Glossary · Phase 11 · Professional practice
At-least-once delivery
A guarantee that each message is delivered, but possibly more than once, so consumers must be idempotent.
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The second "a" is a hit, so it moves to the newest end. When "c" arrives, "b" is the least recently used and is evicted.
Learn it properly
Caches, queues and background jobs · Speed reads with caches, and move slow work to queues that retry safely. About 40 minutes.
Other terms from the same lesson
- cache stampede: Many requests missing the cache at once, such as when a popular entry expires, and all hitting the database.
- cache-aside: Read the cache first; on a miss, load from the source, store the result in the cache, and return it.
- dead-letter queue: Where a queue puts jobs that failed too many times, so a person can inspect them.
- visibility timeout: How long a pulled job stays hidden from other workers. If it isn't acknowledged in time, it's retried.
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