The Lock API

Whenever we process data on a regular basis, especially if it takes a while to complete, we might run into a situation in which parallel requests want to trigger that process again while the first is still running. Most of the time, this is not a good thing, as it can lead to conflicts and/or data corruption. A good example from Drupal core in which this can happen is the cron. If we start it, the process can end up taking a few seconds. Remember, it needs to pull together the hook_cron() implementations and run them all. So, while that is happening, if we trigger another cron run, it will give us a nice message asking us to chill because the cron is already running. It does this with the help of the Lock API.

The Lock API is a low-level Drupal solution for ensuring that processes don’t trample each other. Since in this chapter we are talking about things such as batch operations, queues, and other kinds of potentially time-consuming processes, let’s look...