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System design · interactive

Message queues: at-least-once delivery

A producer drops work on a queue and moves on. Consumers pull it, do it, and delete it. Make the consumers slow, or send a message that always fails, and watch what “at least once” really means.

Producer

1 of 8 sent

Queue

0 visible

Consumer 1

working on m1

Consumer 2

idle

Dead-letter queue

0 parked

  • m1

    in-flight

    try 1

  • m2

    not sent

    ·

  • m3

    not sent

    ·

  • m4

    not sent

    ·

  • m5

    not sent

    ·

  • m6

    not sent

    ·

  • m7

    not sent

    ·

  • m8

    not sent

    ·

0 done · 0 dead-lettered · 0 duplicate charges

Tick 0: Consumer 1 receives m1.

tick 0
  1. 1

  2. 2

    The message stays in the queue but is invisible for the visibility timeout (4 ticks here, 30 seconds by default on SQS).

    1 of 2 consumers busy.

  3. 3

  4. 4

  5. 5

In production

Tools and technologies

Queues decouple producers from consumers in every stack; the delivery guarantees are what differ.

Not tied to one language — use these names when you search docs or design a system.