Continuing on from “CoreOS on a single server” we’ll be setting up a persistent instance of MySQL for your single-server CoreOS (non-)cluster.

Persisting after reboots

Whenever CoreOS receives an update, the /usr directory is swapped out for a fresh copy. This means that any files you put here will be gone as soon as the next update happens. The CoreOS developers have helpfully marked this as a read-only partition. It should therefore be fairly difficult to accidentally lose data between reboots.

localhost / # touch /usr/write-test
touch: cannot touch '/usr/write-test': Read-only file system

For this example we’ll be keeping our data in /home/core/mysql-rubypx. This will survive the dual-partition switch and will be equivalently reliable as storing MySQL data on any other linux distribution.

Creating the unit file

CoreOS uses systemd for system management. We’ll be focusing on learning how to use systemctl for managing services; and journalctl for viewing their logs.

Systemd replaces “/etc/init.d/” scripts with their own “.ini”-like templates. Below is the service unit file we’ll use to get our MySQL container running. Use sudo vi /etc/systemd/system/rubypx-db.service to create the following file:

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[Unit]
Description=Database
After=docker.service
Requires=docker.service

[Service]
ExecStart=/usr/bin/docker run \
  --name rubypx-db \
  -e MYSQL_ROOT_PASSWORD=<password> \
  -v /home/core/mysql-rubypx/:/var/lib/mysql \
  -p 172.17.42.1:3306:3306 \
  mysql
ExecStop=/usr/bin/docker rm -f rubypx-db

[Install]
WantedBy=basic.target

We assign a docker short name of “rubypx-db” so that we can easily stop the container in the “ExecStop” step (otherwise we’d have to keep track of the container ID). The “MYSQL_ROOT_PASSWORD” is set here, as the instance will only ever be accessible from inside the server anyway. The container is set to bind to 172.17.42.1, this is the IP of the “docker0” internal interface, and allows us to prevent the container exposing an external port on the host.

We’ll need to create the directory for MySQL to mount as it’s data volume, do so with: mkdir /home/core/mysql-rubypx.

Starting the service

First of all we’ll need to enable our unit in systemd: sudo systemctl enable rubypx-db. This will now ensure that the service loads once all of it’s dependencies have been satisfied.

The “WantedBy” field allows us to select which dependencies our service has. The “basic.target” will load before the GUI (if applicable), and after all of the basic subsystems (networking, filesystem).

Since we want to start the unit immediately and test it out, we can run sudo systemctl start rubypx-db.

Testing the service

Now that the unit has been started we can use sudo journalctl -u rubypx-db to view the logs (the “-f” flag can be used to tail the log).

We can also use sudo systemctl status rubypx-db to display the status of the unit:

[email protected] ~ $ sudo systemctl status rubypx-db
● rubypx-db.service - Database
   Loaded: loaded (/etc/systemd/system/rubypx-db.service; enabled; vendor preset: disabled)
   Active: active (running) since Fri 2015-07-10 21:51:20 UTC; 3 days ago
 Main PID: 620 (docker)
   Memory: 5.9M
      CPU: 2.242s
   CGroup: /system.slice/rubypx-db.service
           └─620 /usr/bin/docker run --name rubypx-db -e MYSQL_ROOT_PASSWORD=<password> -v /home/core/mysql-rubypx/:/var/lib/mysql -p 172.17.42.1:3306:3306 mysql
Jul 10 21:51:35 localhost docker[620]: 2015-07-10 21:51:35 1 [Note] InnoDB: Waiting for purge to start
...

We can test out our new server by starting the MySQL client in a temporary container using the following command:

docker run -it --rm mysql \
  sh -c 'exec mysql \
    -h172.17.42.1 \
    -P3306 \
    -uroot \
    -p<password>'

We should also be able to be able to connect to our MySQL server manually by using an ssh tunnel, and then connecting to the “docker0” interface ip. Now that we’ve got our MySQL database set up, we’re ready to deploy our application; I’ll cover this in my next post!