In this tutorial, you will learn the ins and outs of using volumes and persistent data with Docker Compose. This topic is crucial for anyone who wishes to maintain data across container lifecycle. You will learn about Docker volumes, how to use them in Docker Compose, and to solidify your understanding, I’ll walk you through an example of creating a persistent MySQL database. Let’s dive right in!
What are Docker volumes?
In the world of Docker, data is usually ephemeral, meaning it only exists for the lifespan of the container. However, what if we need our data to persist beyond the lifespan of a single container instance? That’s where Docker volumes come into play.
Docker volumes are essentially a mechanism that allows data to persist even after a container is stopped or deleted. They are like dedicated folders lying on the host filesystem that can be connected to one or more containers. This means any changes in this folder can be seen by all linked containers, providing a way to share data between containers. Furthermore, the data in the volume continues to exist even if no containers are currently using it.
How to use volumes in Docker Compose
Now that you understand what Docker volumes are, let’s see how to use them in Docker Compose.
Docker Compose is a tool that simplifies the process of managing multi-container Docker applications. It uses a YAML file (usually named
docker-compose.yml) to define the multi-container setup.
To use a volume in Docker Compose, you need to define it in your
docker-compose.yml file. Here is a basic example:
version: '3' services: myservice: image: myimage volumes: - myvolume:/path/in/container volumes: myvolume:
In this configuration, we’re creating a service named
myservice using an image named
volumes field under the
myservice specifies that a volume named
myvolume should be mounted at
/path/in/container within the running container. The
volumes field at the root level of the configuration file declares the
Example: Creating a Persistent MySQL Database
Now let’s put what you’ve learned into practice. I’ll walk you through an example of creating a persistent MySQL database using Docker Compose.
We’re going to create a
docker-compose.yml file with the following content:
version: '3' services: db: image: mysql:8 volumes: - db_data:/var/lib/mysql environment: MYSQL_ROOT_PASSWORD: mysecretpassword volumes: db_data:
Here we’re defining a single service named
db using the
mysql:8 image. The
volumes field mounts a volume named
/var/lib/mysql, which is where MySQL stores its data by default. We’re also setting an environment variable
MYSQL_ROOT_PASSWORD to specify the root password for the MySQL server.
After you’ve created this file, you can start your MySQL container with Docker Compose by running the following command in the same directory as your
docker-compose up -d
-d flag tells Docker Compose to run the containers in the background.
With this configuration, any data that your MySQL server writes to
/var/lib/mysql will be stored in the
db_data volume. If the container is stopped or deleted, the data will remain intact and can be used by new containers.
To verify that the data is persisting, you could create a table in the MySQL database, then stop and remove the containers with
docker-compose down, and then bring them back up again with
docker-compose up -d. After doing this, you should see that the table you created still exists, which means your data has persisted.
How can I view the data inside a Docker volume?
Now that you’re familiar with Docker volumes and how to use them with Docker Compose, you might be wondering, “How can I actually view the data inside a Docker volume?” In this section, I will guide you through the steps to do exactly that.
Firstly, it’s important to understand where Docker volumes are stored. On a typical Linux installation, Docker stores the data in volumes in the following directory on the host machine:
/var/lib/docker/volumes/. Each volume is represented by a directory within this folder. However, the data is not easily accessible and it’s not recommended to directly interact with these directories, as Docker manages them.
So, how can we view the data? The answer is by creating a new Docker container that has the volume attached, and then exploring the data from within this new container.
Let’s say we have a volume named
myvolume and we want to view its contents. We can create and run a temporary container which mounts
myvolume. For this purpose, we can use a lightweight image like
busybox. Here’s the command:
docker run -it --rm -v myvolume:/vol busybox ls /vol
Here’s what’s happening in this command:
docker runis the command to create and start a new container.
-ittells Docker to run the container in interactive mode so you can interact with the BusyBox shell.
--rmtells Docker to remove the container after it exits. This is useful for cleanup since we’re only creating this container to peek into the volume.
-v myvolume:/volmounts the
myvolumevolume to the path
/volinside the container.
busyboxis the name of the image we’re using to create the container. BusyBox is a lightweight Linux distribution that’s useful for debugging and exploring.
ls /volis the command we’re running inside the new container. This will list the contents of the
/voldirectory, which is where we’ve mounted
After you run this command, you should see a list of files and directories. These are the contents of your
Remember, although it’s technically possible to view the data in a Docker volume directly from the host filesystem, it’s generally not recommended because Docker volumes are intended to be managed by Docker. Interacting with them directly could lead to unexpected results. Instead, using a Docker container to explore the volume’s contents, as described above, is the preferred method.
I hope this helps you understand how to view the data inside a Docker volume. With this knowledge, you can now confidently manage your data in Docker!
In this tutorial, you’ve learned about Docker volumes, how to use them with Docker Compose, and you’ve seen an example of how to create a persistent MySQL database. I hope you found this information useful and feel more comfortable with these concepts now. Remember, Docker volumes are a powerful tool for managing and preserving data in your Docker applications. Happy Dockering!
Frequently Asked Questions
- What is the difference between Docker volumes and bind mounts? Bind mounts have been around since the early days of Docker. Bind mounts have limited functionality compared to volumes. When you use a bind mount, a file or directory on the host machine is mounted into a container. The file or directory is referenced by its full or relative path on the host machine. By contrast, a volume is managed by Docker and can be more safely shared among multiple containers.
- How can I view the data inside a Docker volume? Docker volumes are stored in a part of the host filesystem which is managed by Docker (
/var/lib/docker/volumes/...on Linux). You can use Docker commands such as
docker volume lsto list volumes and
docker volume inspectto view the specific path of a volume.
- How can I backup Docker volumes? Docker does not provide direct methods to backup volumes, but you can use a workaround by creating a new container which mounts the volume and then copying the data out of the volume. Another method is to directly copy the data from the host filesystem where Docker stores the volume data.
- Can I share volumes between different Docker Compose files or projects? Yes, you can use the same volume in different Docker Compose files or projects. However, you need to make sure that the containers do not interfere with each other’s operations on the shared data, as this can lead to data inconsistency or loss.
- What happens to my data when I delete a Docker volume? When a Docker volume is deleted, all data in the volume is permanently deleted. Docker volumes are designed to persist data across container restarts, but not beyond the lifespan of the volume itself.
- How can I use Docker volumes with Docker Swarm or Kubernetes? Docker Swarm supports Docker volumes in much the same way as Docker Compose. However, Kubernetes uses a different concept called Persistent Volumes (PV) and Persistent Volume Claims (PVC) for managing persistent data.