Table of contents
- What is a Replicaset?
- Which problem it solves?
- What is an object?
- What is a resource?
- The Anatomy of a ReplicaSet Manifest
- How to scale up/down?
- Scaling down
- Scaling up
- How to delete replicaset?
- How to delete replicaset without deleting pods
😃 In the journey of learning Kubernetes you come up to Replicasets, what does it mean? why we're using it? and where do we use it? I'll try to clear some of the doubts that come up to you as a newbie, Let's see what it is all about, and what's the role it plays inthe world of Kubernetes
A Replicaset simply means, it replicates pods, like you can have an n number of pods,1 replica means it controls one pod, under the hood of a replicaset there is a central important concept called labels, let's see in detail what it is all about and explore what exactly replicaset means, and its use cases...
which problem it solves?
It automatically replicates and it makes sure a certain number of pods run all the time as you mentioned in your manifest file.
Before we dive in we want to understand some terms
What is an object?
simply an object is a design model only we use for a specific purpose there are many objects in Kubernetes
What is a resource?
In simple terms, a resource is a collection of objects for that specific resource
- (pod) -> this is an object
- (pods) -> this is an endpoint, in this case, it stores the collection of pods
Let's see what are the different resources available, there is a command for this
kubectl api-resources
NAME SHORTNAMES APIVERSION NAMESPACED KIND
bindings v1 true Binding
componentstatuses cs v1 false ComponentStatus
configmaps cm v1 true ConfigMap
endpoints ep v1 true Endpoints
events ev v1 true Event
limitranges limits v1 true LimitRange
namespaces ns v1 false Namespace
nodes no v1 false Node
persistentvolumeclaims pvc v1 true PersistentVolumeClaim
persistentvolumes pv v1 false PersistentVolume
pods po v1 true Pod
podtemplates v1 true PodTemplate
replicationcontrollers rc v1 true ReplicationController
resourcequotas quota v1 true ResourceQuota
secrets v1 true Secret
serviceaccounts sa v1 true ServiceAccount
services svc v1 true Service
mutatingwebhookconfigurations admissionregistration.k8s.io/v1 false MutatingWebhookConfiguration
validatingwebhookconfigurations admissionregistration.k8s.io/v1 false ValidatingWebhookConfiguration
customresourcedefinitions crd,crds apiextensions.k8s.io/v1 false CustomResourceDefinition
apiservices apiregistration.k8s.io/v1 false APIService
controllerrevisions apps/v1 true ControllerRevision
daemonsets ds apps/v1 true DaemonSet
deployments deploy apps/v1 true Deployment
replicasets rs apps/v1 true ReplicaSet
statefulsets sts apps/v1 true StatefulSet
tokenreviews authentication.k8s.io/v1 false TokenReview
localsubjectaccessreviews authorization.k8s.io/v1 true LocalSubjectAccessReview
selfsubjectaccessreviews authorization.k8s.io/v1 false SelfSubjectAccessReview
selfsubjectrulesreviews authorization.k8s.io/v1 false SelfSubjectRulesReview
subjectaccessreviews authorization.k8s.io/v1 false SubjectAccessReview
horizontalpodautoscalers hpa autoscaling/v2 true HorizontalPodAutoscaler
cronjobs cj batch/v1 true CronJob
jobs batch/v1 true Job
certificatesigningrequests csr certificates.k8s.io/v1 false CertificateSigningRequest
leases coordination.k8s.io/v1 true Lease
bgpconfigurations crd.projectcalico.org/v1 false BGPConfiguration
bgppeers crd.projectcalico.org/v1 false BGPPeer
blockaffinities crd.projectcalico.org/v1 false BlockAffinity
caliconodestatuses crd.projectcalico.org/v1 false CalicoNodeStatus
clusterinformations crd.projectcalico.org/v1 false ClusterInformation
felixconfigurations
These are some of the resources and types of objects you can see
- Now let's understand the replica set with an example
apiVersion: apps/v1
kind: ReplicaSet
metadata:
name: frontend
labels:
app: guestbook
tier: frontend
spec:
# modify replicas according to your case
replicas: 4
selector:
matchLabels:
tier: frontend
template:
metadata:
labels:
tier: frontend
spec:
containers:
- name: php-redis
image: gcr.io/google_samples/gb-frontend:v3
The Anatomy of a ReplicaSet Manifest
Let's understand what is going inside (step by step)
apiVersion: apps/v1
kind: ReplicaSet
metadata:
name: frontend
labels:
app: guestbook
tier: frontend
There are some resources you want to mention always in you're manifest file such as apiVersion, kind, metadata, spec
apiVersion: this simply means which API version you're using to create the object
kind: This just means, what kind of object you're creating, in this case, it is Replicaset
plz note: what you're providing as an object, that is the desired state and that is what you're workload is going to be
metadata: this is a piece of additional information to identify you're workload, in this case, your workload is a replica set
metadata.name(name: frontend) -> you object name in the cluster
metadata.labels(labels app: guestbook tier: frontend) -> this is like a tag to the replicaset we can see details of the replicaset by using those labels
labels are most important in Replicaset more info coming below
spec:
# modify replicas according to your case
replicas: 4
selector:
matchLabels:
tier: frontend
template:
metadata:
labels:
tier: frontend
spec: refers to specs of replicaset
replicas: how many pods(Instances) want to be created by the replica set
selector: here selector is the thing, the replicaset takes it, and checks the same label to identify what are the pods it wants to control.
you want to make sure (selector.matchLabels.tier: frontend)-> this tier: frontend
taken as a reference by replicaset to check the pod == (template.metadata.labels.tier: frontend)
-> this tag is attached to each pod, this just wants to be the same, then only rs knows what
are the pods it wants to checkout.
template: above we mentioned 4 replicas which means the same number of pods are created by the replica set, when pods are made this is taken as reference by the replica set to create new pods
don't think replicaset means just creating replicas, that's not the main view, it replicates or re-create pods if anyone one of them dies, but how replicaset is identifying its designated pods
just by matching the
selector.tier tag to the template.tier tag
How to scale up/down?
There are 2 ways to scale up or scale down the pods, one is modifying the manifest another is just by giving a command:
Scaling down this is how you do it
$ kubectl get rs
NAME DESIRED CURRENT READY AGE
frontend 4 4 4 19s
$ kubectl scale --replicas=2 rs frontend
replicaset.apps/frontend scaled
$ kubectl get rs
NAME DESIRED CURRENT READY AGE
frontend 2 2 2 23s
Scaling up this is how you do it
$ kubectl get rs
NAME DESIRED CURRENT READY AGE
frontend 2 2 0 32s
$ kubectl scale --replicas=6 rs frontend
replicaset.apps/frontend scaled
$ kubectl get rs
NAME DESIRED CURRENT READY AGE
frontend 6 6 6 45s
Removing replicaset from the cluster
How to delete replicaset?
kubectl delete rs frontend
How to delete replicaset without deleting pods?
kubectl delete rs frontend --cascade=orphan
I hope you find this helpful for your journey, Thank you for reading my blog. If you like my work feel free to connect on LinkedIn or Twitter, see you with another one guys. 😀
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