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Docker path · Running containers · Topic 12 of 12

docker compose for local stacks

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Docker Compose turns a pile of docker run commands into one declarative file. You describe the services (containers), the ports they publish, the volumes they need and how they depend on each other in compose.yaml; docker compose up creates a network for the project, the volumes, and the containers in a sensible order. Compose v2 is a plugin of the Docker CLI (docker compose); the old Python docker-compose command is retired.

Why it matters: A new teammate should get the app, its database and its cache running with one command, the same way every time, and Compose is the standard way to write that down.

  1. 1Services, networks, volumes

    compose.yaml declares containers as services; Compose creates a network for the project and the named volumes for you.

  2. 2Names are hostnames

    Every service can reach every other by its service name, through the project network's DNS.

  3. 3Start order needs health

    depends_on only waits for a container to start. With condition: service_healthy it waits for the dependency's healthcheck to pass.

  4. 4A development tool

    Compose is ideal on laptops and in CI. Production orchestration (rescheduling, rolling updates, many machines) is what Kubernetes is for.

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How it works

The explanation, with examples you can run.

A web service with a cache

# compose.yamlservices:  web:    build: ./node    ports:      - "127.0.0.1:8765:3000"    environment:      REDIS_URL: redis://cache:6379    depends_on:      cache:        condition: service_healthy  cache:    image: redis:7-alpine    healthcheck:      test: ["CMD", "redis-cli", "ping"]      interval: 5s      timeout: 3s      retries: 5    volumes:      - cache-data:/datavolumes:  cache-data:
docker compose up -d --wait#  Container demo-cache-1  Healthy#  Container demo-web-1    Healthydocker compose ps --format '{{.Service}} {{.Status}}'# cache Up 6 seconds (healthy)# web Up Less than a secondcurl -s localhost:8765# hello from d9eb232cfb16docker compose exec cache redis-cli -h cache ping# PONGdocker compose down -v      # containers, network, and the cache-data volume

What happened: Compose built web from ./node, created the network demo_default (the project name defaults to the directory name), created the volume, started cache, waited until its healthcheck passed, then started web. --wait returns once the services are running or healthy, which is handy in CI before running tests.

Details that matter

  • No version: line. The top-level version key is obsolete; current Compose prints the attribute 'version' is obsolete, it will be ignored and uses the Compose Specification regardless.
  • Readiness, not just order. depends_on in short form waits only for the container to start. Give dependencies a healthcheck and use condition: service_healthy. Your app should still retry connections; dependencies restart in real life too.
  • Ports for humans, names for services. Publish only what you open in a browser or a local tool, bound to 127.0.0.1. Services talk to each other over the project network by name, on the container port.
  • Overrides. Compose automatically merges compose.override.yaml, which is a good place for developer-only settings such as bind mounts and debug ports. profiles: keep optional services (an admin UI, a load generator) off unless asked for.
  • Live code. develop.watch with docker compose watch syncs changed files into the container or rebuilds the image when dependency files change, often faster than bind-mounting the whole tree on Docker Desktop.
  • Secrets. Do not commit real secrets to environment:. Use an untracked .env file for local values, or Compose secrets: to mount them as files.

Where Compose stops

Compose runs containers on one Docker engine. It restarts a crashed container if you set restart:, but it does not move work to another machine, roll out new versions gradually, or scale on load. Those are the jobs of an orchestrator, which is where Kubernetes takes over, built on the same images, networking and volume ideas.

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Check yourself

3 questions. Get them all right to finish this topic.

  1. 1.In compose.yaml, the web service needs Redis from the service named cache. Which address does it use?
  2. 2.web has depends_on: [db] in short form. When does web start?
  3. 3.What does docker compose down -v remove that docker compose down keeps?

0 of 3 right so far. Get all three to finish this topic.

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