Backend
What is Background Job?
Work executed asynchronously outside the request–response cycle so slow tasks don't block users, typically via a queue and workers.
Definition
A background job moves slow or deferrable work — sending email, resizing images, generating PDFs, syncing with third-party APIs — out of the web request so the user gets an immediate response. The web process enqueues a job, and separate worker processes pull from the queue and execute it. Sidekiq (Ruby), Celery (Python), and BullMQ (Node.js, backed by Redis) are standard frameworks. Robust setups add retry with exponential backoff, dead-letter queues for repeated failures, and idempotent handlers so a retried job never sends the same email twice.