Concurrent operations#

Psycopg allows to write concurrent code, executing more than one operation at time.

Connection objects are thread-safe: more than one thread at time can use the same connection. Different thread can use the same connection by creating different cursors.
Cursor objects are not thread-safe, and are not designed to be used by several threads at the same time. However, cursors are lightweight objects: different threads can create each one its own cursor to use independently from other threads.

Note

All the cursors that share the same connection will also share the same transaction. This means that, if a thread starts a transaction, every cursor on the same connection will execute their queries in the same transaction and, if one thread causes a database server error, all the other cursors will be in error state until transaction rollback.

It also means that every cursor will see changes made in the same session by other cursors, even if the transaction is still uncommitted. This effect might be desirable or not, and is something to consider when deciding whether to share a connection or not.

Hint

Should you use many cursors or many connections?

Query execution and results retrieval on a connection is serialized: only one cursor at time will be able to run a query on the same connection (the Connection object will coordinate different cursors’ access). If your program runs a mix of database and non-database operations in several threads, then these threads might be able to share the same connection. However, if you expect to execute massively parallel operations on the database, it might be useful to use more than one connection at time, rather than many cursors on the same connection (or a mix of both).

Using several connections, however, has an impact on the server’s performance and usually the number of connections that a server can handle is limited by grumpy sysadmins with long beards and a strict control on the max_connections server setting.

If you want to use more than one connection at time, but still avoid to create too many connections and starve the server, you might want to use a connection pool.

Warning

Connections are not process-safe and cannot be shared across processes, for instance using the facilities of the multiprocessing module.

If you are using Psycopg in a forking framework (for instance in a web server that implements concurrency using multiprocessing), you should make sure that the database connections are created after the worker process is forked. Failing to do so you will probably find the connection in broken state.

Asynchronous operations#

Psycopg Connection and Cursor have counterparts AsyncConnection and AsyncCursor supporting an asyncio interface.

The design of the asynchronous objects is pretty much the same of the sync ones: in order to use them you will only have to scatter the await keyword here and there.

async with await psycopg.AsyncConnection.connect(
        "dbname=test user=postgres") as aconn:
    async with aconn.cursor() as acur:
        await acur.execute(
            "INSERT INTO test (num, data) VALUES (%s, %s)",
            (100, "abc'def"))
        await acur.execute("SELECT * FROM test")
        await acur.fetchone()
        # will return (1, 100, "abc'def")
        async for record in acur:
            print(record)

An AsyncConnection can be used by several asyncio.Task at the same time. However, as with threads, all the AsyncCursor on the same connection will share the same session and will have their access to the connection serialized.

Changed in version 3.1: AsyncConnection.connect() performs DNS name resolution in a non-blocking way.

Warning

Before version 3.1, AsyncConnection.connect() may still block on DNS name resolution. To avoid that you should set the hostaddr connection parameter, or use the resolve_hostaddr_async() to do it automatically.

Warning

On Windows, Psycopg is not compatible with the default ProactorEventLoop. Please use a different loop, for instance the SelectorEventLoop.

For instance, you can use, early in your program:

asyncio.set_event_loop_policy(
    asyncio.WindowsSelectorEventLoopPolicy()
)

`with` async connections#

As seen in the basic usage, connections and cursors can act as context managers, so you can run:

with psycopg.connect("dbname=test user=postgres") as conn:
    with conn.cursor() as cur:
        cur.execute(...)
    # the cursor is closed upon leaving the context
# the transaction is committed, the connection closed

For asynchronous connections it’s almost what you’d expect, but not quite. Please note that connect() and cursor() don’t return a context: they are both factory methods which return an object which can be used as a context. That’s because there are several use cases where it’s useful to handle the objects manually and only close() them when required.

As a consequence you cannot use async with connect(): you have to do it in two steps instead, as in

aconn = await psycopg.AsyncConnection.connect()
async with aconn:
    async with aconn.cursor() as cur:
        await cur.execute(...)

which can be condensed into async with await:

async with await psycopg.AsyncConnection.connect() as aconn:
    async with aconn.cursor() as cur:
        await cur.execute(...)

…but no less than that: you still need to do the double async thing.

Note that the AsyncConnection.cursor() function is not an async function (it never performs I/O), so you don’t need an await on it; as a consequence you can use the normal async with context manager.

Interrupting async operations#

If a long running operation is interrupted by a Ctrl-C on a normal connection running in the main thread, the operation will be cancelled and the connection will be put in error state, from which can be recovered with a normal rollback().

An async connection provides similar behavior in that if the async task is cancelled, any operation on the connection will similarly be cancelled. This can happen either indirectly via Ctrl-C or similar signal, or directly by cancelling the Python Task via the normal way. Psycopg will ask the PostgreSQL postmaster to cancel the operation when it encounters the standard Python CancelledError.

Remember that cancelling the Python Task does not guarantee that the operation will not complete, even if the task ultimately exits prematurely due to CancelledError. If you need to know the ultimate outcome of the statement, then consider calling Connection.cancel() as an alternative to cancelling the task.

Previous versions of Psycopg recommended setting up signal handlers to manually cancel connections. This should no longer be necessary.

Gevent support#

Psycopg 3 supports gevent out of the box. If the select module is found patched by functions such as gevent.monkey.patch_select() or patch_all(), psycopg will behave in a collaborative way.

Unlike with psycopg2, using the psycogreen module is not required.

Warning

gevent support was initially accidental, and was accidentally broken in psycopg 3.1.4.

gevent is officially supported only starting from psycopg 3.1.14.

Server messages#

PostgreSQL can send, together with the query results, informative messages about the operation just performed, such as warnings or debug information. Notices may be raised even if the operations are successful and don’t indicate an error. You are probably familiar with some of them, because they are reported by psql:

$ psql
=# ROLLBACK;
WARNING:  there is no transaction in progress
ROLLBACK

Messages can be also sent by the PL/pgSQL ‘RAISE’ statement (at a level lower than EXCEPTION, otherwise the appropriate DatabaseError will be raised). The level of the messages received can be controlled using the client_min_messages setting.

By default, the messages received are ignored. If you want to process them on the client you can use the Connection.add_notice_handler() function to register a function that will be invoked whenever a message is received. The message is passed to the callback as a Diagnostic instance, containing all the information passed by the server, such as the message text and the severity. The object is the same found on the diag attribute of the errors raised by the server:

>>> import psycopg

>>> def log_notice(diag):
...     print(f"The server says: {diag.severity} - {diag.message_primary}")

>>> conn = psycopg.connect(autocommit=True)
>>> conn.add_notice_handler(log_notice)

>>> cur = conn.execute("ROLLBACK")
The server says: WARNING - there is no transaction in progress
>>> print(cur.statusmessage)
ROLLBACK

Warning

The Diagnostic object received by the callback should not be used after the callback function terminates, because its data is deallocated after the callbacks have been processed. If you need to use the information later please extract the attributes requested and forward them instead of forwarding the whole Diagnostic object.

Asynchronous notifications#

Psycopg allows asynchronous interaction with other database sessions using the facilities offered by PostgreSQL commands LISTEN and NOTIFY. Please refer to the PostgreSQL documentation for examples about how to use this form of communication.

Because of the way transactions interact with notifications (see NOTIFY documentation), you should keep the connection in autocommit mode if you wish to receive or send notifications in a timely manner.

Notifications are received as instances of Notify. If you are reserving a connection only to receive notifications, the simplest way is to consume the Connection.notifies generator. The generator can be stopped using close(). Starting from Psycopg 3.2, the method supports options to receive notifications only for a certain time or up to a certain number.

Note

You don’t need an AsyncConnection to handle notifications: a normal blocking Connection is perfectly valid.

The following example will print notifications and stop when one containing the "stop" message is received.

import psycopg
conn = psycopg.connect("", autocommit=True)
conn.execute("LISTEN mychan")
gen = conn.notifies()
for notify in gen:
    print(notify)
    if notify.payload == "stop":
        gen.close()
print("there, I stopped")

If you run some NOTIFY in a psql session:

=# NOTIFY mychan, 'hello';
NOTIFY
=# NOTIFY mychan, 'hey';
NOTIFY
=# NOTIFY mychan, 'stop';
NOTIFY

You may get output from the Python process such as:

Notify(channel='mychan', payload='hello', pid=961823)
Notify(channel='mychan', payload='hey', pid=961823)
Notify(channel='mychan', payload='stop', pid=961823)
there, I stopped

Alternatively, you can use add_notify_handler() to register a callback function, which will be invoked whenever a notification is received, during the normal query processing; you will be then able to use the connection normally. Please note that in this case notifications will not be received immediately, but only during a connection operation, such as a query.

conn.add_notify_handler(lambda n: print(f"got this: {n}"))

# meanwhile in psql...
# =# NOTIFY mychan, 'hey';
# NOTIFY

print(conn.execute("SELECT 1").fetchone())
# got this: Notify(channel='mychan', payload='hey', pid=961823)
# (1,)

Detecting disconnections#

Sometimes it is useful to detect immediately when the connection with the database is lost. One brutal way to do so is to poll a connection in a loop running an endless stream of SELECT 1… Don’t do so: polling is so out of fashion. Besides, it is inefficient (unless what you really want is a client-server generator of ones), it generates useless traffic and will only detect a disconnection with an average delay of half the polling time.

A more efficient and timely way to detect a server disconnection is to create an additional connection and wait for a notification from the OS that this connection has something to say: only then you can run some checks. You can dedicate a thread (or an asyncio task) to wait on this connection: such thread will perform no activity until awaken by the OS.

In a normal (non asyncio) program you can use the selectors module. Because the Connection implements a fileno() method you can just register it as a file-like object. You can run such code in a dedicated thread (and using a dedicated connection) if the rest of the program happens to have something else to do too.

import selectors

sel = selectors.DefaultSelector()
sel.register(conn, selectors.EVENT_READ)
while True:
    if not sel.select(timeout=60.0):
        continue  # No FD activity detected in one minute

    # Activity detected. Is the connection still ok?
    try:
        conn.execute("SELECT 1")
    except psycopg.OperationalError:
        # You were disconnected: do something useful such as panicking
        logger.error("we lost our database!")
        sys.exit(1)

In an asyncio program you can dedicate a Task instead and do something similar using add_reader:

import asyncio

ev = asyncio.Event()
loop = asyncio.get_event_loop()
loop.add_reader(conn.fileno(), ev.set)

while True:
    try:
        await asyncio.wait_for(ev.wait(), 60.0)
    except asyncio.TimeoutError:
        continue  # No FD activity detected in one minute

    # Activity detected. Is the connection still ok?
    try:
        await conn.execute("SELECT 1")
    except psycopg.OperationalError:
        # Guess what happened
        ...

Concurrent operations#

Asynchronous operations#

with async connections#

Interrupting async operations#

Gevent support#

Server messages#

Asynchronous notifications#

Detecting disconnections#

`with` async connections#