The Entry Point into a Service

A handler is the entry point to a service. It receives instructions from other services, apps, and clients. You might think of them as controllers in MVC terms, but that's a very loose comparison.

This handler does deposits to a bank account.

A handler receives a command, does its work, and when it's done with that work, it reports the status and outcome of that work by publishing an event.

A handler might also respond (or react) to events by other services, or it might respond to events published by its own service (when a service calls itself).