Before a request is sent to the Kube-API server, some sort of client-side validation occurs. During this client-side validation, Kubectl validates requests and ensures that bad requests[ bad resource names, malformed image names, etc] do not go through to the API server. After the client-side validation, requests to the server are then authenticated. The authentication aims to ensure that the user making the request has the right to access that cluster. After the authentication process, Authorization is then carried out to verify if the requestor is allowed to perform the specified action. Once the above actions are completed successfully, the request is allowed to pass through the admission controllers.
Admission controllers intercept authenticated & authorized requests to the Kubernetes API server just before the appropriate objects are persisted to etcd. Admission controllers ensure that the authenticated requests obey the rules of the of the cluster — Such as resource quotas allocated, and limit ranger. Internally, Admission controllers are stored as plugins in this directory.
There are over 30 admission controllers available provided by default along with the Kubernetes binary. When a request is intercepted, All the admission controllers behave like a dependent chain, where if a single admission controller fails, all other controllers fail leading to the entire request failing. The entire list of admission controllers and their functions can be found here. The Admission control process has two phases, The mutating phase which is executed first, and the validating phase. The controllers can act as validating controllers, mutating controllers, or a combination of both mutating and validating.
So far, We have talked quite a bit about the Kubernetes admission controller. I hope you enjoyed reading this and learning about admission controllers. If you have any questions, leave them in the comments below! I will attempt to answer them
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