python连接池原理_python 数据库连接池 DBUtils 源码解析
"""PooledDB - pooling for DB-API 2 connections.
Implements a pool of steady, thread-safe cached connections
to a database which are transparently reused,
using an arbitrary DB-API 2 compliant database interface module.
This should result in a speedup for persistent applications such as the
application server of "Webware for Python," without loss of robustness.
Robustness is provided by using "hardened" SteadyDB connections.
Even if the underlying database is restarted and all connections
are lost, they will be automatically and transparently reopened.
However, since you don't want this to happen in the middle of a database
transaction, you must explicitly start transactions with the begin()
method so that SteadyDB knows that the underlying connection shall not
be replaced and errors passed on until the transaction is completed.
Measures are taken to make the pool of connections thread-safe.
If the underlying DB-API module is thread-safe at the connection level,
the requested connections may be shared with other threads by default,
but you can also request dedicated connections in case you need them.
For the Python DB-API 2 specification, see:
https://www.python.org/dev/peps/pep-0249/
For information on Webware for Python, see:
https://cito.github.io/w4py/
Usage:
First you need to set up the database connection pool by creating
an instance of PooledDB, passing the following parameters:
creator: either an arbitrary function returning new DB-API 2
connection objects or a DB-API 2 compliant database module
mincached: the initial number of idle connections in the pool
(the default of 0 means no connections are made at startup)
maxcached: the maximum number of idle connections in the pool
(the default value of 0 or None means unlimited pool size)
maxshared: maximum number of shared connections allowed
(the default value of 0 or None means all connections are dedicated)
When this maximum number is reached, connections are
shared if they have been requested as shareable.
maxconnections: maximum number of connections generally allowed
(the default value of 0 or None means any number of connections)
blocking: determines behavior when exceeding the maximum
(if this is set to true, block and wait until the number of
connections decreases, but by default an error will be reported)
maxusage: maximum number of reuses of a single connection
(the default of 0 or None means unlimited reuse)
When this maximum usage number of the connection is reached,
the connection is automatically reset (closed and reopened).
setsession: an optional list of SQL commands that may serve to
prepare the session, e.g. ["set datestyle to german", ...]
reset: how connections should be reset when returned to the pool
(False or None to rollback transcations started with begin(),
the default value True always issues a rollback for safety's sake)
failures: an optional exception class or a tuple of exception classes
for which the connection failover mechanism shall be applied,
if the default (OperationalError, InternalError) is not adequate
ping: an optional flag controlling when connections are checked
with the ping() method if such a method is available
(0 = None = never, 1 = default = whenever fetched from the pool,
2 = when a cursor is created, 4 = when a query is executed,
7 = always, and all other bit combinations of these values)
The creator function or the connect function of the DB-API 2 compliant
database module specified as the creator will receive any additional
parameters such as the host, database, user, password etc. You may
choose some or all of these parameters in your own creator function,
allowing for sophisticated failover and load-balancing mechanisms.
For instance, if you are using pgdb as your DB-API 2 database module and
want a pool of at least five connections to your local database 'mydb':
import pgdb # import used DB-API 2 module
from DBUtils.PooledDB import PooledDB
pool = PooledDB(pgdb, 5, database='mydb')
Once you have set up the connection pool you can request
database connections from that pool:
db = pool.connection()
You can use these connections just as if they were ordinary
DB-API 2 connections. Actually what you get is the hardened
SteadyDB version of the underlying DB-API 2 connection.
Please note that the connection may be shared with other threads
by default if you set a non-zero maxshared parameter and the DB-API 2
module allows this. If you want to have a dedicated connection, use:
db = pool.connection(shareable=False)
You can also use this to get a dedicated connection:
db = pool.dedicated_connection()
If you don't need it any more, you should immediately return it to the
pool with db.close(). You can get another connection in the same way.
Warning: In a threaded environment, never do the following:
pool.connection().cursor().execute(...)
This would release the connection too early for reuse which may be
fatal if the connections are not thread-safe. Make sure that the
connection object stays alive as long as you are using it, like that:
db = pool.connection()
cur = db.cursor()
cur.execute(...)
res = cur.fetchone()
cur.close() # or del cur
db.close() # or del db
Note that you need to explicitly start transactions by calling the
begin() method. This ensures that the connection will not be shared
with other threads, that the transparent reopening will be suspended
until the end of the transaction, and that the connection will be rolled
back before being given back to the connection pool.
Ideas for improvement:
* Add a thread for monitoring, restarting (or closing) bad or expired
connections (similar to DBConnectionPool/ResourcePool by Warren Smith).
* Optionally log usage, bad connections and exceeding of limits.
Copyright, credits and license:
* Contributed as supplement for Webware for Python and PyGreSQL
by Christoph Zwerschke in September 2005
* Based on the code of DBPool, contributed to Webware for Python
by Dan Green in December 2000
Licensed under the MIT license.
"""
from threading import Condition
from DBUtils.SteadyDB import connect
__version__ = '1.3'
class PooledDBError(Exception):
"""General PooledDB error."""
class InvalidConnection(PooledDBError):
"""Database connection is invalid."""
class NotSupportedError(PooledDBError):
"""DB-API module not supported by PooledDB."""
class TooManyConnections(PooledDBError):
"""Too many database connections were opened."""
class PooledDB:
"""Pool for DB-API 2 connections.
After you have created the connection pool, you can use
connection() to get pooled, steady DB-API 2 connections.
"""
version = __version__
def __init__(
self, creator, mincached=0, maxcached=0,
maxshared=0, maxconnections=0, blocking=False,
maxusage=None, setsession=None, reset=True,
failures=None, ping=1,
*args, **kwargs):
"""Set up the DB-API 2 connection pool.
creator: either an arbitrary function returning new DB-API 2
connection objects or a DB-API 2 compliant database module
mincached: initial number of idle connections in the pool
(0 means no connections are made at startup)
maxcached: maximum number of idle connections in the pool
(0 or None means unlimited pool size)
maxshared: maximum number of shared connections
(0 or None means all connections are dedicated)
When this maximum number is reached, connections are
shared if they have been requested as shareable.
maxconnections: maximum number of connections generally allowed
(0 or None means an arbitrary number of connections)
blocking: determines behavior when exceeding the maximum
(if this is set to true, block and wait until the number of
connections decreases, otherwise an error will be reported)
maxusage: maximum number of reuses of a single connection
(0 or None means unlimited reuse)
When this maximum usage number of the connection is reached,
the connection is automatically reset (closed and reopened).
setsession: optional list of SQL commands that may serve to prepare
the session, e.g. ["set datestyle to ...", "set time zone ..."]
reset: how connections should be reset when returned to the pool
(False or None to rollback transcations started with begin(),
True to always issue a rollback for safety's sake)
failures: an optional exception class or a tuple of exception classes
for which the connection failover mechanism shall be applied,
if the default (OperationalError, InternalError) is not adequate
ping: determines when the connection should be checked with ping()
(0 = None = never, 1 = default = whenever fetched from the pool,
2 = when a cursor is created, 4 = when a query is executed,
7 = always, and all other bit combinations of these values)
args, kwargs: the parameters that shall be passed to the creator
function or the connection constructor of the DB-API 2 module
"""
try:
threadsafety = creator.threadsafety
except AttributeError:
try:
if not callable(creator.connect):
raise AttributeError
except AttributeError:
threadsafety = 2
else:
threadsafety = 0
if not threadsafety:
raise NotSupportedError("Database module is not thread-safe.")
self._creator = creator
self._args, self._kwargs = args, kwargs
self._blocking = blocking
self._maxusage = maxusage
self._setsession = setsession
self._reset = reset
self._failures = failures
self._ping = ping
if mincached is None:
mincached = 0
if maxcached is None:
maxcached = 0
if maxconnections is None:
maxconnections = 0
if maxcached:
if maxcached < mincached:
maxcached = mincached
self._maxcached = maxcached
else:
self._maxcached = 0
if threadsafety > 1 and maxshared:
self._maxshared = maxshared
self._shared_cache = [] # the cache for shared connections
else:
self._maxshared = 0
if maxconnections:
if maxconnections < maxcached:
maxconnections = maxcached
if maxconnections < maxshared:
maxconnections = maxshared
self._maxconnections = maxconnections
else:
self._maxconnections = 0
self._idle_cache = [] # the actual pool of idle connections
self._lock = Condition()
self._connections = 0
# Establish an initial number of idle database connections:
idle = [self.dedicated_connection() for i in range(mincached)]
while idle:
idle.pop().close()
def steady_connection(self):
"""Get a steady, unpooled DB-API 2 connection."""
return connect(
self._creator, self._maxusage, self._setsession,
self._failures, self._ping, True, *self._args, **self._kwargs)
def connection(self, shareable=True):
"""Get a steady, cached DB-API 2 connection from the pool.
If shareable is set and the underlying DB-API 2 allows it,
then the connection may be shared with other threads.
"""
if shareable and self._maxshared:
self._lock.acquire()
try:
while (not self._shared_cache and self._maxconnections
and self._connections >= self._maxconnections):
self._wait_lock()
if len(self._shared_cache) < self._maxshared:
# shared cache is not full, get a dedicated connection
try: # first try to get it from the idle cache
con = self._idle_cache.pop(0)
except IndexError: # else get a fresh connection
con = self.steady_connection()
else:
con._ping_check() # check this connection
con = SharedDBConnection(con)
self._connections += 1
else: # shared cache full or no more connections allowed
self._shared_cache.sort() # least shared connection first
con = self._shared_cache.pop(0) # get it
while con.con._transaction:
# do not share connections which are in a transaction
self._shared_cache.insert(0, con)
self._wait_lock()
self._shared_cache.sort()
con = self._shared_cache.pop(0)
con.con._ping_check() # check the underlying connection
con.share() # increase share of this connection
# put the connection (back) into the shared cache
self._shared_cache.append(con)
self._lock.notify()
finally:
self._lock.release()
con = PooledSharedDBConnection(self, con)
else: # try to get a dedicated connection
self._lock.acquire()
try:
while (self._maxconnections
and self._connections >= self._maxconnections):
self._wait_lock()
# connection limit not reached, get a dedicated connection
try: # first try to get it from the idle cache
con = self._idle_cache.pop(0)
except IndexError: # else get a fresh connection
con = self.steady_connection()
else:
con._ping_check() # check connection
con = PooledDedicatedDBConnection(self, con)
self._connections += 1
finally:
self._lock.release()
return con
def dedicated_connection(self):
"""Alias for connection(shareable=False)."""
return self.connection(False)
def unshare(self, con):
"""Decrease the share of a connection in the shared cache."""
self._lock.acquire()
try:
con.unshare()
shared = con.shared
if not shared: # connection is idle,
try: # so try to remove it
self._shared_cache.remove(con) # from shared cache
except ValueError:
pass # pool has already been closed
finally:
self._lock.release()
if not shared: # connection has become idle,
self.cache(con.con) # so add it to the idle cache
def cache(self, con):
"""Put a dedicated connection back into the idle cache."""
self._lock.acquire()
try:
if not self._maxcached or len(self._idle_cache) < self._maxcached:
con._reset(force=self._reset) # rollback possible transaction
# the idle cache is not full, so put it there
self._idle_cache.append(con) # append it to the idle cache
else: # if the idle cache is already full,
con.close() # then close the connection
self._connections -= 1
self._lock.notify()
finally:
self._lock.release()
def close(self):
"""Close all connections in the pool."""
self._lock.acquire()
try:
while self._idle_cache: # close all idle connections
con = self._idle_cache.pop(0)
try:
con.close()
except Exception:
pass
if self._maxshared: # close all shared connections
while self._shared_cache:
con = self._shared_cache.pop(0).con
try:
con.close()
except Exception:
pass
self._connections -= 1
self._lock.notifyAll()
finally:
self._lock.release()
def __del__(self):
"""Delete the pool."""
try:
self.close()
except Exception:
pass
def _wait_lock(self):
"""Wait until notified or report an error."""
if not self._blocking:
raise TooManyConnections
self._lock.wait()
# Auxiliary classes for pooled connections
class PooledDedicatedDBConnection:
"""Auxiliary proxy class for pooled dedicated connections."""
def __init__(self, pool, con):
"""
Create a pooled dedicated connection.
pool: the corresponding PooledDB instance
con: the underlying SteadyDB connection
"""
# basic initialization to make finalizer work
self._con = None
# proper initialization of the connection
if not con.threadsafety():
raise NotSupportedError("Database module is not thread-safe.")
self._pool = pool
self._con = con
def close(self):
"""Close the pooled dedicated connection."""
# Instead of actually closing the connection,
# return it to the pool for future reuse.
if self._con:
self._pool.cache(self._con)
self._con = None
def __getattr__(self, name):
"""Proxy all members of the class."""
if self._con:
return getattr(self._con, name)
else:
raise InvalidConnection
def __del__(self):
"""Delete the pooled connection."""
try:
self.close()
except Exception:
pass
class SharedDBConnection:
"""Auxiliary class for shared connections."""
def __init__(self, con):
"""Create a shared connection.
con: the underlying SteadyDB connection
"""
self.con = con
self.shared = 1
def __lt__(self, other):
if self.con._transaction == other.con._transaction:
return self.shared < other.shared
else:
return not self.con._transaction
def __le__(self, other):
if self.con._transaction == other.con._transaction:
return self.shared <= other.shared
else:
return not self.con._transaction
def __eq__(self, other):
return (self.con._transaction == other.con._transaction
and self.shared == other.shared)
def __ne__(self, other):
return not self.__eq__(other)
def __gt__(self, other):
return other.__lt__(self)
def __ge__(self, other):
return other.__le__(self)
def share(self):
"""Increase the share of this connection."""
self.shared += 1
def unshare(self):
"""Decrease the share of this connection."""
self.shared -= 1
class PooledSharedDBConnection:
"""Auxiliary proxy class for pooled shared connections."""
def __init__(self, pool, shared_con):
"""Create a pooled shared connection.
pool: the corresponding PooledDB instance
con: the underlying SharedDBConnection
"""
# basic initialization to make finalizer work
self._con = None
# proper initialization of the connection
con = shared_con.con
if not con.threadsafety() > 1:
raise NotSupportedError("Database connection is not thread-safe.")
self._pool = pool
self._shared_con = shared_con
self._con = con
def close(self):
"""Close the pooled shared connection."""
# Instead of actually closing the connection,
# unshare it and/or return it to the pool.
if self._con:
self._pool.unshare(self._shared_con)
self._shared_con = self._con = None
def __getattr__(self, name):
"""Proxy all members of the class."""
if self._con:
return getattr(self._con, name)
else:
raise InvalidConnection
def __del__(self):
"""Delete the pooled connection."""
try:
self.close()
except Exception:
pass