Persist the DB

In case you didn't notice, your todo list is empty every single time you launch the container. Why is this? In this part, you'll dive into how the container is working.

The container's filesystem

When a container runs, it uses the various layers from an image for its filesystem. Each container also gets its own "scratch space" to create/update/remove files. Any changes won't be seen in another container, even if they're using the same image.

See this in practice

To see this in action, you're going to start two containers. In one container, you'll create a file. In the other container, you'll check whether that same file exists.

1. Start an Alpine container and create a new file in it.

   $ docker run --rm alpine touch greeting.txt
   

   Tip

   Any commands you specify after the image name (in this case, alpine) are executed inside the container. In this case, the command touch greeting.txt puts a file named greeting.txt on the container's filesystem.

2. Run a new Alpine container and use the stat command to check whether the file exists.

   $ docker run --rm alpine stat greeting.txt
   

   You should see output similar to the following that indicates the file does not exist in the new container.

   stat: can't stat 'greeting.txt': No such file or directory
   

The greeting.txt file created by the first container did not exist in the second container. That is because the writeable "top layer" of each container is isolated. Even though both containers shared the same underlying layers that make up the base image, the writable layer is unique to each container.

Container volumes

With the previous experiment, you saw that each container starts from the image definition each time it starts. While containers can create, update, and delete files, those changes are lost when you remove the container and Docker isolates all changes to that container. With volumes, you can change all of this.

There are two main types of volumes. You'll eventually use both, but you'll start with volume mounts.

Persist the todo data

By default, the todo app stores its data in a SQLite database at /etc/todos/todo.db in the container's filesystem. If you're not familiar with SQLite, no worries! It's simply a relational database that stores all the data in a single file. While this isn't the best for large-scale applications, it works for small demos. You'll learn how to switch this to a different database engine later.

With the database being a single file, if you can persist that file on the host and make it available to the next container, it should be able to pick up where the last one left off. By creating a volume and attaching (often called "mounting") it to the directory where you stored the data, you can persist the data. As your container writes to the todo.db file, it will persist the data to the host in the volume.

As mentioned, you're going to use a volume mount. Think of a volume mount as an opaque bucket of data. Docker fully manages the volume, including the storage location on disk. You only need to remember the name of the volume.

Create a volume and start the container

You can create the volume and start the container using the CLI or Docker Desktop's graphical interface.

Verify that the data persists

1. Once the container starts up, open the app and add a few items to your todo list.

   

2. Stop and remove the container for the todo app. Use Docker Desktop or docker ps to get the ID and then docker rm -f <id> to remove it.

3. Start a new container using the previous steps.

4. Open the app. You should see your items still in your list.

5. Go ahead and remove the container when you're done checking out your list.

You've now learned how to persist data.

Dive into the volume

A lot of people frequently ask "Where is Docker storing my data when I use a volume?" If you want to know, you can use the docker volume inspect command.

$ docker volume inspect todo-db

You should see output like the following:

[
    {
        "CreatedAt": "2019-09-26T02:18:36Z",
        "Driver": "local",
        "Labels": {},
        "Mountpoint": "/var/lib/docker/volumes/todo-db/_data",
        "Name": "todo-db",
        "Options": {},
        "Scope": "local"
    }
]

The Mountpoint is the actual location of the data on the disk. Note that on most machines, you will need to have root access to access this directory from the host.

Summary

In this section, you learned how to persist container data.

Related information:

• docker CLI reference
• Next steps

  Next, you'll learn how you can develop your app more efficiently using bind mounts.