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K8s Newbie Series ReplicaSet Resources

Popularity：749 ℃/2025-05-04 20:12:29

Overview

Official website address:/zh-cn/docs/concepts/workloads/controllers/replicaset/

ReplicaSetAbbreviationrs, its main function andReplicationControllerLikewise, ensure that a certain number of pods run normally, it will continuously monitor the running status of these pods. Once the pod fails, it will be restarted or rebuilt. It also supports scaling and mirrored versions of pods.

Compared with ReplicationController, ReplicaSet is lighter and more powerful, and supports regular matching pods.

If you want to learn ReplicationController, you can view this article：ReplicationController resources for K8s newbies series

The role of ReplicaSet

  • Replica Management: The core function of ReplicaSet is to ensure that there are a specified number of Pod replicas running at any time. If a pod terminates unexpectedly, ReplicaSet will automatically create a new pod to replace it; if there are too many pods, it will delete the excess pods.

  • Cluster expansion and reduction: Users can easily expand or reduce applications by modifying the number of replicas in the ReplicaSet.

  • Flexible tag selector

    • Collection selector: Unlike ReplicationController that only supports equation selectors, ReplicaSet supports more flexible collection selectors, such as in, notin, exists, etc. This allows ReplicaSet to select the pods to be managed more accurately, improving the flexibility and expressiveness of tag selection.

    • Dynamic Match Pods: Through the tag selector, ReplicaSet can dynamically match Pods that meet the criteria. Even after the ReplicaSet is created, these Pods will be included in the management scope of the ReplicaSet as long as new Pods are created and their labels are in line with the ReplicaSet selector.

Detailed explanation of ReplicaSet resource configuration file

Can be passedkubectl explain rsView the fields required to create a ReplicaSet

Example:

[root@node01 ~]# kubectl explain rs
 KIND: ReplicaSet
 VERSION: apps/v1

 DESCRIPTION:
      ReplicaSet ensures that a specified number of pod replicas are running at
      any given time.

 # The content in the fileds field has been intercepted
 FIELDS:
    apiVersion <string>
     
    kind <string>
 
    metadata <Object>
 
    spec <Object>
 
    status <Object>

Through the above, we can find that the resource file that defines ReplicaSet is the same as the Pod, and also requires fields such as apiVersion, kind, metadata, spec, etc.

However, there are three fields in the spec field, which should be noted, namely replicas, selector, and template. as follows:

[root@node01 ~]# kubectl explain
 KIND: ReplicaSet
 VERSION: apps/v1
 
 RESOURCE: spec <Object>
 
 DESCRIPTION:
      Spec defines the specification of the desired behavior of the ReplicaSet.
      More info:
      https://git./community/contributors/devel/sig-architecture/#spec-and-status

      ReplicaSetSpec is the specification of a ReplicaSet.

 FIELDS:
    minReadySeconds <integer>
    # minReadySeconds specifies a time threshold that is considered available only if the time when the Pod remains ready reaches or exceeds this threshold, and no containers within the Pod crash during this period.
   
    replicas <integer>
    # Define the number of copies of the Pod
   
    selector <map[string]string>
    # Tag selector, here specifies the tag defined by Pod
   
    template <Object>
    # Define Pod templates

Create a ReplicaSet

Here we create three Pod copies
Example:

# Define resource files
 [root@node01 ~/rs]# cat
 apiVersion: apps/v1
 kind: ReplicaSet
 metadata:
   name: rs-nginx-1
   namespace: default
   labels:
     app: rs-nginx
 spec:
   # Define the number of copies of the Pod
   replicas: 3
   # Tag selector, here is consistent with Pod's Label
   selector:
     # Match the Pod's Label
     matchLabels:
       app: nginx
   # Define the Pod template, just define two fields: metadata and spec
   template:
     metadata:
       name: pod-nginx
       labels:
         app: nginx
     spec:
       restartPolicy: Always
       containers:
       - name: nginx
         image: nginx:latest

 # Create a ReplicaSet
 [root@node01 ~/rs]# kubectl apply -f

View the corresponding ReplicaSet and Pod

View ReplicaSet

[root@node01 ~/rs]# kubectl get rs
NAME         DESIRED   CURRENT   READY   AGE
rs-nginx-1   3         3         3       93s

View Pod

[root@node01 ~/rs]# kubectl get po | grep rs-nginx
rs-nginx-1-6rb9p       1/1     Running            0               2m
rs-nginx-1-ssdhv       1/1     Running            0               2m
rs-nginx-1-vzn2x       1/1     Running            0               2m

Verify Replica Management of ReplicaSet

The core function of ReplicaSet is to ensure that there are a specified number of Pod replicas running at any time. If a pod terminates unexpectedly, ReplicaSet will automatically create a new pod to replace it; if there are too many pods, it will delete the excess pods.

What happens to Replicaset when we delete the created Pod?

# Delete all pods created above
 [root@node01 ~/rs]# kubectl delete po rs-nginx-1-6rb9p rs-nginx-1-ssdhv rs-nginx-1-vzn2x
 pod "rs-nginx-1-6rb9p" deleted
 pod "rs-nginx-1-ssdhv" deleted
 pod "rs-nginx-1-vzn2x" deleted

 # Check rs and find that the number of copies is still 3
 [root@node01 ~/rs]# kubectl get rs
 NAME DESIRED CURRENT READY AGE
 rs-nginx-1 3 3 3 5m17s

 # Check the pod and find out that there are 3 corresponding pods. Through the AGE field, it is found that it is newly created.
 [root@node01 ~/rs]# kubectl get po | grep rs-nginx
 NAME READY STATUS RESTARTS AGE
 rs-nginx-1-6h7fv 1/1 Running 0 22s
 rs-nginx-1-fhkm9 1/1 Running 0 22s
 rs-nginx-1-l5njb 1/1 Running 0 22s

Verify the scaling capacity of Replicaset

Verify the capacity expansion

Let's adjust the number of copies of rs to 5 to see what happens?

[root@node01 ~/rs]# cat
 apiVersion: apps/v1
 kind: ReplicaSet
 metadata:
   name: rs-nginx-1
   namespace: default
   labels:
     app: rs-nginx
 spec:
   # Change the number of copies to 5
   replicas: 5
   selector:
     matchLabels:
       app: nginx
   template:
     metadata:
       name: pod-nginx
       labels:
         app: nginx
     spec:
       restartPolicy: Always
       containers:
       - name: nginx
         image: nginx:latest

 # Reapply it
 [root@node01 ~/rs]# kubectl apply -f
 /rs-nginx-1 configured

Check out the changes in the Pod:

[root@node01 ~/rs]# kubectl get po | grep rs-nginx
NAME                   READY   STATUS             RESTARTS          AGE
rs-nginx-1-6h7fv       1/1     Running            0                4m17s
rs-nginx-1-fhkm9       1/1     Running            0                4m26s
rs-nginx-1-l5njb       1/1     Running            0                4m26s
rs-nginx-1-t75z7       1/1     Running            0                8s
rs-nginx-1-tdvbr       1/1     Running            0                8s

Through the above, it was found that the number of pod copies changed from 3 to 5. Looking at the AGE field, I found that there are two pods in a newly created state.

Verify the size reduction

Similarly, we adjust the number of copies to 1 to see what happens?

[root@node01 ~/rs]# cat
 apiVersion: apps/v1
 kind: ReplicaSet
 metadata:
   name: rs-nginx-1
   namespace: default
   labels:
     app: rs-nginx
 spec:
   # Adjust the number of pods to 1
   replicas: 1
   selector:
     matchLabels:
       app: nginx
   template:
     metadata:
       name: pod-nginx
       labels:
         app: nginx
     spec:
       restartPolicy: Always
       containers:
       - name: nginx
         image: nginx:latest

 # Reapply it
 [root@node01 ~/rs]# kubectl apply -f
 /rs-nginx-1 configured

Check it out:

# View rs
 [root@node01 ~/rs]# kubectl get rs
 NAME DESIRED CURRENT READY AGE
 rs-nginx-1 1 1 1 11m
 # View Pod
 [root@node01 ~/rs]# kubectl get po | grep rs-nginx
 rs-nginx-1-fhkm9 1/1 Running 0 6m33s

Through the above findings, the number of Pods has been reduced from 5 to 1

Replicaset's tag selector

Replicaset's tag selector is divided into two categories: one is the equation selector and the other is the set selector.

Equation selector

The equation selector filters labels by =, == (the two have the same meanings) or !=. It passesTo achieve it.

Example:

apiVersion: apps/v1
 kind: ReplicaSet
 metadata:
   name: rs-nginx-1
   namespace: default
   labels:
     app: rs-nginx
 spec:
   replicas: 1
   selector:
   # Equation selector
     matchLabels:
       app: nginx
   template:
     metadata:
       name: pod-nginx
       labels:
         app: nginx
     spec:
       restartPolicy: Always
       containers:
       - name: nginx
         image: nginx:latest

Collection selector

The collection selector uses in, notin, exists, and DoesNotExist to filter tags.

  • in: Used to select the pod with the label value in the specified set.
  • notin: Used to select Pods whose label value is not in the specified set.
  • exists: Used to select a Pod with the specified tag, regardless of the value of the tag.
  • DoesNotExist: Used to select Pods that do not have the specified tag, regardless of the value of the tag.

It passesTo achieve

Example:

apiVersion: apps/v1
 kind: ReplicaSet
 metadata:
   name: nginx-replicaset-set
 spec:
   replicas: 3
   selector:
    # Collection selector
     matchExpressions:
         # key: specify the key of the Label, operator: represents the operator, value: represents the value of the Label
       - {key: app, operator: In, values: [nginx]}
       - {key: env, operator: NotIn, values: [development, testing]}
   template:
     metadata:
       labels:
         app: nginx
         env: production
     spec:
       containers:
       - name: nginx
         image: nginx:1.14.2
         Ports:
         - containerPort: 80

Manage Replicaset

View Replicaset

grammar:

kubectl get rs <rs-name> -n <namespace-name>

Example:

[root@node01 ~/rs]# kubectl get rs
NAME         DESIRED   CURRENT   READY   AGE
rs-nginx-1   1         1         1       25m

Modify Replicaset

In two ways, one is throughkubectl edit rc <rc-name>To modify it, a similarvim, just modify its corresponding value, and finallywqSave to apply your configuration.
Another way is to modify the corresponding resource file and finally use itkubectl apply -f <file>Just do it.

Delete Replicaset

grammar:

kubectl delete rc <rc-name> -n <namespace-name>
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