API Server:

• All the incoming/outgoing communication with Kubernetes cluster happening through API Server.
• APIServer is a RESTful server.
• Workstations/Client uses Kubectl tool for communicating with the API Server.
• API Server expects the inputs in JSON format.
• All the communication with the Master node worker nodes are also happening through API Server.
• Scheduler/Controller Manager uses API Server for all the communications with the worker nodes.
• Authentication and authorization.

Etcd:

• This is the storage mechanism Kubernetes Master node has.
• All the details about the cluster configurations and deployments details will be stored in it.
• Stores in key-value format.
• Communicates with API Server only.
• Duties: Config management, Service discovery, Cordinating distributed work.
• With out this we can say Kubernetes cluster is literally dead.

Scheduler:

• Scheduler is responsible for scheduling works to the available pods.
• It intelligently distribute the work to different nodes based on the current work load of the nodes.
• It also consider contraints, available resources etc, before assigning a work.
• Scheduler will provide the self healing fecility on receiving request from controller manager.

Controller Manager:

• Controller manager controlls/monitors the state of each and everything in the cluster.
• It repeatedly checks the health of each nodes and pods.
• It used the watch mechansim provided by the API Server.
• It will communicate with Scheduler if anything went wrong.