Protobuf definition

Learn to define messages with a .proto file.

Protobuf definition

In gRPC the contract is written in a .proto file: messages (data structures) and a service (callable methods).

syntax = "proto3";
package selam;
option go_package = "example.com/app/selampb";

message SelamIstek { string ad = 1; }
message SelamYanit { string mesaj = 1; }

service Selamlayici {
  rpc Selam(SelamIstek) returns (SelamYanit);
}

Field numbers (= 1) are the field's identity in the binary encoding.

Types and repeated fields

Protobuf types map directly to Go: string → string, int32 → int32, bool → bool. A repeated field becomes a slice and a nested message a pointer:

message Sepet {
  string sahip = 1;
  repeated string urunler = 2;  // []string
}

When removing a field, marking its number reserved prevents accidental reuse.

Exercise Message counterpart

Write the Go counterpart of message Sepet { string sahip = 1; repeated string urunler = 2; } by hand: type Sepet struct { Sahip string; Urunler []string }. Print the number of products for Sepet{"ada", []string{"kalem", "defter"}}.

Show example solution

package main

import "fmt"

type Sepet struct {
    Sahip string
    Urunler   []string
}

func main() {
    s := Sepet{"ada", []string{"kalem", "defter"}}
    fmt.Println(len(s.Urunler))
}

Test yourself

Why is changing a field number in a published message dangerous?
- It has no effect
- Old clients decode the data into the wrong field
- The message gets bigger
- Compilation slows down
Show answer

Old clients decode the data into the wrong field — The binary format sends the number, not the name; if it changes, old and new code interpret the data differently.

List field

message Siparis {
  ____ string urunler = 1;
}

Show answer

message Siparis {
  repeated string urunler = 1;
}

proto and Go

string ad = 1

Ad string
repeated int32 puan = 2

Puan []int32
message

struct
service

Interface

Next lesson: Compiling with protoc

We Are Enhancing Your Experience

Cookie Preferences

Strictly Necessary Cookies

Analytical Cookies