Docker Compose

Docker Compose

Opinionated conventions for consistent, maintainable Docker Compose files.

Best Practices

File naming and override strategy

• Prefer compose.yaml (or compose.yml) as the canonical base file.
• Keep local-only changes in compose.override.yaml (auto-merged by Compose).
  • If an override file is local-only, add it to .gitignore unless the team explicitly wants it committed (optionally keep a .example file).
• For environment-specific configs, use compose.<env>.yaml and combine with -f.
• Avoid the top-level version key (obsolete).

Service naming and identity

• Use kebab-case service names; they become internal DNS names.
• Avoid container_name unless strictly required; it prevents scaling.
• Pin image tags (avoid latest); prefer digests for production.

Env var conventions

• Use .env for interpolation, env_file for container runtime values.
• Keep secrets out of Compose files; commit .env.example only.
• Prefer mapping style for environment when values include special chars.
• If the same variable is set in both, environment overrides env_file (key order does not change behavior).

Ports, volumes, and paths

• Quote HOST:CONTAINER port mappings to avoid YAML base-60 parsing issues.
• Prefer relative bind mounts starting with ./.
• Use named volumes for persistent data.

Readiness and dependencies

• Use depends_on for ordering; add healthcheck and condition: service_healthy for readiness-sensitive dependencies.

Profiles for optional services

• Use profiles to gate optional services (e.g., debug, seed, admin) instead of maintaining separate files.
• If a service is profile-gated, any depends_on targets must be in the same profile or always enabled; otherwise the model is invalid.

Project naming and file composition

• Set top-level name when you need stable project identifiers (network/volume names) across directories or CI.
• Use include/fragments to split large Compose files, but only if your Compose implementation supports them.

DRY with extensions and anchors

• Use x- extension fields plus YAML anchors/merge to share repeated blocks (env, volumes, labels) across services.

Secrets and configs

• Declare secrets/configs at top level and explicitly grant access per service; top-level declaration alone is not enough.
• Prefer secrets/configs for sensitive or shared files when the platform supports them.

Deploy portability

• Treat deploy as platform-specific; some Compose implementations ignore it in local runs.
• Document when deploy is required so users do not assume it applies everywhere.

Init for PID 1

• Set init: true for long-running services to handle PID 1 signal and zombie reaping correctly.

Ordering Conventions

Top-level order

1. name (optional)
2. services
3. networks
4. volumes
5. configs
6. secrets

Service key order

Use this exact order for keys within each service to keep the file consistent.

1. Identity: image / build, container_name, hostname
2. Runtime: command, entrypoint, restart
3. Environment: env_file, environment, labels
4. Resources: ports, expose, volumes
5. Connectivity: depends_on, networks, network_mode
6. Health: healthcheck

Validation

• Always run docker compose config before shipping changes.

References

• For the official specification and detailed semantics, see https://compose-spec.io