Event Notifications
Event Notifications
Our application is becoming more sophisticated. A customer places an order. Several services need to know about it. The inventory service must reduce stock. The email service must send a confirmation. The analytics service must record the sale. The loyalty service must award reward points. One event... Many interested services. The engineering problem became obvious. How do we notify multiple services without sending separate messages to each one? The engineering concept that solved this problem is Publish–Subscribe Messaging, often called Pub/Sub. Instead of sending individual messages, the application publishes a single event. Every service that has subscribed to that event automatically receives a copy. The sender doesn't need to know who is listening. It simply announces that something happened. Amazon Web Services provides this through Simple Notification Service (SNS). Microsoft Azure provides Azure Event Grid. Google Cloud provides Pub/Sub. Different names. One engineering concept. Applications became event-driven. New services could subscribe to existing events without changing the application that generated them. But another challenge soon emerged. Some applications weren't generating thousands of events. They were generating millions every second. How do we continuously process data that never stops arriving?