Two phase admission documentation (kubernetes-sigs#1181)

* Two phase admission documentation

* Review Remarks

* Review Remarks

site/content/en/docs/concepts/_index.md

@@ -40,18 +40,25 @@ Kueue. Sometimes referred to as _job_.
 independently from [pod priority](https://kubernetes.io/docs/concepts/scheduling-eviction/pod-priority-preemption/).  
 This priority value from a `WorkloadPriorityClass` is only used for managing the queueing and preemption of [Workloads](#workload).
 
+### [Admission Check](/docs/concepts/admission_check)
+
+A mechanism allowing internal or external components to influence the timing of workloads admission.
+
 ![Components](/images/queueing-components.svg)
 
 ## Glossary
 
-### Admission
-
-The process of admitting a Workload to start (Pods to be created). A Workload
-is admitted by a ClusterQueue according to the available resources and gets
-resource flavors assigned for each requested resource.
+### Quota Reservation
 
 Sometimes referred to as _workload scheduling_ or _job scheduling_
 (not to be confused with [pod scheduling](https://kubernetes.io/docs/concepts/scheduling-eviction/assign-pod-node/)).
+Is the process during which the kueue scheduler locks the resources needed by a workload within the targeted [ClusterQueues ResourceGroups](/docs/concepts/cluster_queue/#resource-groups)
+
+### Admission
+
+The process of admitting a Workload to start (Pods to be created). A Workload
+is admitted when it has a Quota Reservation and all its [AdmissionCheckStates](/docs/concepts/admission_check)
+are `Ready`.
 
 ### [Cohort](/docs/concepts/cluster_queue#cohort)
 

site/content/en/docs/concepts/admission_check.md

@@ -0,0 +1,85 @@
+---
+title: "Admission Check"
+date: 2023-10-05
+weight: 6
+description: >
+  A mechanism allowing internal or external components to influence the timing of
+  workloads admission.
+---
+
+AdmissionChecks are a mechanism that allows Kueue to consider additional criteria before admitting a Workload.
+When a ClusterQueue has AdmissionChecks configured, each of the checks has to provide a
+positive signal to the Workload before it can be [Admitted](docs/concepts/#admission).
+
+## Components
+
+You can configure the mechanism using the following components:
+
+### Admission Check
+
+Admission check is a non-namespaced API object used to define details about an `AdmissionCheck` like:
+
+- **controllerName** - It's an identifier for the controller that processes this AdmissionCheck, not necessarily a Kubernetes Pod or Deployment name. Cannot be empty.
+- **retryDelayMinutes** - Specifies how long to keep the workload suspended after a failed check (after it transitioned to False). After that the check state goes to "Unknown". The default is 15 min.
+- **parameters** - Identifies an additional resource providing additional parameters for the check.
+
+An AdmissionCheck object looks like the following:
+```yaml
+apiVersion: kueue.x-k8s.io/v1beta1
+kind: AdmissionCheck
+metadata:
+  name: prov-test
+spec:
+  controllerName: kueue.x-k8s.io/provisioning-request
+  retryDelayMinutes: 15
+  parameters:
+    apiGroup: kueue.x-k8s.io
+    kind: ProvisioningRequestConfig
+    name: prov-test-config
+```
+
+### ClusterQueue admissionChecks
+
+Once defined, an AdmissionCheck can be referenced in the ClusterQueues' spec. All Workloads associated with the queue need to be evaluated by the AdmissionCheck's controller before being admitted.
+Similarly to `ResourceFlavors`, if an `AdmissionCheck` is not found or its controller has not marked it as `Active`, the ClusterQueue will be marked as Inactive.
+
+### AdmissionCheckState
+
+AdmissionCheckState is the way the state of an AdmissionCkeck for a specific Workload is tracked.
+
+AdmissionCheckStates are listed in the Workload's `.status.admissionCheckStates` field.
+
+The status of a Workload that has pending AdmissionChecks looks like the following:
+```yaml
+status:
+  admission:
+    <...>
+  admissionChecks:
+  - lastTransitionTime: "2023-10-20T06:40:14Z"
+    message: ""
+    name: sample-prov
+    podSetUpdates:
+    - annotations:
+        cluster-autoscaler.kubernetes.io/consume-provisioning-request: job-prov-job-9815b-sample-prov
+      name: main
+    state: Ready
+  <...>
+```
+
+A list of states being maintained in the Status of all the monitored Workloads.
+
+Kueue ensures that the list of the Workloads AdmissionCheckStates is in sync with the list of its associated ClusterQueue, Kueue adds new checks with the `Pending` state.
+
+- Once a workload has QuotaReservation and all its AdmissionChecks are in "Ready" state it will become Admitted.
+- If at least one of the Workloads AdmissionCheck is in the `Retry` state.
+  - If `Admitted` the workload is evicted.
+  - If the workload has `QuotaReservation` it will be release released.
+- If at least one of the Workloads AdmissionCheck is in the `Rejected`:
+  - If `Admitted` the workload is evicted.
+  - If the workload has `QuotaReservation` it will be release released.
+  - The workload is marked as 'Finished' with a relevant failure message.
+
+### Admission Check Controller
+
+Is a component that monitors Workloads maintaining the content of its specific `admissionCheckStates` and the `Active` condition of the AdmissionChecks it's  controlling.
+The logic for how an AdmissionCheck changes states is not part of Kueue.

site/static/examples/sample-admission-check.yaml

@@ -0,0 +1,21 @@
+apiVersion: kueue.x-k8s.io/v1beta1
+kind: AdmissionCheck
+metadata:
+  name: prov-test
+spec:
+  controllerName: ProvisioningRequestController
+  parameters:
+    apiGroup: kueue.x-k8s.io
+    kind: ProvisioningRequestConfig
+    name: prov-test-config
+
+---
+
+apiVersion: kueue.x-k8s.io/v1beta1
+kind: ProvisioningRequestConfig
+metadata:
+  name: prov-test-config
+spec:
+  provisioningClassName: test-class
+  parameters:
+    parma1: value1