CKAD Preparation - ReplicationSets

Controllers (yes multiple) keep track of all the activities and monitors each of them to respond accordingly in case of change of behavior. All the controllers run inside kube-control-manager which act as reflexes in the cluster.

Replication Controller

To prevent users from loosing connection to a running pod that crashes. So either by creating a replicate of existing POD or by redeploying the crashed POD. This nature therefore helps in addressing High Availability, Load Balancing and Scaling.

Replication Controller is one of the Master node components that spans across all nodes in the cluster.

Definition file

Start with the base definition keywords:

Note The pods created using ReplicationController definition is from its name not from the metadata under spec → template → metadata

Replica Set

Replica Set is a new recommended way of working as it originates from Replication Controller.

Additional feature of a ReplicaSet is managing Pods those are not part the current replica set. Use the selector tag in the definition file under spec section to add other pods.

Learning with example

Consider, within a K8S cluster there are already PODs, labeled login-api, running even before there exists a ReplicaSet. Now when a replicaSet, named authentication-rs, is created with template corresponding to same container image, as the running POD but labeled authentication-api on the selector.

POD from ReplicaSet Perspective

The definition file for the ReplicaSet contains template of POD and selector fields. Based on this information, a replicaSet instance keep track of pods. In above scenario by default, the authentication-rs keeps track of only the PODs labeled authentication-api to make sure desired number of these PODs are running but not the PODs labeled login-api. By using the command kubectl describe replicaset authentication-rs, the Events sections will list all the PODs which are being tracked by the replicaSet. All the pods created by the replicaSet adds a metadata label ownerreferences.

ReplicaSet from POD perspective

The pods deployed from the replicaSet can be identified by the names, as the start with the replicaSet name. In the above scenario, the pods names turnup something similar to authentication-rs-ab1c2d. Another option to find the details are by using the command kubectl describe pod authentication-rs-ab1c2d. The Events sections will list all the POD details, one of which will be Controlled By: ReplicaSet/authentication-rs. Also notice that the metadata section of the POD contains ownerReferences with the replicaSet name authentication-rs.

Definition Files

Following below are sample Definition files to create a ReplicationController and a ReplicaSet

ReplicationController Definition File

# replicationController-definition.yaml 

apiVersion: v1
kind: ReplicationController
metadata:
    name: myapp-rc
    labels:
        app: myapp
        type: frontend
spec:
    template:
        # Pod definition
        metadata:
            name: sample-app
            labels:
                app: api
                type: frontend
        spec:
            containers:
                - name: nginx-container
                  image: nginx

    replicas: 5

ReplicaSet Definition File

# replicaSet-definition.yaml 

apiVersion: apps/v1
kind: ReplicaSet
metadata:
    name: myapp-rc
    labels:
        app: myapp
        type: frontend
spec:
    template:
        # Pod definition
        metadata:
            name: sample-app
            labels:
                app: api
                type: frontend
        spec:
            containers:
                - name: nginx-container
                  image: nginx

    replicas: 5
    selector: 
        matchLabels:
            type: frontend

Commands

• Create without yaml

kubectl create replicaSet authentication-rs --image nginx --replicas=3

• Get details of ReplicaSet

kubectl describe replicaSet authentication-rs

• To fix existing erroneous ReplicaSet

kubectl edit replicaset <name-of-replicaset>

• Scale up or down pods in the replica sets:

## Option 1
kubectl scale --replicas <number> rs/<name-of-replicaset>

## Option 2
kubectl scale replicaset --replicas=<number> <name-of-replicaset>

Debugging issue

• A erroneous ReplicaSet can still create PODs but in error state. To fix the issue there are two options:
• Option 1: Delete the ReplicaSet → Fix the YAML → Apply updated file to create new ReplicaSet
• Option 2: Using kubectl edit fix the YAML → Delete all the PODs which were created erroneously