How to fix ERROR: for app Container depends_on condition on db is invalid in Docker

# Remove this line or update to a higher version
# version: "3"  # <-- Delete this

services:
  web:
    build: .
    depends_on:
      db:
        condition: service_healthy

  db:
    image: postgres:15
    healthcheck:
      test: ["CMD-SHELL", "pg_isready -U postgres"]
      interval: 10s
      timeout: 5s
      retries: 5

# Check your version
docker compose version   # Modern v2 (plugin)
docker-compose version   # Legacy v1 (standalone)

# Use the modern command
docker compose up        # ✅ Use this
docker-compose up        # ❌ Avoid if possible

# On Ubuntu/Debian
sudo apt-get update
sudo apt-get install docker-compose-plugin

# Or via Docker Desktop (includes it by default)

services:
  app:
    build: .
    depends_on:
      db:
        condition: service_healthy
      redis:
        condition: service_healthy

  db:
    image: postgres:15
    healthcheck:
      test: ["CMD-SHELL", "pg_isready -U postgres"]
      interval: 10s
      timeout: 5s
      retries: 5
      start_period: 30s

  redis:
    image: redis:7
    healthcheck:
      test: ["CMD", "redis-cli", "ping"]
      interval: 10s
      timeout: 5s
      retries: 5

version: "2.4"

services:
  app:
    build: .
    depends_on:
      db:
        condition: service_healthy

  db:
    image: postgres:15
    healthcheck:
      test: ["CMD-SHELL", "pg_isready -U postgres"]
      interval: 10s
      timeout: 5s
      retries: 5

# ❌ WRONG - Array syntax (no conditions)
depends_on:
  - db
  - redis

# ❌ WRONG - Incorrect nesting
depends_on:
  db:
  condition: service_healthy

# ✅ CORRECT - Long-form syntax
depends_on:
  db:
    condition: service_healthy
  redis:
    condition: service_started