Queue

A buffer of work to be done later, in order, by someone else — a request writes a job and returns; a worker takes the job when it can; the queue holds what is between them and survives either side being slow or down. Retries, dead letters and idempotent jobs are where the design goes

The seam between a fast answer and a slow job. BullMQ over Redis is the Node standard and is in the tech registry as evaluated; a database table with a status column is the honest first version and would serve this site’s two candidates — the e-mail send and the AI reindex — for a long time. The rule that matters more than the tool: a job must be safe to run twice, because it will be.