Kubernetes is a powerful container orchestration system designed to manage containerized applications across clusters of machines. At the heart of Kubernetes are several key components that work together to ensure the cluster operates efficiently, providing automated deployment, scaling, and management of containerized applications.

Here’s an overview of Kubernetes core components, organized by the role they play in the system:

1. Master Components

The master components are responsible for the control plane, making decisions about the cluster, and maintaining the overall state of the system. These components run on the control plane nodes and are critical for the functioning of the Kubernetes cluster.

a) API Server (kube-apiserver)

• Role: The API server is the central component of the Kubernetes control plane and exposes the Kubernetes REST API. It acts as a gateway for all communication with the Kubernetes cluster, and all interactions with the cluster (e.g., creating/deleting Pods, Services, etc.) go through the API server.

• Responsibilities:

  • Handles RESTful requests to the cluster.
  • Manages the state of the cluster and acts as the front-end to the cluster’s shared state (stored in etcd).
  • Provides the API for clients (e.g., kubectl, dashboards, or other internal components) to interact with Kubernetes resources.

• How it works:

  • The API server communicates with etcd (the cluster's state store), the scheduler, controller manager, and other components to ensure the desired state is maintained.

b) Scheduler (kube-scheduler)

• Role: The scheduler is responsible for deciding which node (worker machine) will run each Pod. When new Pods are created, the scheduler selects a suitable node based on various factors like resource availability, affinity rules, and taints/tolerations.

• Responsibilities:

  • Watches for newly created Pods without assigned nodes and schedules them onto appropriate nodes.
  • Considers factors like resource requests (CPU, memory), node availability, and any node-specific constraints.

• How it works:

  • The scheduler watches the API server for unscheduled Pods and finds a node that meets the resource requirements (e.g., CPU, memory).
  • Once the right node is found, it updates the Pod’s specification with the node's name, and the scheduler passes this information to the API server.

c) Controller Manager (kube-controller-manager)

• Role: The controller manager runs a set of controllers that regulate the state of the cluster. Controllers are responsible for maintaining the desired state of the system, such as ensuring that the correct number of Pods are running or that failed Pods are recreated.

• Responsibilities:

  • Handles routine control loops (like replication, deployment, and scaling).
  • Responsible for maintaining desired state, like ensuring Pods are running, replication controllers are ensuring the correct number of replicas, etc.

• How it works:

  • Each controller continuously watches for changes in the cluster (such as a pod failing or being deleted) and then acts accordingly.
  • Examples of controllers include:
    • Replication Controller: Ensures the desired number of replicas of a Pod are running.
    • Deployment Controller: Manages deployments, including rolling updates.
    • StatefulSet Controller: Manages stateful applications.

d) etcd

• Role: etcd is a distributed key-value store used as the backing store for all cluster data. It stores the entire state of the Kubernetes cluster and is used by the API server to retrieve the current state of resources and by other components to ensure data consistency across the cluster.

• Responsibilities:

  • Stores persistent cluster state data, including configurations, metadata, and the overall state of resources.
  • Serves as a consistent and highly available source of truth for all components in the cluster.

• How it works:

  • etcd stores data in key-value pairs and is highly available and fault-tolerant, typically running in a clustered mode.
  • Kubernetes components (like the API server) interact with etcd to read or write the cluster’s state.

2. Node Components

The node components are responsible for running the applications and workloads on worker nodes. These components run on each worker node in the cluster and ensure the proper execution of containers.

a) Kubelet

• Role: The kubelet is an agent that runs on each node in the Kubernetes cluster. It ensures that containers (Pods) are running in the desired state by managing the lifecycle of containers on the node.

• Responsibilities:

  • Watches for Pods scheduled to the node via the API server and makes sure the containers in those Pods are running.
  • Reports the node’s status and resource usage back to the API server.
  • Executes commands defined in Pods (e.g., liveness and readiness probes).

• How it works:

  • The kubelet continuously monitors the state of the node and its associated containers, taking actions if necessary (e.g., restarting failed containers).
  • Communicates with the API server to ensure the Pod is running correctly.

b) Kube Proxy

• Role: The kube-proxy is a network proxy that runs on each worker node and maintains network rules to allow Pods to communicate with each other and with external services.

• Responsibilities:

  • Maintains network rules for Pod communication, enabling services like load balancing and network forwarding.
  • Provides Service abstraction: kube-proxy manages network traffic for services in the cluster by directing it to the appropriate Pods.

• How it works:

  • Kube-proxy watches for Service changes and updates iptables rules or uses IPVS (depending on the mode) to manage the traffic routing for the Service endpoints.
  • It allows the abstraction of Services, enabling communication between Pods in a secure and reliable manner.

c) Container Runtime

• Role: The container runtime is responsible for running and managing containers on each node. Kubernetes can work with several container runtimes, with Docker being the most common, though other runtimes like containerd or CRI-O are also supported.

• Responsibilities:

  • Pulls container images.
  • Starts, stops, and manages containers.

• How it works:

  • The container runtime interacts directly with the operating system to launch containers, as specified by the kubelet.
  • Kubernetes supports multiple runtimes through the Container Runtime Interface (CRI), which allows it to integrate with any container runtime that conforms to the interface.

3. Add-on Components

These are additional components that extend the functionality of Kubernetes and provide essential features for managing the cluster and workloads.

a) CoreDNS

• Role: CoreDNS is the DNS service that resolves domain names to IP addresses for Pods in the Kubernetes cluster. It enables service discovery in Kubernetes.

• Responsibilities:

  • Resolves service names to IP addresses for Pods and Services.
  • Provides DNS-based service discovery for applications in the cluster.

• How it works:

  • CoreDNS runs as a Pod inside the cluster and uses the Kubernetes API to query for service names and IP addresses.
  • It provides DNS resolution for internal service discovery.

b) Network Plugin (CNI)

• Role: The Container Network Interface (CNI) is a set of standards used to configure networking for containers in Kubernetes. It enables networking for Pods, Services, and external communication.

• Responsibilities:

  • Handles networking between Pods, including setting up IP addresses and routing.
  • Enables communication between Pods on different nodes.

• How it works:

  • Kubernetes supports multiple CNI plugins, including Calico, Flannel, Weave, and more. These plugins configure network interfaces and routes for Pods and Services.

Summary of Kubernetes Core Components

Conclusion

The Kubernetes architecture is highly modular, with various components working together to ensure efficient management and orchestration of containerized applications. The master components control the cluster, while the node components ensure containers run efficiently on worker nodes. Add-on components like DNS and network plugins provide additional capabilities to support communication and resource management in the cluster. Understanding how each of these components interacts is key to managing a Kubernetes cluster effectively.