Compose watch for development in Docker - Mini Project: Build & Apply

Compose watch for development

📖 Scenario: You are building a simple development environment using Docker Compose. You want to automatically restart your application container whenever you change the source code files. This helps you see your changes immediately without manually restarting the container.

🎯 Goal: Create a Docker Compose setup that watches for file changes in your app directory and restarts the container automatically during development.

📋 What You'll Learn

Create a Docker Compose file with a service named app

Mount the local ./app directory into the container

Use the restart: always policy initially

Add a command to run a simple watch script inside the container

Print a message showing the container restarts on file changes

💡 Why This Matters

🌍 Real World

Developers often want their code changes to reflect immediately without restarting containers manually. This setup helps automate that during development.

💼 Career

Knowing how to configure Docker Compose for live development is a valuable skill for DevOps engineers and developers working with containerized applications.

1

Create the initial Docker Compose file

Create a file named docker-compose.yml with a service called app that uses the image alpine. Mount the local directory ./app to /app inside the container.

Docker

# Create docker-compose.yml with service app using image alpine
# Mount ./app to /app

Hint

Use volumes to mount the local folder ./app into the container path /app.

2

Add restart policy and working directory

In the app service, add restart: always and set working_dir to /app.

Docker

version: '3.8'
services:
  app:
    image: alpine
    volumes:
      - ./app:/app
    # Add restart policy and working directory here

Hint

The restart key controls container restart behavior. working_dir sets the default directory inside the container.

3

Add a command to watch for file changes

Add a command to the app service that runs sh -c "while true; do inotifywait -e modify,create,delete -r /app && echo 'Files changed, restarting...'; sleep 1; done". This will watch for file changes in /app and print a message.

Docker

version: '3.8'
services:
  app:
    image: alpine
    volumes:
      - ./app:/app
    restart: always
    working_dir: /app
    # Add command to watch files here

Hint

Use inotifywait inside a shell loop to watch for file changes and print a message.

4

Print a message when container starts

Add a command that first prints "Starting watch on /app" then runs the watch loop. Use sh -c with echo followed by the while true loop.

Docker

version: '3.8'
services:
  app:
    image: alpine
    volumes:
      - ./app:/app
    restart: always
    working_dir: /app
    # Update command to print start message and then watch files

Hint

Chain commands in sh -c using semicolon ; to print a message before the loop.

Practice

1. What is the main purpose of using docker-compose watch during development?

easy

A. To deploy the app to production servers

B. To build the Docker image only once

C. To automatically reload the application when code changes

D. To monitor container resource usage

Solution

1. Step 1: Understand the role of watch in development

   Watching means tracking file changes to trigger actions automatically.

2. Step 2: Connect watch with docker-compose usage

   Compose watch reloads the app inside the container when source code changes on the host.

3. Final Answer:

   To automatically reload the application when code changes -> Option C

4. Quick Check:

   Compose watch = auto reload on code change [OK]

Hint: Watch means auto reload on code change [OK]

Common Mistakes:

• Confusing watch with building images
• Thinking watch deploys to production
• Assuming watch monitors resource usage

2. Which of the following docker-compose.yml volume configurations correctly syncs your local source code to the container for live reload?

easy

A. volumes:\n - ./app:/app

B. volumes:\n - /app:/app

C. volumes:\n - ./app

D. volumes:\n - /app

Solution

1. Step 1: Understand volume syntax in docker-compose

   Volumes map host paths to container paths using host_path:container_path.

2. Step 2: Identify correct host to container mapping

   ./app:/app maps local folder app to container's /app for syncing code.

3. Final Answer:

   volumes:\n - ./app:/app -> Option A

4. Quick Check:

   Host path before colon, container path after [OK]

Hint: Volume syntax: host_path:container_path [OK]

Common Mistakes:

• Reversing host and container paths
• Omitting container path
• Using absolute container path as host path

3. Given this docker-compose.yml snippet and a Node.js app using nodemon:

services:
  web:
    volumes:
      - ./src:/usr/src/app
    command: nodemon index.js

What happens when you edit a file inside ./src on your host machine?

medium

A. Nothing happens until you rebuild the image

B. The container deletes the edited file

C. The container crashes due to volume conflict

D. The container restarts the Node.js app automatically

Solution

1. Step 1: Understand volume syncing

   The volume syncs ./src on host to /usr/src/app in container, so changes reflect inside container.

2. Step 2: Understand nodemon behavior

   nodemon watches files and restarts the app when files change.

3. Final Answer:

   The container restarts the Node.js app automatically -> Option D

4. Quick Check:

   Volume sync + nodemon = auto restart on code change [OK]

Hint: Nodemon restarts app on synced file changes [OK]

Common Mistakes:

• Thinking image rebuild is needed for code changes
• Assuming container crashes on volume use
• Believing files are deleted on sync

4. You set up docker-compose with volumes and watchmedo to reload a Python app, but changes don't trigger reload. What is the most likely cause?

medium

A. The volume path is incorrect or missing

B. The Dockerfile lacks a CMD instruction

C. The container has no network access

D. The image was not built with --no-cache

Solution

1. Step 1: Check volume configuration

   If volumes don't map host code to container, changes won't appear inside container.

2. Step 2: Understand watchmedo needs code inside container

   Without synced code, watchmedo cannot detect changes to reload app.

3. Final Answer:

   The volume path is incorrect or missing -> Option A

4. Quick Check:

   Missing volume = no code sync = no reload [OK]

Hint: Check volumes if reload doesn't work [OK]

Common Mistakes:

• Blaming CMD instead of volumes
• Assuming network affects file watching
• Thinking image cache affects reload

5. You want to set up a development environment with docker-compose that automatically reloads a React app on code changes. Which combination below is the best practice?

hard

A. Build image with source code baked in, run npm run build once

B. Use volumes to sync source code, run npm start with react-scripts watch mode inside container

C. Use volumes but disable hot reload to avoid conflicts

D. Run npm start on host and use container only for database

Solution

1. Step 1: Sync source code with volumes

   Volumes keep container code updated with host edits for live reload.

2. Step 2: Use React's built-in watch mode

   npm start with react-scripts runs dev server that reloads on changes.

3. Final Answer:

   Use volumes to sync source code, run npm start with react-scripts watch mode inside container -> Option B

4. Quick Check:

   Volumes + watch mode = best dev reload setup [OK]

Hint: Sync code + run dev server with watch mode [OK]

Common Mistakes:

• Baking code into image prevents live reload
• Disabling hot reload loses dev speed
• Running app only on host misses container benefits