Data Management in Docker

Separation of concerns

Volumes	Bind mounting
Volumes are stored in a part of the host file system which is managed by docker	Bind mounts are stored anywhere on the host system
Volumes are the best way to persist data in docker	Non Docker process can modify this part of the system
Non docker process should not modify this part of the system	To use bind mounts , the file or directory does not need to exist on your docker host already . If it doesn't exist, it will be created on demand
we can link a volume to multiple containers
we can link (mount) a volume to multiple containers

• Name a volume

-v mariavolume:/var/lib/mysql

mariavolume: This is the name of the volume. If the volume does not exist, Docker will create it.

/var/lib/mysql: This is the directory inside the container where the volume will be mounted.

Imagine you have a MariaDB database running in a Docker container. The database stores all its data in the /var/lib/mysql directory inside the container. By using the -v mariavolume:/var/lib/mysql option:

• When you start the container, Docker mounts the mariavolume volume to /var/lib/mysql .
• Any data written by MariaDB to /var/lib/mysql is actually written to the mariavolume on the host system.
• If you stop and remove the container, the data remains intact in the mariavolume .
• When you start a new container with the same volume mount, the new container can access the existing data.

Reason: By default, data created inside a Docker container is not persistent. If the container is stopped or removed, all the data inside it is lost.

Benefit: Using a volume ensures that the data stored in /var/lib/mysql (which is the default data directory for MariaDB) is persisted across container restarts and even if the container is deleted.

Basic Commands for volumes

# Create a Volume
docker volume create VOLUME_NAME

# Inspect a Volume
docker volume inspect VOLUME_NAME

# List Volumes
docker volume ls

# Remove a Volume
docker volume rm VOLUME_NAME

# Prune Unused Volumes
docker volume prune -f

• Bind Mount

docker run -v ${PWD}/data.txt:/mounted_folder my_container_image

-v ${PWD}/data.txt:/mounted_folder option in Docker is used to bind mount a file from your host system into a Docker container. Let's break down the components of this command:

Components of the Command

• -v : This flag stands for "volume" and is used to mount a volume.
• ${PWD}/data.txt : This specifies the source path on the host machine. ${PWD} is a shell variable that holds the present working directory (the directory you are currently in). data.txt is a file in this directory.
• /mounted_folder : This specifies the target path inside the container where the file will be mounted.

What This Does

• Bind Mount: This command creates a bind mount, which means it links a specific file or directory on the host system (${PWD}/data.txt ) to a file or directory in the Docker container (/mounted_folder ).
• File Accessibility: The file data.txt from your current working directory on the host will be accessible inside the container at /mounted_folder . Any changes made to the file in either the host or the container will be reflected in both places.

Example Scenario

Host Setup:

• Assume you have a file called data.txt in your current working directory on your host machine.

• The file path on your host is /path/to/your/directory/data.txt . Inside the Container:

• Within the container, you can access the contents of data.txt by referring to /mounted_folder .

• For example, you can read the file using a command like cat /mounted_folder or write to it with a text editor.

Benefits

• Persistence: Changes to the file are preserved between container restarts because the file exists on the host.
• Data Sharing: Multiple containers can access the same file if the same bind mount is used.
• Simplicity: Easily share configuration files, datasets, or other resources between your host and containers.