Synchronous communication is a blocking call in which client

blocks itself from doing anything else, till the response comes back.

In Asynchronous communication, client can move ahead with its

work after making an asynchronous call. Therefore client is not

blocked.

In synchronous communication, a Microservice can provide instant

response about success or failure. In real-time systems, synchronous

service is very useful. In Asynchronous communication, a service

has to react based on the response received in future.

Synchronous systems are also known as request/response based.

Asynchronous systems are event-based.

Synchronous Microservices are not loosely coupled.

Depending on the need and critical nature of business domain,

Microservices can choose synchronous or asynchronous form of

communication.

https://learn.microsoft.com/en-us/dotnet/architecture/microservices/architect-microservice-container-applications/communication-in-microservice-architecture