Basic Concepts

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System Components

For more information see: https://kubernetes.io/docs/concepts/overview/components/.

Master Components

These components run typical on a seperate machine, even if they don't have to. They are responsible for:

Global decisions about the cluster (e.g. scheduling)
In AKS high available managed server

kube-apiserver

Github: kube-apiserver
Kubernetes API
Frontend for the Kubernetes Control Plane
Receives commands from kubectl tool
Exposed by AKS (see Azure Portal or kubectl cluster-info)

etcd

Github: etcd
Key-Value Store for all cluster data

kube-scheduler

Github: kube-scheduler
Starts new pods as containers on nodes
Takes several metrics into consideration, like resources, etc.

kube-controller-manager

Github: kube-controller-manager
Runs controllers in a single process
Controllers:
- Node Controller: Handles Node Shutdowns
- Replication Controller: Ensure correct number of pods
- Endpoints Controller: Populates endpoints (pods, services)
- Service Account & Token Controllers

cloud-controller-manager

Github: cloud-controller-manager
Interaction with the underlying cloud providers
Controllers
- Node: Lifecycle management of cloud nodes
- Route: Routing in the Cloud Infrastructure
- Service: Handles Load Balancers
- Volume: Mounting and interacting with Cloud Storages

Node Components

kubelet

Github: https://github.com/kubernetes/kubernetes/tree/master/cmd/kubectl
Manages Pods on nodes
Also Virtual Kubelet: "Kubernetes API on top, programmable bac": Github virtual-kubelet

Used for connecting AKS to ACI (e.g. burst traffic spikes).

kube-proxy

Runs on the nodes, takes the traffic e.g. from Load-Balancer and sends it to the pods round robin.

Github: kube-proxy
Networking Abstraction
Kubernetes service abstraction
Connection-Forwarding

Container Runtime

Runtime to run containers, e.g. docker

cAdvisor

Integrated into kubelet
Collects CPU, memory, filesystem, and network usage statistics of containers and exposes them to kubelet

Addons

To further enhance functionality there are several addons that can be included:

DNS
- DNS Server in the cluster
Web UI (Dashboard)
- Simple Web UI for managing kubernetes
Container Resource Monitoring
- Metric collection and monitoring
Cluster-level Logging
- Store logs in a central log store