You’re a developer who has to write a user migration tool for a client. The right answer is to the use the surprisingly simple Batch API. Everything is going great. Except that for every migration, there are seven (seemingly random) users that aren’t migrated. Hmmm.... What is going on and how do you find out? Batch API is an abstraction layer for performing tasks, in this case migrating a block of users. Instead of loading a drupal page that runs a function to migrate all users, it loads a page that makes AJAX calls to run that function. We do this to avoid PHP timeouts and provide a better user experience. Instead of an endlessly loading white page we can see a themed page that includes a progress bar.