SQLAlchemy 2.1 Documentation
SQLAlchemy Core
- SQL Statements and Expressions API
- Schema Definition Language
- SQL Datatype Objects
- Engine and Connection Use
- Engine Configuration
- Working with Engines and Connections
- Connection Pooling¶
- Connection Pool Configuration
- Switching Pool Implementations
- Using a Custom Connection Function
- Constructing a Pool
- Reset On Return
- Pool Events
- Dealing with Disconnects
- Using FIFO vs. LIFO
- Using Connection Pools with Multiprocessing or os.fork()
- Using a pool instance directly
- API Documentation - Available Pool Implementations
Pool
QueuePool
AsyncAdaptedQueuePool
SingletonThreadPool
AssertionPool
NullPool
StaticPool
ManagesConnection
ConnectionPoolEntry
PoolProxiedConnection
PoolProxiedConnection.close()
PoolProxiedConnection.dbapi_connection
PoolProxiedConnection.detach()
PoolProxiedConnection.driver_connection
PoolProxiedConnection.info
PoolProxiedConnection.invalidate()
PoolProxiedConnection.is_detached
PoolProxiedConnection.is_valid
PoolProxiedConnection.record_info
_ConnectionFairy
_ConnectionRecord
- Core Events
- Core API Basics
Project Versions
- Previous: Working with Engines and Connections
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- Up: Home
- On this page:
- Connection Pooling
- Connection Pool Configuration
- Switching Pool Implementations
- Using a Custom Connection Function
- Constructing a Pool
- Reset On Return
- Pool Events
- Dealing with Disconnects
- Using FIFO vs. LIFO
- Using Connection Pools with Multiprocessing or os.fork()
- Using a pool instance directly
- API Documentation - Available Pool Implementations
Pool
QueuePool
AsyncAdaptedQueuePool
SingletonThreadPool
AssertionPool
NullPool
StaticPool
ManagesConnection
ConnectionPoolEntry
PoolProxiedConnection
PoolProxiedConnection.close()
PoolProxiedConnection.dbapi_connection
PoolProxiedConnection.detach()
PoolProxiedConnection.driver_connection
PoolProxiedConnection.info
PoolProxiedConnection.invalidate()
PoolProxiedConnection.is_detached
PoolProxiedConnection.is_valid
PoolProxiedConnection.record_info
_ConnectionFairy
_ConnectionRecord
Connection Pooling¶
A connection pool is a standard technique used to maintain long running connections in memory for efficient re-use, as well as to provide management for the total number of connections an application might use simultaneously.
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.
Connection Pool Configuration¶
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 only to learn how to 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+psycopg2://me@localhost/mydb", pool_size=20, max_overflow=0
)
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.
Note
The QueuePool
class is not compatible with asyncio.
When using create_async_engine
to create an instance of
AsyncEngine
, the AsyncAdaptedQueuePool
class,
which makes use of an asyncio-compatible queue implementation, is used
instead.
Switching Pool Implementations¶
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. A common use case here is when
connection pooling is to be disabled, which can be achieved by using
the NullPool
implementation:
from sqlalchemy.pool import NullPool
engine = create_engine(
"postgresql+psycopg2://scott:tiger@localhost/test", poolclass=NullPool
)
Using a Custom Connection Function¶
See the section Custom DBAPI connect() arguments / on-connect routines for a rundown of the various connection customization routines.
Constructing a Pool¶
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(user="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_obj = conn.cursor()
cursor_obj.execute("select foo")
The purpose of the transparent proxy is to intercept the close()
call,
such that instead of the DBAPI connection being closed, it is 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). This usage is not recommended however and in particular is not supported with asyncio DBAPI drivers.
Reset On Return¶
The pool includes “reset on return” behavior which will call the rollback()
method of the DBAPI connection when the connection is returned to the pool.
This is so that any existing transactional state is removed from the
connection, which includes not just uncommitted data but table and row locks as
well. For most DBAPIs, the call to rollback()
is inexpensive, and if the
DBAPI has already completed a transaction, the method should be a no-op.
Disabling Reset on Return for non-transactional connections¶
For very specific cases where this rollback()
is not useful, such as when
using a connection that is configured for
autocommit or when using a database
that has no ACID capabilities such as the MyISAM engine of MySQL, the
reset-on-return behavior can be disabled, which is typically done for
performance reasons. This can be affected by using the
Pool.reset_on_return
parameter of Pool
, which
is also available from create_engine()
as
create_engine.pool_reset_on_return
, passing a value of None
.
This is illustrated in the example below, in conjunction with the
create_engine.isolation_level
parameter setting of
AUTOCOMMIT
:
non_acid_engine = create_engine(
"mysql://scott:tiger@host/db",
pool_reset_on_return=None,
isolation_level="AUTOCOMMIT",
)
The above engine won’t actually perform ROLLBACK when connections are returned to the pool; since AUTOCOMMIT is enabled, the driver will also not perform any BEGIN operation.
Custom Reset-on-Return Schemes¶
“reset on return” consisting of a single rollback()
may not be sufficient
for some use cases; in particular, applications which make use of temporary
tables may wish for these tables to be automatically removed on connection
checkin. Some (but notably not all) backends include features that can “reset”
such tables within the scope of a database connection, which may be a desirable
behavior for connection pool reset. Other server resources such as prepared
statement handles and server-side statement caches may persist beyond the
checkin process, which may or may not be desirable, depending on specifics.
Again, some (but again not all) backends may provide for a means of resetting
this state. The two SQLAlchemy included dialects which are known to have
such reset schemes include Microsoft SQL Server, where an undocumented but
widely known stored procedure called sp_reset_connection
is often used,
and PostgreSQL, which has a well-documented series of commands including
DISCARD
RESET
, DEALLOCATE
, and UNLISTEN
.
The following example illustrates how to replace reset on return with the
Microsoft SQL Server sp_reset_connection
stored procedure, using the
PoolEvents.reset()
event hook. The
create_engine.pool_reset_on_return
parameter is set to None
so that the custom scheme can replace the default behavior completely. The
custom hook implementation calls .rollback()
in any case, as it’s usually
important that the DBAPI’s own tracking of commit/rollback will remain
consistent with the state of the transaction:
from sqlalchemy import create_engine
from sqlalchemy import event
mssql_engine = create_engine(
"mssql+pyodbc://scott:tiger^5HHH@mssql2017:1433/test?driver=ODBC+Driver+17+for+SQL+Server",
# disable default reset-on-return scheme
pool_reset_on_return=None,
)
@event.listens_for(mssql_engine, "reset")
def _reset_mssql(dbapi_connection, connection_record, reset_state):
if not reset_state.terminate_only:
dbapi_connection.execute("{call sys.sp_reset_connection}")
# so that the DBAPI itself knows that the connection has been
# reset
dbapi_connection.rollback()
Changed in version 2.0.0b3: Added additional state arguments to
the PoolEvents.reset()
event and additionally ensured the event
is invoked for all “reset” occurrences, so that it’s appropriate
as a place for custom “reset” handlers. Previous schemes which
use the PoolEvents.checkin()
handler remain usable as well.
See also
Temporary Table / Resource Reset for Connection Pooling - in the Microsoft SQL Server documentation
Temporary Table / Resource Reset for Connection Pooling in the PostgreSQL documentation
Logging reset-on-return events¶
Logging for pool events including reset on return can be set
logging.DEBUG
log level along with the sqlalchemy.pool
logger, or by setting
create_engine.echo_pool
to "debug"
when using
create_engine()
:
>>> from sqlalchemy import create_engine
>>> engine = create_engine("postgresql://scott:tiger@localhost/test", echo_pool="debug")
The above pool will show verbose logging including reset on return:
>>> c1 = engine.connect()
DEBUG sqlalchemy.pool.impl.QueuePool Created new connection <connection object ...>
DEBUG sqlalchemy.pool.impl.QueuePool Connection <connection object ...> checked out from pool
>>> c1.close()
DEBUG sqlalchemy.pool.impl.QueuePool Connection <connection object ...> being returned to pool
DEBUG sqlalchemy.pool.impl.QueuePool Connection <connection object ...> rollback-on-return
Pool Events¶
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 PoolEvents
for details.
Dealing with Disconnects¶
The connection pool has the ability to refresh individual connections as well as its entire set of connections, setting the previously pooled connections as “invalid”. A common use case is allow the connection pool to gracefully recover when the database server has been restarted, and all previously established connections are no longer functional. There are two approaches to this.
Disconnect Handling - Pessimistic¶
The pessimistic approach refers to emitting a test statement on the SQL connection at the start of each connection pool checkout, to test that the database connection is still viable. The implementation is dialect-specific, and makes use of either a DBAPI-specific ping method, or by using a simple SQL statement like “SELECT 1”, in order to test the connection for liveness.
The approach adds a small bit of overhead to the connection checkout process, however is otherwise the most simple and reliable approach to completely eliminating database errors due to stale pooled connections. The calling application does not need to be concerned about organizing operations to be able to recover from stale connections checked out from the pool.
Pessimistic testing of connections upon checkout is achievable by
using the Pool.pre_ping
argument, available from create_engine()
via the create_engine.pool_pre_ping
argument:
engine = create_engine("mysql+pymysql://user:pw@host/db", pool_pre_ping=True)
The “pre ping” feature operates on a per-dialect basis either by invoking a DBAPI-specific “ping” method, or if not available will emit SQL equivalent to “SELECT 1”, catching any errors and detecting the error as a “disconnect” situation. If the ping / error check determines that the connection is not usable, the connection will be immediately recycled, and all other pooled connections older than the current time are invalidated, so that the next time they are checked out, they will also be recycled before use.
If the database is still not available when “pre ping” runs, then the initial connect will fail and the error for failure to connect will be propagated normally. In the uncommon situation that the database is available for connections, but is not able to respond to a “ping”, the “pre_ping” will try up to three times before giving up, propagating the database error last received.
It is critical to note that the pre-ping approach does not accommodate for
connections dropped in the middle of transactions or other SQL operations. If
the database becomes unavailable while a transaction is in progress, the
transaction will be lost and the database error will be raised. While the
Connection
object will detect a “disconnect” situation and
recycle the connection as well as invalidate the rest of the connection pool
when this condition occurs, the individual operation where the exception was
raised will be lost, and it’s up to the application to either abandon the
operation, or retry the whole transaction again. If the engine is
configured using DBAPI-level autocommit connections, as described at
Setting Transaction Isolation Levels including DBAPI Autocommit, a connection may be reconnected transparently
mid-operation using events. See the section How Do I “Retry” a Statement Execution Automatically? for
an example.
For dialects that make use of “SELECT 1” and catch errors in order to detect
disconnects, the disconnection test may be augmented for new backend-specific
error messages using the DialectEvents.handle_error()
hook.
Custom / Legacy Pessimistic Ping¶
Before create_engine.pool_pre_ping
was added, the “pre-ping”
approach historically has been performed manually using
the ConnectionEvents.engine_connect()
engine event.
The most common recipe for this is below, for reference
purposes in case an application is already using such a recipe, or special
behaviors are needed:
from sqlalchemy import exc
from sqlalchemy import event
from sqlalchemy import select
some_engine = create_engine(...)
@event.listens_for(some_engine, "engine_connect")
def ping_connection(connection, branch):
if branch:
# this parameter is always False as of SQLAlchemy 2.0,
# but is still accepted by the event hook. In 1.x versions
# of SQLAlchemy, "branched" connections should be skipped.
return
try:
# run a SELECT 1. use a core select() so that
# the SELECT of a scalar value without a table is
# appropriately formatted for the backend
connection.scalar(select(1))
except exc.DBAPIError as err:
# catch SQLAlchemy's DBAPIError, which is a wrapper
# for the DBAPI's exception. It includes a .connection_invalidated
# attribute which specifies if this connection is a "disconnect"
# condition, which is based on inspection of the original exception
# by the dialect in use.
if err.connection_invalidated:
# run the same SELECT again - the connection will re-validate
# itself and establish a new connection. The disconnect detection
# here also causes the whole connection pool to be invalidated
# so that all stale connections are discarded.
connection.scalar(select(1))
else:
raise
The above recipe has the advantage that we are making use of SQLAlchemy’s
facilities for detecting those DBAPI exceptions that are known to indicate
a “disconnect” situation, as well as the Engine
object’s ability
to correctly invalidate the current connection pool when this condition
occurs and allowing the current Connection
to re-validate onto
a new DBAPI connection.
Disconnect Handling - Optimistic¶
When pessimistic handling is not employed, as well as when the database is
shutdown and/or restarted in the middle of a connection’s period of use within
a transaction, the other approach to dealing with stale / closed connections is
to let SQLAlchemy handle disconnects as they occur, at which point all
connections in the pool are invalidated, meaning they are assumed to be
stale and will be refreshed upon next checkout. This behavior assumes the
Pool
is used in conjunction with a Engine
.
The Engine
has logic which can detect
disconnection events and refresh the pool automatically.
When the Connection
attempts to use a DBAPI connection, and an
exception is raised that corresponds to a “disconnect” event, the connection
is invalidated. The Connection
then calls the Pool.recreate()
method, effectively invalidating all connections not currently checked out so
that they are replaced with new ones upon next checkout. This flow is
illustrated by the code example below:
from sqlalchemy import create_engine, exc
e = create_engine(...)
c = e.connect()
try:
# suppose the database has been restarted.
c.execute(text("SELECT * FROM table"))
c.close()
except exc.DBAPIError as e:
# an exception is raised, Connection is invalidated.
if e.connection_invalidated:
print("Connection was invalidated!")
# after the invalidate event, a new connection
# starts with a new Pool
c = e.connect()
c.execute(text("SELECT * FROM table"))
The above example illustrates that no special intervention is needed to refresh the pool, which continues normally after a disconnection event is detected. However, one database exception is raised, per each connection that is in use while the database unavailability event occurred. In a typical web application using an ORM Session, the above condition would correspond to a single request failing with a 500 error, then the web application continuing normally beyond that. Hence the approach is “optimistic” in that frequent database restarts are not anticipated.
Setting Pool Recycle¶
An additional setting that can augment the “optimistic” approach is to set the pool recycle parameter. This parameter prevents the pool from using a particular connection that has passed a certain age, and is appropriate for database backends such as MySQL that automatically close connections that have been stale after a particular period of time:
from sqlalchemy import create_engine
e = create_engine("mysql+mysqldb://scott:tiger@localhost/test", pool_recycle=3600)
Above, any DBAPI connection that has been open for more than one hour will be invalidated and replaced,
upon next checkout. Note that the invalidation only occurs during checkout - not on
any connections that are held in a checked out state. pool_recycle
is a function
of the Pool
itself, independent of whether or not an Engine
is in use.
More on Invalidation¶
The Pool
provides “connection invalidation” services which allow
both explicit invalidation of a connection as well as automatic invalidation
in response to conditions that are determined to render a connection unusable.
“Invalidation” means that a particular DBAPI connection is removed from the
pool and discarded. The .close()
method is called on this connection
if it is not clear that the connection itself might not be closed, however
if this method fails, the exception is logged but the operation still proceeds.
When using a Engine
, the Connection.invalidate()
method is
the usual entrypoint to explicit invalidation. Other conditions by which
a DBAPI connection might be invalidated include:
a DBAPI exception such as
OperationalError
, raised when a method likeconnection.execute()
is called, is detected as indicating a so-called “disconnect” condition. As the Python DBAPI provides no standard system for determining the nature of an exception, all SQLAlchemy dialects include a system calledis_disconnect()
which will examine the contents of an exception object, including the string message and any potential error codes included with it, in order to determine if this exception indicates that the connection is no longer usable. If this is the case, the_ConnectionFairy.invalidate()
method is called and the DBAPI connection is then discarded.When the connection is returned to the pool, and calling the
connection.rollback()
orconnection.commit()
methods, as dictated by the pool’s “reset on return” behavior, throws an exception. A final attempt at calling.close()
on the connection will be made, and it is then discarded.When a listener implementing
PoolEvents.checkout()
raises theDisconnectionError
exception, indicating that the connection won’t be usable and a new connection attempt needs to be made.
All invalidations which occur will invoke the PoolEvents.invalidate()
event.
Supporting new database error codes for disconnect scenarios¶
SQLAlchemy dialects each include a routine called is_disconnect()
that is
invoked whenever a DBAPI exception is encountered. The DBAPI exception object
is passed to this method, where dialect-specific heuristics will then determine
if the error code received indicates that the database connection has been
“disconnected”, or is in an otherwise unusable state which indicates it should
be recycled. The heuristics applied here may be customized using the
DialectEvents.handle_error()
event hook, which is typically
established via the owning Engine
object. Using this hook, all
errors which occur are delivered passing along a contextual object known as
ExceptionContext
. Custom event hooks may control whether or not a
particular error should be considered a “disconnect” situation or not, as well
as if this disconnect should cause the entire connection pool to be invalidated
or not.
For example, to add support to consider the Oracle Database driver error codes
DPY-1001
and DPY-4011
to be handled as disconnect codes, apply an event
handler to the engine after creation:
import re
from sqlalchemy import create_engine
engine = create_engine(
"oracle+oracledb://scott:tiger@localhost:1521?service_name=freepdb1"
)
@event.listens_for(engine, "handle_error")
def handle_exception(context: ExceptionContext) -> None:
if not context.is_disconnect and re.match(
r"^(?:DPY-1001|DPY-4011)", str(context.original_exception)
):
context.is_disconnect = True
return None
The above error processing function will be invoked for all Oracle Database errors raised, including those caught when using the pool pre ping feature for those backends that rely upon disconnect error handling (new in 2.0).
See also
Using FIFO vs. LIFO¶
The QueuePool
class features a flag called
QueuePool.use_lifo
, which can also be accessed from
create_engine()
via the flag 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 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 QueuePool
use.
New in version 1.3.
See also
Using Connection Pools with Multiprocessing or os.fork()¶
It’s critical that when using a connection pool, and by extension when
using an Engine
created via create_engine()
, that
the pooled connections are not shared to a forked process. TCP connections
are represented as file descriptors, which usually work across process
boundaries, meaning this will cause concurrent access to the file descriptor
on behalf of two or more entirely independent Python interpreter states.
Depending on specifics of the driver and OS, the issues that arise here range from non-working connections to socket connections that are used by multiple processes concurrently, leading to broken messaging (the latter case is typically the most common).
The SQLAlchemy Engine
object refers to a connection pool of existing
database connections. So when this object is replicated to a child process,
the goal is to ensure that no database connections are carried over. There
are four general approaches to this:
Disable pooling using
NullPool
. This is the most simplistic, one shot system that prevents theEngine
from using any connection more than once:from sqlalchemy.pool import NullPool engine = create_engine("mysql+mysqldb://user:pass@host/dbname", poolclass=NullPool)
Call
Engine.dispose()
on any givenEngine
, passing theEngine.dispose.close
parameter with a value ofFalse
, within the initialize phase of the child process. This is so that the new process will not touch any of the parent process’ connections and will instead start with new connections. This is the recommended approach:from multiprocessing import Pool engine = create_engine("mysql+mysqldb://user:pass@host/dbname") def run_in_process(some_data_record): with engine.connect() as conn: conn.execute(text("...")) def initializer(): """ensure the parent proc's database connections are not touched in the new connection pool""" engine.dispose(close=False) with Pool(10, initializer=initializer) as p: p.map(run_in_process, data)
New in version 1.4.33: Added the
Engine.dispose.close
parameter to allow the replacement of a connection pool in a child process without interfering with the connections used by the parent process.Call
Engine.dispose()
directly before the child process is created. This will also cause the child process to start with a new connection pool, while ensuring the parent connections are not transferred to the child process:engine = create_engine("mysql://user:pass@host/dbname") def run_in_process(): with engine.connect() as conn: conn.execute(text("...")) # before process starts, ensure engine.dispose() is called engine.dispose() p = Process(target=run_in_process) p.start()
An event handler can be applied to the connection pool that tests for connections being shared across process boundaries, and invalidates them:
from sqlalchemy import event from sqlalchemy import exc import os engine = create_engine("...") @event.listens_for(engine, "connect") def connect(dbapi_connection, connection_record): connection_record.info["pid"] = os.getpid() @event.listens_for(engine, "checkout") def checkout(dbapi_connection, connection_record, connection_proxy): pid = os.getpid() if connection_record.info["pid"] != pid: connection_record.dbapi_connection = connection_proxy.dbapi_connection = None raise exc.DisconnectionError( "Connection record belongs to pid %s, " "attempting to check out in pid %s" % (connection_record.info["pid"], pid) )
Above, we use an approach similar to that described in Disconnect Handling - Pessimistic to treat a DBAPI connection that originated in a different parent process as an “invalid” connection, coercing the pool to recycle the connection record to make a new connection.
The above strategies will accommodate the case of an Engine
being shared among processes. The above steps alone are not sufficient for the
case of sharing a specific Connection
over a process boundary;
prefer to keep the scope of a particular Connection
local to a
single process (and thread). It’s additionally not supported to share any kind
of ongoing transactional state directly across a process boundary, such as an
ORM Session
object that’s begun a transaction and references
active Connection
instances; again prefer to create new
Session
objects in new processes.
Using a pool instance directly¶
A pool implementation can be used directly without an engine. This could be used
in applications that just wish to use the pool behavior without all other
SQLAlchemy features.
In the example below the default pool for the MySQLdb
dialect is obtained using
create_pool_from_url()
:
from sqlalchemy import create_pool_from_url
my_pool = create_pool_from_url(
"mysql+mysqldb://", max_overflow=5, pool_size=5, pre_ping=True
)
con = my_pool.connect()
# use the connection
...
# then close it
con.close()
If the type of pool to create is not specified, the default one for the dialect
will be used. To specify it directly the poolclass
argument can be used,
like in the following example:
from sqlalchemy import create_pool_from_url
from sqlalchemy import NullPool
my_pool = create_pool_from_url("mysql+mysqldb://", poolclass=NullPool)
API Documentation - Available Pool Implementations¶
Object Name | Description |
---|---|
Proxies a DBAPI connection and provides return-on-dereference support. |
|
Maintains a position in a connection pool which references a pooled connection. |
|
A |
|
An asyncio-compatible version of |
|
Interface for the object that maintains an individual database
connection on behalf of a |
|
Common base for the two connection-management interfaces
|
|
A Pool which does not pool connections. |
|
Abstract base class for connection pools. |
|
A connection-like adapter for a PEP 249 DBAPI connection, which
includes additional methods specific to the |
|
A |
|
A Pool that maintains one connection per thread. |
|
A Pool of exactly one connection, used for all requests. |
- class sqlalchemy.pool.Pool¶
Abstract base class for connection pools.
Members
Class signature
class
sqlalchemy.pool.Pool
(sqlalchemy.log.Identified
,sqlalchemy.event.registry.EventTarget
)-
method
sqlalchemy.pool.Pool.
__init__(creator: _CreatorFnType | _CreatorWRecFnType, recycle: int = -1, echo: log._EchoFlagType = None, logging_name: str | None = None, reset_on_return: _ResetStyleArgType = True, events: List[Tuple[_ListenerFnType, str]] | None = None, dialect: _ConnDialect | Dialect | None = None, pre_ping: bool = False, _dispatch: _DispatchCommon[Pool] | None = None)¶ Construct a Pool.
- Parameters:
creator¶ – a callable function that returns a DB-API connection object. The function will be called with parameters.
recycle¶ – If set to a value other than -1, number of seconds between connection recycling, which means upon checkout, if this timeout is surpassed the connection will be closed and replaced with a newly opened connection. Defaults to -1.
logging_name¶ – String identifier which will be used within the “name” field of logging records generated within the “sqlalchemy.pool” logger. Defaults to a hexstring of the object’s id.
echo¶ –
if True, the connection pool will log informational output such as when connections are invalidated as well as when connections are recycled to the default log handler, which defaults to
sys.stdout
for output.. If set to the string"debug"
, the logging will include pool checkouts and checkins.The
Pool.echo
parameter can also be set from thecreate_engine()
call by using thecreate_engine.echo_pool
parameter.See also
Configuring Logging - further detail on how to configure logging.
reset_on_return¶ –
Determine steps to take on connections as they are returned to the pool, which were not otherwise handled by a
Connection
. Available fromcreate_engine()
via thecreate_engine.pool_reset_on_return
parameter.Pool.reset_on_return
can have any of these values:"rollback"
- call rollback() on the connection, to release locks and transaction resources. This is the default value. The vast majority of use cases should leave this value set."commit"
- call commit() on the connection, to release locks and transaction resources. A commit here may be desirable for databases that cache query plans if a commit is emitted, such as Microsoft SQL Server. However, this value is more dangerous than ‘rollback’ because any data changes present on the transaction are committed unconditionally.None
- don’t do anything on the connection. This setting may be appropriate if the database / DBAPI works in pure “autocommit” mode at all times, or if a custom reset handler is established using thePoolEvents.reset()
event handler.True
- same as ‘rollback’, this is here for backwards compatibility.False
- same as None, this is here for backwards compatibility.
For further customization of reset on return, the
PoolEvents.reset()
event hook may be used which can perform any connection activity desired on reset.events¶ – a list of 2-tuples, each of the form
(callable, target)
which will be passed tolisten()
upon construction. Provided here so that event listeners can be assigned viacreate_engine()
before dialect-level listeners are applied.dialect¶ – a
Dialect
that will handle the job of calling rollback(), close(), or commit() on DBAPI connections. If omitted, a built-in “stub” dialect is used. Applications that make use ofcreate_engine()
should not use this parameter as it is handled by the engine creation strategy.pre_ping¶ –
if True, the pool will emit a “ping” (typically “SELECT 1”, but is dialect-specific) on the connection upon checkout, to test if the connection is alive or not. If not, the connection is transparently re-connected and upon success, all other pooled connections established prior to that timestamp are invalidated. Requires that a dialect is passed as well to interpret the disconnection error.
New in version 1.2.
-
method
sqlalchemy.pool.Pool.
connect() → PoolProxiedConnection¶ 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.
-
method
sqlalchemy.pool.Pool.
dispose() → None¶ Dispose of this pool.
This method leaves the possibility of checked-out connections remaining open, as it only affects connections that are idle in the pool.
See also
-
method
- class sqlalchemy.pool.QueuePool¶
A
Pool
that imposes a limit on the number of open connections.QueuePool
is the default pooling implementation used for allEngine
objects other than SQLite with a:memory:
database.The
QueuePool
class is not compatible with asyncio andcreate_async_engine()
. TheAsyncAdaptedQueuePool
class is used automatically when usingcreate_async_engine()
, if no other kind of pool is specified.See also
Members
Class signature
-
method
sqlalchemy.pool.QueuePool.
__init__(creator: _CreatorFnType | _CreatorWRecFnType, pool_size: int = 5, max_overflow: int = 10, timeout: float = 30.0, use_lifo: bool = False, **kw: Any)¶ Construct a QueuePool.
- Parameters:
creator¶ – a callable function that returns a DB-API connection object, same as that of
Pool.creator
.pool_size¶ – The size of the pool to be maintained, defaults to 5. This is the largest number of connections that will be kept persistently in the pool. Note that the pool begins with no connections; once this number of connections is requested, that number of connections will remain.
pool_size
can be set to 0 to indicate no size limit; to disable pooling, use aNullPool
instead.max_overflow¶ – The maximum overflow size of the pool. When the number of checked-out connections reaches the size set in pool_size, additional connections will be returned up to this limit. When those additional connections are returned to the pool, they are disconnected and discarded. It follows then that the total number of simultaneous connections the pool will allow is pool_size + max_overflow, and the total number of “sleeping” connections the pool will allow is pool_size. max_overflow can be set to -1 to indicate no overflow limit; no limit will be placed on the total number of concurrent connections. Defaults to 10.
timeout¶ – The number of seconds to wait before giving up on returning a connection. Defaults to 30.0. This can be a float but is subject to the limitations of Python time functions which may not be reliable in the tens of milliseconds.
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.
New in version 1.3.
**kw¶ – Other keyword arguments including
Pool.recycle
,Pool.echo
,Pool.reset_on_return
and others are passed to thePool
constructor.
-
method
sqlalchemy.pool.QueuePool.
dispose() → None¶ Dispose of this pool.
This method leaves the possibility of checked-out connections remaining open, as it only affects connections that are idle in the pool.
See also
-
method
- class sqlalchemy.pool.AsyncAdaptedQueuePool¶
An asyncio-compatible version of
QueuePool
.This pool is used by default when using
AsyncEngine
engines that were generated fromcreate_async_engine()
. It uses an asyncio-compatible queue implementation that does not usethreading.Lock
.The arguments and operation of
AsyncAdaptedQueuePool
are otherwise identical to that ofQueuePool
.Class signature
class
sqlalchemy.pool.AsyncAdaptedQueuePool
(sqlalchemy.pool.impl.QueuePool
)
- class sqlalchemy.pool.SingletonThreadPool¶
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.
Warning
the
SingletonThreadPool
will call.close()
on arbitrary connections that exist beyond the size setting ofpool_size
, e.g. if more unique thread identities than whatpool_size
states are used. This cleanup is non-deterministic and not sensitive to whether or not the connections linked to those thread identities are currently in use.SingletonThreadPool
may be improved in a future release, however in its current status it is generally used only for test scenarios using a SQLite:memory:
database and is not recommended for production use.The
SingletonThreadPool
class is not compatible with asyncio andcreate_async_engine()
.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.
SingletonThreadPool
is used by the SQLite dialect automatically when a memory-based database is used. See SQLite.Members
Class signature
class
sqlalchemy.pool.SingletonThreadPool
(sqlalchemy.pool.base.Pool
)-
method
sqlalchemy.pool.SingletonThreadPool.
connect() → PoolProxiedConnection¶ 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.
-
method
sqlalchemy.pool.SingletonThreadPool.
dispose() → None¶ Dispose of this pool.
-
method
sqlalchemy.pool.SingletonThreadPool.
recreate() → SingletonThreadPool¶ Return a new
Pool
, of the same class as this one and configured with identical creation arguments.This method is used in conjunction with
dispose()
to close out an entirePool
and create a new one in its place.
- class sqlalchemy.pool.AssertionPool¶
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.
The
AssertionPool
class is compatible with asyncio andcreate_async_engine()
.Members
Class signature
class
sqlalchemy.pool.AssertionPool
(sqlalchemy.pool.base.Pool
)-
method
sqlalchemy.pool.AssertionPool.
dispose() → None¶ Dispose of this pool.
This method leaves the possibility of checked-out connections remaining open, as it only affects connections that are idle in the pool.
See also
-
method
sqlalchemy.pool.AssertionPool.
recreate() → AssertionPool¶ Return a new
Pool
, of the same class as this one and configured with identical creation arguments.This method is used in conjunction with
dispose()
to close out an entirePool
and create a new one in its place.
-
method
- class sqlalchemy.pool.NullPool¶
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.The
NullPool
class is compatible with asyncio andcreate_async_engine()
.Members
Class signature
-
method
sqlalchemy.pool.NullPool.
dispose() → None¶ Dispose of this pool.
This method leaves the possibility of checked-out connections remaining open, as it only affects connections that are idle in the pool.
See also
-
method
- class sqlalchemy.pool.StaticPool¶
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 only partially supported right now and may not yield good results.The
StaticPool
class is compatible with asyncio andcreate_async_engine()
.Members
Class signature
class
sqlalchemy.pool.StaticPool
(sqlalchemy.pool.base.Pool
)-
method
sqlalchemy.pool.StaticPool.
dispose() → None¶ Dispose of this pool.
This method leaves the possibility of checked-out connections remaining open, as it only affects connections that are idle in the pool.
See also
-
method
sqlalchemy.pool.StaticPool.
recreate() → StaticPool¶ Return a new
Pool
, of the same class as this one and configured with identical creation arguments.This method is used in conjunction with
dispose()
to close out an entirePool
and create a new one in its place.
-
method
- class sqlalchemy.pool.ManagesConnection¶
Common base for the two connection-management interfaces
PoolProxiedConnection
andConnectionPoolEntry
.These two objects are typically exposed in the public facing API via the connection pool event hooks, documented at
PoolEvents
.Members
dbapi_connection, driver_connection, info, invalidate(), record_info
New in version 2.0.
-
attribute
sqlalchemy.pool.ManagesConnection.
dbapi_connection: DBAPIConnection | None¶ A reference to the actual DBAPI connection being tracked.
This is a PEP 249-compliant object that for traditional sync-style dialects is provided by the third-party DBAPI implementation in use. For asyncio dialects, the implementation is typically an adapter object provided by the SQLAlchemy dialect itself; the underlying asyncio object is available via the
ManagesConnection.driver_connection
attribute.SQLAlchemy’s interface for the DBAPI connection is based on the
DBAPIConnection
protocol object
-
attribute
sqlalchemy.pool.ManagesConnection.
driver_connection: Any | None¶ The “driver level” connection object as used by the Python DBAPI or database driver.
For traditional PEP 249 DBAPI implementations, this object will be the same object as that of
ManagesConnection.dbapi_connection
. For an asyncio database driver, this will be the ultimate “connection” object used by that driver, such as theasyncpg.Connection
object which will not have standard pep-249 methods.New in version 1.4.24.
-
attribute
sqlalchemy.pool.ManagesConnection.
info¶ Info dictionary associated with the underlying DBAPI connection referred to by this
ManagesConnection
instance, allowing user-defined data to be associated with the connection.The data in this dictionary is persistent for the lifespan of the DBAPI connection itself, including across pool checkins and checkouts. When the connection is invalidated and replaced with a new one, this dictionary is cleared.
For a
PoolProxiedConnection
instance that’s not associated with aConnectionPoolEntry
, such as if it were detached, the attribute returns a dictionary that is local to thatConnectionPoolEntry
. Therefore theManagesConnection.info
attribute will always provide a Python dictionary.See also
-
method
sqlalchemy.pool.ManagesConnection.
invalidate(e: BaseException | None = None, soft: bool = False) → None¶ Mark the managed connection as invalidated.
- Parameters:
See also
-
attribute
sqlalchemy.pool.ManagesConnection.
record_info¶ Persistent info dictionary associated with this
ManagesConnection
.Unlike the
ManagesConnection.info
dictionary, the lifespan of this dictionary is that of theConnectionPoolEntry
which owns it; therefore this dictionary will persist across reconnects and connection invalidation for a particular entry in the connection pool.For a
PoolProxiedConnection
instance that’s not associated with aConnectionPoolEntry
, such as if it were detached, the attribute returns None. Contrast to theManagesConnection.info
dictionary which is never None.See also
-
attribute
- class sqlalchemy.pool.ConnectionPoolEntry¶
Interface for the object that maintains an individual database connection on behalf of a
Pool
instance.The
ConnectionPoolEntry
object represents the long term maintainance of a particular connection for a pool, including expiring or invalidating that connection to have it replaced with a new one, which will continue to be maintained by that sameConnectionPoolEntry
instance. Compared toPoolProxiedConnection
, which is the short-term, per-checkout connection manager, this object lasts for the lifespan of a particular “slot” within a connection pool.The
ConnectionPoolEntry
object is mostly visible to public-facing API code when it is delivered to connection pool event hooks, such asPoolEvents.connect()
andPoolEvents.checkout()
.New in version 2.0:
ConnectionPoolEntry
provides the public facing interface for the_ConnectionRecord
internal class.Members
close(), dbapi_connection, driver_connection, in_use, info, invalidate(), record_info
Class signature
class
sqlalchemy.pool.ConnectionPoolEntry
(sqlalchemy.pool.base.ManagesConnection
)-
method
sqlalchemy.pool.ConnectionPoolEntry.
close() → None¶ Close the DBAPI connection managed by this connection pool entry.
-
attribute
sqlalchemy.pool.ConnectionPoolEntry.
dbapi_connection: DBAPIConnection | None¶ A reference to the actual DBAPI connection being tracked.
This is a PEP 249-compliant object that for traditional sync-style dialects is provided by the third-party DBAPI implementation in use. For asyncio dialects, the implementation is typically an adapter object provided by the SQLAlchemy dialect itself; the underlying asyncio object is available via the
ManagesConnection.driver_connection
attribute.SQLAlchemy’s interface for the DBAPI connection is based on the
DBAPIConnection
protocol object
-
attribute
sqlalchemy.pool.ConnectionPoolEntry.
driver_connection: Any | None¶ The “driver level” connection object as used by the Python DBAPI or database driver.
For traditional PEP 249 DBAPI implementations, this object will be the same object as that of
ManagesConnection.dbapi_connection
. For an asyncio database driver, this will be the ultimate “connection” object used by that driver, such as theasyncpg.Connection
object which will not have standard pep-249 methods.New in version 1.4.24.
-
attribute
sqlalchemy.pool.ConnectionPoolEntry.
in_use¶ Return True the connection is currently checked out
-
attribute
sqlalchemy.pool.ConnectionPoolEntry.
info¶ inherited from the
ManagesConnection.info
attribute ofManagesConnection
Info dictionary associated with the underlying DBAPI connection referred to by this
ManagesConnection
instance, allowing user-defined data to be associated with the connection.The data in this dictionary is persistent for the lifespan of the DBAPI connection itself, including across pool checkins and checkouts. When the connection is invalidated and replaced with a new one, this dictionary is cleared.
For a
PoolProxiedConnection
instance that’s not associated with aConnectionPoolEntry
, such as if it were detached, the attribute returns a dictionary that is local to thatConnectionPoolEntry
. Therefore theManagesConnection.info
attribute will always provide a Python dictionary.See also
-
method
sqlalchemy.pool.ConnectionPoolEntry.
invalidate(e: BaseException | None = None, soft: bool = False) → None¶ inherited from the
ManagesConnection.invalidate()
method ofManagesConnection
Mark the managed connection as invalidated.
- Parameters:
See also
-
attribute
sqlalchemy.pool.ConnectionPoolEntry.
record_info¶ inherited from the
ManagesConnection.record_info
attribute ofManagesConnection
Persistent info dictionary associated with this
ManagesConnection
.Unlike the
ManagesConnection.info
dictionary, the lifespan of this dictionary is that of theConnectionPoolEntry
which owns it; therefore this dictionary will persist across reconnects and connection invalidation for a particular entry in the connection pool.For a
PoolProxiedConnection
instance that’s not associated with aConnectionPoolEntry
, such as if it were detached, the attribute returns None. Contrast to theManagesConnection.info
dictionary which is never None.See also
-
method
- class sqlalchemy.pool.PoolProxiedConnection¶
A connection-like adapter for a PEP 249 DBAPI connection, which includes additional methods specific to the
Pool
implementation.PoolProxiedConnection
is the public-facing interface for the internal_ConnectionFairy
implementation object; users familiar with_ConnectionFairy
can consider this object to be equivalent.New in version 2.0:
PoolProxiedConnection
provides the public- facing interface for the_ConnectionFairy
internal class.Members
close(), dbapi_connection, detach(), driver_connection, info, invalidate(), is_detached, is_valid, record_info
Class signature
class
sqlalchemy.pool.PoolProxiedConnection
(sqlalchemy.pool.base.ManagesConnection
)-
method
sqlalchemy.pool.PoolProxiedConnection.
close() → None¶ Release this connection back to the pool.
The
PoolProxiedConnection.close()
method shadows the PEP 249.close()
method, altering its behavior to instead release the proxied connection back to the connection pool.Upon release to the pool, whether the connection stays “opened” and pooled in the Python process, versus actually closed out and removed from the Python process, is based on the pool implementation in use and its configuration and current state.
-
attribute
sqlalchemy.pool.PoolProxiedConnection.
dbapi_connection: DBAPIConnection | None¶ A reference to the actual DBAPI connection being tracked.
This is a PEP 249-compliant object that for traditional sync-style dialects is provided by the third-party DBAPI implementation in use. For asyncio dialects, the implementation is typically an adapter object provided by the SQLAlchemy dialect itself; the underlying asyncio object is available via the
ManagesConnection.driver_connection
attribute.SQLAlchemy’s interface for the DBAPI connection is based on the
DBAPIConnection
protocol object
-
method
sqlalchemy.pool.PoolProxiedConnection.
detach() → None¶ Separate this connection from its Pool.
This means that the connection will no longer be returned to the pool when closed, and will instead be literally closed. The associated
ConnectionPoolEntry
is de-associated from this DBAPI connection.Note that any overall connection limiting constraints imposed by a Pool implementation may be violated after a detach, as the detached connection is removed from the pool’s knowledge and control.
-
attribute
sqlalchemy.pool.PoolProxiedConnection.
driver_connection: Any | None¶ The “driver level” connection object as used by the Python DBAPI or database driver.
For traditional PEP 249 DBAPI implementations, this object will be the same object as that of
ManagesConnection.dbapi_connection
. For an asyncio database driver, this will be the ultimate “connection” object used by that driver, such as theasyncpg.Connection
object which will not have standard pep-249 methods.New in version 1.4.24.
-
attribute
sqlalchemy.pool.PoolProxiedConnection.
info¶ inherited from the
ManagesConnection.info
attribute ofManagesConnection
Info dictionary associated with the underlying DBAPI connection referred to by this
ManagesConnection
instance, allowing user-defined data to be associated with the connection.The data in this dictionary is persistent for the lifespan of the DBAPI connection itself, including across pool checkins and checkouts. When the connection is invalidated and replaced with a new one, this dictionary is cleared.
For a
PoolProxiedConnection
instance that’s not associated with aConnectionPoolEntry
, such as if it were detached, the attribute returns a dictionary that is local to thatConnectionPoolEntry
. Therefore theManagesConnection.info
attribute will always provide a Python dictionary.See also
-
method
sqlalchemy.pool.PoolProxiedConnection.
invalidate(e: BaseException | None = None, soft: bool = False) → None¶ inherited from the
ManagesConnection.invalidate()
method ofManagesConnection
Mark the managed connection as invalidated.
- Parameters:
See also
-
attribute
sqlalchemy.pool.PoolProxiedConnection.
is_detached¶ Return True if this
PoolProxiedConnection
is detached from its pool.
-
attribute
sqlalchemy.pool.PoolProxiedConnection.
is_valid¶ Return True if this
PoolProxiedConnection
still refers to an active DBAPI connection.
-
attribute
sqlalchemy.pool.PoolProxiedConnection.
record_info¶ inherited from the
ManagesConnection.record_info
attribute ofManagesConnection
Persistent info dictionary associated with this
ManagesConnection
.Unlike the
ManagesConnection.info
dictionary, the lifespan of this dictionary is that of theConnectionPoolEntry
which owns it; therefore this dictionary will persist across reconnects and connection invalidation for a particular entry in the connection pool.For a
PoolProxiedConnection
instance that’s not associated with aConnectionPoolEntry
, such as if it were detached, the attribute returns None. Contrast to theManagesConnection.info
dictionary which is never None.See also
-
method
- class sqlalchemy.pool._ConnectionFairy¶
Proxies a DBAPI connection and provides return-on-dereference support.
This is an internal object used by the
Pool
implementation to provide context management to a DBAPI connection delivered by thatPool
. The public facing interface for this class is described by thePoolProxiedConnection
class. See that class for public API details.The name “fairy” is inspired by the fact that the
_ConnectionFairy
object’s lifespan is transitory, as it lasts only for the length of a specific DBAPI connection being checked out from the pool, and additionally that as a transparent proxy, it is mostly invisible.Class signature
class
sqlalchemy.pool._ConnectionFairy
(sqlalchemy.pool.base.PoolProxiedConnection
)
- class sqlalchemy.pool._ConnectionRecord¶
Maintains a position in a connection pool which references a pooled connection.
This is an internal object used by the
Pool
implementation to provide context management to a DBAPI connection maintained by thatPool
. The public facing interface for this class is described by theConnectionPoolEntry
class. See that class for public API details.Class signature
class
sqlalchemy.pool._ConnectionRecord
(sqlalchemy.pool.base.ConnectionPoolEntry
)
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