Docker Cookbook

5.5 Taking Advantage of Labels for Querying Kubernetes Objects | 141

---

See Also
• Introduction, motivation, and syntax of 
labels
5.6 Using a Replication Controller to Manage the Number
of Replicas of a Pod
Problem
You need to make sure that several replicas of your pod exist at any time in
the cluster.
Solution
Kubernetes is a declarative system: users express what they want the system to do and
not how to do it. Using replication controllers, you can specify the number of replicas
that you want for a pod. This helps with high load and availability by serving part of
an application through a service proxy (see 
Recipe 5.8
).
Replication controllers are one of the three key objects in a Kubernetes cluster (with
pods and services). You can list all running replication controllers with 
kubectl
:
$ kubectl get replicationcontrollers
...
$ kubectl get rc
To create a replication controller, you write a JSON or YAML file following the repli‐
cation controller API specification. It can contain metadata, the number of replicas
that you want, a selector to target specific pods, and a template for a pod. Currently,
the template is embedded within the replication controller definition, but this may
change in future version of Kubernetes.
For example, if you want to create a replication controller for the 2048 game that you
ran in a single pod in 
Recipe 5.4
, you can write the following 
rc2048.yaml
file:
apiVersion: v1
kind: ReplicationController
metadata:
labels:
name: rcgame
name: rcgame
spec:
replicas: 1
selector:
name: game
template:
metadata:
142 | Chapter 5: Kubernetes

---

labels:
name: game
spec:
containers:
- image: cpk1224/docker-2048
name: test
ports:
- containerPort: 80
The controller itself will have the 
rcgame
label, and will target pods with the label
game
. Once started, the controller will ensure that one pod is running at all times. You
launch it with 
kubectl create
:
$ kubectl create -f rc2048.yml
replicationcontrollers/rcgame
$ kubectl get rc
CONTROLLER CONTAINER(S) IMAGE(S) SELECTOR REPLICAS
rcgame test cpk1224/docker-2048 name=game 1
Try killing the pod that was created. You will see that a new one automatically starts
again.
You do not need to have an existing pod running before starting a
replication controller. It will automatically start a pod that matches
the label specified in the definition if it does not exist yet. You can
also set the number of replicas to zero.
The magic happens when you want to increase the number of replicas. You can use
the 
kubectl resize
command, and the number of pods will automatically be adjus‐
ted:
$ kubectl resize --replicas=4 rc rcgame
resized