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
