Publisher and Subscriber Model

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Arjit Sharma
01 Min

Publisher-Subscriber model is a messaging pattern where message senders (publishers) do not send messages directly to specific receivers (subscribers). Instead, messages go through something called a message broker, which acts like a middleman. Subscribers just “subscribe” to the topics they care about, and the broker sends relevant messages to them.

Note - Think of a messaging queue as the foundation (the “what”), while Pub-Sub is the pattern (the “how”) for exchanging messages.

Why Use Pub-Sub?

Imagine you have tons of microservices talking to each other. If each service tries to connect directly, things get messy.

Key Advantages -

  • Loose Coupling: Publishers and subscribers don’t need to know each other.
  • Asynchronous Communication: No need to wait for responses.
  • Reliability: Messages can be stored temporarily, reducing data loss.
  • Scalability: You can add more services without changing existing ones.

Disagvantages -

  • Latency: There can be delays between publishing and receiving messages.
  • Eventual Consistency: Subscribers may not get data instantly.
  • Single Point of Failure: If the broker fails, the whole system could break.

Example: E-Commerce System

Let’s say you’re building an online store. A customer places an order, what happens next?

  1. Order Service (Publisher) - Publishes an OrderPlaced event with order details.
  2. Message Broker - Receives the event and routes it to all interested subscribers.
  3. Subscriber Services Respond:
    • Inventory Service: Updates stock levels.
    • Payment Service: Initiates billing.
    • Notification Service: Sends a confirmation email or SMS.
    • Shipping Service: Prepares the order for delivery.

Thanks to Pub-Sub, these services act independently yet stay in sync, all without needing to directly call one another.

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