pelicin › devops › Kubernetes

Replica Sets

You don't usually create ReplicaSet directly. Instead, you would normally create them using Deployments.

ReplicaSet ensures there are a certain number of pod-replicas (that match the given pod-label selectors) running in the cluster at any given time.

If there are too many pods, ReplicaSet will delete the excess pods; if there are too few, ReplicaSet will create new ones using the template.

If you change or add a pod's label when ReplicaSet is in effect: the number of matched pods may differ, and thus, pods will be deleted or created to accomodate.

Relationship with ReplicationController

ReplicationController was the "older version" of ReplicaSet. Its selector capability is much limited (could only use the equality operator).

You should use ReplicaSet going forward.

ReplicaSet descriptor

A sample ReplicaSet looks like the following. It mainly contains of 3 parts:

  • replicas — how many replica of pods
  • selector — pod label selectors
  • template — how to create new pods
# Note that this is not part of the core 'v1' API
# So you would use "apiVersion: apps/v1"
apiVersion: apps/v1
kind: ReplicaSet
metadata:
  name: kubia
spec:
  replicas: 3
  selector:
    matchLabels:
      app: kubia
  template:
    metadata:
      labels:
        app: kubia
    spec:
      containers:
        - name: kubia
          image: luksa/kubia

Type of selectors

You can use matchLabels and/or matchExpressions. If you specify both, it will select pods matching ALL the selectors.

matchLabels

This will match pods whose app label value is kubia:

# Under .spec.selector:
matchLabels:
  app: kubia

matchExpressions

Specify an array of expressions (it will select pods matching all the expressions):

  • key — the label's key
  • operator — could be In, NotIn, Exists, or DoesNotExist
  • values — required if you use the In or NotIn operator
# Under .spec.selector:
matchExpressions:
  - key: app
    operator: In
    values:
      - kubia
      - kubiu

Working with ReplicaSets

ReplicaSet's abbreviation is rs.

Creating a ReplicaSet

kubectl create -f <filename>

Listing ReplicaSets

kubectl get rs
# NAME      DESIRED   CURRENT   READY     AGE
# kubia     3         3         3         3s

Describing a ReplicaSet

kubectl describe rs <replicaSetName>
# Name:           kubia
# Namespace:      default
# Selector:       app=kubia
# Labels:         app=kubia
# Annotations:    <none>
# Replicas:       3 current / 3 desired
# Pods Status:    3 Running / 0 Waiting / 0 Succeeded / 0 Failed
# Pod Template:
#   Labels:       app=kubia
#   Containers:   ...
#   Volumes:      <none>
# Events:         <none>

Deleting a ReplicaSet

Deleting a ReplicaSet will also delete all the matching pods.
kubectl delete rs <replicaSetName>

References