How to: Kubernetes Components <a name="components"></a> (Ref: Kubernetes.io)
- **Control Plane:** User enters commands and configuration files from control plane. It controls all cluster.
    - **API Server:** "It exposes the Kubernetes API. The API server is the front end for the Kubernetes control plane."
    - **Etcd:** "Consistent and highly-available key value store used as Kubernetes' backing store for all cluster data (meta data, objects, etc.)."
    - **Scheduler:** "It watches for newly created Pods with no assigned node, and selects a node for them to run on. 
        -  Factors taken into account for scheduling decisions include: 
            -  individual and collective resource requirements, 
            -  hardware/software/policy constraints, 
            -  affinity and anti-affinity specifications, 
            -  data locality, 
            -  inter-workload interference,
            -  deadlines."
    - **Controller Manager:** "It runs controller processes.
        - Logically, each controller is a separate process, but to reduce complexity, they are all compiled into a single binary and run in a single process.
        - Some types of these controllers are:
            - Node controller: Responsible for noticing and responding when nodes go down.
            - Job controller: Watches for Job objects that represent one-off tasks, then creates Pods to run those tasks to completion.
            - Endpoints controller: Populates the Endpoints object (that is, joins Services & Pods).
            - Service Account & Token controllers: Create default accounts and API access tokens for new namespaces"
     - **Cloud Controller Manager:** "It embeds cloud-specific control logic. The cloud controller manager lets you link your cluster into your cloud provider's API, and separates out the components that interact with that cloud platform from components that only interact with your cluster. The cloud-controller-manager only runs controllers that are specific to your cloud provider
        -  The following controllers can have cloud provider dependencies:
            - Node controller: For checking the cloud provider to determine if a node has been deleted in the cloud after it stops responding
            - Route controller: For setting up routes in the underlying cloud infrastructure
            - Service controller: For creating, updating and deleting cloud provider load balancers."
- **Node:** "Node components run on every node, maintaining running pods and providing the Kubernetes runtime environment."
    - **Kubelet:** "An agent that runs on each node in the cluster. It makes sure that containers are running in a Pod. The kubelet takes a set of PodSpecs that are provided through various mechanisms and ensures that the containers described in those PodSpecs are running and healthy."
    - **Kube-proxy:** "It is a network proxy that runs on each node in your cluster, implementing part of the Kubernetes Service concept.
        - It maintains network rules on nodes. These network rules allow network communication to your Pods from network sessions inside or outside of your cluster.
        - It uses the operating system packet filtering layer if there is one and it's available. Otherwise, kube-proxy forwards the traffic itself."
    - **Container Runtime:** "The container runtime is the software that is responsible for running containers.
        -  Kubernetes supports several container runtimes: **Docker, containerd, CRI-O,** and any implementation of the Kubernetes CRI (Container Runtime Interface)"  

![image](https://user-images.githubusercontent.com/10358317/146250916-a9298521-526b-451a-9810-6813e4165db5.png)