why we need kube-controller-manager in Kubernetes (k8s)

The Kubernetes kube-controller-manager is a critical component responsible for managing various controllers that regulate the state of the cluster. It runs multiple controller processes, each handling different aspects of the cluster's desired state. Here’s why kube-controller-manager is essential in Kubernetes:

Key Responsibilities of kube-controller-manager:

1. Node Controller:

   • Node Lifecycle: Monitors the status of nodes in the cluster (e.g., node addition, removal, and status changes).
   • Node Health: Detects and responds to node failures by initiating actions such as rescheduling Pods to healthy nodes.

2. Replication Controller:

   • Pod Replication: Ensures that the specified number of Pods for each ReplicationController, ReplicaSet, or Deployment is running at all times.
   • Scaling: Scales the number of Pods up or down based on defined replication settings.

3. Endpoints Controller:

   • Service Endpoints: Updates the endpoints object in the API server when a Service selector changes.
   • Service Discovery: Ensures that network traffic is routed to available Pods matching a Service.

4. Service Account & Token Controllers:

   • Security: Manages Service Accounts for Pods and generates tokens required for secure API access.
   • Automates: Automates the creation and deletion of Service Account tokens.

5. Persistent Volume Controller:

   • Volume Management: Manages the lifecycle of PersistentVolumes and PersistentVolumeClaims.
   • Provisioning: Dynamically provisions PersistentVolumes based on StorageClass specifications.

6. Namespace Controller:

   • Namespace Lifecycle: Manages the lifecycle of namespaces within the cluster.
   • Access Control: Enforces namespace-based access controls and resource quotas.

7. DaemonSet Controller:

   • DaemonSet Management: Ensures that a specified Pod runs on all or a subset of nodes in the cluster.
   • Node-Level Services: Useful for running node-level agents or services (e.g., monitoring agents, log collectors).

8. Job Controller:

   • Job Management: Ensures the successful completion of batch jobs and maintains the desired number of completed job instances.

Example Scenario:

Consider a scenario where a Deployment needs to scale up due to increased traffic:

1. Deployment Scale-Up:

   • The kube-controller-manager monitors the Deployment's desired state and observed state.
   • Upon detecting that more replicas are needed (e.g., due to increased demand or failure recovery), it updates the desired number of replicas.

2. ReplicaSet Creation:

   • If the Deployment uses a ReplicaSet, the kube-controller-manager creates or updates the ReplicaSet specification to match the desired number of Pods.

3. Pod Creation:

   • The kube-scheduler schedules new Pods to nodes based on the updated Deployment or ReplicaSet specification.

4. Monitoring and Scaling:

   • The kube-controller-manager continues to monitor the state of the cluster and ensures that the actual state matches the desired state.
   • If a node fails or the load increases, it may trigger rescheduling of Pods, creation of new Pods, or scaling of existing deployments to maintain availability and performance.