Updated April 13, 2023
Introduction to Kubernetes Replica Set
The following article provides an outline for Kubernetes Replica Set. The replica set here is related to the availability of identical pods. It is basically used, or its purpose is to maintain a specified set of pods running at the specified time. It is also used to assure or guarantee the availability of pods that are identical.
We can define replica set by using fields, including selectors; it is an overview of this. As the name suggests, it also guarantees how many replicas that means a copy of the pods should be running at a time; it helps us replace the replica controller for us. We have significant differences between both of them: replica set based on the set-based controller and replication controller based on the equality-based selector.
What is Kubernetes Replica Set?
As the name suggests, replica set, which basically means an exact copy of the pods. It decides how many replicas of the pods will be created at the same time. It is defined with the fields, which basically include the selector. This selector is used to identify the pods it can acquire; we also specify the number of replica, representing the number of pods it will maintain. Once the configuration is done properly, Replica sets to identify and start deleting and creating the pods as per the criteria to reach the desired number it has to maintain.
Given below are some of the key points of what is Kubernetes replica set is all about:
- First, it creates a copy or replica of the pods.
- Then, it keeps on creating and deleting the pods to reach the desired number of pods which it has to maintain.
- Finally, it guarantees the availability of the pods by creating its replica, which all the pods are running at the specified time.
- It can be defined by the use of fields and selectors, which will decide the number of pods.
Creating and Setup Kubernetes Replica Set
To create and set up the Kubernetes replica set, we have to follow the below steps:
1. We will create one .yml file called ‘frontend.yaml’, and we will be submitting this file to the Kubernetes cluster.
2. Once it is submitted, the Kubernetes cluster will create the pods and ReplicaSet.
Finally, 3. Run the below command.
Code:
kubectl apply -f https://kubernetes.io/examples/controllers/frontend.yaml4. Now, we can check the deployed replica set by using the below command.
Code:
kubectl get rs5. If we want to check the status of the replica set, then run the below command.
Code:
kubectl describe rs/frontend6. At the end, we can also check for the pods that are brought up by running the below command.
Code:
kubectl get podsBelow is the frontend.yml file for your reference:
apiVersion: apps/v1
kind: ReplicaSet
metadata:
name: frontend
labels:
app: guestbook
tier: frontend
spec:
# modify replicas according to your case
replicas: 3
selector:
matchLabels:
tier: frontend
template:
metadata:
labels:
tier: frontend
spec:
containers:
- name: php-redis
image: gcr.io/google_samples/gb-frontend:v3Scaling your Application Kubernetes Replica Set
To scale up and scale down the replica set, we can use to of the fields, that is, .spec.replicas; here, the replica controller guarantees us that the desired number of pods will be created that match the selector will always be available and operational. As the name suggests, m scale down means the specified replica set will not be available r needed at the moment. So the ReplicaSet controller will try to choose the pods to delete them, it will sort and prioritize them in order to scale down the specified pods, but for this, it generally follows one algorithm.
- First, we have to scale down the pending and unschedulable pods.
- If the ReplicaSet controller has the following annotation, that is .kubernetes.io/pod-deletion-cost, it will choose the pods with the lower value first.
- It will try to calculate the pods with fewer replicas than pods with high replicas, and higher will come first to scale down.
- Now it will check the pods created recently with the older pods, so they will come before the pods that are created old.
- It will match all these points to scale down; if all the criteria match, the selection is random.
Alternatives of Kubernetes Replica Set
Below is the list that shows the alternative to Kubernetes replica sets:
- Deployments: It is recommended to own the replica and uses it to update them and the pods using the declarative server-side updates. On the other side, replica sets can be used independently. But when we are trying to use Deployments, we should not worry about maintaining the replicas.
- Jobs: We can use jobs when the pods are terminating by their own.
- DaemonSet: DaemonSet provides us machine-level functions such as machine logging, monitoring, etc., because the lifetime of pods is tied with the lifetime of the machine because pods are safe to terminate when the machine is rebooted/shutdown.
- ReplicationController: If we talked about replica sets, so these are the successors of the ReplicationController, both of them behave similarly and serve the same purpose; the only difference between them is ReplicationController does not support the set bases selector; it helps equality selector. But ReplicaSets are preferred over the ReplicationController.
Use of Kubernetes Replica Set
Key points of using Kubernetes replica sets are mentioned below:
- It creates the replica of pods that are running at the moment.
- It helps in scale up and scale down the application easily.
- Check the status; pods are deployed using simple commands.
Conclusion
As we have seen all the necessary points and steps to maintain the Kubernetes replica sets, go through the tutorial once, you will get a good understanding about the replica set in Kubernetes, how it works and how we can create it, it is easy to use and maintain as well by the developers.
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