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

Message Protocol

Task messages

Version 2


properties = {
    'correlation_id': uuid task_id,
    'content_type': string mimetype,
    'content_encoding': string encoding,

    # optional
    'reply_to': string queue_or_url,
headers = {
    'lang': string 'py'
    'task': string task,
    'id': uuid task_id,
    'root_id': uuid root_id,
    'parent_id': uuid parent_id,
    'group': uuid group_id,

    # optional
    'meth': string method_name,
    'shadow': string alias_name,
    'eta':  iso8601 ETA,
    'expires'; iso8601 expires,
    'retries': int retries,
    'timelimit': (soft, hard),
    'argsrepr': str repr(args),
    'kwargsrepr': str repr(kwargs),
    'origin': str nodename,

body = (
    object[] args,
    Mapping kwargs,
    Mapping embed {
        'callbacks': Signature[] callbacks,
        'errbacks': Signature[] errbacks,
        'chain': Signature[] chain,
        'chord': Signature chord_callback,


This example sends a task message using version 2 of the protocol:

# chain: add(add(add(2, 2), 4), 8) == 2 + 2 + 4 + 8

import json
import os
import socket

task_id = uuid()
args = (2, 2)
kwargs = {}
    message=json.dumps((args, kwargs, None),
        'lang': 'py',
        'task': 'proj.tasks.add',
        'argsrepr': repr(args),
        'kwargsrepr': repr(kwargs),
        'origin': '@'.join([os.getpid(), socket.gethostname()])
        'correlation_id': task_id,
        'content_type': 'application/json',
        'content_encoding': 'utf-8',

Changes from version 1

  • Protocol version detected by the presence of a task message header.

  • Support for multiple languages via the lang header.

    Worker may redirect the message to a worker that supports the language.

  • Meta-data moved to headers.

    This means that workers/intermediates can inspect the message and make decisions based on the headers without decoding the payload (that may be language specific, for example serialized by the Python specific pickle serializer).

  • Always UTC

    There’s no utc flag anymore, so any time information missing timezone will be expected to be in UTC time.

  • Body is only for language specific data.

    • Python stores args/kwargs and embedded signatures in body.
    • If a message uses raw encoding then the raw data will be passed as a single argument to the function.
    • Java/C, etc. can use a Thrift/protobuf document as the body
  • origin is the name of the node sending the task.

  • Dispatches to actor based on task, meth headers

    meth is unused by Python, but may be used in the future to specify class+method pairs.

  • Chain gains a dedicated field.

    Reducing the chain into a recursive callbacks argument causes problems when the recursion limit is exceeded.

    This is fixed in the new message protocol by specifying a list of signatures, each task will then pop a task off the list when sending the next message:

    chain = embed['chain']
    if chain:
        sig = maybe_signature(chain.pop())
  • correlation_id replaces task_id field.

  • root_id and parent_id fields helps keep track of work-flows.

  • shadow lets you specify a different name for logs, monitors can be used for concepts like tasks that calls a function specified as argument:

    from celery.utils.imports import qualname
    class PickleTask(Task):
        def unpack_args(self, fun, args=()):
            return fun, args
        def apply_async(self, args, kwargs, **options):
            fun, real_args = self.unpack_args(*args)
            return super(PickleTask, self).apply_async(
                (fun, real_args, kwargs), shadow=qualname(fun), **options
    def call(fun, args, kwargs):
        return fun(*args, **kwargs)

Version 1

In version 1 of the protocol all fields are stored in the message body: meaning workers and intermediate consumers must deserialize the payload to read the fields.

Message body

  • task

    Name of the task. required

  • id

    Unique id of the task (UUID). required

  • args

    List of arguments. Will be an empty list if not provided.

  • kwargs

    Dictionary of keyword arguments. Will be an empty dictionary if not provided.

  • retries

    Current number of times this task has been retried. Defaults to 0 if not specified.

  • eta
    string (ISO 8601):

    Estimated time of arrival. This is the date and time in ISO 8601 format. If not provided the message isn’t scheduled, but will be executed asap.

  • expires
    string (ISO 8601):

    New in version 2.0.2.

    Expiration date. This is the date and time in ISO 8601 format. If not provided the message will never expire. The message will be expired when the message is received and the expiration date has been exceeded.

  • taskset

    The group this task is part of (if any).

  • chord

    New in version 2.3.

    Signifies that this task is one of the header parts of a chord. The value of this key is the body of the cord that should be executed when all of the tasks in the header has returned.

  • utc

    New in version 2.5.

    If true time uses the UTC timezone, if not the current local timezone should be used.

  • callbacks

    New in version 3.0.

    A list of signatures to call if the task exited successfully.

  • errbacks

    New in version 3.0.

    A list of signatures to call if an error occurs while executing the task.

  • timelimit
    <tuple>(float, float):

    New in version 3.1.

    Task execution time limit settings. This is a tuple of hard and soft time limit value (int/float or None for no limit).

    Example value specifying a soft time limit of 3 seconds, and a hard time limit of 10 seconds:

    {'timelimit': (3.0, 10.0)}

Example message

This is an example invocation of a task in json format:

{"id": "4cc7438e-afd4-4f8f-a2f3-f46567e7ca77",
 "task": "celery.task.PingTask",
 "args": [],
 "kwargs": {},
 "retries": 0,
 "eta": "2009-11-17T12:30:56.527191"}

Task Serialization

Several types of serialization formats are supported using the content_type message header.

The MIME-types supported by default are shown in the following table.

Scheme MIME Type
json application/json
yaml application/x-yaml
pickle application/x-python-serialize
msgpack application/x-msgpack

Event Messages

Event messages are always JSON serialized and can contain arbitrary message body fields.

Since version 4.0. the body can consist of either a single mapping (one event), or a list of mappings (multiple events).

There are also standard fields that must always be present in an event message:

Standard body fields

  • string type

    The type of event. This is a string containing the category and action separated by a dash delimiter (e.g., task-succeeded).

  • string hostname

    The fully qualified hostname of where the event occurred at.

  • unsigned long long clock

    The logical clock value for this event (Lamport time-stamp).

  • float timestamp

    The UNIX time-stamp corresponding to the time of when the event occurred.

  • signed short utcoffset

    This field describes the timezone of the originating host, and is specified as the number of hours ahead of/behind UTC (e.g., -2 or +1).

  • unsigned long long pid

    The process id of the process the event originated in.

Standard event types

For a list of standard event types and their fields see the Event Reference.

Example message

This is the message fields for a task-succeeded event:

properties = {
    'routing_key': 'task.succeeded',
    'exchange': 'celeryev',
    'content_type': 'application/json',
    'content_encoding': 'utf-8',
    'delivery_mode': 1,
headers = {
    'hostname': '',
body = {
    'type': 'task-succeeded',
    'hostname': '',
    'pid': 6335,
    'clock': 393912923921,
    'timestamp': 1401717709.101747,
    'utcoffset': -1,
    'uuid': '9011d855-fdd1-4f8f-adb3-a413b499eafb',
    'retval': '4',
    'runtime': 0.0003212,