.. _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 won't 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` ETA "Estimated Time of Arrival", in Celery and Google Task Queue, etc., used as the term for a delayed message that should not be processed until the specified ETA time. See :ref:`calling-eta`. 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 ` 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 `. apply Originally a synonym to :term:`call ` 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'll 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 isn't the same as :term:`idempotence ` as the return value doesn't 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 shouldn't deliver any more messages to that consumer. See :ref:`optimizing-prefetch-limit`. pidbox A process mailbox, used to implement remote control commands.