Distributed systems are built out of services that need to communicate, and the simplest way to do that is for one service to call another directly and wait for a response. This pattern works well for small systems and predictable workloads.

However, as systems grow, it tends to produce tight coupling between services, fragile failure behavior, and bottlenecks at the slowest component in any chain of calls.

Event-driven architecture is an alternative communication model where services publish events when something meaningful happens, and other services react to those events on their own time. The patterns related to this architecture are the established techniques for handling the new problems that the model introduces.

In this article, we will start with the basics of how event-driven systems are structured, look at why synchronous communication starts to break down at scale, and then walk through six patterns that solve specific problems EDA introduces.