How gRPC handles backward compatibility with proto files

The most important point about gRPC.

Both Server and Client MUST have the proto file — or at least the generated classes from the proto file.

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Why is this necessary?

Because gRPC doesn’t send raw JSON or XML over the wire.

Instead, it sends binary data based on the structure defined in the .proto file.

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How gRPC Works (Step-by-Step)

1. You create the .proto file describing:
   • Services
   • RPC methods
   • Request & Response message types
   • Data types
2. Both client and server must generate code from this .proto using:
   • protoc (Protocol Buffer Compiler)
   • Language-specific plugins (Java, Python, Go, etc.)

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Example .proto file:

protoCopyEditsyntax = "proto3";

message User {
  string name = 1;
  int32 age = 2;
}

service UserService {
  rpc GetUser(User) returns (User);
}

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What Happens Under the Hood 🔍

Step	Server	Client
Define .proto	✅ Yes	✅ Yes
Generate Code	protoc --java_out=...	protoc --java_out=...
Implement Logic	Writes UserServiceImpl.java	Calls userService.getUser()
Communication	Encodes User into binary	Decodes binary into User

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The BIG Difference 🔥

In REST APIs:

• Client only needs the API URL
• No code sharing is required
• Only JSON schema is needed (optional)

In gRPC:

• Client must know the data structure before calling the API
• Server cannot send unknown fields to the client
• The proto file is the contract between client and server

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How Client Gets the Proto File? 🤔

✅ Usually, server provides:

• SDK (pre-generated client classes)
• .proto file + instructions
• Git repo with client library

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Example Folder Structure

pgsqlCopyEditservice-repo/
├─ user.proto        # Proto file
├─ server/
│  ├─ UserService.java
│  └─ pom.xml
└─ client/
   ├─ UserClient.java
   └─ pom.xml

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What If Client Doesn’t Have Proto File?

❌ It won’t work
Because without the .proto file, the client can’t:

• Serialize requests
• Deserialize responses
• Understand what fields exist

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Why Did Google Design It This Way?

gRPC is built for microservices + high-performance systems — where contract enforcement is critical.

✅ With gRPC, client and server ALWAYS agree on:

• Data types
• Field names
• Field order
• Optional vs Required fields

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Can There Be a Generic gRPC Client?

No 🚫.

You always need generated code or the proto file to interact with a specific gRPC service.

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This is What Makes gRPC:

Feature	REST	gRPC
Loose Coupling	✅ Yes	❌ No
Performance	🔴 Slow	🔥 Fast
Type Safety	🔴 No	✅ Yes
Automatic Code Generation	❌ No	✅ Yes
Contract-Based	❌ Optional	🔥 Mandatory
Unknown Fields	✅ Ignored	❌ Error

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Summary 💪

Tech	When to Use?
REST	Public APIs, Simple Services
gRPC	Microservices, High Performance, Internal APIs

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Final Answer 🔥🔥

If anyone wants to call a gRPC service — they MUST either:

1. Use the generated client SDK (from the server)
2. Have the .proto file and generate client code themselves

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Why People Get Confused?

Because HTTP APIs like REST are open — anyone can call them without needing extra files.

But gRPC is closed by default — only those who have the proto file (or generated SDK) can talk to the server.