What is an .proto file and how do you define a gRPC service?

Expected Interview Answer

Step-by-Step Explanation

1. Step 1

   Set the syntax

   Declare proto3 at the top of the file so the compiler knows the language version.

2. Step 2

   Define messages

   Create message types with typed, uniquely numbered fields for request and response payloads.

3. Step 3

   Declare the service

   Add a service block listing rpc methods, each mapping one request message to one response message.

4. Step 4

   Compile with protoc

   Run protoc with the gRPC plugin for your target language to generate client stubs and server interfaces.

5. Step 5

   Implement and call

   Implement the generated server interface and use the generated client stub to invoke the RPCs.

Code Example

syntax = "proto3";

package greet;

message HelloRequest {
  string name = 1;
}

message HelloReply {
  string message = 1;
}

service Greeter {
  rpc SayHello (HelloRequest) returns (HelloReply);
}

Follow-up Questions

• Why do Protocol Buffers use numbered fields instead of names on the wire?
• How do you evolve a .proto file without breaking existing clients?
• What is the difference between proto2 and proto3?
• How does protoc generate code for different languages?
• What happens if a client and server use mismatched .proto versions?