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DOCKER COMPOSE

What it is:

  • Tool for defining and running multi-container applications
  • Uses YAML configuration file
  • Single command starts entire application stack with dependencies, networks, volumes

Problem it solves:

  • Running multi-container apps with docker run means typing long commands for each service
  • Must manually create networks, manage startup order, remember all flags
  • Compose replaces this with declarative config file and simple commands

Structure​

A docker-compose.yml defines:

Services​

  • Containers to run
  • Each service specifies image/build context, ports, environment variables, dependencies, health checks

Networks​

  • How containers communicate

Volumes​

  • Persistent data

When to Use​

Use for:

  • Multi-container applications (app + database + cache)
  • Local development environments with multiple services
  • Integration testing requiring full stack
  • Small single-host production deployments

Skip for:

  • Single containers (just use docker run)
  • Production at scale (use docker_orchestration like Kubernetes)
  • Multi-host deployments needing auto-scaling and rolling updates

Dependency Management​

depends_on controls startup order:

  • Only waits for containers to start, NOT for services to be ready
  • Combine with condition: service_healthy to wait for docker_healthcheck to pass
  • Without health checks, app might try connecting to database that's still initializing

Environment Variables and Config​

Configuration options:

  • Set inline in YAML
  • Load from .env files
  • Substitute from host environment
  • Mount config files via volumes

Pattern: Use environment variables for secrets and runtime config, mount files for complex configuration.

Compose vs Orchestration​

Differences:

  • Compose: Single-host, development-focused
  • docker_orchestration: Multi-host, production-grade
  • Compose orchestrates containers but doesn't manage clusters
  • Many start with Compose for dev, move to Kubernetes for prod

For production needing auto-scaling, rolling updates, self-healing: use orchestration platforms.

Common Pitfalls​

WARNING

Ignoring health checks: Using depends_on without health checks means dependent services start before dependencies are ready. Always add docker_healthcheck to services that need startup time.

DANGER

Using Compose for production scale: Compose has no auto-scaling, no rolling updates, no multi-host support. Great for single-host deployments but not designed for production scale. Use docker_orchestration platforms instead.

References​