Use bind mounts

In part 5, we talked about and used a volume mount to persist the data in our database. A volume mount is a great choice when you need somewhere persistent to store your application data.

A bind mount is another type of mount, which lets you share a directory from the host’s filesystem into the container. When working on an application, you can use a bind mount to mount source code into the container. The container sees the changes you make to the code immediately, as soon as you save a file. This means that you can run processes in the container that watch for filesystem changes and respond to them.

In this chapter, we’ll see how we can use bind mounts and a tool called nodemon to watch for file changes, and then restart the application automatically. There are equivalent tools in most other languages and frameworks.

Quick volume type comparisons

The following table outlines the main differences between volume mounts and bind mounts.

  Named volumes Bind mounts
Host location Docker chooses You decide
Mount example (using --mount) type=volume,src=my-volume,target=/usr/local/data type=bind,src=/path/to/data,target=/usr/local/data
Populates new volume with container contents Yes No
Supports Volume Drivers Yes No

Trying out bind mounts

Before looking at how we can use bind mounts for developing our application, let’s run a quick experiment to get a practical understanding of how bind mounts work.

If you’re following these steps on Windows, make sure to use PowerShell and not command prompt (cmd).

  1. Open a terminal and make sure your current working directory is in the app directory of the getting started repository.

  2. Run the following command to start bash in an ubuntu container with a bind mount.

    $ docker run -it --mount type=bind,src="$(pwd)",target=/src ubuntu bash

    The --mount option tells Docker to create a bind mount, where src is the current working directory on your host machine (getting-started/app), and target is where that directory should appear inside the container (/src).

  3. After running the command, Docker starts an interactive bash session in the root directory of the container’s filesystem.

    root@ac1237fad8db:/# pwd
    root@ac1237fad8db:/# ls
    bin   dev  home  media  opt   root  sbin  srv  tmp  var
    boot  etc  lib   mnt    proc  run   src   sys  usr
  4. Now, change directory in the src directory.

    This is the directory that you mounted when starting the container. Listing the contents of this directory displays the same files as in the getting-started/app directory on your host machine.

    root@ac1237fad8db:/# cd src
    root@ac1237fad8db:/src# ls
    Dockerfile  node_modules  package.json  spec  src  yarn.lock
  5. Create a new file named myfile.txt.

    root@ac1237fad8db:/src# touch myfile.txt
    root@ac1237fad8db:/src# ls
    Dockerfile  myfile.txt  node_modules  package.json  spec  src  yarn.lock
  6. Now if you open this directory on the host, you’ll see the myfile.txt file has been created in the directory.

    File viewer on the host machine that sees the file created from the container

  7. From the host, delete the myfile.txt file.
  8. In the container, list the contents of the app directory once more. You’ll see that the file is now gone.

    root@ac1237fad8db:/src# ls
    Dockerfile  node_modules  package.json  spec  src  yarn.lock
  9. Stop the interactive container session with Ctrl + D.

And that’s all for a brief introduction to bind mounts. This procedure demonstrated how files are shared between the host and the container, and how changes are immediately reflected on both sides. Now let’s see how we can use bind mounts to develop software.

Run your app in a development container

The following steps describe how to run a development container with a bind mount that does the following:

  • Mount our source code into the container
  • Install all dependencies
  • Start nodemon to watch for filesystem changes

So, let’s do it!

  1. Make sure you don’t have any getting-started containers currently running.

  2. Run the following command from the getting-started/app directory.

    If you are using an Mac or Linux device, then use the following command.

    $ docker run -dp 3000:3000 \
        -w /app --mount type=bind,src="$(pwd)",target=/app \
        node:18-alpine \
        sh -c "yarn install && yarn run dev"

    If you are using Windows, then use the following command in PowerShell.

    $ docker run -dp 3000:3000 `
        -w /app --mount type=bind,src="$(pwd)",target=/app `
        node:18-alpine `
        sh -c "yarn install && yarn run dev"
    • -dp 3000:3000 - same as before. Run in detached (background) mode and create a port mapping
    • -w /app - sets the “working directory” or the current directory that the command will run from
    • --mount type=bind,src="$(pwd)",target=/app - bind mount the current directory from the host into the /app directory in the container
    • node:18-alpine - the image to use. Note that this is the base image for our app from the Dockerfile
    • sh -c "yarn install && yarn run dev" - the command. We’re starting a shell using sh (alpine doesn’t have bash) and running yarn install to install packages and then running yarn run dev to start the development server. If we look in the package.json, we’ll see that the dev script starts nodemon.
  3. You can watch the logs using docker logs <container-id>. You’ll know you’re ready to go when you see this:

    $ docker logs -f <container-id>
    nodemon src/index.js
    [nodemon] 2.0.20
    [nodemon] to restart at any time, enter `rs`
    [nodemon] watching dir(s): *.*
    [nodemon] starting `node src/index.js`
    Using sqlite database at /etc/todos/todo.db
    Listening on port 3000

    When you’re done watching the logs, exit out by hitting Ctrl+C.

  4. Now, make a change to the app. In the src/static/js/app.js file, on line 109, change the “Add Item” button to simply say “Add”:

    - {submitting ? 'Adding...' : 'Add Item'}
    + {submitting ? 'Adding...' : 'Add'}

    Save the file.

  5. Refresh the page in your web browser, and you should see the change reflected almost immediately. It might take a few seconds for the Node server to restart. If you get an error, try refreshing after a few seconds.

    Screenshot of updated label for Add button

  6. Feel free to make any other changes you’d like to make. Each time you make a change and save a file, the nodemon process restarts the app inside the container automatically. When you’re done, stop the container and build your new image using:

    $ docker build -t getting-started .

Using bind mounts is common for local development setups. The advantage is that the development machine doesn’t need to have all of the build tools and environments installed. With a single docker run command, dependencies and tools are pulled and ready to go. We’ll talk about Docker Compose in a future step, as this will help simplify our commands (we’re already getting a lot of flags).

In addition to volume mounts and bind mounts, Docker also supports other mount types and storage drivers for handling more complex and specialized use cases. To learn more about the advanced storage concepts, see Manage data in Docker.

Next steps

At this point, you can persist your database and respond rapidly to the needs and demands of your investors and founders. Hooray! But, guess what? You received great news! Your project has been selected for future development!

In order to prepare for production, you need to migrate your database from working in SQLite to something that can scale a little better. For simplicity, you’ll keep with a relational database and switch your application to use MySQL. But, how should you run MySQL? How do you allow the containers to talk to each other? You’ll learn about that next!

Multi container apps