Introduction
We are aware of the fact that Docker images are Read-only. When you run a container in Docker, it creates a writable layer on top of the image file system. This layer is used to store any changes made to the container during its lifetime, such as installed software or modified files. When a container is terminated, the writable layer is removed, and any data or settings that were stored in the layer are lost.
To solve this, we can use Docker volumes. They allow us to mount directories from the host machine or other containers into the container. By storing data and settings in a volume, they can be easily accessed and reused between container runs. There are three types of volume management:
- Host Volume
- Container Volume
- Docker Volume
π I will be going through each of them along with a short hands on examples
Host Volumes
Host volumes are directories on the host machine that are mounted into containers, allowing the container to read and write files directly to the host file system
β¨ Useful for scenarios where we need to persist data outside of the container, or share data between the host and container
Simple Test
Creating a mysql_DB container
docker run -d \
> --name mysql_db \
> -e MYSQL_ROOT_PASSWORD=root \
> -e MYSQL_DATABASE=testDB \
> -v /docker_dir/con_volume_1:/var/lib/mysql \
> mysql:5.7
π The -e option lets us create root user's password and a test DB as well
π The line containing -v option means that any data written to the /var/lib/mysql directory within the container will be stored in the /docker_dir/con_volume_1 directory on the host, allowing the data to persist even if the container is deleted
π‘ In Kubernetes, credentials are handled through a secret file
Checking docker process
docker ps
π We will be able to see the mysql5.7 container that we just created and started
In our host system,
ls -l /docker_dir/con_volume_1/
drwxr-x--- 2 polkitd input 4096 Mar 5 19:34 mysql
drwxr-x--- 2 polkitd input 20 Mar 5 19:34 testDB
The data is saved in our local host
We can also find the same data inside our container
docker exec -it mysql_db /bin/bash
bash-4.2# ls -l /var/lib/mysql
drwxr-x--- 2 mysql mysql 4096 Mar 5 10:34 mysql
drwxr-x--- 2 mysql mysql 20 Mar 5 10:34 testDB
Returning to the host and after deleting the container, we can still see the database data still remains
docker stop mysql_db
docker rm mysql_db
ls -l /docker_dir/con_volume_1
drwxr-x--- 2 polkitd input 4096 Mar 5 19:34 mysql
drwxr-x--- 2 polkitd input 20 Mar 5 19:34 testDB
π₯ But using the -v option has a slight issue
Creating a new container with a different DB name
docker run -d \
> --name mysql_db \
> -e MYSQL_ROOT_PASSWORD=root \
> -e MYSQL_DATABASE=MYSQLDB \
> -v /docker_dir/con_volume_1:/var/lib/mysql \
> mysql:5.7
π The above should create a MYSQLDB but instead it shows the previous database data
From the host
ls -l /docker_dir/con_volume_1
drwxr-x--- 2 polkitd input 4096 Mar 5 19:34 mysql
drwxr-x--- 2 polkitd input 20 Mar 5 19:34 testDB
Inside the container
ls -l /var/lib/mysql
drwxr-x--- 2 mysql mysql 4096 Mar 5 10:34 mysql
drwxr-x--- 2 mysql mysql 20 Mar 5 10:34 testDB
π We can confirm that the container directory replicates the same data from the host directory. This means that the -v option is actually mounting the container's directory into the host directory.
Container Volume
Container volumes are volumes created and managed by Docker, and are associated with a specific container
β¨ Useful when we need to isolate and share data between containers
Simple Test
Creating an index.html file inside the directory
cat > /docker_dir/con_volume_1/index.html
Nginx Main Page !!
Starting a container on port 8001
docker run -d --name Nginx_1 -p 8001:80 \
> -v /docker_dir/con_volume_1:/usr/share/nginx/html \
> nginx:latest
On port 8002
docker run -d --name Nginx_2 -p 8002:80 \
> --volumes-from Nginx_1 \
> nginx:latest
On port 8003
docker run -d --name Nginx_3 -p 8003:80 \
> --volumes-from Nginx_1 \
> nginx:latest
π Here --volumes-from shares the host directory that is connected to the specified container with another container
Now from the host system
curl localhost:8001
Nginx Main Page !!
curl localhost:8002
Nginx Main Page !!
curl localhost:8003
Nginx Main Page !!
If we add something to the existing file,
cat >> /docker_dir/con_volume_1/index.html
Volumes From Test !!
curl localhost:8001
Nginx Main Page !!
Volumes From Test !!
curl localhost:8002
Nginx Main Page !!
Volumes From Test !!
curl localhost:8003
Nginx Main Page !!
Volumes From Test !!
π We can see that one volume directory is shared across several containers
Docker Volume
Docker volumes are volumes created and managed by Docker, and can be used by multiple containers, not just a single container.
β¨ Useful for scenarios where we need to share data between multiple containers, or when we need to back up or restore volumes
Simple Test
Creating a docker volume
docker volume create myVolume_1
myVolume_1
To check current volumes
docker volume ls
DRIVER VOLUME NAME
local 97e467c79d320138ef408f661ea6ce37d8294ce7e58524ae018d4225ad5ba914
local db9eb4a8b3eb12c36056d398b65c6385315a28e3d3a43b7a7913afc94efc86aa
local myVolume_1
π We can also use docker volume inspect myVolume_1 to inspect further details such as mountpoint of the volume
Now we can start a new container connected to the docker volume
docker run -d \
> --name mysql_DB \
> -e MYSQL_ROOT_PASSWORD=root \
> -e MYSQL_DATABASE=testDB \
> -v myVolume_1:/var/lib/mysql \
> mysql:5.7
Accessing the CLI inside the container
docker exec -it mysql_DB /bin/bash
Inside the container
mysql -u root -p testDB
mysql> create database testDB2;
Query OK, 1 row affected (0.00 sec)
mysql> show databases;
+--------------------+
| Database |
+--------------------+
| information_schema |
| mysql |
| performance_schema |
| sys |
| testDB |
| testDB2 |
+--------------------+
6 rows in set (0.00 sec)
mysql> exit
π I created a testDB2 database to check results
From the host system
ls -l /var/lib/docker/volumes/myVolume_1/_data/
drwxr-x--- 2 polkitd input 4096 6μ 23 11:53 mysql
drwxr-x--- 2 polkitd input 20 6μ 23 11:53 testDB
drwxr-x--- 2 polkitd input 20 6μ 23 11:55 testDB2
π We can confirm the database that we created inside the container can be seen from the host system
Read-only Docker Volume
Creating another docker volume
docker volume create myVolume_2
myVolume_2
Creating a new index.html inside this volume
cat > /var/lib/docker/volumes/myVolume_2/_data/index.html
My Nginx Web Site !!
ls -l /var/lib/docker/volumes/myVolume_2/_data/
-rw-r--r-- 1 root root 41 6μ 23 12:06 index.html
Creating a new nginx container and checking
docker run -d --name Nginx_1 -p 80:80 \
> -v myVolume_2:/usr/share/nginx/html:ro \
> nginx:latest
curl localhost:80
My Nginx Web Site !!
π The ro is used to connect it as read-only
Now from inside this container if we try to edit the file
cat >> /usr/share/nginx/html/index.html
bash: /usr/share/nginx/html/index.html: Read-only file system
π As we mounted as read-only, we cannot write from inside the container
However, we can add contents from the host system
cat >> /var/lib/docker/volumes/myVolume_2/_data/index.html
Change Content !!
curl localhost:80
My Nginx Web Site !!
Change Content !!
Conclusion
In conclusion, Docker volumes provide a way to manage and persist data within containers. The three types of Docker volumes - host volumes, container volumes, and Docker volumes - offer different approaches to managing data, each with its own strengths and limitations β
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