In a Kubernetes (K8s) cluster, several essential services run on nodes to ensure proper cluster management, networking, and communication between components. Here are some of the key services and their roles:
- **kubelet**: The kubelet is an agent that runs on each node and is responsible for managing the containers running on that node. It communicates with the Kubernetes control plane and ensures that the containers specified in Pod manifests are running and healthy.
- **kube-proxy**: The kube-proxy is responsible for network proxying and load balancing for services running in the cluster. It enables communication between Pods and services and maintains network rules to forward traffic to the appropriate destinations.
- **container runtime**: The container runtime is the software responsible for pulling container images and running containers on the node. Kubernetes supports various container runtimes, such as Docker, containerd, and others.
- **kube-dns/coredns**: The kube-dns or CoreDNS service provides DNS resolution within the cluster. It allows Pods to discover and communicate with each other using DNS names instead of direct IP addresses.
- **kubelet-certificate-controller**: This service ensures that each node has the necessary TLS certificates required for secure communication with the control plane.
- **kubelet-eviction-manager**: The kubelet-eviction-manager monitors the resource usage of the node and triggers Pod eviction when there is a lack of resources, helping to maintain node stability and prevent node resource exhaustion.
- **kube-proxy (IPVS mode)**: In clusters running with IPVS (IP Virtual Server) mode, kube-proxy uses IPVS to handle the load balancing of services more efficiently.
- **metrics-server**: The metrics-server collects resource usage metrics (CPU, memory, etc.) from nodes and Pods and provides them to Kubernetes Horizontal Pod Autoscaler (HPA) and other components for scaling decisions.
- **node-problem-detector**: The node-problem-detector detects and reports node-level issues, such as kernel panics or unresponsive nodes, to the Kubernetes control plane for further actions.
- **kube-reserved and kube-system-reserved cgroups**: These are control groups that reserve CPU and memory resources for the kubelet and critical system components to ensure their stability and proper functioning.
These services, running on every node, play a crucial role in maintaining the health, networking, and performance of the Kubernetes cluster. They ensure seamless communication, resource management, and container orchestration, providing the foundation for deploying and managing containerized applications effectively in the Kubernetes environment.
