Kubernetes is a popular platform for managing containerized applications. It offers various features and components to automate the deployment, scaling and management of your applications. However, understanding the differences between some of these components can be confusing for beginners. In this blog post, we will explore the difference between two essential components of Kubernetes: ReplicaSet and Deployment. We will also learn about their features, advantages, use cases and key differences.

What is a ReplicaSet?

A ReplicaSet is a Kubernetes object that ensures that a specified number of replicas of a pod are running at any given time. A pod is the smallest unit of deployment in Kubernetes, consisting of one or more containers that share resources and networks. A ReplicaSet provides basic scaling mechanisms for pods, such as creating new pods if any pod fails or deleting excess pods if the desired number is exceeded.

A ReplicaSet is defined with fields such as:

• selector: A label selector that specifies how to identify pods it can acquire

• replicas: The number of pods that should be running at any given time

• template: The pod template used to create new pods

Here is an example YAML file for a ReplicaSet:

apiVersion: apps/v1
kind: ReplicaSet
metadata:
  name: myapp-replicaset
  labels:
    app: myapp
spec:
  replicas: 3
  selector:
    matchLabels:
      app: myapp
  template:
    metadata:
      labels:
        app: myapp
    spec:
      containers:
      - name: myapp-container
        image: myapp:latest
        ports:
        - containerPort: 80

This ReplicaSet specifies that we want to run three replicas of the myapp container. The selector field selects the pods controlled by the ReplicaSet based on the app label. The template field specifies the pod template used to create new pods.

We can create the ReplicaSet using the kubectl apply command:

$ kubectl apply -f replicaset.yaml

The above command will create a ReplicaSet with three replicas and manage the lifecycle of the pods.

What is a Deployment?

A Deployment is a Kubernetes object that manages a set of identical pods, ensuring that a specified number of replicas of the pod are running at any given time. It provides a declarative approach to managing Kubernetes objects, allowing for automated rollouts and rollbacks of containerized applications.

Moreover, a Deployment manages the deployment of a new version of an application. It also helps us roll back to a previous version by creating a replica set and updating it with the new configuration. A replica set is a lower-level abstraction that provides basic scaling mechanisms for pods.

A Deployment is defined with fields such as:

• selector: A label selector that specifies how to identify pods it can acquire

• replicas: The number of pods that should be running at any given time

• template: The pod template used to create new pods

• strategy: The strategy used to replace old pods with new ones

Here is an example YAML file for a Deployment:

apiVersion: apps/v1
kind: Deployment
metadata:
  name: myapp-deployment
  labels:
    app: myapp
spec:
  replicas: 3
  selector:
    matchLabels:
      app: myapp
  template:
    metadata:
      labels:
        app: myapp
    spec:
      containers:
      - name: myapp-container
        image: myapp:v1
        ports:
        - containerPort: 80
  strategy:
    type: RollingUpdate
    rollingUpdate:
      maxUnavailable: 1
      maxSurge: 1

This Deployment specifies that we want to run three replicas of the myapp container with version v1. The selector field selects the pods controlled by the Deployment based on the app label. The template field specifies the pod template used to create new pods. The strategy field specifies the type and parameters of the rolling update strategy, which replaces old pods with new ones gradually.

We can create the Deployment using the kubectl apply command:

$ kubectl apply -f deployment.yaml

The above command will create a Deployment with three replicas and manage the lifecycle of the pods.

If we need to update the application to a new version, we can change the image field in the Deployment YAML file:

...
spec:
  containers:
  - name: myapp-container
    image: myapp:v2 # change from v1 to v2 
    ports:
    - containerPort: 80
...

Then, we can apply the changes using the kubectl apply command:

$ kubectl apply -f deployment.yaml

The above command will create a new ReplicaSet with the new configuration and update the Deployment to use it. The Deployment will then perform a rolling update, replacing old pods with new ones one by one, until all pods are updated.

We can also roll back to a previous version of the application by using the kubectl rollout undo command:

$ kubectl rollout undo deployment/myapp-deployment

The above command will revert the Deployment to the previous ReplicaSet and perform a rolling update to restore the old pods.

What are the differences between ReplicaSet and Deployment?

Now that we have learned about ReplicaSet and Deployment, let’s summarize their key differences:

• Abstraction level: A Deployment is a higher-level abstraction that manages ReplicaSets and provides declarative updates to pods along with other useful features. A ReplicaSet is a lower-level abstraction that provides basic scaling mechanisms for pods.

• Update mechanism: A Deployment can perform automated rollouts and rollbacks of containerized applications, using different strategies such as rolling update, recreate, or blue-green. A ReplicaSet can only scale up or down the number of pods, but cannot update them with a new configuration.

• Versioning: A Deployment can keep track of the history and versions of the application, allowing us to roll back to a previous version if needed. A ReplicaSet does not have this capability and only maintains the current state of the pods.

• Use cases: A Deployment is recommended for most scenarios where we need to deploy and update containerized applications in a Kubernetes environment. A ReplicaSet is mainly used as a building block for higher-level abstractions such as Deployment, or for scenarios where we don’t need updates at all.

Conclusion

In this blog post, we have explored the difference between ReplicaSet and Deployment in Kubernetes. We have learned that they are both used to manage the lifecycle of pods, but they have different features, advantages, and use cases. We have also seen how to create and use them with YAML files and kubectl commands.

We hope that this blog post has helped you understand the difference between ReplicaSet and Deployment in Kubernetes. If you have any questions or feedback, please feel free to leave a comment below.

References:

• ReplicaSet | Kubernetes

• Deployments | Kubernetes