How to Make a Docker Container Persistent

Docker is a powerful containerization platform known for its ability to create lightweight, portable, and self-sufficient containers from application packages. However, the default behavior of Docker containers is to be volatile. This means that any data within a container is lost if the container stops. But, if you want to maintain persistent storage, you can use a feature called VOLUME mapping. VOLUME mapping allows you to link the storage of a running container with a directory on the host system. This ensures that data persists even if the container is stopped or restarted.

Volumes vs. Bind Mounts

Before diving into the VOLUME feature, it's important to understand the difference between volumes and bind mounts. Both can be used to persist data, but they have different characteristics and use cases.

Volumes: These are managed by Docker and their filesystem is optimized for Docker. Volumes are best for data that needs to be shared among containers and for data that should be restored as part of the Docker image lifecycle. They are also portable and can be backed up and restored easily. Bind Mounts: These map directly to a path on the host filesystem. They are useful for files needed in a container but not necessarily committed back to the Docker image. They can be used to share files between the host and the container, or to share files among multiple containers. They are not managed by Docker and are not as portable as volumes.

Using VOLUME to Make Your Docker Container Persistent

To create a persistent Docker container using VOLUME, you can add the VOLUME instruction to your Dockerfile or the -v option in the docker run command.

Adding a VOLUME to Your Dockerfile

Inside your Dockerfile, you can include a line like this:

FROM httpd:latestVOLUME /usr/local/apache/htdocs

This tells Docker to create a named volume mounted at /usr/local/apache/htdocs inside the container. When you build the image and run the container, Docker will create a volume and mount it at this location.

Using the -v Option in docker run

Alternatively, you can specify a volume at runtime using the -v option:

docker run -v /home/vishal/apache/data:/usr/local/apache/htdocs httpd:latest

This does the same thing as the VOLUME line in the Dockerfile, but it allows you to specify a custom volume name or use an existing host directory as the volume.

Example: Persistent Apache Data with VOLUME

Here's a step-by-step guide to setting up a persistent Apache container using VOLUME:

Define the VOLUME in your Dockerfile: Create a Dockerfile with the following content:

FROM httpd:latestVOLUME /usr/local/apache/htdocs

Build the Docker image:

docker build -t apache-persistent .

Run the container, specifying a volume on the host:

docker run -v /home/vishal/apache/data:/usr/local/apache/htdocs -p 8080:80 apache-persistent

This will launch a new Apache container, mounting the local folder /home/vishal/apache/data as /usr/local/apache/htdocs inside the container. Any changes made in the container will be reflected back in the host folder, and vice versa.

Best Practices for Using VOLUME

To ensure that your persistent storage works as expected, follow these best practices:

Unmount Volumes on Container Delete: When deleting a container, make sure to unmap the volumes to avoid any conflicts in the host filesystem. Use Named Volumes: Named volumes are easier to manage and can be shared among multiple containers. Backup and Restore Volumes: Regularly back up your volumes to ensure data safety and integrity.

Conclusion

While Docker containers are designed to be volatile, you can achieve persistent storage using VOLUME mapping. By understanding the differences between volumes and bind mounts, and by following best practices, you can ensure that your containers maintain their state across runs and restarts. This not only improves the stability of your applications but also facilitates easier data management and backup.