In this section, we will learn how to develop non-blocking versions of the read APIs. The composite service will make reactive, that is, non-blocking, calls in parallel to the three core services. When the composite service has received responses from the core services, it will create a composite response and send it back to the caller. This is illustrated in the following diagram:

We will cover the following:

• An introduction to Spring Reactor
• Non-blocking persistence using Spring Data for MongoDB
• Non-blocking REST APIs in the core services, including how to handle blocking code for the JPA-based persistence layer
• Non-blocking REST APIs in the composite service