So we can use the Kafka Streams API to piece together complex business systems as a collection of asynchronously executing, event-driven services. The differentiator here is the API itself, which is far richer than, say, the Kafka Producer or Consumer. It makes code more readable, provides reusable implementations of common patterns like joins, aggregates, and filters and wraps the whole ecosystem with a transparent level of correctness.
Systems built in this way, in the real world, come in a variety of guises. They can be fine grained and fast executing, completing in the context of an HTTP request, or complex and long-running, manipulating the stream of events that map a whole company’s business flow. This post focusses on the former, building up a real-world example of a simple order management system that executes within the context of a HTTP request, and is entirely built with Kafka Streams. Each service is a small function, with well-defined inputs and outputs. As we build this ecosystem up, we will encounter problems such as blending streams and tables, reading our own writes, and managing consistency in a distributed and autonomous environment.
This post stays high-level and covers a lot of ground. I’m wishy-washy on the idea of microservices, but if you are going to do them, it’s better to do them right.