Tell me about application layer protocols. What is gRPC? How does it differ from REST?

Application layer protocols are a set of rules and formats that define how applications exchange data. They operate over the transport layer (e.g., TCP) and provide message structure and interaction.

gRPC is a modern remote procedure call (RPC) protocol developed by Google. It uses HTTP/2 for transport, Protocol Buffers (protobuf) for data serialization, and supports bidirectional streams, enabling efficient message exchange.

REST (Representational State Transfer) is an architectural style based on using standard HTTP methods (GET, POST, PUT, DELETE) and URIs to access resources. Data is usually transmitted in JSON or XML format.

Main differences between gRPC and REST:

• Data format: gRPC uses binary protobuf, which is faster and more compact than JSON in REST.
• Transport: gRPC operates over HTTP/2 with support for multiplexing and streaming, while REST typically uses HTTP/1.1.
• Interaction type: gRPC supports synchronous and asynchronous calls, including streaming data in both directions; REST mainly uses request-response.
• Contracts: gRPC requires defining services and messages in protobuf, ensuring strict typing and auto-generated code; REST is more flexible and does not require strict contracts.

Example of a gRPC service in Go:

syntax = "proto3";

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

message HelloRequest {
  string name = 1;
}

message HelloReply {
  string message = 1;
}

This service generates server and client code for calling the SayHello method.