API Server (kube-apiserver)

• API server exposes a REST interface to the Kubernetes cluster. It is the front end for the Kubernetes control plane.
• All operations against pods, services, and so forth, are executed programmatically by communicating with the endpoints provided by it.
• It tracks the state of all cluster components and manages the interaction between them.
• It is designed to scale horizontally.
• It consumes YAML/JSON manifest files.
• It validates and processes the requests made via API.

etcd (key-value store)

• Etcd is a consistent, distributed, and highly-available key-value store.
• is stateful, persistent storage that stores all of Kubernetes cluster data (cluster state and config).
• is the source of truth for the cluster.
• can be part of the control plane, or, it can be configured externally.
• ETCD benefits include
  • Fully replicated: Every node in an etcd cluster has access to the full data store.
  • Highly available: etcd is designed to have no single point of failure and gracefully tolerate hardware failures and network partitions.
  • Reliably consistent: Every data ‘read’ returns the latest data ‘write’ across all clusters.
  • Fast: etcd has been benchmarked at 10,000 writes per second.
  • Secure: etcd supports automatic Transport Layer Security (TLS) and optional secure socket layer (SSL) client certificate authentication.
  • Simple: Any application, from simple web apps to highly complex container orchestration engines such as Kubernetes, can read or write data to etcd using standard HTTP/JSON tools.

Scheduler (kube-scheduler)

• The scheduler is responsible for assigning work to the various nodes. It keeps watch over the resource capacity and ensures that a worker node’s performance is within an appropriate threshold.
• It schedules pods to worker nodes.
• It watches api-server for newly created Pods with no assigned node, and selects a healthy node for them to run on.
• If there are no suitable nodes, the pods are put in a pending state until such a healthy node appears.
• It watches API Server for new work tasks.
• 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 and taints.

Controller Manager (kube-controller-manager)

• Controller manager is responsible for making sure that the shared state of the cluster is operating as expected.
• It watches the desired state of the objects it manages and watches their current state through the API server.
• It takes corrective steps to make sure that the current state is the same as the desired state.
• It is a controller of controllers.
• 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 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

• The cloud controller manager integrates with the underlying cloud technologies in your cluster when the cluster is running in a cloud environment.
• The cloud-controller-manager only runs controllers that are specific to your cloud provider.
• Cloud controller lets you link your cluster into cloud provider’s API, and separates out the components that interact with that cloud platform from components that only interact with your cluster.
• 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.