.. currentmodule:: psycopg


.. index:: threads

.. _concurrency:

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
    :ref:`connection pool <connection-pools>`.

    .. __: https://www.postgresql.org/docs/current/runtime-config-connection.html#GUC-MAX-CONNECTIONS

.. 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.


.. index:: asyncio

.. _async:

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.

.. code:: python

    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.


.. versionchanged:: 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 `~psycopg._dns.resolve_hostaddr_async()` to
        do it automatically.

        .. __: https://www.postgresql.org/docs/current/libpq-connect.html
               #LIBPQ-PARAMKEYWORDS

.. warning::

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

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

    .. parsed-literal::

        `asyncio.set_event_loop_policy`\ (
            `asyncio.WindowsSelectorEventLoopPolicy`\ ()
        )



.. index:: with

.. _async-with:

`!with` async connections
-------------------------

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

.. code:: python

    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 `~Connection.connect()` and `~Connection.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

.. code:: python

    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`:

.. code:: python

    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.


.. index:: Ctrl-C

.. _async-ctrl-c:

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
`~Connection.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.


.. __: https://docs.python.org/3/library/asyncio-task.html#task-cancellation


.. index:: gevent

.. _gevent:

Gevent support
--------------

Psycopg 3 supports `gevent <https://www.gevent.org/>`__ 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.

.. __: http://www.gevent.org/api/gevent.monkey.html#gevent.monkey.patch_select
.. __: http://www.gevent.org/api/gevent.monkey.html#gevent.monkey.patch_all

.. 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.


.. index::
    pair: Asynchronous; Notifications
    pair: LISTEN; SQL command
    pair: NOTIFY; SQL command

.. _async-messages:

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 :program:`psql`::

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

.. __: https://www.postgresql.org/docs/current/runtime-config-logging.html
    #RUNTIME-CONFIG-SEVERITY-LEVELS

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.

.. __: https://www.postgresql.org/docs/current/plpgsql-errors-and-messages.html
.. __: https://www.postgresql.org/docs/current/runtime-config-client.html
    #GUC-CLIENT-MIN-MESSAGES


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 `~errors.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 `~psycopg.Error.diag`
attribute of the errors raised by the server:

.. code:: python

    >>> 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.


.. index::
    pair: Asynchronous; Notifications
    pair: LISTEN; SQL command
    pair: NOTIFY; SQL command

.. _async-notify:

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.

.. |LISTEN| replace:: :sql:`LISTEN`
.. _LISTEN: https://www.postgresql.org/docs/current/sql-listen.html
.. |NOTIFY| replace:: :sql:`NOTIFY`
.. _NOTIFY: https://www.postgresql.org/docs/current/sql-notify.html

Because of the way sessions interact with notifications (see |NOTIFY|_
documentation), you should keep the connection in `~Connection.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.

.. code:: python

    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 :sql:`NOTIFY` in a :program:`psql` session:

.. code:: psql

    =# 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 `~Connection.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.

.. code:: python

    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,)


.. index:: disconnections

.. _disconnections:

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 :sql:`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 `~Connection.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.

.. code:: python

    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 `~asyncio.Task` instead and do
something similar using `~asyncio.loop.add_reader`:

.. code:: python

    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
            ...