rcs.md

+++ title = "Replication Controllers" subtitle = "Kubernetes replication controllers by example" date = "2019-02-27" url = "/rcs/" +++

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.

Let's create an RC that supervises a single replica of a pod:

kubectl apply -f https://raw.githubusercontent.com/openshift-evangelists/kbe/main/specs/rcs/rc.yaml

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

kubectl get rc

NAME                DESIRED   CURRENT   READY     AGE
rcex                1         1         1         3m

kubectl get pods --show-labels

NAME           READY     STATUS    RESTARTS   AGE    LABELS
rcex-qrv8j     1/1       Running   0          4m     app=sise

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

NAME         READY     STATUS    RESTARTS   AGE
rcex-1rh9r   1/1       Running   0          54s
rcex-lv6xv   1/1       Running   0          54s
rcex-qrv8j   1/1       Running   0          10m

Finally, to get rid of the RC and the pods it is supervising, use:

kubectl delete rc rcex

replicationcontroller "rcex" deleted

Note that, going forward, the RCs are called replica sets (RS), supporting set-based selectors. The RS are already in use in the context of deployments.

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