version: 1
id: docker-networking
title: Repair Service Networking
summary: Fix a Compose topology so an API uses service discovery instead of loopback.
difficulty: intermediate
estimatedMinutes: 25
prerequisites: [docker-debugging]
image: alpine:3.21
shell: /bin/sh
setup:
  - "printf 'services:\\n  api:\\n    image: alpine:3.21\\n    environment:\\n      DATABASE_URL: postgres://app@localhost:5432/app\\n  database:\\n    image: postgres:17-alpine\\n' > /workspace/compose.yaml"
tasks:
  - id: discovery
    title: Use Compose DNS
    description: Change the API database hostname from localhost to the database service and add a dependency.
    hints:
      - Inside a container, localhost refers to that same container.
      - Compose service names are DNS names on the default network.
    checks:
      - type: command
        name: API targets database service
        command: grep -q 'postgres://app@database:5432/app' /workspace/compose.yaml
      - type: command
        name: API declares its dependency
        command: grep -q 'depends_on:' /workspace/compose.yaml
limits: {cpus: "0.5", memory: 128m, pids: 64, timeout: 1800, network: false}