Did someone understand the last slides about the differences between transactions in tasks with and without celery ? I'm using celery, and i have been using django in the past, and I really didn't get the point.
He's trying to avoid the situation where you enqueue a background job inside a transaction and the worker gets started on that job before the transaction is committed. If the job needs to hit the database for any reason your likely to get errors as the new data hasn't been committed yet.
It looks like the work around he used is to cache the job queue locally and only flush it to the real job queue after the database commit, so your guaranteed whatever data may be needed for the job has been committed to the database.
Basically if you modify data in a transaction, add a task to the queue that relies on that data, and the worker pulls the task off the queue BEFORE the transaction is committed, then the task tries to access data that doesn't exist in the database yet.
It looks like the work around he used is to cache the job queue locally and only flush it to the real job queue after the database commit, so your guaranteed whatever data may be needed for the job has been committed to the database.