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Step 3: Persist Data Using Volumes
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.. contents:: Table of Contents

**Objective**: Demonstrate the separation of volumes and containers.

A challenge in working with Docker containers is how to back up container
data. Or, what happens if you accidentally remove a container! Any data on
the inside of the container gets destroyed if the container is stopped and
removed. Docker makes it easy to remove containers, which increases the level
of risk when using containers. Mounting volumes keeps the data outside of the
container. Our containers become disposable when we use volumes to mount the
container data in our VPS.

   + We can stop and delete the containers in the project because the data
     remains on the VPS.
   + We can destroy a container and pull in an updated version without
     worrying about data loss.
   + We can migrate the data to a new machine and launch new containers.

Let's look at how volumes work through trial and error using Docker Compose.

Verifying Volume Data
=======================

#. Let's take another look at Docker Compose commands that we have used.

   .. code-block:: bash

       docker-compose up -d     # Creates and starts the Docker project in daemon mode.
       docker-compose start     # Starts images that were running but are now stopped.
       docker-compose stop      # Stops the project but keeps the containers.
       docker-compose down      # Stops the project and removes the containers.
       docker-compose down -v   # 1) Stops the project,
                                # 2) removes the containers,
                                # 3) and remove the volume data.

  There are two commands that stop a running project (``stop`` and ``down``).
  However, one also removes the containers. Let's do both!

#. Stop the running containers using ``stop``, but don't remove them.

   .. code-block:: bash

       docker-compose stop

#. Try to access the webpage. You will get a `502 Bad Gateway` error.

   + The project is stopped but the containers still exist. View the stopped
     containers through command ``docker ps -a``. They have the status
     of `Exited`.


   .. code-block:: bash
       :caption: Output
       :emphasize-lines: 1,4,6,7

       root@vps298933:~/wordpress-docker# docker-compose stop
       Stopping wordpressdocker_wordpress_1 ... done
       Stopping wordpressdocker_db_1        ... done
       root@vps298933:~/wordpress-docker# docker ps -a
       CONTAINER ID        IMAGE                COMMAND                  CREATED             STATUS                      PORTS                     NAMES
       28822cf1444c        wordpress:latest     "docker-entrypoint.s…"   About an hour ago   Exited (0) 16 seconds ago                             wordpressdocker_wordpress_1
       cdc959eb5014        mariadb              "docker-entrypoint.s…"   About an hour ago   Exited (0) 15 seconds ago                             wordpressdocker_db_1
       2175f0a86466        nextcloud            "/entrypoint.sh apac…"   7 days ago          Up 6 days                   0.0.0.0:20850->80/tcp     condescending_agnesi
       root@vps298933:~/wordpress-docker#


#. Start the project and then refresh the webpage. It will be back where you
   left it.

   .. code-block:: bash

       docker-compose start

   .. code-block:: bash
       :caption: Output
       :emphasize-lines: 1

       root@vps298933:~/wordpress-docker# docker-compose start
       Starting db        ... done
       Starting wordpress ... done
       root@vps298933:~/wordpress-docker#

#. Now, let's remove the containers entirely!

   .. Note:: Running this command without having volumes will remove the contain
      and all data.

   .. code-block:: bash

       docker-compose down

#. View the stopped containers through command ``docker ps -a``.
   You will see that the containers are gone. However, our data is still there.
   You can verify that the volume data still exists using ``ls -lh`` to view
   the directory contents.

   .. code-block:: bash
       :caption: Output
       :emphasize-lines: 1,8,12

       root@vps298933:~/wordpress-docker# docker-compose down
       Stopping wordpressdocker_wordpress_1 ... done
       Stopping wordpressdocker_db_1        ... done
       Removing wordpressdocker_wordpress_1 ... done
       Removing wordpressdocker_db_1        ... done
       Removing network wordpressdocker_default
       root@vps298933:~/wordpress-docker#
       root@vps298933:~/wordpress-docker# docker ps -a
       CONTAINER ID        IMAGE                COMMAND                  CREATED             STATUS              PORTS                          NAMES
       2175f0a86466        nextcloud            "/entrypoint.sh apac…"   7 days ago          Up 6 days           0.0.0.0:20850->80/tcp          condescending_agnesi
       root@vps298933:~/wordpress-docker#
       root@vps298933:~/wordpress-docker# ls -lh
       total 12K
       drwxr-xr-x 5      999 root     4.0K Mar 30 11:27 db_data
       -rw-r--r-- 1 root     root      660 Mar 30 10:10 docker-compose.yml
       drwxr-xr-x 5 www-data www-data 4.0K Mar 30 11:27 wordpress
       root@vps298933:~/wordpress-docker#


#. Now, rebuild the project using ``docker-compose up -d``

   .. code-block:: bash

       docker-compose up -d


   .. code-block:: bash
       :caption: Output
       :emphasize-lines: 1

       root@vps298933:~/wordpress-docker# docker-compose up -d
       Creating network "wordpressdocker_default" with the default driver
       Creating wordpressdocker_db_1 ...
       Creating wordpressdocker_db_1 ... done
       Creating wordpressdocker_wordpress_1 ...
       Creating wordpressdocker_wordpress_1 ... done
       root@vps298933:~/wordpress-docker#


#. Refresh the webpage.

   It should be back where you left it because the data and the applications
   are separated. Creating backups of the project data is simple. You need to
   save the project directory only. The container data are disposable.