docs/get-started/05_persisting_data.md

title	keywords	description
Persist the DB	get started, setup, orientation, quickstart, intro, concepts, containers, docker desktop	Making our DB persistent in our application

In case you didn't notice, our todo list is being wiped clean every single time we launch the container. Why is this? Let's 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 are using the same image.

See this in practice

To see this in action, we're going to start two containers and create a file in each. What you'll see is that the files created in one container aren't available in another.

1. Start an ubuntu container that will create a file named /data.txt with a random number between 1 and 10000.

   $ docker run -d ubuntu bash -c "shuf -i 1-10000 -n 1 -o /data.txt && tail -f /dev/null"
   

   In case you're curious about the command, we're starting a bash shell and invoking two commands (why we have the &&). The first portion picks a single random number and writes it to /data.txt. The second command is simply watching a file to keep the container running.

2. Validate that you can see the output by accessing the terminal in the container. To do so, go to Containers in Docker Desktop, hover over the container running the ubuntu image, and select the Show container actions menu. From the dropdown menu, select Open in terminal.

   You will see a terminal that is running a shell in the Ubuntu container. Run the following command to see the content of the /data.txt file. Close this terminal afterwards again.

   $ cat /data.txt
   

   If you prefer the command line you can use the docker exec command to do the same. You need to get the container's ID (use docker ps to get it) and get the content with the following command.

   $ docker exec <container-id> cat /data.txt
   

   You should see a random number!

3. Now, let's start another ubuntu container (the same image) and we'll see we don't have the same file.

   $ docker run -it ubuntu ls /
   

   In this case the command lists the files in the root directory of the container. Look, there's no data.txt file there! That's because it was written to the scratch space for only the first container.

4. Go ahead and remove the first container using the docker rm -f <container-id> command.

Container volumes

With the previous experiment, we 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 the container is removed and all changes are isolated to that container. With volumes, we can change all of this.

Volumes provide the ability to connect specific filesystem paths of the container back to the host machine. If a directory in the container is mounted, changes in that directory are also seen on the host machine. If we mount that same directory across container restarts, we'd see the same files.

There are two main types of volumes. We will eventually use both, but we will 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 in which all of the data is stored in a single file. While this isn't the best for large-scale applications, it works for small demos. We'll talk about switching this to a different database engine later.

With the database being a single file, if we 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 the data is stored in, we can persist the data. As our container writes to the todo.db file, it will be persisted to the host in the volume.

As mentioned, we are going to use a volume mount. Think of a volume mount as an opaque bucket of data. Docker fully manages the volume, including where it is stored on disk. You only need to remember the name of the volume.

1. Create a volume by using the docker volume create command.

   $ docker volume create todo-db
   

2. Stop and remove the todo app container once again in the Dashboard (or with docker rm -f <id>), as it is still running without using the persistent volume.

3. Start the todo app container, but add the --mount option to specify a volume mount. We will give the volume a name, and mount it to /etc/todos in the container, which will capture all files created at the path.

   $ docker run -dp 3000:3000 --mount type=volume,src=todo-db,target=/etc/todos getting-started
   

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

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

6. Start a new container using the same command from above.

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

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

Hooray! 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
[
    {
        "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 on the disk where the data is stored. Note that on most machines, you will need to have root access to access this directory from the host. But, that's where it is!

Accessing volume data directly on Docker Desktop

While running in Docker Desktop, the Docker commands are actually running inside a small VM on your machine. If you wanted to look at the actual contents of the Mount point directory, you would need to look inside of that VM.

Next steps

At this point, you have a functioning application that can survive restarts! You can show it off to your investors and hope they can catch your vision!

However, you saw earlier that rebuilding images for every change takes quite a bit of time. There's got to be a better way to make changes, right? With bind mounts (which was hinted at earlier), there is a better way!

Use bind mounts{: .button .primary-btn}