Queues
It's finally time to talk a bit about the Queue API, how it works, and what its main components are; the theory, basically. We will do this before diving into code examples, which we all thoroughly enjoy.
Introduction to the Queue API
The main purpose of the Queue API is to provide a way for us to add items to a queue in order to have them processed at a later time. In charge of processing these items are the queue worker plugins, which can be enlisted either automatically by the Drupal cron, manually (programmatically) by us, or by Drush. We will look at examples of all three.
The central player in this API is an implementation of the QueueInterface, which is the actual queue into which we put items. There are two types of queues Drupal can handle: reliable and unreliable. The first preserves the order in which the items are processed (first in, first out) and guarantees that each item gets processed at least once. In this chapter, we will focus only on this type...