Significance of volume in docker

When you save changes to a local file in a host machine, these changes do not automatically reflect in a running Docker container unless you have set up a way to synchronize those changes. There are a few scenarios to consider, depending on how your Docker environment is configured:

1. Using Docker Volumes

Docker volumes are a powerful way to share data between your host machine and your Docker containers. If you set up a volume to mount a local directory to a directory in the container, any changes made to files in the local directory will immediately reflect in the container.

Example

Let's say you have a local directory myapp and you want to mount it to /app inside the container:

docker run -v /path/to/myapp:/app -d myimage

In this case:

• /path/to/myapp is the local directory.
• /app is the directory inside the container.

Any changes you make to files in /path/to/myapp on your local machine will be reflected in the /app directory inside the container.

2. Using docker-compose with Volumes

If you're using docker-compose, you can achieve the same effect by specifying volumes in your docker-compose.yml file.

Example docker-compose.yml

version: '3'
services:
  web:
    image: myimage
    volumes:
      - /path/to/myapp:/app

Run docker-compose up to start the services defined in the docker-compose.yml file. This will mount the local myapp directory to /app in the container.

3. Rebuilding the Docker Image

If you are not using volumes, changes to local files will not be reflected in the running container. You will need to rebuild the Docker image and restart the container to see the changes.

Example

1. Make changes to your local files.
2. Build a new Docker image:

   docker build -t myimage .
   

3. Stop the existing container:

   docker stop mycontainer
   docker rm mycontainer
   

4. Run a new container with the updated image:

   docker run -d --name mycontainer myimage
   

4. Live Reloading with Development Tools

For development environments, you can use tools that automatically reload the application when file changes are detected. This often involves combining Docker volumes with a file-watching tool.

Example with Node.js and nodemon

1. Install nodemon in your project:

   npm install --save-dev nodemon
   

2. Update your Dockerfile to use nodemon:

   FROM node:14
   WORKDIR /app
   COPY package*.json ./
   RUN npm install
   COPY . .
   CMD ["npx", "nodemon", "main.js"]
   

3. Use a volume to mount your local directory:

   docker run -v /path/to/myapp:/app -d myimage
   

With nodemon monitoring the files, any changes you make to your local files will cause the application inside the container to automatically reload, reflecting the changes immediately.