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In this blog post we are going to discuss one of the concepts in Kubernetes called “ReplicaSets”. We will explain what it is, what it is used for and how to create it with a simple example. This post assumes you have a basic understanding of what a “Pod” is and its purpose, minikube and kubectl.

What is ReplicaSets?

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”.

Labels” are key value pair used to specify attributes of objects that are meaningful and useful to users, so keep in mind that It doesn’t change the way the core system works. “Label Selectors” is used to identify a set of objects in Kubernetes.

ReplicaSets allow us to use “set-based” label selector(e.g environment in (production, qa) or tier notin (frontend, backend)) as opposed to “equality-based”(e.g environment = production or tier != frontend) which is what you use with replication controller. There are 3 kinds of operator we can use with ReplicaSets. In, notin and exists. ReplicaSets manages all the pods with labels that match the selector. When pods are created, it does not differentiate between all pods so if you create another ReplicaSets with the same label selector, the previous ReplicaSets will think it created those pods therefore causing issues. This is important to note, therefore it is recommended to make sure label selectors do not match from one ReplicaSets to another.

What does the manifest look like?

apiVersion: apps/v1 # our API version
kind: ReplicaSet   # The kind we are creating
Metadata: # Specify all Metadata like name, labels
  name: some-name
  labels:
    app: some-App
    tier: some-Tier
Spec: 
  replicas: 3 # Here is where we tell k8s how many replicas we want
  Selector: # This is our label selector field. 
    matchLabels:
      tier: some-Tier
    matchExpressions:
      - {key: tier, operator: In, values: [some-Tier]} # we are using the set-based operators
  template:
    metadata:
      labels:
        app: some-App
        tier: someTier
    Spec: # This spec section should look like spec in a pod definition
      Containers: 

APIVersion, kind and Metadata look similar to any other object in Kubernetes but the Spec section look slightly different from other objects. As a matter of fact there are two Spec’s in the manifest example above. The first Spec lets you declare what the replicaset should look like and the second spec is for containers. “.spec.template” is the only required field of the the first spec section. If you do not specify replicas, it will deploy just 1 pod. It is also worth noting that the “.spec.template.metadata.labels” must match exactly what you have in the “.spec.selector”(You can see this in our manifest above), or Kubernetes will not allow it to be created.

ReplicaSet by example?

Ok now that we understand what a replicaset is, let us create one. In this example, we will deploy our replica.yaml file which will create a simple frontend nginx app with 3 replicas.

apiVersion: apps/v1
kind: ReplicaSet
metadata:
  name: myapp-replicas
  labels:
    app: myapp
    tier: frontend
spec:
  replicas: 3
  selector:
    matchLabels:
      tier: frontend
    matchExpressions:
      - {key: tier, operator: In, values: [frontend]}
  template:
    metadata:
      labels:
        app: myapp
        tier: frontend
    spec:
      containers:
      - name: nginx
        image: nginx
        ports:
        - containerPort: 80

First we run kubectl create command to apply our manifest

$ kubectl create -f replica.yaml
replicaset.apps "myapp-replicas" created

Next, we make sure it is created

$ kubectl get replicaset
NAME                            DESIRED   CURRENT   READY     AGE
myapp-replicas                  3         3         3         15s

We see that there are 3 deployed and 3 ready. We can also just check the pods.

$ kubectl get pod
NAME                            READY     STATUS    RESTARTS   AGE
myapp-replicas-67rkp            1/1       Running   0          33s
myapp-replicas-6kfd8            1/1       Running   0          33s
myapp-replicas-s96sg            1/1       Running   0          33s

We see all 3 running with 0 restarts which mean our application is not crashing. We can also describe the object which will give us more details about our replicas.

$ kubectl describe replicaset myapp-replicas
Name:         myapp-replicas
Namespace:    default
Selector:     tier=frontend,tier in (frontend)
Labels:       app=myapp
              tier=frontend
Annotations:  
Replicas:     3 current / 3 desired
Pods Status:  3 Running / 0 Waiting / 0 Succeeded / 0 Failed
Pod Template:
  Labels:  app=myapp
           tier=frontend
  Containers:
   nginx:
    Image:        nginx
    Port:         80/TCP
    Host Port:    0/TCP
    Environment:  
    Mounts:       
  Volumes:        
Events:
  Type    Reason            Age   From                   Message
  ----    ------            ----  ----                   -------
  Normal  SuccessfulCreate  12m   replicaset-controller  Created pod: myapp-replicas-6kfd8
  Normal  SuccessfulCreate  12m   replicaset-controller  Created pod: myapp-replicas-67rkp
  Normal  SuccessfulCreate  12m   replicaset-controller  Created pod: myapp-replicas-s96sg

What if we no longer need 3 replicas, we now only need 1?

All we have to do is change the replica field to the value we want and k8s will scale it to that number. For example “replicas: 1”. If we make the change and re apply, we will only have one replica running.

Can we remove pod from a ReplicaSets?

Yes we can. It is as simple as removing the label from the pod and it will be removed from the Set.

How to cleanup?

To delete replicaset, all we have to do is run the “kubectl delete replicaset myapp-replicas” this command will delete the replicasets and the pods.

In general Kubernetes recommend that we use deployment (a higher-level concept that manages ReplicaSets and provides declarative updates to pods along with a lot of other useful features) controller instead of ReplicaSets.

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Last updated: 01/09/2019

These postings are my own and do not necessarily represent BMC's position, strategies, or opinion.

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About the author

Toye Idowu

Toye Idowu

Olatoye is a Certified Kubernetes Administrator and experienced DevOps/Platform engineering Consultant with a demonstrated history of working in the Information Technology and Services industry.