Replication Controller

A replication controller (RC) is a supervisor for long-running pods. An RC will launch a specified number of pods called replicas and makes sure that they keep running, for example when a node fails or something inside of a pod, that is, in one of its containers goes wrong.

It is one of the Kubernetes controllers used to make sure that we have a specified number of pod replicas running (A controller in Kubernetes is what takes care of tasks to make sure the desired state of the cluster matches the observed state). Without it, we will have to create multiple manifests for the number of pods we need which is a lot of work to deploy replicas of a single application. In previous versions of Kubernetes, it was called “Replication Controller”. The main difference between the two is that ReplicaSets allow us to use something called “Label Selector”.

Lab:

kubectl create -f https://raw.githubusercontent.com/mhausenblas/kbe/master/specs/rcs/rc.yaml

You can see the RC and the pod it looks after like so:

kubectl get rc

kubectl get pods --show-labels

Note two things here:

• the supervised pod got a random name assigned (rcex-qrv8j)
• the way the RC keeps track of its pods is via the label, here app=sise

To scale up, that is, to increase the number of replicas, do:

kubectl scale --replicas=3 rc/rcex

kubectl get pods -l app=sise

To delete the rc

kubectl delete rc rcex

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