Kubernetes ReplicaSet

Another requirement is to keep a predefined number of pods running. If more pods are up, the additional ones are terminated. Similarly, of one or more pods failed, new pods are activated until the desired count is reached.

A Kubernetes ReplicaSet resource was designed to address both of those requirements. It creates and maintains a specific number of similar pods (replicas).

Under this lab, we’ll discuss how we can define a ReplicaSet and what are the different options that can be used for fine-tuning it.

How Does ReplicaSet Manage Pods?

To be able to create new pods if necessary, the ReplicaSet definition includes a template part containing the definition for new pods.

Creating Your First ReplicaSet

git clone https://github.com/collabnix/dockerlabs
cd dockerlabs/kubernetes/workshop/replicaset101
kubectl apply -f nginx_replicaset.yaml
kubectl get rs
NAME   DESIRED   CURRENT   READY   AGE
web	4     	4     	4   	2m

A Peep into the ReplicaSet definition file

Let’s examine the definition file that was used to create our ReplicaSet:

Is Our ReplicaSet the Owner of Those Pods?

OK, so we do have four pods running, and our ReplicaSet reports that it is controlling four pods. In a busier environment, you may want to verify that a particular pod is actually managed by this ReplicaSet and not by another controller. By simply querying the pod, you can get this info:

kubectl get pods web-6n9cj -o yaml | grep -A 5 owner

The first part of the command will get all the pod information, which may be too verbose. Using grep with the -A flag (it takes a number and prints that number of lines after the match) will get us the required information as in the example:

ownerReferences:
  - apiVersion: extensions/v1beta1
	blockOwnerDeletion: true
	controller: true
	kind: ReplicaSet
	name: web

Removing a Pod From a ReplicaSet

You can remove (not delete) a pod that is managed by a ReplicaSet by simply changing its label. Let’s isolate one of the pods created in our previous example:

kubectl edit pods web-44cjb

Then, once the YAML file is opened, change the pod label to be role=isolated or anything different than role=web. In a few moments, run kubectl get pods. You will notice that we have five pods now. That’s because the ReplicaSet dutifully created a new pod to reach the desired number of four pods. The isolated one is still running, but it is no longer managed by the ReplicaSet.

Scaling the Replicas to 5

[node1 replicaset101]$ kubectl scale --replicas=5 -f nginx_replicaset.yaml

Scaling and Autoscaling ReplicaSets

You can easily change the number of pods a particular ReplicaSet manages in one of two ways:

kubectl autoscale rs web --max=5

This will use the Horizontal Pod Autoscaler (HPA) with the ReplicaSet to increase the number of pods when the CPU load gets higher, but it should not exceed five pods. When the load decreases, it cannot have less than the number of pods specified before (two in our example).

Best Practices

The recommended practice is to always use the ReplicaSet’s template for creating and managing pods. However, because of the way ReplicaSets work, if you create a bare pod (not owned by any controller) with a label that matches the ReplicaSet selector, the controller will automatically adopt it. This has a number of undesirable consequences. Let’s have a quick lab to demonstrate them.

Deploy a pod by using a definition file like the following:

apiVersion: v1
kind: Pod
metadata:
  name: orphan
  labels:
	role: web
spec:
  containers:
  - name: orphan
	image: httpd

It looks a lot like the other pods, but it is using Apache (httpd) instead of Nginx for an image. Using kubectl, we can apply this definition like:

kubectl apply -f orphan.yaml

Give it a few moments for the image to get pulled and the container is spawned then run kubectl get pods. You should see an output that looks like the following:

NAME    	READY   STATUS    	RESTARTS   AGE
orphan  	0/1 	Terminating   0      	1m
web-6n9cj   1/1 	Running   	0      	25m
web-7kqbm   1/1 	Running   	0      	25m
web-9src7   1/1 	Running   	0      	25m
web-fvxzf   1/1 	Running   	0      	25m

The pod is being terminated by the ReplicaSet because, by adopting it, the controller has more pods than it was configured to handle. So, it is killing the excess one.

Another scenario where the ReplicaSet won’t terminate the bare pod is that the latter gets created before the ReplicaSet does. To demonstrate this case, let’s destroy our ReplicaSet:

kubectl delete -f nginx_replicaset.yaml

Now, let’s create it again (our orphan pod is still running):

kubectl apply -f nginx_replicaset.yaml

Let’s have a look at our pods status by running kubectl get pods. The output should resemble the following:

orphan  	1/1 	Running   0      	29s
web-44cjb   1/1 	Running   0      	12s
web-hcr9j   1/1 	Running   0      	12s
web-kc4r9   1/1 	Running   0      	12s

The situation now is that we’re having three pods running Nginx, and one pod running Apache (the httpd image). As far as the ReplicaSet is concerned, it is handling four pods (the desired number), and their labels match its selector. But what if the Apache pod went down?

Let’s do just that:

kubectl delete pods orphan

Now, let’s see how the ReplicaSet responded to this event:

kubectl get pods

The output should be something like:

NAME    	READY   STATUS          	RESTARTS   AGE
web-44cjb   1/1 	Running         	0      	24s
web-5kjwx   0/1 	ContainerCreating   0      	3s
web-hcr9j   1/1 	Running         	0      	24s
web-kc4r9   1/1 	Running         	0      	24s

The ReplicaSet is doing what is was programmed to: creating a new pod to reach the desired state using the template that was added in its definition. Obviously, it is creating a new Nginx container instead of the Apache one that was deleted.

So, although the ReplicaSet is supposed to maintain the state of the pods it manages, it failed to respawn the Apache web server. It replaced it with an Nginx one.

The bottom line: you should never create a pod with a label that matches the selector of a controller unless its template matches the pod definition. The more-encouraged procedure is to always use a controller like a ReplicaSet or, even better, a Deployment to create and maintain your pods.

Deleting Replicaset

kubectl delete rs ReplicaSet_name

Alternatively, you can also use the file that was used to create the resource (and possibly, other resource definitions as well) to delete all the resources defined in the file as follows:

kubectl delete -f definition_file.yaml

The above commands will delete the ReplicaSet and all the pods that it manges. But sometimes you may want to just delete the ReplicaSet resource, keeping the pods unowned (orphaned). Maybe you want to manually delete the pods and you don’t want the ReplicaSet to restart them. This can be done using the following command:

kubectl delete rs ReplicaSet_name --cascade=false

If you run kubectl get rs now you should see that there are no ReplicaSets there. Yet if you run kubectl get pods, you should see all the pods that were managed by the destroyed ReplicaSet still running.

The only way to get those pods managed by a ReplicaSet again is to create this ReplicaSet with the same selector and pod template as the previous one. If you need a different pod template, you should consider using a Deployment instead, which will handle replacing pods in a controlled way.

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