Docker Compose — Dependencies — Practice Exercises

1 minute read

Test your understanding of depends_on, health checks, and wait strategies.

Exercise 1: Basic depends_on

Create a stack where api depends on db. Verify db starts first.

Solution 
services:
  api:
    image: alpine
    command: sleep 3600
    depends_on:
      - db
  db:
    image: postgres:16
    environment:
      POSTGRES_PASSWORD: secret

docker compose up -d
docker compose ps

db starts before api.

Exercise 2: Health Check

Add a health check to PostgreSQL and verify it shows (healthy) in docker compose ps.

Solution 
services:
  db:
    image: postgres:16
    environment:
      POSTGRES_PASSWORD: secret
    healthcheck:
      test: ["CMD-SHELL", "pg_isready -U postgres"]
      interval: 5s
      timeout: 3s
      retries: 5
      start_period: 10s

docker compose up -d
sleep 15
docker compose ps

NAME        STATUS                PORTS
myapp-db-1  Up 15 seconds (healthy)  5432/tcp

Exercise 3: Wait for Healthy

Make api wait until db is healthy before starting.

Solution 
services:
  api:
    image: alpine
    command: sleep 3600
    depends_on:
      db:
        condition: service_healthy
  db:
    image: postgres:16
    environment:
      POSTGRES_PASSWORD: secret
    healthcheck:
      test: ["CMD-SHELL", "pg_isready -U postgres"]
      interval: 5s
      timeout: 3s
      retries: 5
      start_period: 10s

Exercise 4: Init Container

Create a migrate service that runs and exits, then api starts after it completes successfully.

Solution 
services:
  migrate:
    image: alpine
    command: sh -c "echo 'Running migrations...' && sleep 3 && echo 'Done!'"
    depends_on:
      db:
        condition: service_healthy

  api:
    image: alpine
    command: sleep 3600
    depends_on:
      migrate:
        condition: service_completed_successfully

  db:
    image: postgres:16
    environment:
      POSTGRES_PASSWORD: secret
    healthcheck:
      test: ["CMD-SHELL", "pg_isready -U postgres"]
      interval: 5s
      timeout: 3s
      retries: 5
      start_period: 10s

Exercise 5: Multi-Dependency with Health

Create a full stack: web → api → db + cache, all with health checks and condition: service_healthy.

Solution 
services:
  web:
    image: nginx:alpine
    depends_on:
      api:
        condition: service_healthy

  api:
    image: alpine
    command: sh -c "while true; do echo ok | nc -l -p 3000; done"
    healthcheck:
      test: ["CMD-SHELL", "nc -z localhost 3000"]
      interval: 5s
      timeout: 3s
      retries: 5
    depends_on:
      db:
        condition: service_healthy
      cache:
        condition: service_healthy

  db:
    image: postgres:16
    environment:
      POSTGRES_PASSWORD: secret
    healthcheck:
      test: ["CMD-SHELL", "pg_isready -U postgres"]
      interval: 5s
      timeout: 3s
      retries: 5
      start_period: 10s

  cache:
    image: redis:7
    healthcheck:
      test: ["CMD", "redis-cli", "ping"]
      interval: 5s
      timeout: 3s
      retries: 5

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