# Task Cookbook {#cookbook-tasks}

::: {.contents local=""}
:::

## Ensuring a task is only executed one at a time {#cookbook-task-serial}

You can accomplish this by using a lock.

In this example we\'ll be using the cache framework to set a lock
that\'s accessible for all workers.

It\'s part of an imaginary RSS feed importer called
[djangofeeds]{.title-ref}. The task takes a feed URL as a single
argument, and imports that feed into a Django model called
[Feed]{.title-ref}. We ensure that it\'s not possible for two or more
workers to import the same feed at the same time by setting a cache key
consisting of the MD5 check-sum of the feed URL.

The cache key expires after some time in case something unexpected
happens, and something always will\...

For this reason your tasks run-time shouldn\'t exceed the timeout.

::: note
::: title
Note
:::

In order for this to work correctly you need to be using a cache backend
where the `.add` operation is atomic. `memcached` is known to work well
for this purpose.
:::

``` python
import time
from celery import task
from celery.utils.log import get_task_logger
from contextlib import contextmanager
from django.core.cache import cache
from hashlib import md5
from djangofeeds.models import Feed

logger = get_task_logger(__name__)

LOCK_EXPIRE = 60 * 10  # Lock expires in 10 minutes

@contextmanager
def memcache_lock(lock_id, oid):
    timeout_at = time.monotonic() + LOCK_EXPIRE - 3
    # cache.add fails if the key already exists
    status = cache.add(lock_id, oid, LOCK_EXPIRE)
    try:
        yield status
    finally:
        # memcache delete is very slow, but we have to use it to take
        # advantage of using add() for atomic locking
        if time.monotonic() < timeout_at and status:
            # don't release the lock if we exceeded the timeout
            # to lessen the chance of releasing an expired lock
            # owned by someone else
            # also don't release the lock if we didn't acquire it
            cache.delete(lock_id)

@task(bind=True)
def import_feed(self, feed_url):
    # The cache key consists of the task name and the MD5 digest
    # of the feed URL.
    feed_url_hexdigest = md5(feed_url).hexdigest()
    lock_id = '{0}-lock-{1}'.format(self.name, feed_url_hexdigest)
    logger.debug('Importing feed: %s', feed_url)
    with memcache_lock(lock_id, self.app.oid) as acquired:
        if acquired:
            return Feed.objects.import_feed(feed_url).url
    logger.debug(
        'Feed %s is already being imported by another worker', feed_url)
```