Docker Cookbook

5.8 Using Cluster IP Services for Dynamic Linking of
Containers
Problem
You want to 
link
containers across multiple hosts in your cluster instead of running
multiple containers per pod. This is the more cloud-native way of designing an appli‐
146 | Chapter 5: Kubernetes

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cation, where layers that can scale and can operate separately from each other run as
separate replication controllers.
Solution
In 
Recipe 5.7
, you started WordPress by running the MySQL and WordPress contain‐
ers in a single pod. The two containers started on the same host. You took advantage
of the fact that a pod has a single IP address to set the WordPress MySQL host to
localhost
. However, you could imagine running a replicated MySQL service and/or
a replicated WordPress frontend. This would mean that the containers would run on
different hosts in the cluster.
Kubernetes services are smart proxies that keep track of changes in pod cluster allo‐
cation and update their port mapping dynamically when pods get rescheduled.
A better way of running our canonical WordPress example would be to run MySQL
as a single pod or replication controller (glancing over the issues with database repli‐
cation and data persistence) and then exposing this MySQL service through a Kuber‐
netes service definition.
The replication controller would look something like this:
apiVersion: v1
kind: ReplicationController
metadata:
name: wp-mysql
spec:
replicas: 1
selector:
tier: wp-mysql
template:
metadata:
labels:
tier: wp-mysql
spec:
containers:
- name: mysql
image: mysql
ports:
- containerPort: 3306
env:
- name: MYSQL_ROOT_PASSWORD
value: wordpressdocker
- name: MYSQL_DATABASE
value: wordpress
- name: MYSQL_USER
value: wordpress
- name: MYSQL_PASSWORD
value: wordpresspwd