Messaging Patterns

Should one message go to one person, or to everyone? Choosing the right messaging pattern determines how easily you can add new features in the future.

1. Point-to-Point (Queue)

One producer sends a message to exactly one consumer. If multiple consumers exist, they "Competing Consumer" pattern is used—only one gets each message. Best for: Processing payments or sending a single email.

2. Publish/Subscribe (Topic)

The producer publishes to a "Topic." Multiple "Subscribers" can listen to that topic. Every subscriber gets a copy of the message. Best for: Notification systems, Updating multiple caches, or logging.

4. Interview Mastery

Q: "How does Pub/Sub enable 'Plug-and-Play' microservices?"

Architect Answer: "Because the Producer doesn't know who is listening. If you have an 'OrderPlaced' event, today only the 'Shipping' service listens. Tomorrow, if you want to add a 'Marketing' service that sends a coupon, you just point it to the topic. You don't have to change a single line of code in the 'Order' service. This is the ultimate in architectural decoupling."