Synchronous

A synchronous microservice is one in which data moves to and from an endpoint of a service in a blocking interaction. A typical example of a synchronous data exchange is an HTTP request/response interaction, seen in Figure 1 below. When a request is made to an endpoint under HTTP, the caller is locked in the interaction until a response is received.

The caller might receive the response in a mere millisecond or in a few seconds. Regardless of the application latency, the caller cannot move forward to the next task until the response is received. REST is a good example of a synchronous microservice.

Asynchronous

An asynchronous microservice is one in which a request to a service and the subsequent response occur independently from each other. The general practice for implementing an asynchronous microservice is to use a message broker technology, such as Kafka or RabbitMQ, to act as a go-between for services, as seen in Figure 2 below. One service will publish a message to another service using the message broker.

https://www.theserverside.com/answer/Synchronous-vs-asynchronous-microservices-communication-patterns