This document describes the current stable version of Celery (3.1). For development docs, go here.

Glossary

ack

Short for acknowledged.

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).

apply

Originally a synonym to call but used to signify that a function is executed by the current process.

billiard

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

calling

Sends a task message so that the task function is executed by a worker.

cipater

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

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 Context

executing

Workers execute task requests.

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 unintented effects, but not necessarily side-effect free in the pure sense (compare to nullipotent).

kombu

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

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 idempotent.

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 Prefetch Limits.

prefetch multiplier

The prefetch count is configured by using the CELERYD_PREFETCH_MULTIPLIER setting, which is multiplied by the number of pool slots (threads/processes/greenthreads).

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 idempotence 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.

request

Task messages are converted to requests within the worker. The request information is also available as the task’s context (the task.request attribute).