• A physical or virtual machine on which Kubernetes is installed
• Nodes are cluster scoped. They are not scoped within a namespace.
• When you install Kubernetes on a node, the following components are installed. Some of them are used in worker nodes and the rest are used in master nodes.
  • API Server
  • etcd Service
  • Kubelet Service
  • Container Runtime
  • Controller
  • Scheduler
• A cluster is a collection of nodes grouped together

Worker Nodes

• These nodes do the actual work so they need to have more resources
• Each worker node has multiple pods running on it
• 3 processes must be installed on every worker node
  • Container Runtime (eg. docker)
  • Kubelet
    • process of Kubernetes
    • starts pods and runs containers inside them
    • allocates resources from the node to the container
  • Kubeproxy
    • process of Kubernetes
    • forwards the requests to pods intelligently
    • Image

      • Kubeproxy forwards requests to the DB pod running on the same node to minimize network overhead.

        

Master Nodes

• Control the cluster state & manage worker nodes
• Need less resources as they don't do the actual work
• Multi-master setup is often used for fault tolerance
• 4 processes run on every master node

  • API Server

    • User interacts with the cluster via the API server using a client (Kubernetes Dashboard, CLI, or Kubernetes API)
    • Cluster gateway (acts as the entry point into the cluster)
    • Can be used for authentication

    

  • Scheduler

    • Decides the node where the new pod should be scheduled and sends a request to the Kubelet to start a pod.

    

  • Controller

    • Detects state changes like crashing of pods
    • If a pod dies, it requests scheduler to schedule starting up of a new pod

    

  • etcd

    • Key-value store of the cluster state (also known as cluster brain)
    • Cluster changes get stored in the etcd
    • In multi-master configuration, etcd is a distributed key-value store
    • Application data is not stored in the etcd