Understanding docker volume and path mapping

What is docker path mapping

Docker path mapping, or volume mounting, is the process of linking a directory on your host machine to a directory inside a Docker container. This is achieved using Docker volumes or bind mounts. Volumes are managed by Docker, whereas bind mounts are dependent on the directory structure of the host machine.

Why is it needed

Docker can be thought of as a form of virtualization, but rather than creating a full virtual machine, it uses containers to package applications and their dependencies into isolated environments.

Each Docker container has its own file system, which is distinct from the host machine’s file system. This container file system operates in a virtualized space, managed by Docker, and contains everything the application needs to run.

To ensure that data and files are accessible between the Docker container and the host machine, we need to map paths between the two. This process involves specifying "volume mounts" or "bind mounts" when running Docker containers.

By mapping a directory on the host machine to a directory within the container, we create a link that allows data to be shared and persisted across both environments. This mapping is crucial for tasks such as persisting logs, configuration files, or any other data that needs to be consistent or accessible from both the host and the container.

Type of path mapping

There are two main types of path mapping in Docker:

1. Volumes
   Managed by Docker and stored in a part of the host filesystem that is managed by Docker (/var/lib/docker/volumes on Linux). Volumes are the best way to persist data in Docker.

2. Bind Mounts
   Any directory on the host machine can be mounted into a container. This method is less portable since it depends on the host's directory structure.

How to use path mapping

Volumes

To create and use a Docker volume, you can use the docker volume create command and then map it to a container using the -v or --mount flag.

Example:

# Create a Docker volume
docker volume create my_volume

# Run a container with the volume mounted
docker run -d -v my_volume:/app/data my_image

In this example, my_volume is the Docker volume and /app/data is the directory inside the container where the volume is mounted.

Bind Mounts

To use a bind mount, you specify the full path to the directory on the host machine and the path inside the container.

Example:

# Run a container with a bind mount
docker run -d -v /path/on/host:/app/data my_image

In this example, /path/on/host is the directory on the host machine and /app/data is the directory inside the container.

Docker compose

Path mapping

Docker Compose simplifies the process of defining and running multi-container Docker applications. Path mappings can be easily defined in a docker-compose.yml file.

Example:

version: '3'
services:
  web:
    image: my_web_image
    volumes:
      - ./host_data:/app/data

In this docker-compose.yml file, the volumes key maps the ./host_data directory on the host machine to the /app/data directory inside the web service container.

Volume mapping

You can also use docker volume in docker compose.

Example:

version: '3'
services:
  web:
    image: my_web_image
    volumes:
      - web_volume:/app/data
      
volumes:
  web_volume:

Best Practices

1. Use Volumes for Persistent Data
   Whenever you need to persist data, use Docker volumes instead of bind mounts. Volumes are managed by Docker and are more portable. Unless you want to have more control on where you put your data directories.

2. Avoid Hardcoding Paths
   When using bind mounts, avoid hardcoding absolute paths. Use relative paths or environment variables to make your configurations more portable.

3. Separate Configuration from Code
   Map configuration files separately to allow for easy updates without rebuilding the container image.

4. Backup Volumes Regularly
   Ensure that you have a backup strategy for your Docker volumes to prevent data loss.

Tags

• docker