What is the difference between Pub/Sub and Message Queue?

Pub/Sub (Publish/Subscribe) and Message Queue are two messaging patterns that solve different problems.

Message Queue is a queue of messages where the sender places a message into the queue, and the receiver reads them sequentially. It is typically used for load balancing and asynchronous task processing. A message is usually processed by a single consumer.

Pub/Sub is a publish-subscribe model where the publisher publishes a message to a topic, and all subscribers of that topic receive copies of the messages. This allows for broadcast messaging and reactive systems.

Example:

• In Message Queue: the task is to process orders, each order is processed by one worker.
• In Pub/Sub: the task is to notify multiple services about an event (e.g., data update), all subscribers receive the notification.

Thus, the main difference is that Message Queue ensures message delivery to a single consumer, while Pub/Sub delivers to multiple subscribers.