Three teams. One queue. Everyone missed messages.

Understand it one step at a time

The short runs these in order in about 32 seconds. Here they are written out — pick any step to jump the short straight to it.

1. 1

   One message. Who gets it?

   Billing, analytics and audit all need to know an order was placed. All three are listening.

   OrderPlaced · 3 interested teams

2. 2

   A queue is a DM

   Exactly one consumer receives each message, completes it, and it is deleted. Nobody else sees it.

   queue · one message, one reader

3. 3

   Add a second consumer

   The other teams attach to the same queue expecting a copy. Instead all three start competing.

   3 consumers · 6 messages · 2 each

4. 4

   That's scale, not fan-out

   Competing consumers exist to go faster, not to go wider. It is the right tool for the wrong goal here.

   more consumers = throughput, not copies

5. 5

   A topic is a group broadcast

   Same message, same sender code. The topic hands a private copy to every subscription.

   orders · topic · 3 subscriptions

6. 6

   Each subscription is its own queue

   Its own backlog, its own delivery count, its own dead-letter queue. One broken reader hurts nobody else.

   analytics down · billing unaffected

7. 7

   Filters let you mute

   A subscription rule decides which messages it even receives — like muting the parts of the group you ignore.

   audit-sub · amount > 10000 only

8. 8

   One line of difference

   The sender does not change at all. A topic is addressed exactly like a queue — the fan-out is infrastructure.

   CreateQueue → CreateTopic + subscriptions

9. 9

   Queue or topic

   One is work that must happen once. The other is news that several teams need to hear.

   point-to-point · or · pub/sub

10. 10

    One reader? Queue. Many? Topic.

    And if you are not sure yet, a topic with one subscription costs almost nothing and saves the migration.

    Azure Service Bus · queues and topics