Decoding Rails Magic: How Does ActiveJob work?

Executing background jobs is quite a common feature in many of the web applications. Switching between different background processing frameworks used to be quite painful as most of them had different API for enqueuing jobs, enqueuing mailers and scheduling jobs. One of the great addition in Rails 4.2 was a solution to this problem: ActiveJob, which provides extra layer on top of background jobs framework and unifies the API regardless of the queue adapter you use. But how exactly does it work? What are the requirements for adding new queue adapters? What kind of API does ActiveJob provide? Let’s dive deep into the codebase and answer these and some other questions.