Connection pools

A connection pool is an object managing a set of connections and allowing their use in functions needing one. Because the time to establish a new connection can be relatively long, keeping connections open can reduce latency.

This page explains a few basic concepts of Psycopg connection pool’s behaviour. Please refer to the ConnectionPool object API for details about the pool operations.

Note

The connection pool objects are distributed in a package separate from the main psycopg package: use pip install psycopg[pool] or pip install psycopg_pool to make the psycopg_pool package available. See Installing the connection pool.

Pool life cycle

A simple way to use the pool is to create a single instance of it, as a global object, and to use this object in the rest of the program, allowing other functions, modules, threads to use it:

# module db.py in your program
from psycopg_pool import ConnectionPool

pool = ConnectionPool(conninfo, **kwargs)
# the pool starts connecting immediately.

# in another module
from .db import pool

def my_function():
    with poolconnection() as conn:
        conn.execute(...)

Ideally you may want to call close() when the use of the pool is finished. Failing to call close() at the end of the program is not terribly bad: probably it will just result in some warnings printed on stderr. However, if you think that it’s sloppy, you could use the atexit module to have close() called at the end of the program.

If you want to avoid starting to connect to the database at import time, and want to wait for the application to be ready, you can create the pool using open = False, and call the open() and close() methods when the conditions are right. Certain frameworks provide callbacks triggered when the program is started and stopped (for instance FastAPI startup/shutdown events): they are perfect to initiate and terminate the pool operations:

pool = ConnectionPool(conninfo, open=False, **kwargs)

@app.on_event("startup")
def open_pool():
    pool.open()

@app.on_event("shutdown")
def close_pool():
    pool.close()

Creating a single pool as a global variable is not the mandatory use: your program can create more than one pool, which might be useful to connect to more than one database, or to provide different types of connections, for instance to provide separate read/write and read-only connections. The pool also acts as a context manager and is open and closed, if necessary, on entering and exiting the context block:

from psycopg_pool import ConnectionPool

with ConnectionPool(conninfo, **kwargs) as pool:
    run_app(pool)

# the pool is now closed

When the pool is open, the pool’s background workers start creating the requested min_size connections, while the constructor (or the open() method) returns immediately. This allows the program some leeway to start before the target database is up and running. However, if your application is misconfigured, or the network is down, it means that the program will be able to start, but the threads requesting a connection will fail with a PoolTimeout only after the timeout on connection() is expired. If this behaviour is not desirable (and you prefer your program to crash hard and fast, if the surrounding conditions are not right, because something else will respawn it) you should call the wait() method after creating the pool, or call open(wait=True): these methods will block until the pool is full, or will raise a PoolTimeout exception if the pool isn’t ready within the allocated time.

Connections life cycle

The pool background workers create connections according to the parameters conninfo, kwargs, and connection_class passed to ConnectionPool constructor, invoking something like connection_class(conninfo, **kwargs). Once a connection is created it is also passed to the configure() callback, if provided, after which it is put in the pool (or passed to a client requesting it, if someone is already knocking at the door).

If a connection expires (it passes max_lifetime), or is returned to the pool in broken state, or is found closed by check()), then the pool will dispose of it and will start a new connection attempt in the background.

Using connections from the pool

The pool can be used to request connections from multiple threads or concurrent tasks - it is hardly useful otherwise! If more connections than the ones available in the pool are requested, the requesting threads are queued and are served a connection as soon as one is available, either because another client has finished using it or because the pool is allowed to grow (when max_size > min_size) and a new connection is ready.

The main way to use the pool is to obtain a connection using the connection() context, which returns a Connection or subclass:

with my_pool.connection() as conn:
    conn.execute("what you want")

The connection() context behaves like the Connection object context: at the end of the block, if there is a transaction open, it will be committed, or rolled back if the context is exited with as exception.

At the end of the block the connection is returned to the pool and shouldn’t be used anymore by the code which obtained it. If a reset() function is specified in the pool constructor, it is called on the connection before returning it to the pool. Note that the reset() function is called in a worker thread, so that the thread which used the connection can keep its execution without being slowed down by it.

Pool connection and sizing

A pool can have a fixed size (specifying no max_size or max_size = min_size) or a dynamic size (when max_size > min_size). In both cases, as soon as the pool is created, it will try to acquire min_size connections in the background.

If an attempt to create a connection fails, a new attempt will be made soon after, using an exponential backoff to increase the time between attempts, until a maximum of reconnect_timeout is reached. When that happens, the pool will call the reconnect_failed() function, if provided to the pool, and just start a new connection attempt. You can use this function either to send alerts or to interrupt the program and allow the rest of your infrastructure to restart it.

If more than min_size connections are requested concurrently, new ones are created, up to max_size. Note that the connections are always created by the background workers, not by the thread asking for the connection: if a client requests a new connection, and a previous client terminates its job before the new connection is ready, the waiting client will be served the existing connection. This is especially useful in scenarios where the time to establish a connection dominates the time for which the connection is used (see this analysis, for instance).

If a pool grows above min_size, but its usage decreases afterwards, a number of connections are eventually closed: one every time a connection is unused after the max_idle time specified in the pool constructor.

What’s the right size for the pool?

Big question. Who knows. However, probably not as large as you imagine. Please take a look at this analysis for some ideas.

Something useful you can do is probably to use the get_stats() method and monitor the behaviour of your program to tune the configuration parameters. The size of the pool can also be changed at runtime using the resize() method.

Null connection pools

New in version 3.1.

Sometimes you may want leave the choice of using or not using a connection pool as a configuration parameter of your application. For instance, you might want to use a pool if you are deploying a “large instance” of your application and can dedicate it a handful of connections; conversely you might not want to use it if you deploy the application in several instances, behind a load balancer, and/or using an external connection pool process such as PgBouncer.

Switching between using or not using a pool requires some code change, because the ConnectionPool API is different from the normal connect() function and because the pool can perform additional connection configuration (in the configure parameter) that, if the pool is removed, should be performed in some different code path of your application.

The psycopg_pool 3.1 package introduces the NullConnectionPool class. This class has the same interface, and largely the same behaviour, of the ConnectionPool, but doesn’t create any connection beforehand. When a connection is returned, unless there are other clients already waiting, it is closed immediately and not kept in the pool state.

A null pool is not only a configuration convenience, but can also be used to regulate the access to the server by a client program. If max_size is set to a value greater than 0, the pool will make sure that no more than max_size connections are created at any given time. If more clients ask for further connections, they will be queued and served a connection as soon as a previous client has finished using it, like for the basic pool. Other mechanisms to throttle client requests (such as timeout or max_waiting) are respected too.

Note

Queued clients will be handed an already established connection, as soon as a previous client has finished using it (and after the pool has returned it to idle state and called reset() on it, if necessary).

Because normally (i.e. unless queued) every client will be served a new connection, the time to obtain the connection is paid by the waiting client; background workers are not normally involved in obtaining new connections.

Connection quality

The state of the connection is verified when a connection is returned to the pool: if a connection is broken during its usage it will be discarded on return and a new connection will be created.

Warning

The health of the connection is not checked when the pool gives it to a client.

Why not? Because doing so would require an extra network roundtrip: we want to save you from its latency. Before getting too angry about it, just think that the connection can be lost any moment while your program is using it. As your program should already be able to cope with a loss of a connection during its process, it should be able to tolerate to be served a broken connection: unpleasant but not the end of the world.

Warning

The health of the connection is not checked when the connection is in the pool.

Does the pool keep a watchful eye on the quality of the connections inside it? No, it doesn’t. Why not? Because you will do it for us! Your program is only a big ruse to make sure the connections are still alive…

Not (entirely) trolling: if you are using a connection pool, we assume that you are using and returning connections at a good pace. If the pool had to check for the quality of a broken connection before your program notices it, it should be polling each connection even faster than your program uses them. Your database server wouldn’t be amused…

Can you do something better than that? Of course you can, there is always a better way than polling. You can use the same recipe of Detecting disconnections, reserving a connection and using a thread to monitor for any activity happening on it. If any activity is detected, you can call the pool check() method, which will run a quick check on each connection in the pool, removing the ones found in broken state, and using the background workers to replace them with fresh ones.

If you set up a similar check in your program, in case the database connection is temporarily lost, we cannot do anything for the threads which had taken already a connection from the pool, but no other thread should be served a broken connection, because check() would empty the pool and refill it with working connections, as soon as they are available.

Faster than you can say poll. Or pool.

Pool stats

The pool can return information about its usage using the methods get_stats() or pop_stats(). Both methods return the same values, but the latter reset the counters after its use. The values can be sent to a monitoring system such as Graphite or Prometheus.

The following values should be provided, but please don’t consider them as a rigid interface: it is possible that they might change in the future. Keys whose value is 0 may not be returned.

Metric

Meaning

pool_min

Current value for min_size

pool_max

Current value for max_size

pool_size

Number of connections currently managed by the pool (in the pool, given to clients, being prepared)

pool_available

Number of connections currently idle in the pool

requests_waiting

Number of requests currently waiting in a queue to receive a connection

usage_ms

Total usage time of the connections outside the pool

requests_num

Number of connections requested to the pool

requests_queued

Number of requests queued because a connection wasn’t immediately available in the pool

requests_wait_ms

Total time in the queue for the clients waiting

requests_errors

Number of connection requests resulting in an error (timeouts, queue full…)

returns_bad

Number of connections returned to the pool in a bad state

connections_num

Number of connection attempts made by the pool to the server

connections_ms

Total time spent to establish connections with the server

connections_errors

Number of failed connection attempts

connections_lost

Number of connections lost identified by check()