Kubernetes Concepts
External
Internal
TODO
Deplete Kubernetes Concepts TO DEPLETE.
Overview
Kubernetes is a container orchestrator. To understand how Kubernetes works is to understand a set of high level concepts, briefly mentioned here. More details on individual concepts are available on their respective pages. All interactions with a Kubernetes cluster are performed by sending REST requests into an API server. The API server is responsible with managing and exposing the state of the cluster, most commonly via a command line client named kubectl. The cluster consists in a set of nodes. Nodes are used to run Pods - Pods are scheduled to nodes and monitored closely by the cluster. A Pod is the atomic unit of deployment in Kubernetes, and contains one or more containers. Pods come and go - if one Pod dies, it is not resurrected, but another Pod might be scheduled as replacement. In consequence, the IP address of an individual Pod cannot be relied on. To provide a stable access point to a set of equivalent Pods - which is how applications are deployed on Kubernetes, Kubernetes uses the concept of Service, which can be thought of as stable networking for a continuously changing set of Pods. A Service's IP address and port can be relied on to be stable for the life of the Service. All live Pods represented by a Service at a moment in time are known as the Service's endpoint. There are several types of Services: ClusterIP, NodePort and LoadBalancer. The association between Services and Pods they expose is loose, established logically by the Service's Selector, which is a label-based mechanism: a Pod "belongs" to a Service if the Service's Selector matches the Pod's Labels. A Pod by itself has no built-in resilience: if it fails for any reason, it is gone. A higher level concept - the Deployment - is used to manage a set of Pods from a high availability perspective: the Deployment insures that a specific number of equivalent Pods is always running, and if one of more Pods fail, the Deployment brings up replacement Pods. The deployment relies on an intermediary concept - the Replica Set. Deployments are used to implement rolling updates and rollbacks.
Declarative versus Imperative Approach
API Resources
The full list of API resources available to interact with in Kubernetes is generated by
kubectl api-resources
Link from it.