diff --git a/exercises/docker/docker-debugging.md b/exercises/docker/docker-debugging.md new file mode 100644 index 000000000..1b1d8f502 --- /dev/null +++ b/exercises/docker/docker-debugging.md @@ -0,0 +1,121 @@ +# Docker Scenario-Based Exercises + +This file contains scenario-based Docker questions to help DevOps engineers practice real-world troubleshooting and configuration tasks. Each question simulates a practical scenario with a step-by-step answer. + +## Question 1: Debugging a Docker Container Failure + +### Question +You’re a DevOps engineer deploying a Node.js application using Docker. You run `docker run -d -p 3000:3000 my-node-app`, but the container exits immediately. Using `docker ps -a`, you see the container status as `Exited`. How would you troubleshoot and resolve this issue? + +### Answer +To troubleshoot a container exiting immediately: +1. **Check Logs**: Run `docker logs my-node-app` to view error messages. Common issues include missing dependencies (e.g., `npm install` failed) or an incorrect command. +2. **Inspect the Container**: Use `docker inspect my-node-app` to check `Config.Cmd` or `Config.Entrypoint`. Ensure the command (e.g., `node app.js`) is valid. +3. **Verify the Dockerfile**: Check if `CMD` or `ENTRYPOINT` is correct, e.g., `CMD ["node", "app.js"]`. Update and rebuild if needed: `docker build -t my-node-app .`. +4. **Test Interactively**: Run `docker run -it my-node-app sh` to debug manually (e.g., `node app.js`). +5. **Check Resources**: Ensure the host has enough memory/CPU using `docker stats`. + +**Example Fix**: If logs show `node: command not found`, update the Dockerfile to use `FROM node:18`, rebuild, and rerun. + +### Additional Notes +- Always start with `docker logs` for error clues. +- Use `docker ps -a` to check container status and ID. +- Common issues include missing dependencies or crashing apps. + + +--- + + +## Question 2: Configuring a Multi-Container Application + +### Question +As a DevOps engineer, you need to deploy a web application with a Node.js backend and a MySQL database using Docker. The Node.js app connects to MySQL on `localhost:3306`, but running `docker run` for each container separately fails because they can’t communicate. How would you set up these containers to work together? + +### Answer +To make the Node.js and MySQL containers communicate: +1. **Use Docker Compose**: Create a `docker-compose.yml` file to define and link both services: + ```yaml + version: '3.8' + services: + node-app: + image: my-node-app + build: . + ports: + - "3000:3000" + depends_on: + - mysql-db + environment: + - DB_HOST=mysql-db + - DB_PORT=3306 + mysql-db: + image: mysql:8.0 + environment: + - MYSQL_ROOT_PASSWORD=secret + ports: + - "3306:3306" + ``` +2. **Run the Application**: Execute `docker-compose up -d` to start both containers. The `node-app` service connects to `mysql-db` using the service name (`mysql-db`) as the hostname, not `localhost`. +3. **Verify Connectivity**: Check logs with `docker-compose logs node-app` to ensure the Node.js app connects to MySQL. If it fails, verify the environment variables and MySQL’s readiness. +4. **Alternative Without Compose**: Use a custom network: + - Create a network: `docker network create my-app-network` + - Run MySQL: `docker run -d --name mysql-db --network my-app-network -e MYSQL_ROOT_PASSWORD=secret mysql:8.0` + - Run Node.js: `docker run -d --name node-app --network my-app-network -p 3000:3000 -e DB_HOST=mysql-db my-node-app` + +### Additional Notes +- Docker Compose simplifies multi-container setups by managing networks and dependencies. +- Always set environment variables for database credentials to avoid hardcoding. + + +--- + + +## Question 3: Optimizing a Dockerfile for CI/CD + +### Question +You’re a DevOps engineer integrating a Dockerized Python application into a Jenkins CI/CD pipeline. The Dockerfile builds slowly, causing pipeline delays. How would you optimize the Dockerfile to speed up builds while maintaining functionality? + +### Answer +To optimize a Dockerfile for faster CI/CD builds: +1. **Use a Smaller Base Image**: Replace heavy images like `python:3.9` with `python:3.9-slim` to reduce size and download time. + ```dockerfile + FROM python:3.9-slim + ``` +2. **Leverage Layer Caching**: Order instructions from least to most likely to change. Copy `requirements.txt` and install dependencies before copying the app code: + ```dockerfile + COPY requirements.txt . + RUN pip install -r requirements.txt + COPY . . + ``` +3. **Minimize Layers**: Combine related commands with `&&` to reduce layers: + ```dockerfile + RUN pip install -r requirements.txt && rm -rf /root/.cache/pip + ``` +4. **Use Multi-Stage Builds**: If the app needs build tools, use a multi-stage build to keep the final image small: + ```dockerfile + FROM python:3.9 AS builder + COPY requirements.txt . + RUN pip install -r requirements.txt + FROM python:3.9-slim + COPY --from=builder /usr/local/lib/python3.9 /usr/local/lib/python3.9 + COPY . . + CMD ["python", "app.py"] + ``` +5. **Test in Jenkins**: Update the Jenkins pipeline to rebuild the image only when `Dockerfile` or code changes, using a cached image otherwise: + ```groovy + pipeline { + agent any + stages { + stage('Build Docker Image') { + when { changeset "Dockerfile,**.py" } + steps { sh 'docker build -t my-python-app .' } + } + } + } + ``` + +### Additional Notes +- Use `.dockerignore` to exclude unnecessary files (e.g., `.git`, `tests/`) from the build context. +- Monitor build times in Jenkins to confirm improvements. + + +*Contributed by Lahiru Galhena*