Day 19 - Docker Volumes

What is Docker Volume?

If we are creating docker containers and adding data to that containers then the data will be there when containers are running, that data can go away anytime or can get destroyed anytime when our docker container get exited or get crashed. So, we need to persist that data from that crash, we need "Volumes". They are completely managed by Docker.

There is a concept called Bind Mounts. Bind mounts are a way to access and share files or directories between the host machine and a Docker container. Unlike volumes, which are managed by Docker and stored within the Docker environment, bind mounts link specific locations on the host file system directly to the container.

Features of Docker Volumes

Volumes are easier to back up or migrate.
You can manage volumes using Docker CLI commands or the Docker API.
Volumes work on both Linux and Windows containers.
Volumes can be more safely shared among multiple containers.
Volume drivers let you store volumes on remote hosts or cloud providers, encrypt the contents of volumes, or add other functionality.
New volumes can have their content pre-populated by a container.
Volumes on Docker Desktop have much higher performance from Mac and Windows hosts.

Create a Volume

Creates a new volume that containers can consume and store data in.

docker volume create --name <volumename> --opt type=none --opt device=location --opt o=bind

Where, location = Where we want to store data.
       --opt or -o = Set driver specific options.
       --type = Cluster Volume access type(mount, block).

Multiple containers can use the same volume in the same time period. This is useful if two containers need access to shared data.

Volume names must be unique among drivers. This means you cannot use the same volume name with two different drivers. If you attempt this docker returns an error.
If you specify a volume name already in use on the current driver, Docker assumes you want to re-use the existing volume and does not return an error.

List of Volumes

To list all the volumes present.

docker volume ls

Start a Container with Volumes

If you start a container with a volume that doesn't yet exist, Docker creates the volume for you.

The below command will mount volume to container:

docker run -d --mount source=<volumename>,target=<workdir of container> -p <hostport:containerport> appname

--mount: Consists of multiple key-value pairs, separated by commas and each consisting of a <key>=<value> tuple.

The type of the mount, which can be bind, volume, or tmpfs. This topic discusses volumes, so the type is always volume.
The source of the mount. For named volumes, this is the name of the volume. For anonymous volumes, this field is omitted. Can be specified as source or src.
The destination takes as its value the path where the file or directory is mounted in the container. Can be specified as destination, dst, or target.
The readonly option, if present, causes the bind mount to be mounted into the container as read-only. Can be specified as readonly or ro.
The volume-opt option, which can be specified more than once, takes a key-value pair consisting of the option name and its value.

Volume Details

To get the volume details, use the below command:

docker volume inspect <volumename>

By default, this command renders all results in a JSON array.

Delete Volume

To delete the volume attached to a container, use the below command:

docker stop <containername>
docker rm <containername>
docker volume rm <volumename>

docker-compose with Volume

version : "3.3"
services:
  web:
    image: nginx:latest
    ports:
      - "80:80"
    depends_on:
      - db
  db:
    image: mysql
    ports:
      - "3306:3306"
    environment:
      - "MYSQL_ROOT_PASSWORD=test@123"

In this example, I have defined two services: web and db. The web service represents the application container and the db service represents the database container.

Now using docker-compose up we can start both containers:

docker-compose up

Use the docker-compose scale command to increase or decrease the number of replicas for a specific service. You can also add replicas in deployment file for auto-scaling.

docker-compose up --scale web=4

To scale down you can use:

docker-compose up --scale=2

To down the docker-compose:

docker-compose down

To create a docker volume:

sudo docker volume create --name myvol_1

To mount the volume in a container:

docker run -d -v myvol_1:/app/data <image_id>

For shared volumes, include it in the docker-compose.yaml file:

To see list of volumes:

docker volume ls

To inspect the volume:

docker volume inspect myvol_1

By using the docker exec command, I have verified that the data in both the containers is same:

docker exec -it <cont_ID> <command_oryou_can_start_bash>

I have used ls for the listing of all directories.

Example 2:

Conclusion

In conclusion, Docker volumes provide a way to persist and share data between containers in a Docker environment. They offer several advantages over bind mounts. When using Docker volumes, you can either create anonymous volumes or named volumes. Anonymous volumes are created automatically by Docker, while named volumes are explicitly defined in the Dockerfile or Docker Compose file.

Unlike container file systems, volumes exist independently of containers, allowing data to persist even when containers are stopped or removed. Volumes facilitate data sharing among containers and enable easy backup, restoration, and migration. They also enhance collaboration by separating application code from data storage concerns.

Docker volumes contribute to the scalability and maintainability of containerized applications, making them a fundamental component in the Docker ecosystem.

*👆The information presented above is based on my interpretation. Suggestions are always welcome.*😊

~Smriti Sharma✌