What is the difference between synchronous and asynchronous communication in microservices?

In synchronous communication (typically REST/gRPC over HTTP), the calling service blocks and waits for an immediate response, creating tight temporal coupling — if the downstream service is slow or down, the caller is directly affected. In asynchronous communication (typically via a message broker like Kafka or RabbitMQ), the caller publishes an event and moves on without waiting, decoupling the services in time — the consumer processes the message whenever it's ready, which improves resilience at the cost of not getting an immediate result.