ReplicationController is a Kubernetes resource that ensures a specified number of replica pods is always running. It’s responsible for maintaining the desired state of a replicated application by creating or deleting pods as necessary to match the desired replica count.
If we want to create a ReplicationController for an Apache container image to run three replicas of it on a cluster, we’ll define the kind as ReplicationController in the configuration YAML file. Additionally, we’ll specify the desired number of replicas, which is three, to ensure that there are always three instances of the Apache pod running. Then the ReplicationController will manage the lifecycle of these replicas, ensuring their availability and the desired state, as given in the following file.
apiVersion: v1kind: ReplicationControllermetadata:name: apachespec:replicas: 3selector:app: apachetemplate:metadata:name: apachelabels:app: apachespec:containers:- name: apacheimage: httpdports:- containerPort: 80
Line 2: We used the resource type of ReplicationController, ensuring the number of replica pods running.
Line 6: We specified the number of replica pods we wanted to apply to our cluster.
Line 17: We specified the container image as httpd, indicating that the Apache HTTP Server image will be used for the container.
To create the Kubernetes resources based on the configuration specified in the YAML file replicationController.yaml, we can execute the following command in the terminal after clicking the “Run” button in the provided widget. This will initiate the creation of the cluster.
kubectl apply -f replicationController.yaml
Once the configuration is applied in the Kubernetes cluster defined in the YAML file, ensuring the desired state of the application, we can verify the number of replicas by using the following command:
kubectl describe replicationcontroller/apache
The output of this command will display the current count of running pods and the desired number of pods we aim to have in the cluster.
We can try it out ourselves below.
apiVersion: v1
kind: ReplicationController
metadata:
name: apache
spec:
replicas: 3
selector:
app: apache
template:
metadata:
name: apache
labels:
app: apache
spec:
containers:
- name: apache
image: httpd
ports:
- containerPort: 80
Unlock your potential: Kubernetes Essentials series, all in one place!
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