This document describes the current stable version of Celery (5.6). For development docs, go here.

# Message Protocol

## Task messages

### Version 2

#### Definition

    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,
        'replaced_task_nesting': int,
        'compression': string compression_method (optional; omitted when no compression is used, matches kombu compressor names such as 'zlib', 'bzip2', 'gzip'),
    }

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

#### Example

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 = {}
    basic_publish(
        message=json.dumps((args, kwargs, None)),
        application_headers={
            'lang': 'py',
            'task': 'proj.tasks.add',
            'argsrepr': repr(args),
            'kwargsrepr': repr(kwargs),
            'origin': '@'.join([os.getpid(), socket.gethostname()])
        }
        properties={
            '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:

  >     execute_task(message)
  >     chain = embed['chain']
  >     if chain:
  >         sig = maybe_signature(chain.pop())
  >         sig.apply_async(chain=chain)

- `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().apply_async(
  >                 (fun, real_args, kwargs), shadow=qualname(fun), **options
  >             )

  >     @app.task(base=PickleTask)
  >     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`
    string:

    Name of the task. required

- `id`
    string:

    Unique id of the task (UUID). required

- `args`
    list:

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

- `kwargs`
    dictionary:

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

- `retries`
    int:

    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):

    Added 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`
    string:

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

- `chord`
    Signature:

    Added 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`
    bool:

    Added in version 2.5.

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

- `callbacks`
    <list>Signature:

    Added in version 3.0.

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

- `errbacks`
    <list>Signature:

    Added in version 3.0.

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

- `timelimit`
    <tuple>(float, float):

    Added 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 celery.task.ping 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': 'worker1@george.vandelay.com',
    }
    body = {
        'type': 'task-succeeded',
        'hostname': 'worker1@george.vandelay.com',
        'pid': 6335,
        'clock': 393912923921,
        'timestamp': 1401717709.101747,
        'utcoffset': -1,
        'uuid': '9011d855-fdd1-4f8f-adb3-a413b499eafb',
        'retval': '4',
        'runtime': 0.0003212,
    )