This document describes the current stable version of Celery (4.0). For development docs, go here.


Short for 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).
Originally a synonym to call but used to signify that a function is executed by the current process.
Fork of the Python multiprocessing library containing improvements required by Celery.
Sends a task message so that the task function is executed by a worker.
Celery release 3.1 named after song by Autechre (
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 Task Request
early ack
Short for early acknowledgment
early acknowledgment

Task is 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 task_acks_late.

“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 ETA and Countdown.
Workers execute task requests.

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 nullipotent).

Further reading:

Python messaging library used by Celery to send and receive messages.
late ack
Short for late acknowledgment
late acknowledgment

Task is 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 task_acks_late.

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 idempotent.
A process mailbox, used to implement remote control commands.
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 Prefetch Limits.
prefetch multiplier
The prefetch count is configured by using the worker_prefetch_multiplier setting, which is multiplied by the number of pool slots (threads/processes/greenthreads).
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 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.
Task messages are converted to requests within the worker. The request information is also available as the task’s context (the task.request attribute).