1.26 blog: pod scheduling readiness to alpha

content/en/blog/_posts/2022-11-29-pod-scheduling-readiness/index.md

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+---
+layout: blog
+title: "Kubernetes 1.26: Pod Scheduling Readiness to Alpha"
+date: 2022-11-29
+slug: pod-scheduling-readiness-alpha-1-26
+---
+
+**Author:** Wei Huang (Apple), Abdullah Gharaibeh (Google)
+
+Kubernetes 1.26 introduced a new Pod feature named SchedulingGates. SchedulingGates are keys that
+tell the scheduler when a Pod is ready to be considered for scheduling.
+
+## What problem does it solve?
+
+When a Pod is created, the scheduler will continuously attempt to find a node that fits it. This
+infinite loop continues until the scheduler either finds a node for the Pod, or the Pod gets deleted.
+
+Pods that remain unschedulable for long periods of time (e.g., one that is blocked on some external event) 
+waste scheduling cycles. A scheduling cycle may take ~20ms or more depending on the complexity of
+the Pod's scheduling constraints. Therefore, at scale, those wasted cycles significantly impact the scheduler's performance. See the cycle in red arrows below.
+
+![](./scheduler-workflow-1.png)
+
+SchedulingGates helps address this problem. It allows declaring that newly created Pods are not
+ready for scheduling. When SchedulingGates are present on a Pod, the scheduler ignores the Pod
+and therefore saves unnecessary scheduling attempts. Clearing the gates is the responsibility of
+external controllers with knowledge of when the Pod should be considered for scheduling (e.g., 
+a resource provisioner).
+
+![](scheduler-workflow-2.png)
+
+## How does it work?
+
+SchedulingGates in general works very similar to Finalzers. Pods with non-empty 
+`spec.schedulingGates` field will carry the status `SchedulingGated` and will be blocked from
+scheduling. Note that more than one gate can be added, but they all should be added upon Pod
+creation (e.g., you can add them as part of the spec of via a mutating webhook).
+
+```
+NAME       READY   STATUS            RESTARTS   AGE
+test-pod   0/1     SchedulingGated   0          10s
+```
+
+To clear the gates, you update the Pod by removing the `schedulingGates` string literals.
+The gates do not need to be removed all at once, but only when all the gates are removed the
+scheduler will start to consider the Pod for scheduling.
+
+Under the hood, SchedulingGates is implemented as a PreEnqueue scheduler plugin, a new scheduler
+framework extension point that is invoked at the beginning of each scheduling cycle.
+
+## Use Cases
+
+An important use case this feature enables is dynamic quota management. Kubernetes supports
+ResourceQuota, however quotas are enforced by the api-server on Pod creation. For example, if a
+new Pod exceeds the cpu quota, it gets rejected. The api-server doesn't queue the Pod; therefore,
+whoever created the Pod needs to continuously attempt to recreate it again!
+
+SchedulingGates allows an external quota manager to address the above limitation of ResourceQuota.
+Specifically, the manager could add a "quotaCheck" scheduling gate to all Pods created in the
+cluster (using a mutating webhook). The manager would then remove the gate when there is quota to
+start the Pod.
+
+## Whats next?
+
+To use this feature, the `PodSchedulingReadiness` feature gate must be enabled in the API Server
+and Scheduler. You're more than welcome to test it out and tell us (sig-scheduling) what you think!
+
+## Additional resources
+
+- [Official PodSchedulingReadiness docs](https://kubernetes.io/docs/concepts/scheduling-eviction/pod-scheduling-readiness/)
+- [Official Kubernetes Enhancement Proposal](https://github.com/kubernetes/enhancements/blob/master/keps/sig-scheduling/3521-pod-scheduling-readiness/README.md)