On the SitePoint PHP blog Daniel Sipos has written up a tutorial spotlighting a powerful feature of Drupal 8, the latest major release of this popular project: the Queue API. Queueing in Drupal allows you to offload tasks to be handled outside of the current web request.
The Queue API in Drupal allows us to handle a number of tasks at a later stage. What this means is that we can place items into a queue which will run some time in the future and process each individual item at that point and at least once. Usually, this happens on CRON runs, and Drupal 8 allows for a quick set up for cronjob based queues. It doesn’t necessarily have to be CRON, however.
They use two examples to help illustrate how to use the queueing system: one that uses the cron-based approach and another that's more manually triggered. They start out with the theory behind it all, talking about the different pieces (objects/classes) and how they fit together to make the queueing system. With that out of the way the article starts in on the code and the "Node Publish" queue, defining its basic structure and hooking it in to the framework. It shows you how to create the cron worker to process the queue and how to build the manual worker to do the same but only when specifically called.