Remarks#

“Celery is an asynchronous task queue/job queue based on distributed message passing.” – https://www.celeryproject.org/

Celery is great for asychronous and scheduled background tasks. It is commonly used for long-running tasks that are part of a Django or Flask application.

Installation or Setup

You can install Celery either via the Python Package Index (PyPI) or from source.

To install the latest version using pip:

$ pip install celery

To install using easy_install:

$ easy_install celery

Downloading and installing from source

Download the latest version of Celery from https://pypi.python.org/pypi/celery/

You can install it by doing the following:

$ tar xvfz celery-0.0.0.tar.gz
$ cd celery-0.0.0
$ python setup.py build
# python setup.py install # as root

Celery + Redis

Installation

Additional dependencies are required for Redis support. Install both Celery and the dependencies in one go using the celery[redis] bundle:

$ pip install -U celery[redis]

Configuration

Configure the location of your Redis database:

BROKER_URL = 'redis://localhost:6379/0'

The URL should be in the format of:

redis://:password@hostname:port/db_number

Application

Create the file tasks.py:

from celery import Celery

BROKER_URL = 'redis://localhost:6379/0'
app = Celery('tasks', broker=BROKER_URL)

@app.task
def add(x, y):
    return x + y

The first argument to Celery is the name of the current module. This way names can be automatically generated. The second argument is the broker keyword which specifies the URL of the message broker.

Running the celery worker server

Run the worker by executing with the worker argument:

$ celery -A tasks worker --loglevel=info

Calling the task

To call the task, use the delay() method.

>>> from tasks import add
>>> add.delay(4, 4)

Calling a task returns an AsyncResult instance, which can check the state of the task, wait for the task to finish, or get its return value. (If the task failed, it gets the exception and traceback).

Keeping Results

To keep track of the task’s states, Celery needs to store or send the states somewhere. Use Redis as the result backend:

BROKER_URL = 'redis://localhost:6379/0'
BACKEND_URL = 'redis://localhost:6379/1'
app = Celery('tasks', broker=BROKER_URL, backend=BACKEND_URL)

To read more about result backends please see Result Backends.

Now with the result backend configured, call the task again. This time hold on to the AsyncResult instance returned from the task:

>>> result = add.delay(4, 4)

The ready() method returns whether the task has finished processing or not:

>>> result.ready()
False

It is possible to wait for the result to complete, but this is rarely used since it turns the asynchronous call into a synchronous one:

>>> result.get(timeout=1)
8

Based on celery official document