How to distinguish between synchronous and asynchronous interaction between services, and what are the main differences?

Synchronous interaction between services means that the calling service sends a request and waits for a response, blocking further execution until the result is received. Asynchronous interaction is when a service sends a request and continues working without waiting for a response, which may arrive later via callback, event, or message queue.

Main differences:

• Waiting time: In synchronous calls, the client waits for a response; in asynchronous calls, it does not.
• Coupling: Synchronous interaction is more tightly coupled in time, asynchronous is more loosely coupled.
• Error handling: In synchronous cases, errors are handled immediately; in asynchronous, later, upon receiving the response.
• Performance: Asynchrony allows better resource utilization and scalability.

Example of a synchronous call in Java:

String response = restTemplate.getForObject("http://service/api/data", String.class);
System.out.println(response);

Asynchronous example with CompletableFuture:

CompletableFuture.supplyAsync(() -> restTemplate.getForObject("http://service/api/data", String.class))
    .thenAccept(response -> System.out.println(response));
// main thread is not blocked