Background job processing system design sounds simple until you try to build one that doesn't lose a job or send a duplicate email.
We take a basic signup flow and push it through four iterations, starting with a version that works fine on a laptop and falls apart the moment the email service slows down. Each fix (a queue, an outbox table, a visibility timeout, idempotent workers, a dead letter queue) exists because the previous fix left a gap. This is the version of the system you'd actually ship, not the one from a textbook.
In this video:
?The exact failure mode that kills the naive synchronous approach
?Why writing to a database and a queue in two steps is dangerous
?How visibility timeouts prevent jobs from disappearing when a worker crashes
?Why at least once delivery means your workers must handle duplicates
?When to stop retrying a job and send it to a dead letter queue
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