• Kashomon
  • bprashanth
  • madhusudancs title: Replica Sets

  • TOC {:toc}

What is a ReplicaSet?

ReplicaSet is the next-generation Replication Controller. The only difference between a ReplicaSet and a Replication Controller right now is the selector support. ReplicaSet supports the new set-based selector requirements as described in the labels user guide whereas a Replication Controller only supports equality-based selector requirements.

Most kubectl commands that support Replication Controllers also support ReplicaSets. One exception is the rolling-update command. If you want the rolling update functionality please consider using Deployments instead. Also, the rolling-update command is imperative whereas Deployments are declarative, so we recommend using Deployments through the rollout command.

While ReplicaSets can be used independently, today it's mainly used by Deployments as a mechanism to orchestrate pod creation, deletion and updates. When you use Deployments you don't have to worry about managing the ReplicaSets that they create. Deployments own and manage their ReplicaSets.

When to use a ReplicaSet?

A ReplicaSet ensures that a specified number of pod “replicas” are running at any given time. However, a Deployment is a higher-level concept that manages ReplicaSets and provides declarative updates to pods along with a lot of other useful features. Therefore, we recommend using Deployments instead of directly using ReplicaSets, unless you require custom update orchestration or don't require updates at all.

This actually means that you may never need to manipulate ReplicaSet objects: use directly a Deployment and define your application in the spec section.


{% include code.html language="yaml" file="frontend.yaml" ghlink="/docs/concepts/workloads/controllers/frontend.yaml" %}

Saving this config into frontend.yaml and submitting it to a Kubernetes cluster should create the defined ReplicaSet and the pods that it manages.

$ kubectl create -f frontend.yaml
replicaset "frontend" created
$ kubectl describe rs/frontend
Name:          frontend
Namespace:     default
Image(s):      gcr.io/google_samples/gb-frontend:v3
Selector:      tier=frontend,tier in (frontend)
Labels:        app=guestbook,tier=frontend
Replicas:      3 current / 3 desired
Pods Status:   3 Running / 0 Waiting / 0 Succeeded / 0 Failed
No volumes.
  FirstSeen    LastSeen    Count    From                SubobjectPath    Type        Reason            Message
  ---------    --------    -----    ----                -------------    --------    ------            -------
  1m           1m          1        {replicaset-controller }             Normal      SuccessfulCreate  Created pod: frontend-qhloh
  1m           1m          1        {replicaset-controller }             Normal      SuccessfulCreate  Created pod: frontend-dnjpy
  1m           1m          1        {replicaset-controller }             Normal      SuccessfulCreate  Created pod: frontend-9si5l
$ kubectl get pods
frontend-9si5l   1/1       Running   0          1m
frontend-dnjpy   1/1       Running   0          1m
frontend-qhloh   1/1       Running   0          1m

ReplicaSet as an Horizontal Pod Autoscaler target

A ReplicaSet can also be a target for Horizontal Pod Autoscalers (HPA), i.e. a ReplicaSet can be auto-scaled by an HPA. Here is an example HPA targeting the ReplicaSet we created in the previous example.

{% include code.html language="yaml" file="hpa-rs.yaml" ghlink="/docs/concepts/workloads/controllers/hpa-rs.yaml" %}

Saving this config into hpa-rs.yaml and submitting it to a Kubernetes cluster should create the defined HPA that autoscales the target ReplicaSet depending on the CPU usage of the replicated pods.

kubectl create -f hpa-rs.yaml

Alternatively, you can just use the kubectl autoscale command to accomplish the same (and it's easier!)

kubectl autoscale rs frontend