In synchronous communication, the sender sends a request and waits (blocks) until it receives a response before doing anything else. The two parties must be available at the same time.

Think of it like a phone call:

• You dial → you wait → the other person answers → you talk → you get your answer.

Common Protocols for Synchronous Communication

REST Example

A browser asks a server: "Give me user #42's profile."

GET /users/42 HTTP/1.1 Host: api.example.com

Server responds:

{
  "id": 42,
  "name": "Alice",
  "email": "alice@example.com"
}

The browser waits for this response before showing the page.

gRPC Example

gRPC uses .proto files to define services:

// user.proto
service UserService {
  rpc GetUser (UserRequest) returns (UserResponse);
}

message UserRequest {
  int32 user_id = 1;
}

message UserResponse {
  int32 id = 1;
  string name = 2;
  string email = 3;
}

The client calls GetUser(user_id=42) and blocks until the server returns a UserResponse.

Synchronous Communication Diagram