Anything slow, flaky or bursty belongs off the request path: email, webhooks, AI calls, exports, image processing. Queues make that safe, provided your workers expect the same message twice. How SQS works, the settings that matter, and the failure modes of jobs built with setTimeout and cron.
A request that sends an email, calls an AI model and resizes an image is slow and fails whenever any of those does. Put the slow parts on a queue, return immediately, and let workers do them with retries.
At-least-once delivery: a standard queue can deliver a message more than once. Workers must be idempotent, so a duplicate does no harm.
Visibility timeout: when a worker receives a message, it is hidden for a period. If the worker does not delete it before the timeout, it becomes visible again and another worker retries it. Set the timeout longer than the job takes.
Dead-letter queue: after a set number of failed receives, the message moves to a separate queue for inspection, instead of retrying forever.
Ordering: standard queues do not guarantee order. FIFO queues do, within a message group, at lower throughput.
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Work in memory is lost on every deploy, crash or scale-in.
Two web servers run the same cron job twice.
There is no record of what failed and no way to replay it.
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