Having developed a basic understanding of Kafka Streams, now let’s look at the techniques needed to build a small streaming services application. We will base this chapter around a simple order processing workflow that validates and processes orders in response to HTTP requests, mapping the synchronous world of a standard REST interface to the asynchronous world of events, and back again.
Starting from the lefthand side of the Figure 15-1, the REST interface provides methods to POST and GET orders. Posting an order creates an Order Created event in Kafka. Three validation engines (Fraud, Inventory, Order Details) subscribe to these events and execute in parallel, emitting a PASS or FAIL based on whether each validation succeeds. The result of these validations is pushed through a separate topic, Order Validations, so that we retain the single writer relationship between the orders service and the Orders topic.1 The results of the various validation checks are aggregated back in the orders service, which then moves the order to a Validated or Failed state, based on the combined result. Validated orders accumulate in the Orders view, where they can be queried historically. This is an implementation of the CQRS design pattern (see “Command Query Responsibility Segregation” in Chapter 7). The email service sends confirmation emails.
Figure 15-1. An order processing ...
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