Docker Compose Fundamentals¶

Overview¶

Author a compose.yaml with services, a shared network, a volume, environment variables, and a health check — then bring the stack up and down.

Compose declares multi-container apps as code. Ideal for local DevOps stacks and simple single-host deploys before Kubernetes.

This is a core tutorial in Module 9 · Docker Compose of the REBASH Academy Docker for Cloud & DevOps Engineers series — written for Cloud, DevOps, Platform, and SRE engineers.

Prerequisites¶

Docker Networking Fundamentals

Learning Objectives¶

By the end of this tutorial, you will be able to:

Architecture¶

This topic’s control points and relationships are shown below.

Theory¶

What¶

Docker Compose defines multi-container applications as YAML (compose.yaml). Compose v2 is the docker compose CLI plugin (space, not hyphen). Services, networks, and volumes are declared together; up creates them as a project. On the default project network, service names become DNS names.

Why¶

Real systems need an app plus database, cache, or sidecar. Running five independent docker run commands is brittle. Compose is the standard local and small-server orchestration file format and a stepping stone to Kubernetes manifests.

How it works¶

A Compose file lists services: with images or build contexts, environment, ports, volumes, and depends_on. docker compose up -d creates a project-prefixed network and starts containers. docker compose logs -f, ps, exec, and down manage the stack. Profiles and multiple compose files (-f) specialise environments. Prefer explicit healthchecks over blind depends_on for readiness. Secrets and env files keep configuration out of images.

Compose v2 command shape:

Terminal

docker compose version
docker compose -f compose.yaml up -d

Key concepts¶

Idea	Detail
Project name	Prefixes resources; set via folder or `-p`
Service DNS	`http://api:8080` from another service
Build vs image	Local Dockerfile or registry image
Override files	`compose.override.yaml` for local deltas

Keep Compose files readable: one service per concern, explicit image tags or digests, and comments only where behaviour is non-obvious. Use .env for local non-secret defaults and never commit production credentials beside compose.yaml. When a stack grows beyond a single host, treat Compose as the development contract and plan the Kubernetes translation deliberately.

Common pitfalls¶

Using obsolete docker-compose v1 behaviour docs blindly
Publishing database ports to the world on shared networks
Relying on depends_on without health conditions
Hard-coding hostpaths that only exist on one laptop

Hands-on Lab¶

Objective¶

Define a two-service Compose stack with a named volume, start it detached, verify with curl, and tear down including volumes.

Prerequisites¶

Docker Engine with Compose plugin (docker compose version)
curl on the host

Lab environment¶

Workspace: ~/rebash-docker/module-09

Host port 18087 maps to the web service.

Terminal

mkdir -p ~/rebash-docker/module-09 && cd ~/rebash-docker/module-09

Real-world scenario¶

Locally you replicate a minimal production pair: nginx serves a static page while Redis holds session keys. Compose wires service DNS, mounts a volume for Redis data, publishes only the web port, and lets you run the same file in CI with docker compose up -d.

Step-by-step tasks¶

Task 1 – Create Compose file and static page¶

Create html/index.html:

index.html

<!DOCTYPE html>
<html lang="en">
<head><meta charset="utf-8"><title>REBASH mod09</title></head>
<body><h1>compose stack ok</h1></body>
</html>

Create compose.yaml:

compose.yaml

services:
  web:
    image: nginx:1.27-alpine
    ports:
      - "18087:80"
    depends_on:
      - redis
    volumes:
      - ./html:/usr/share/nginx/html:ro
  redis:
    image: redis:7.4-alpine
    volumes:
      - redis-data:/data
    command: ["redis-server", "--save", "", "--appendonly", "no"]

volumes:
  redis-data:

Expected output

compose.yaml and html/index.html exist in the lab directory.

Task 2 – Start stack and check status¶

Terminal

cd ~/rebash-docker/module-09
docker compose up -d
docker compose ps | tee compose-ps.txt
grep -E 'web|redis' compose-ps.txt
curl -s http://127.0.0.1:18087/ | tee compose-curl.txt
grep -q 'compose stack ok' compose-curl.txt

Expected output

compose-ps.txt shows web and redis running; curl returns the custom heading.

Task 3 – Prove Redis volume and fetch logs¶

Terminal

cd ~/rebash-docker/module-09
docker compose exec redis redis-cli PING | tee redis-ping.txt
grep -q 'PONG' redis-ping.txt
docker compose logs --tail=10 web | tee compose-web-logs.txt
test -s compose-web-logs.txt

Expected output

redis-ping.txt is PONG; web logs captured.

Validation steps¶

docker compose ps shows web and redis services up
compose-curl.txt serves custom HTML on port 18087
redis-ping.txt confirms Redis responds inside the stack

Common errors and fixes¶

Error	Cause	Fix
port is already allocated	18087 in use	Edit host port in `compose.yaml`
service "web" depends on undefined service	Typo in `depends_on`	Match service names exactly
bind mount not found	Missing `html/`	Create `html/index.html` before `up`

Challenge exercise¶

Scale web to two replicas is not valid with static published ports — instead add a healthcheck to web and prove Compose reports healthy state:

Add under web: in compose.yaml:

    healthcheck:
      test: ["CMD", "wget", "-qO-", "http://127.0.0.1/"]
      interval: 5s
      timeout: 3s
      retries: 5

Reconcile and verify:

Terminal

cd ~/rebash-docker/module-09
docker compose up -d
docker compose ps | tee compose-ps-health.txt
grep -i 'healthy\|running' compose-ps-health.txt

Expected output

After a short wait, compose-ps-health.txt shows web as healthy or running with healthcheck configured.

Learning outcomes¶

Authored a multi-service compose.yaml with pinned images
Started and inspected a stack with docker compose ps and logs
Verified HTTP and Redis connectivity before teardown

Cleanup¶

Terminal

cd ~/rebash-docker/module-09
docker compose down -v
docker rmi nginx:1.27-alpine redis:7.4-alpine 2>/dev/null || true

Validation¶

Lab commands run under ~/rebash-docker/module-09/
compose-ps.txt and compose-curl.txt prove the stack ran correctly
You can explain each Theory section in your own words
You can describe one production failure mode for this topic

Code Walkthrough¶

Production practice for Docker Compose Fundamentals always combines:

Inspect before you change (status, plan, logs, dry-run)
Prefer reversible, documented changes (Git, IaC, drop-ins, version pins)
Capture evidence (command output, pipeline logs) for handovers
Prefer current tools and APIs over legacy shortcuts
Least privilege — escalate credentials only when required

Keep runbooks short enough to follow under pressure. Automate checks; keep humans for judgement.

Security Considerations¶

Treat credentials and tokens for docker as privileged — never commit them
Prefer short-lived auth (OIDC, roles, SSO) over long-lived keys
Validate blast radius before apply/deploy/delete operations
Restrict who can approve production changes
Collect audit logs; limit who can read sensitive traces

Common Mistakes¶

Using obsolete docker-compose v1 behaviour docs blindly

Validate assumptions against the Theory section and official docs before changing production.

Publishing database ports to the world on shared networks

Lab shortcuts (open security groups, admin roles, skip approvals) must not ship unchanged.

Changing production without a rollback path

Always know how to revert (previous artefact, prior release, state rollback, DNS failback).

Best Practices¶

Encode Docker Compose Fundamentals changes as code and review them in pull requests
Pin versions (images, modules, actions, provider plugins)
Separate environments with clear promotion gates
Alert on symptoms with runbooks attached
Destroy lab resources; tag everything with owner and expiry where possible

Troubleshooting¶

Symptom	Likely cause	Fix
Auth / permission denied	Wrong identity, policy, or scope	Check caller identity, roles, and least-privilege policies
Timeout / no route	Network, DNS, security group, or endpoint	Trace path, DNS, and allow-lists before retrying
Drift / unexpected plan	Manual change or wrong state/workspace	Reconcile desired vs actual; avoid click-ops on managed resources
Pipeline/job red	Flaky step, cache, or missing secret	Read failing step logs; bisect recent workflow/config changes
Cost spike	Idle load balancer, NAT, oversized compute	Inventory billable resources; stop/delete labs promptly

Summary¶

Docker Compose Fundamentals is essential for Cloud and DevOps engineers working with docker. Practise the lab until the inspection and change path is muscle memory, then continue the track.

Interview Questions¶

What problem does Compose solve locally?
How do you tear down including volumes?
Service DNS names in Compose?
Compose versus Swarm/Kubernetes for production?
How do you override config per environment?

Sample answer — question 2

Run docker compose ps and docker compose logs for the failing service.

Sample answer — question 4

Do not commit .env secrets.

References¶

Compose specification

Docker Compose Fundamentals¶

Overview¶

Prerequisites¶

Learning Objectives¶

Architecture¶

Theory¶

What¶

Why¶

How it works¶

Key concepts¶

Common pitfalls¶

Hands-on Lab¶

Objective¶

Prerequisites¶

Lab environment¶

Real-world scenario¶

Step-by-step tasks¶

Task 1 – Create Compose file and static page¶

Task 2 – Start stack and check status¶

Task 3 – Prove Redis volume and fetch logs¶

Validation steps¶

Common errors and fixes¶

Challenge exercise¶

Learning outcomes¶

Cleanup¶

Validation¶

Code Walkthrough¶

Security Considerations¶

Common Mistakes¶

Best Practices¶

Troubleshooting¶

Summary¶

Interview Questions¶

Related Tutorials¶

References¶