Compose watch for development in Docker - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of using docker-compose in development?

It helps to define and run multi-container Docker applications easily, allowing developers to start all services with a single command.

intermediate

How does docker-compose help with watching file changes during development?

By mounting local source code into containers, docker-compose allows the app inside the container to see file changes immediately, enabling live reload or hot reload.

beginner

What is the role of volumes in docker-compose.yml for development?

Volumes map local files or directories into the container, so changes made locally are reflected inside the container instantly.

beginner

Which command starts docker-compose in a way that shows real-time logs and keeps the process running?

The command docker-compose up starts the containers and streams their logs to your terminal, helping you watch the app during development.

intermediate

Why is it important to use depends_on in docker-compose.yml during development?

It ensures that services start in the correct order, for example, the database starts before the app, preventing errors during development.

What does mounting a volume in docker-compose do during development?

A Deletes local files automatically

B Creates a backup of container files

C Syncs local files with the container for live updates

D Compresses container data

Mounting a volume syncs local files with the container, allowing live updates during development.

Which command shows live logs of your containers in docker-compose?

A docker-compose build

B docker-compose logs -f

C docker-compose stop

D docker-compose down

docker-compose logs -f streams live logs from running containers.

What is the benefit of using depends_on in docker-compose.yml?

A Controls the order services start

B Increases container speed

C Limits container memory

D Encrypts container data

depends_on controls the order services start to avoid errors.

How do you keep containers running and watch changes during development?

A Use <code>docker-compose up</code> with volume mounts

B Use <code>docker-compose down</code>

C Use <code>docker-compose rm</code>

D Use <code>docker-compose build</code> only

Using docker-compose up with volume mounts keeps containers running and watches changes.

What happens if you don't mount your source code as a volume in development?

A Container will run faster

B Docker will crash

C Container will automatically update code

D Code changes won't reflect inside the container

Without volume mounts, code changes on your machine won't appear inside the container.

Explain how to set up docker-compose to watch for code changes during development.

Think about syncing your local files with the container and keeping it running.

Describe the benefits of using docker-compose watch setup in a development workflow.

Focus on how it helps you work faster and avoid manual steps.

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