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At-least-once delivery

A queue's promise that every job is delivered, possibly more than once.

Taught in Chapter 11: Queues and Workers.

More from the same chapter

• Backlog: The jobs waiting in a queue. If the oldest keeps getting older, the workers can't keep up.
• Change data capture: Reading a database's own log of committed changes and publishing them, so other systems hear of every change without the app sending it twice.
• Dead-letter queue: Where a job goes after failing every attempt, for someone to look at.
• Message queue: Stores jobs durably and acknowledges them at once, so workers can do the work later.
• Publish/subscribe: Sends each message to every subscriber, so several services react to one event.
• Transactional outbox: Saving a message in an outbox table in the same transaction as the data, for a relay to publish afterwards, so the save and the send can't come apart.
• Visibility timeout: How long a queue waits for a worker to finish a job before handing it out again.
• Worker: A process that takes jobs from a queue and does them in the background.