This example describes how to run a persistent installation of Wordpress.

We'll use the mysql and wordpress official Docker images for this installation. (The wordpress image includes an Apache server).

We'll create two Kubernetes pods to run mysql and wordpress, both with associated persistent disks, then set up a Kubernetes service to front each pod.

This example demonstrates several useful things, including: how to set up and use persistent disks with Kubernetes pods; how to define Kubernetes services to leverage docker-links-compatible service environment variables; and use of an external load balancer to expose the wordpress service externally and make it transparent to the user if the wordpress pod moves to a different cluster node.

Some of the example's details, such as the Persistent Disk setup, require that Kubernetes is running on Google Compute Engine.

First, if you have not already done so, create a Google Cloud Platform project, and install the gcloud SDK.

Then, set the gcloud default project name to point to the project you want to use for your Kubernetes cluster:

gcloud config set project <project-name>

Next, grab the Kubernetes release binary. You can do this via:

wget -q -O - https://get.k8s.io | bash

or

curl -sS https://get.k8s.io | bash

Then, start up a Kubernetes cluster as described here.

$ <kubernetes>/cluster/kube-up.sh

where <kubernetes> is the path to your Kubernetes installation.

For this WordPress installation, we're going to configure our Kubernetes pods to use persistent disks. This means that we can preserve installation state across pod shutdown and re-startup.

Before doing anything else, we'll create the persistent disks that we'll use for the installation: one for the mysql pod, and one for the wordpress pod. The general series of steps required is as described here, where $ZONE is the zone where your cluster is running, and $DISK_SIZE is specified as, e.g. '500GB'. In future, this process will be more streamlined.

So for the two disks used in this example, do the following. First create the mysql disk, setting the disk size to meet your needs:

gcloud compute disks create --size=$DISK_SIZE --zone=$ZONE mysql-disk

Then create the wordpress disk. Note that you may not want as large a disk size for the wordpress code as for the mysql disk.

gcloud compute disks create --size=$DISK_SIZE --zone=$ZONE wordpress-disk

Now that the persistent disks are defined, the Kubernetes pods can be launched. We'll start with the mysql pod.

First, edit mysql.yaml, the mysql pod definition, to use a database password that you specify. mysql.yaml looks like this:

apiVersion: v1beta1
id: mysql
desiredState:
  manifest:
    version: v1beta1
    id: mysql
    containers:
      - name: mysql
        image: mysql
        env:
          - name: MYSQL_ROOT_PASSWORD
           # change this
            value: yourpassword
        cpu: 100
        ports:
          - containerPort: 3306
        volumeMounts:
            # name must match the volume name below
          - name: mysql-persistent-storage
            # mount path within the container
            mountPath: /var/lib/mysql
    volumes:
      - name: mysql-persistent-storage
        source:
          persistentDisk:
            # This GCE PD must already exist
            pdName: mysql-disk
            fsType: ext4
labels:
  name: mysql
kind: Pod

Note that we've defined a volume mount for /var/lib/mysql, and specified a volume that uses the persistent disk (mysql-disk) that you created. Once you've edited the file to set your database password, create the pod as follows, where <kubernetes> is the path to your Kubernetes installation:

$ <kubernetes>/cluster/kubectl.sh create -f mysql.yaml

It may take a short period before the new pod reaches the Running state. List all pods to see the status of this new pod and the cluster node that it is running on:

$ <kubernetes>/cluster/kubectl.sh get pods

You can take a look at the logs for a pod by using kubectl.sh log. For example:

$ <kubernetes>/cluster/kubectl.sh log mysql

If you want to do deeper troubleshooting, e.g. if it seems a container is not staying up, you can also ssh in to the node that a pod is running on. There, you can run sudo -s, then docker ps -a to see all the containers. You can then inspect the logs of containers that have exited, via docker logs <container_id>. (You can also find some relevant logs under /var/log, e.g. docker.log and kubelet.log).

We'll define and start a service that lets other pods access the mysql database on a known port and host. We will specifically name the service mysql. This will let us leverage the support for Docker-links-compatible service environment variables when we up the wordpress pod. The wordpress Docker image expects to be linked to a mysql container named mysql, as you can see in the "How to use this image" section on the wordpress docker hub page.

So if we label our Kubernetes mysql service mysql, the wordpress pod will be able to use the Docker-links-compatible environment variables, defined by Kubernetes, to connect to the database.

The mysql-service.yaml file looks like this:

kind: Service
apiVersion: v1beta1
id: mysql
# the port that this service should serve on
port: 3306
# just like the selector in the replication controller,
# but this time it identifies the set of pods to load balance
# traffic to.
selector:
  name: mysql
# the container on each pod to connect to, can be a name
# (e.g. 'www') or a number (e.g. 80)
containerPort: 3306
labels:
  name: mysql

Start the service like this:

$ <kubernetes>/cluster/kubectl.sh create -f mysql-service.yaml

You can see what services are running via:

$ <kubernetes>/cluster/kubectl.sh get services

Once the mysql service is up, start the wordpress pod, specified in wordpress.yaml. Before you start it, edit wordpress.yaml and set the database password to be the same as you used in mysql.yaml. Note that this config file also defines a volume, this one using the wordpress-disk persistent disk that you created.

apiVersion: v1beta1
id: wordpress
desiredState:
  manifest:
    version: v1beta1
    id: frontendController
    containers:
      - name: wordpress
        image: wordpress
        ports:
          - containerPort: 80
        volumeMounts:
            # name must match the volume name below
          - name: wordpress-persistent-storage
            # mount path within the container
            mountPath: /var/www/html
        env:
          - name: WORDPRESS_DB_PASSWORD
            # change this - must match mysql.yaml password
            value: yourpassword
    volumes:
      - name: wordpress-persistent-storage
        source:
          # emptyDir: {}
          persistentDisk:
            # This GCE PD must already exist
            pdName: wordpress-disk
            fsType: ext4
labels:
  name: wpfrontend
kind: Pod

Create the pod:

$ <kubernetes>/cluster/kubectl.sh create -f wordpress.yaml

And list the pods to check that the status of the new pod changes to Running. As above, this might take a minute.

$ <kubernetes>/cluster/kubectl.sh get pods

Once the wordpress pod is running, start its service, specified by wordpress-service.yaml.

The service config file looks like this:

kind: Service
apiVersion: v1beta1
id: wpfrontend
# the port that this service should serve on.
port: 80
# just like the selector in the replication controller,
# but this time it identifies the set of pods to load balance
# traffic to.
selector:
  name: wpfrontend
# the container on each pod to connect to, can be a name
# (e.g. 'www') or a number (e.g. 80)
containerPort: 80
labels:
  name: wpfrontend
createExternalLoadBalancer: true

Note the createExternalLoadBalancer setting. This will set up the wordpress service behind an external IP. createExternalLoadBalancer only works on GCE. Note also that we've set the service port to 80. We'll return to that shortly.

Start the service:

$ <kubernetes>/cluster/kubectl.sh create -f wordpress-service.yaml

and see it in the list of services:

$ <kubernetes>/cluster/kubectl.sh get services

Then, find the external IP for your WordPress service by listing the forwarding rules for your project:

$ gcloud compute forwarding-rules list

Look for the rule called wpfrontend, which is what we named the wordpress service, and note its IP address.

To access your new installation, you first may need to open up port 80 (the port specified in the wordpress service config) in the firewall for your cluster. You can do this, e.g. via:

$ gcloud compute firewall-rules create sample-http --allow tcp:80

This will define a firewall rule called sample-http that opens port 80 in the default network for your project.

Now, we can visit the running WordPress app. Use the external IP that you obtained above, and visit it on port 80:

http://<external_ip>

You should see the familiar WordPress init page.

Set up your WordPress blog and play around with it a bit. Then, take down its pods and bring them back up again. Because you used persistent disks, your blog state will be preserved.

If you are just experimenting, you can take down and bring up only the pods:

$ <kubernetes>/cluster/kubectl.sh delete -f wordpress.yaml
$ <kubernetes>/cluster/kubectl.sh delete -f mysql.yaml

When you restart the pods again (using the create operation as described above), their services will pick up the new pods based on their labels.

If you want to shut down the entire app installation, you can delete the services as well.

If you are ready to turn down your Kubernetes cluster altogether, run:

$ <kubernetes>/cluster/kube-down.sh