What Is ReplicationController In Kubernetes
Hello Everyone
Welcome to CloudAffaire and this is Debjeet.
In the last blog post, we have discussed pod in Kubernetes.
https://cloudaffaire.com/what-is-pod-in-kubernetes/
In this blog post, we will discuss ReplicationController in Kubernetes. In real world sceanrios we hardly deploy a pod directly. Instead pods are deployed using pod controller. ReplicationController is one of the pod controller which ensures that a specified number of pod replicas are running at any one time.
What Is ReplicationController?
A ReplicationController ensures that a specified number of pod replicas are running at any one time. In other words, a ReplicationController makes sure that a pod or a homogeneous set of pods is always up and available.
Note: ReplicationController is replaced by the ReplicaSet controller.
How a ReplicationController Works?
If there are too many pods, the ReplicationController terminates the extra pods. If there are too few, the ReplicationController starts more pods. Unlike manually created pods, the pods maintained by a ReplicationController are automatically replaced if they fail, are deleted, or are terminated. For example, your pods are re-created on a node after disruptive maintenance such as a kernel upgrade. For this reason, you should use a ReplicationController even if your application requires only a single pod. A ReplicationController is similar to a process supervisor, but instead of supervising individual processes on a single node, the ReplicationController supervises multiple pods across multiple nodes.
ReplicationController is often abbreviated to “rc” in discussion, and as a shortcut in kubectl commands. A simple case is to create one ReplicationController object to reliably run one instance of a Pod indefinitely. A more complex use case is to run several identical replicas of a replicated service, such as web servers.
ReplicationController manifest file syntax:
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## --------------------------------- ## ReplicationController YAML syntax ## --------------------------------- apiVersion: v1 kind: ReplicationController metadata: <replicationcontrollerMetadata> spec: <replicationcontrollerSpec> |
replicationcontrollerMetadata attributes:
- annotations: Key value map stored with a resource for arbitrary metadata.
- clusterName: The name of the cluster which the object belongs to.
- creationTimestamp: Timestamp representing the server time when this object was created.
- deletionGracePeriodSeconds: Seconds allowed for gracefully terminate before forcefully removed from the system.
- deletionTimestamp: RFC 3339 date and time at which this resource will be deleted. Is not directly settable by a client.
- finalizers: Must be empty before the object is deleted from the registry.
- generateName: Optional prefix, used by the server, to generate a unique name ONLY IF the Name field has not been provided.
- generation: A sequence number representing a specific generation of the desired state. Populated by the system. Read-only.
- initializers: An initializer is a controller which enforces some system invariant at object creation time.
- labels: Map of string keys and values that can be used to organize and categorize (scope and select) objects.
- name: Name must be unique within a namespace.
- namespace: Namespace defines the space within each name must be unique, default namespace is default.
- ownerReferences: List of objects depended by this object.
- resourceVersion: An opaque value that represents the internal version of this object used to determine when objects have changed.
- selfLink: SelfLink is a URL representing this object. Populated by the system. Read-only.
- uid: UID is the unique in time and space value for this object
replicationcontrollerSpec attributes:
- minReadySeconds: Minimum number of seconds for which a newly created pod should be ready without any of its container crashing, for it to be considered available. Defaults to 0
- replicas: Replicas is the number of desired replicas. This is a pointer to distinguish between explicit zero and unspecified. Defaults to 1.
- selector: Selector is a label query over pods that should match the Replicas count. If Selector is empty, it is defaulted to the labels present on the Pod template. Label keys and values that must match in order to be controlled by this replication controller, if empty defaulted to labels on Pod template.
- template: Template is the object that describes the pod that will be created if insufficient replicas are detected.
ReplicationController Demo:
Step 1: Create the replication controller.
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######################################### ## ReplicationController In Kubernetes ## ######################################### ## Prerequisites ## One Kubernetes Cluster with at least one node ## You can install your own or use online versions using below links ## https://cloudaffaire.com/how-to-install-kubernetes-cluster-in-linux/ ## https://www.katacoda.com/courses/kubernetes/playground ## http://labs.play-with-k8s.com/ ## ----------------------------- ## Create replication controller ## ----------------------------- ## Create the replicationcontroller manifest file mkdir myapp && vi myapp/myreplicationcontrollerv1.yaml -------------------- apiVersion: v1 kind: ReplicationController metadata: name: myreplicationcontroller labels: app: myapp spec: replicas: 2 selector: app: myapp template: metadata: labels: app: myapp spec: containers: - image: nginx name: mycontainer ports: - containerPort: 80 name: http --------------------- :wq ## Validate the yaml configuration file kubectl create --dry-run --validate -f myapp/myreplicationcontrollerv1.yaml ## Create the replication controller kubectl apply -f myapp/myreplicationcontrollerv1.yaml |
Step 2: View replication controller details.
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## ----------------------------------- ## View replication controller details ## ----------------------------------- ## View the replication controller kubectl get replicationcontroller myreplicationcontroller kubectl get rc ## abbreviation of replicationcontroller kubectl get rc -l app=myapp ##list pod by lebel kubectl get rc -o wide ##get detailed output ## View rc configuration kubectl get rc myreplicationcontroller -o yaml ## View rc details kubectl describe rc myreplicationcontroller ## View rc lebels kubectl get rc --show-labels ## View the pod kubectl get pod -l app=myapp -o wide ## NAME READY STATUS RESTARTS AGE IP NODE NOMINATED NODE READINESS GATES ## myreplicationcontroller-g7rp4 1/1 Running 0 5m25s 10.244.1.14 system3 <none> <none> ## myreplicationcontroller-vh79k 1/1 Running 0 5m25s 10.244.2.18 system2 <none> <none> |
Step 3: Scale up and scale down the number of replicas.
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## ------------------------------------------ ## Scale up and scale down number of replicas ## ------------------------------------------ ## Scale up number of pods to three kubectl scale rc myreplicationcontroller --replicas=3 ## View pods kubectl get pod -l app=myapp -o wide ## Scale down number of pods to one kubectl scale rc myreplicationcontroller --replicas=1 ## View pods kubectl get pod -l app=myapp -o wide ## Check the difference between configuration file and actual state kubectl diff -f myapp/myreplicationcontrollerv1.yaml ## Note the count of replica is 1 in actual but 2 in config file. ## Edit the rc to make consistent with manifest file ## vi editor will be opened, change replicas: 1 from 2 and save kubectl edit -f myapp/myreplicationcontrollerv1.yaml ## View pods kubectl get pod -l app=myapp -o wide |
Step 4: Delete a pod managed by replication controller.
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## --------------------------- ## Delete a pod managed by rc ## --------------------------- ## View pods kubectl get pod ## NAME READY STATUS RESTARTS AGE ## myreplicationcontroller-6cfsb 1/1 Running 0 112s ## myreplicationcontroller-g7rp4 1/1 Running 0 20m ## Delete a pod kubectl delete pod myreplicationcontroller-6cfsb #replace pod name ## Get the pod kubectl get pod ## NAME READY STATUS RESTARTS AGE ## myreplicationcontroller-78rth 1/1 Running 0 10s ## myreplicationcontroller-g7rp4 1/1 Running 0 21m ## One new pod has been autometically spinned up to make the replica count 2 |
Step 5: Make a connection to the pod managed by replication controller.
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## ------------------------------------------------- ## Connect to your pods under replication controller ## ------------------------------------------------- ## Expose your rc kubectl expose rc myreplicationcontroller --type=NodePort --port=80 ## Get pod ip address kubectl get pod -o wide ## NAME READY STATUS RESTARTS AGE IP NODE NOMINATED NODE READINESS GATES ## myreplicationcontroller-78rth 1/1 Running 0 2m37s 10.244.1.15 system3 <none> <none> ## myreplicationcontroller-g7rp4 1/1 Running 0 23m 10.244.1.14 system3 <none> <none> ## Call your pod uisng pod ip address (nginx web server) curl 10.244.1.15:80 curl 10.244.1.14:80 |
Step 6: Cleanup.
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## ------- ## Cleanup ## ------- ## delete the nodeport service kubectl delete service myreplicationcontroller ## delete the replication controller kubectl delete rc myreplicationcontroller ## delete myapp directory rm -rf myapp |
Hope you enjoyed this article. In real world sceanrios we hardly deploy a pod directly. Instead pods are deployed using pod controller. In the next blog post, we will discuss one of the pod controller replicaset.
To get more details on Kubernetes you can follow the below link.
https://kubernetes.io/docs/home/
To get more details on Docker you can follow the below links.
https://cloudaffaire.com/category/devops/docker/