Kubernetes Concepts - ReplicationController and Replica Sets

Replication Controller

A ReplicationController in Kubernetes ensures that a specified number of pod replicas are running at any given time. Essentially, it makes sure that a pod or a homogeneous set of pods is always up and available. Let’s dive into how it works:

Pod Management:
- If there are too many pods, the ReplicationController terminates the extra ones.
- Conversely, if there are too few pods, the ReplicationController starts additional replicas.
- Unlike manually created pods, those maintained by a ReplicationController are automatically replaced if they fail, are deleted, or are terminated. For instance, when disruptive maintenance (such as a kernel upgrade) occurs, your pods are re-created on a node.
- Even if your application requires only a single pod, it’s recommended to use a ReplicationController.
Supervision Across Nodes:
- Think of a ReplicationController as a process supervisor, but instead of overseeing individual processes on a single node, it supervises multiple pods across multiple nodes.
- In discussions and kubectl commands, ReplicationController is often abbreviated as “rc.”

Example Configuration:

Let’s create a simple ReplicationController that reliably runs three instances of the nginx web server:

  apiVersion: v1
  kind: ReplicationController
  metadata:
    name: nginx
  spec:
    replicas: 3
    selector:
      app: nginx
    template:
      metadata:
        name: nginx
        labels:
          app: nginx
      spec:
        containers:
          - name: nginx
            image: nginx
            ports:
              - containerPort: 80

You can run this example by downloading the replication.yaml file and applying it using:
```
  kubectl apply -f replication.yaml
```

Check the status of the ReplicationController with:

  kubectl describe replicationcontrollers/nginx

The output will show the desired replicas and their status.

Replica Sets

A ReplicaSet (RS) in Kubernetes is a crucial resource that ensures a stable set of running pods for a specific workload. Let’s dive into the details:

Purpose of a ReplicaSet:
- A ReplicaSet maintains a specified number of identical pods at all times.
- If a pod fails or is evicted, the ReplicaSet compensates by creating additional pods to maintain the desired count.
- It acts as a safety net, ensuring that your application remains available even if individual pods encounter issues.
How It Works:
- A ReplicaSet defines several fields:
  - Selector: Specifies how to identify pods it can acquire.
  - Replicas: Indicates the desired number of pods.
  - Pod Template: Describes the data for new pods created to meet the replica criteria.
- The ReplicaSet continuously monitors the state of its pods.
- When needed, it creates or deletes pods based on the template.
- Pods acquired by a ReplicaSet have their identifying information linked via the metadata.ownerReferences field.
When to Use a ReplicaSet:
- While you can directly use ReplicaSets, it’s recommended to use Deployments instead.
- Deployments manage ReplicaSets and provide declarative updates to pods.
- Unless you require custom update orchestration or don’t need updates at all, use Deployments.
- Define your application in the Deployment’s spec section.

Here’s an example of creating a ReplicaSet for a PHP-Redis guestbook application:

apiVersion: apps/v1
kind: ReplicaSet
metadata:
  name: frontend
spec:
  replicas: 3
  selector:
    matchLabels:
      tier: frontend
  template:
    metadata:
      labels:
        tier: frontend
    spec:
      containers:
        - name: php-redis
          image: gcr.io/google_samples/gb-frontend:v3

Apply this manifest using kubectl apply -f frontend.yaml. You’ll see the ReplicaSet and managed pods:

kubectl get rs
NAME       DESIRED   CURRENT   READY   AGE
frontend   3         3         3       6s

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Kubernetes Concepts - ReplicationController and Replica Sets

Table of contents

Replication Controller

Replica Sets