• Kubernetes is an open-source container orchestration tool developed by Google.
• It helps us manage containerized applications in different deployment environments.

Container Orchestration

• The automated deployment and management of containers is called container orchestration.
• Containers act as perfect hosts for micro-services (running independently).
• The rise of micro-services architecture led to applications using 1000s of containers that need to be smartly managed.
• Container orchestration offers:
  • High Availability (no downtime)
  • Horizontal Scalability
  • Disaster Recovery
  • And so much more..

Kubernetes Node

• 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.
  • Kube API Server
  • etcd Service
  • Kubelet Service
  • Container Runtime (Docker Engine, CRI-O, containerd)
  • Kube Controller
  • Kube 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 can have multiple pods running on it.
• 3 processes must be installed on every worker node.
  • Container Runtime (eg. Docker Engine, CRI-O, containerd)
  • 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
    • Example: 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 typically only run core components of kubernetes
• Multi-master setup is often used for fault tolerance
• 4 processes run on every master node

  • Kube API Server

    • User interacts with the cluster via the Kube 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
      

  • Kube Scheduler

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

  • Kube 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