Pub-Sub vs Message Queues

When building event-driven systems, you need a way to handle communication between different services. Two popular approaches are Message Queues and Pub-Sub systems. Let's understand when to use each one.

Message Queues work like a line at a coffee shop - messages wait in a queue and are processed one by one. When a service (producer) sends a message, it goes to the back of the queue. Another service (consumer) picks up messages from the front of the queue and processes them. Once a message is processed, it's removed from the queue.

In Pub-Sub systems, messages are broadcast to all interested services. Services that want to receive messages (subscribers) tell the system what types of messages they want. When a service (publisher) sends a message, all interested subscribers receive a copy of that message.

Message Queues are great when you want each message to be processed exactly once. For example, in an e-commerce system, when a customer places an order, you want exactly one service to process that order. You don't want multiple services trying to process the same order.

Pub-Sub systems shine when multiple services need to react to the same event. For instance, when a user creates a new account, you might want one service to send a welcome email, another to update analytics, and a third to check for fraud - all based on the same "new user" event.

Both approaches can be combined in modern systems. Tools like Apache Kafka and RabbitMQ support both patterns, letting you choose the right approach for each use case.

Here's a comparison of Message Queues vs Pub-Sub: