.. _glossary:

Glossary
========

.. glossary::
    :sorted:

    acknowledged
        Workers acknowledge messages to signify that a message has been
        handled.  Failing to acknowledge a message
        will cause the message to be redelivered.   Exactly when a
        transaction is considered a failure varies by transport.  In AMQP the
        transaction fails when the connection/channel is closed (or lost),
        but in Redis/SQS the transaction times out after a configurable amount
        of time (the ``visibility_timeout``).

    ack
        Short for :term:`acknowledged`.

    early acknowledgment
        Task is :term:`acknowledged` just-in-time before being executed,
        meaning the task will not be redelivered to another worker if the
        machine loses power, or the worker instance is abruptly killed,
        mid-execution.

        Configured using :setting:`task_acks_late`.

    late acknowledgment
        Task is :term:`acknowledged` after execution (both if successful, or
        if the task is raising an error), which means the task will be
        redelivered to another worker in the event of the machine losing
        power, or the worker instance being killed mid-execution.

        Configured using :setting:`task_acks_late`.

    early ack
        Short for :term:`early acknowledgment`

    late ack
        Short for :term:`late acknowledgment`

    request
        Task messages are converted to *requests* within the worker.
        The request information is also available as the task's
        :term:`context` (the ``task.request`` attribute).

    calling
        Sends a task message so that the task function is
        :term:`executed <executing>` by a worker.

    kombu
        Python messaging library used by Celery to send and receive messages.

    billiard
        Fork of the Python multiprocessing library containing improvements
        required by Celery.

    executing
        Workers *execute* task :term:`requests <request>`.

    apply
        Originally a synonym to :term:`call <calling>` but used to signify
        that a function is executed by the current process.

    context
        The context of a task contains information like the id of the task,
        it's arguments and what queue it was delivered to.
        It can be accessed as the tasks ``request`` attribute.
        See :ref:`task-request-info`

    idempotent
        Idempotence is a mathematical property that describes a function that
        can be called multiple times without changing the result.
        Practically it means that a function can be repeated many times without
        unintended effects, but not necessarily side-effect free in the pure
        sense (compare to :term:`nullipotent`).

        Further reading: https://en.wikipedia.org/wiki/Idempotent

    nullipotent
        describes a function that will have the same effect, and give the same
        result, even if called zero or multiple times (side-effect free).
        A stronger version of :term:`idempotent`.

    reentrant
        describes a function that can be interrupted in the middle of
        execution (e.g. by hardware interrupt or signal) and then safely
        called again later.  Reentrancy is not the same as
        :term:`idempotence <idempotent>` as the return value does not have to
        be the same given the same inputs, and a reentrant function may have
        side effects as long as it can be interrupted;  An idempotent function
        is always reentrant, but the reverse may not be true.

    cipater
        Celery release 3.1 named after song by Autechre
        (http://www.youtube.com/watch?v=OHsaqUr_33Y)

    prefetch multiplier
        The :term:`prefetch count` is configured by using the
        :setting:`worker_prefetch_multiplier` setting, which is multiplied
        by the number of pool slots (threads/processes/greenthreads).

    `prefetch count`
        Maximum number of unacknowledged messages a consumer can hold and if
        exceeded the transport should not deliver any more messages to that
        consumer.  See :ref:`optimizing-prefetch-limit`.

    pidbox
        A process mailbox, used to implement remote control commands.
