Integration with RabbitMQ and Celery

First update taiga-vagrant vm if you're using it, you will have access
to the rabbit management console at port 8001 of the host machine.

* Defining tasks
- Tasks must be defined in a `deferred` module of an app, for example,
`taiga.projects.deferred` module.
- Tasks must be decorated and given the name "<app>.<task-name>", for
example in `taiga.projects.deferred` module:
```
from taiga.celery import app

@app.task(name="projects.add")
def add(x, y):
    return x + y
```
- Tasks should be at most just wrappers around service functions to
promote re-usability of those functions from other parts of the code,
say, management commands and the like.

* Calling tasks
Tasks should be called using one of the three functions defined in
`taiga.deferred` module:

- `defer`: Use this function if you need to perform some task
asynchronously and GET THE RESULT back later, for example:
```
result = defer("projects.add", x=1, y=2)
...
result.get() # => 3
```

- `call_async`: Use this function when you want to fire off some
task. No result is get back. For example:
```
call_async("projects.email_user", user)
```

- `apply_async`: Is the same as `call_async` but since it's a function
application and you must pass the args and kwargs together as one
parameter each you are allowed to pass celery-specific
extra-options (but bear in mind this is not recommended!)
```
apply_async("projects.email_user", args=(user,), kwargs={}, routing_key="tasks.email")
```

requirements.txt

@@ -26,3 +26,5 @@ enum34==1.0
 
 django-reversion==1.8.1
 easy-thumbnails==2.0
+celery==3.1.12
+redis==2.10

settings/__init__.py

@@ -17,6 +17,8 @@
 from __future__ import absolute_import, print_function
 import os, sys
 
+from .celery import *
+
 try:
     print("Trying import local.py settings...", file=sys.stderr)
     from .local import *

settings/celery.py

@@ -0,0 +1,16 @@
+from kombu import Exchange, Queue
+
+BROKER_URL = 'amqp://guest:guest@localhost:5672//'
+CELERY_RESULT_BACKEND = 'redis://localhost:6379/0'
+
+CELERY_TIMEZONE = 'Europe/Madrid'
+CELERY_ENABLE_UTC = True
+
+CELERY_DEFAULT_QUEUE = 'tasks'
+CELERY_QUEUES = (
+    Queue('tasks', routing_key='task.#'),
+    Queue('transient', routing_key='transient.#', delivery_mode=1)
+)
+CELERY_DEFAULT_EXCHANGE = 'tasks'
+CELERY_DEFAULT_EXCHANGE_TYPE = 'topic'
+CELERY_DEFAULT_ROUTING_KEY = 'task.default'

taiga/__init__.py

@@ -0,0 +1 @@
+from . import celery

taiga/celery.py

@@ -0,0 +1,12 @@
+import os
+
+from celery import Celery
+
+from django.conf import settings
+
+os.environ.setdefault('DJANGO_SETTINGS_MODULE', 'settings')
+
+app = Celery('taiga')
+
+app.config_from_object('django.conf:settings')
+app.autodiscover_tasks(lambda: settings.INSTALLED_APPS, related_name="deferred")

taiga/deferred.py

@@ -0,0 +1,43 @@
+from .celery import app
+
+
+def defer(task: str, *args, **kwargs):
+    """Defer the execution of a task.
+
+    Defer the execution of a task and returns a future objects with the following methods among
+    others:
+    - `failed()` Returns `True` if the task failed.
+    - `ready()` Returns `True` if the task has been executed.
+    - `forget()` Forget about the result.
+    - `get()` Wait until the task is ready and return its result.
+    - `result` When the task has been executed the result is in this attribute.
+    More info at Celery docs on `AsyncResult` object.
+
+    :param task: Name of the task to execute.
+
+    :return: A future object.
+    """
+    return app.send_task(task, args, kwargs, routing_key="transient.deferred")
+
+
+def call_async(task: str, *args, **kwargs):
+    """Run a task and ignore its result.
+
+    This is just a star argument version of `apply_async`.
+
+    :param task: Name of the task to execute.
+    :param args: Arguments for the task.
+    :param kwargs: Keyword arguments for the task.
+    """
+    apply_async(task, args, kwargs)
+
+
+def apply_async(task: str, args=None, kwargs=None, **options):
+    """Run a task and ignore its result.
+
+    :param task: Name of the task to execute.
+    :param args: Tupple of arguments for the task.
+    :param kwargs: Dict of keyword arguments for the task.
+    :param options: Celery-specific options when running the task. See Celery docs on `apply_async`
+    """
+    app.send_task(task, args, kwargs, **options)

taiga/services/celery.py

@@ -0,0 +1,8 @@
+from celery import Celery
+
+from django.conf import settings
+
+app = Celery('taiga')
+
+app.config_from_object('django.conf:settings')
+app.autodiscover_tasks(lambda: settings.INSTALLED_APPS)

tests/unit/test_deferred.py

@@ -0,0 +1,35 @@
+from unittest import mock
+from taiga.deferred import defer, call_async, apply_async
+
+
+@mock.patch("taiga.deferred.app")
+def test_defer(app):
+    name = "task name"
+    args = (1, 2)
+    kwargs = {"kw": "keyword argument"}
+
+    defer(name, *args, **kwargs)
+
+    app.send_task.assert_called_once_with(name, args, kwargs, routing_key="transient.deferred")
+
+
+@mock.patch("taiga.deferred.app")
+def test_apply_async(app):
+    name = "task name"
+    args = (1, 2)
+    kwargs = {"kw": "keyword argument"}
+
+    apply_async(name, args, kwargs)
+
+    app.send_task.assert_called_once_with(name, args, kwargs)
+
+
+@mock.patch("taiga.deferred.app")
+def test_call_async(app):
+    name = "task name"
+    args = (1, 2)
+    kwargs = {"kw": "keyword argument"}
+
+    call_async(name, *args, **kwargs)
+
+    app.send_task.assert_called_once_with(name, args, kwargs)