docker-compose

Learn multi-service with docker-compose.

docker-compose

Compose defines several containers in one YAML file and starts them together. A Go service and a database:

services:
  api:
    build: .
    ports: ["8080:8080"]
    environment:
      DATABASE_URL: postgres://app:gizli@db:5432/app?sslmode=disable
    depends_on: [db]
  db:
    image: postgres:16
    environment:
      POSTGRES_USER: app
      POSTGRES_PASSWORD: gizli

docker compose up starts both on the same network; the service reaches the database by the name db.

depends_on and readiness

depends_on only sets the start order; it doesn't guarantee the database is ready to accept connections. Either use healthcheck + condition: service_healthy or retry Ping on the Go side:

for i := 0; i < 10; i++ {
    if err = db.Ping(); err == nil {
        break
    }
    time.Sleep(time.Second)
}

Exercise Retry

Write retry(n int, f func() error) error: try f at most n times, return nil on success and the last error otherwise. Test it with a function that succeeds on the third try and print the number of attempts.

Show example solution

package main

import (
    "errors"
    "fmt"
)

func retry(n int, f func() error) error {
    var err error
    for i := 0; i < n; i++ {
        if err = f(); err == nil {
            return nil
        }
    }
    return err
}

func main() {
    deneme := 0
    err := retry(5, func() error {
        deneme++
        if deneme < 3 {
            return errors.New("hazır değil")
        }
        return nil
    })
    fmt.Println(deneme, err)
}

Test yourself

Which address does the api container use to reach the database?
- localhost:5432
- 127.0.0.1:5432
- postgres.local:5432
- db:5432
Show answer

db:5432 — Compose turns service names into DNS names on the network; inside a container, localhost is the container itself.

Port mapping

services:
  api:
    build: .
    ____: ["8080:8080"]

Show answer

services:
  api:
    build: .
    ports: ["8080:8080"]

Compose keys

build

Image from a Dockerfile
image

Ready-made image
environment

Environment variables
depends_on

Start order

Next lesson: Build args and labels

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