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What is a Replica Set?
Replica Set is the next-generation Replication Controller. The only difference between a Replica Set and a Replication Controller right now is the selector support. Replica Set 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 Replica Sets. 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 Replica Sets 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 Replica Sets that they create. Deployments own and manage their Replica Sets.
When to use a Replica Set?
A Replica Set ensures that a specified number of pod “replicas” are running at any given time. However, a Deployment is a higher-level concept that manages Replica Sets and provides declarative updates to pods along with a lot of other useful features. Therefore, we recommend using Deployments instead of directly using Replica Sets, unless you require custom update orchestration or don't require updates at all.
This actually means that you may never need to manipulate Replica Set objects: use directly a Deployment and define your application in the spec section.
Example
{% include code.html language="yaml" file="replicasets/frontend.yaml" ghlink="/docs/user-guide/replicasets/frontend.yaml" %}
Saving this config into frontend.yaml and submitting it to a Kubernetes cluster should
create the defined Replica Set 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.
Events:
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
NAME READY STATUS RESTARTS AGE
frontend-9si5l 1/1 Running 0 1m
frontend-dnjpy 1/1 Running 0 1m
frontend-qhloh 1/1 Running 0 1m
Replica Set as an Horizontal Pod Autoscaler target
A Replica Set can also be a target for Horizontal Pod Autoscalers (HPA), i.e. a Replica Set can be auto-scaled by an HPA. Here is an example HPA targeting the Replica Set we created in the previous example.
{% include code.html language="yaml" file="replicasets/hpa-rs.yaml" ghlink="/docs/user-guide/replicasets/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 Replica Set 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 acommplish the same
(and it's easier!)
kubectl autoscale rs frontend