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Messaging

Basic System Design
≈ 2 min read

A Broker Becomes the Intermediary Between Services

With messaging, the sender does not call a specific receiver instance directly. It publishes a message to a broker, and a receiver retrieves and processes it separately. 

Producer -> Broker -> Consumer
               |
               +-> stores messages temporarily

The broker helps break the direct dependency on participant addresses and availability: the producer does not have to know which exact instance will process the message right now.

A Message Should Be a Standalone Contract

The receiver needs enough data for processing and a stable format. If a message contains only internal details of the sender, changes to its implementation will begin breaking consumers. 

{
  "eventId": "8b7b...",
  "orderId": 421,
  "type": "ORDER_CREATED"
}

A Queue Smooths Out Speed Differences

If a producer temporarily creates messages faster than a consumer can process them, the broker can hold the accumulated work. Consumers then process it at their own pace.

But the queue itself becomes an important part of the system: backlog size, processing errors, and messages that still fail after retries all need to be monitored.

Message delivery and successful business execution are not the same thing. A broker can deliver data to a consumer, but correct processing still remains the responsibility of the application.
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