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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.

Practice 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"}}.

Starter code

main.go
package main

import "fmt"

func main() {
    // buraya yaz
}

Sample solution

main.go
package main

import "fmt"

type Sepet struct {
    Sahip string
    Urunler   []string
}

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

Expected output

2

Questions and answers

Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.

1 Why is changing a field number in a published message dangerous?

  • It has no effect
  • Old clients decode the data into the wrong field Correct answer
  • The message gets bigger
  • Compilation slows down

Explanation: The binary format sends the number, not the name; if it changes, old and new code interpret the data differently.

2 List field

Question 
message Siparis {
  ____ string urunler = 1;
}
Answer 
message Siparis {
  repeated string urunler = 1;
}

Explanation: The repeated keyword makes a field a list; it becomes []string in Go.

3 proto and Go

Term Meaning
string ad = 1 Ad string
repeated int32 puan = 2 Puan []int32
message struct
service Interface

All lessons in Advanced

1 Reflection

  1. The reflect package
  2. reflect.Type
  3. reflect.Value
  4. Reflecting struct fields
  5. Dynamic call
  6. Summary - Reflection

2 unsafe

  1. The unsafe package
  2. unsafe.Pointer
  3. Sizeof and Alignof
  4. Offsetof
  5. unsafe and slice
  6. Summary - unsafe

3 cgo

  1. What is cgo?
  2. Calling C code
  3. C data types
  4. Go-C conversion
  5. cgo and memory
  6. Summary - cgo

4 plugin

  1. The plugin package
  2. Building a plugin
  3. Loading a plugin
  4. Plugin symbol
  5. Plugin and error
  6. Summary - plugin

5 database/sql

  1. The database/sql package
  2. sql.Open
  3. Query and Exec
  4. Prepared statement
  5. Transaction
  6. Summary - database/sql

6 Middleware

  1. The middleware concept
  2. HTTP middleware
  3. Middleware chain
  4. Logging middleware
  5. Auth middleware
  6. Summary - Middleware

7 gRPC

  1. What is gRPC?
  2. Protobuf definition
  3. Compiling with protoc
  4. gRPC server
  5. gRPC client
  6. Summary - gRPC

8 Project: Microservice

  1. Service skeleton
  2. Database layer
  3. HTTP API
  4. Docker and deployment

9 Docker

  1. Dockerfile
  2. Multi-stage build
  3. scratch image
  4. docker-compose
  5. Build args and labels
  6. Summary - Docker

10 pprof

  1. What is pprof?
  2. CPU profiling
  3. Memory profiling
  4. go test -cpuprofile
  5. The trace tool
  6. Summary - pprof

11 Observability

  1. log/slog
  2. Metrics
  3. Tracing
  4. OpenTelemetry
  5. Health check
  6. Summary - Observability

12 Advanced final

  1. Challenge 1 - Reflection
  2. Challenge 2 - Microservice
  3. Challenge 3 - Performance
  4. Challenge 4 - Database