What is ReplicaSet ?
ReplicaSet object is used to maintain a stable set of replicated pods running within a cluster at any given time. Its purpose is to maintain the specified number of pods and prevent the user from loosing the control of the application in case of any pod failure or inaccessible. Whenever any pod gets failed replicaset immediately launches another pod and replacing it with failed pod.
Replicaset has following features:
- A pod template is used to create a new pod whenever an existing pod fails and also replica count is also maintained by defining the desired number of replicas that a controller needs to be running.
- A replica set also ensures that additional pod needs to be created or deleted whenever instance with same label is created.
- Replcaset allows to have multiple replicas of pod which means that the traffic is sent to different instances which prevents single instance from being overloaded.
- Replcaset ensures it has multiple replicas of application so that it won't fail because of one pod fails.
Replicaset Configuration
Below is the yaml file which will create multi replicas of pod.
---
apiVersion: apps/v1
kind: ReplicaSet
metadata:
name: httpd-replica
namespace: facebook
spec:
replicas: 4
minReadySeconds: 10
selector:
matchLabels:
role: web
matchExpressions:
- {key: version, operator: In, values: [v1, v2, v3]}
template:
metadata:
name: web
labels:
role: web
version: v1
tier: frond
spec:
containers:
- name: web
image: httpd
ports:
- containerPort: 80
protocol: TCP
Fields that are use to define replicaset are as follows:
- Selector: It is used to identify for which pod the replicaset is responsible for.
- Replicas: It specifies the number of pods which replicaset has to maintain.
- Template: It defines the pod template that replicaset will use in order to create new pods.
- ApiVersion: It defines Kubernetes API that supports ReplicaSet.
What is Deployment ?
Deployment is use to maintain the versions of your application running in the pod. It helps to efficiently scale the number of replica pods and enable the rollout and rollback to an earlier deployment version. Deployments runs on multiple replicas of the pod and automatically replaces a pod in case of any failure or unresponsive. In this way Deployments helps to ensure that one or more instances of your application are available to serve user requests.
Deployments use a Pod template, which contains a specification for its Pods. The Pod specification determines how each Pod should look like: what applications should run inside its containers, which volumes the Pods should mount, its labels, and more. When a Deployment's Pod template is changed, new Pods are automatically created one at a time.
What are Rolling Update and Rollback in Deployment ?
Basically, Rolling update provides the orderly migration from one version to a newer version. Rolling update is used in such situation when a new version of application came and you have to switch to a newer version. In the rolling update all the running pod will be replaced with the newer version of application by systematically terminating the older version of application pods.
Rollback on the other hand is used to roll back to older version of your application. Suppose you have a new bug in the application that needs to be solved in that case you might need to go to the older version of the application with rollback you can achieve that. With rollback all the replicas of the pod which are running on the new version of the application will be rollback again to previous version.
Deployment with Replicaset
Deployments are generallyused with replicaset as they are used to manage replicsets. With the help of deployment You can simply roll back to a previous Deployment revision. When you are managing ReplicaSet using Deployment You can also use a Deployment to create a new revision of a ReplicaSet and then migrate existing pods from an older revision into the new revision. After that, the Deployment can take care of cleaning up old, unused ReplicaSets.
So, Replicaset ensure replicas of pods are available whereas deployment are reponsible for managing different versions of the application. Like deployment replicaset cant rollout or rollback to different version of application nor maintain any revisions for the same.
Deployemnt Configuration
Below is the yaml file for Deployment.
---
apiVersion: apps/v1
kind: Deployment
metadata:
name: nginx
namespace: facebook
spec:
strategy:
type: RollingUpdate
rollingUpdate:
maxSurge: 2
maxUnavailable: 1
revisionHistoryLimit: 10
paused: false
replicas: 3
minReadySeconds: 10
selector:
matchLabels:
role: webserver
jane: akil
matchExpressions:
- {key: version, operator: In, values: [v1, v2, v3]}
template:
metadata:
name: web
labels:
role: webserver
version: v3
tier: frond
jane: akil
spec:
containers:
- name: web
image: nginx:1.20-perl
ports:
- containerPort: 80
protocol: TCP
---
apiVersion: v1
kind: Service
metadata:
name: web-service
namespace: facebook
labels:
role: web-service
spec:
selector:
role: webserver
type: NodePort
ports:
- port: 80
nodePort: 32001
Key elements of above YAML file:
- We are creating deployment type file Strategy is which means if any changes is there it will apply those changes.
- The RollingUpdate will check if any request is server by the container after the request is being served it will apply the changes.
- Revision history limit means number of changes that can perform it will keep track of all revision in the form of revisions.
- MaxSurge means whenever a revision is applied it will create specified number of container according to the values is given it will apply it to the pod.
