The connection proxy supports all of the methods on the original connection object, most of which are proxied via __getattr__(). The close() method will return the connection to the pool, and the cursor() method will return a proxied cursor object. Both the connection proxy and the cursor proxy will also return the underlying connection to the pool after they have both been garbage collected, which is detected via weakref callbacks (__del__ is not used).
Additionally, when connections are returned to the pool, a rollback() is issued on the connection unconditionally. This is to release any locks still held by the connection that may have resulted from normal activity.
By default, the connect() method will return the same connection that is already checked out in the current thread. This allows a particular connection to be used in a given thread without needing to pass it around between functions. To disable this behavior, specify use_threadlocal=False to the manage() function.
Return a proxy for a DB-API module that automatically pools connections.
Given a DB-API 2.0 module and pool management parameters, returns a proxy for the module that will automatically pool connections, creating new connection pools for each distinct set of connection arguments sent to the decorated module’s connect() function.
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Remove all current DB-API 2.0 managers.
All pools and connections are disposed.