Docker Compose: Multi-Container Apps

Introduction

Real apps have several parts: a web app, a database, a cache. Starting each with a long docker run command — plus a network and volumes — quickly becomes error-prone. Docker Compose describes the whole app in one YAML file and starts or stops everything with a single command.

The App: Quotes API with a Visit Counter

Priya adds a visit counter to her Quotes API. The count is stored in Redis, a fast in-memory database, so it is shared by every copy of the app. Her app now uses the redis Python package, and finds Redis through the REDIS_HOST variable:

import os
import socket

from flask import Flask, jsonify
from redis import Redis

app = Flask(__name__)
redis = Redis(host=os.environ.get("REDIS_HOST", "localhost"), port=6379)

QUOTES = [
    "Talk is cheap. Show me the code.",
    "First, solve the problem. Then, write the code.",
    "Simplicity is prerequisite for reliability.",
]


@app.route("/")
def home():
    visits = redis.incr("visits")
    return jsonify(message="Hello from Docker Compose!", visits=visits,
                   container=socket.gethostname())


@app.route("/quotes")
def quotes():
    return jsonify(QUOTES)


if __name__ == "__main__":
    app.run(host="0.0.0.0", port=5000)

flask==3.1.3
redis==8.1.0

The Dockerfile and .dockerignore are the same as in lesson 6.

Writing compose.yaml

Next to the Dockerfile, Priya creates compose.yaml:

services:
  web:
    build: .
    ports:
      - "5000:5000"
    environment:
      REDIS_HOST: redis
    depends_on:
      redis:
        condition: service_healthy

  redis:
    image: redis:8-alpine
    volumes:
      - redis-data:/data
    healthcheck:
      test: ["CMD", "redis-cli", "ping"]
      interval: 5s
      timeout: 3s
      retries: 5

volumes:
  redis-data:

Starting the App: up

From the project folder, start everything in the background:

docker compose up -d

Compose pulls redis:8-alpine, builds the web image, creates a network (quotes-api_default), the volume and both containers, and starts web once Redis reports healthy. Names are prefixed with the project name, which defaults to the folder name. Check the status:

docker compose ps

Both services are listed as running, with Redis marked (healthy). Now call the API a few times — the visits count goes 1, 2, 3:

curl http://localhost:5000/
curl http://localhost:5000/
curl http://localhost:5000/

Managing the App

docker compose logs web --tail 3     # recent output of one service
docker compose logs -f               # follow all services live (Ctrl+C to stop)
docker compose exec redis redis-cli get visits   # run a command in a service

After a code change, rebuild and recreate only what changed with docker compose up -d --build. To restart a single service, use docker compose restart web.

Stopping: down and down -v

docker compose down

This stops and removes the containers and the network, but keeps the named volume. Run docker compose up -d again and the counter carries on from where it was, because Redis saved its data to the volume. To remove the volume too — and lose the data — add -v:

docker compose down -v

Compose Command Reference

Common Mistakes

Frequently Asked Questions

Key Takeaways

Summary

You can now run a complete multi-container app with one command. Next you will make your images dramatically smaller with multi-stage builds.