PodUnavailableBudget

FEATURE STATE: Kruise v0.10.0

在诸多Voluntary Disruption 场景中 Kubernetes Pod Disruption Budget 通过限制同时中断的Pod数量，来保证应用的高可用性。然而，PDB只能防控通过 Eviction API 来触发的Pod Disruption，例如：kubectl drain驱逐node上面的所有Pod。

但在如下voluntary disruption场景中，即便有kubernetes PDB防护依然将会导致业务中断、服务降级：

  1. 应用owner通过deployment正在进行版本升级，与此同时集群管理员由于机器资源利用率过低正在进行node缩容。
  2. 中间件团队利用sidecarSet正在原地升级集群中的sidecar版本（例如：ServiceMesh envoy），同时HPA正在对同一批应用进行缩容。
  3. 应用owner和中间件团队利用cloneSet、sidecarSet原地升级的能力，正在对同一批Pod进行升级。

在上面这些 kubernetes PDB 无法很好防护的场景中，Kruise PodUnavailableBudget 通过对Pod Mutating Webhook的拦截，能够覆盖更多的Voluntary Disruption场景，进而提供应用更加强大的防护能力。

一个简单的例子如下：

  1. apiVersion: apps.kruise.io/v1alpha1
  2. kind: PodUnavailableBudget
  3. metadata:
  4.   name: web-server-pub
  5.   namespace: web
  6. spec:
  7.   targetRef:
  8.     apiVersion: apps.kruise.io/v1alpha1
  9.     # cloneset, deployment, statefulset etc.
  10.     kind: CloneSet
  11.     name: web-server
  12.   # selector label query over pods managed by the budget
  13.   # selector and TargetReference are mutually exclusive, targetRef is priority to take effect.
  14.   # selector is commonly used in scenarios where applications are deployed using multiple workloads,
  15.   # and targetRef is used for protection against a single workload.
  16. # selector:
  17. #   matchLabels:
  18. #     app: web-server
  19.   # maximum number of Pods unavailable for the current cloneset, the example is cloneset.replicas(5) * 60% = 3
  20.   # maxUnavailable and minAvailable are mutually exclusive, maxUnavailable is priority to take effect
  21.   maxUnavailable: 60%
  22.   # Minimum number of Pods available for the current cloneset, the example is cloneset.replicas(5) * 40% = 2
  23. # minAvailable: 40%
  24. -----------------------
  26. apiVersion: apps.kruise.io/v1alpha1
  27. kind: CloneSet
  28. metadata:
  29.   labels:
  30.     app: web-server
  31.   name: web-server
  32.   namespace: web
  33. spec:
  34.   replicas: 5
  35.   selector:
  36.     matchLabels:
  37.       app: web-server
  38.   template:
  39.     metadata:
  40.       labels:
  41.         app: web-server
  42.     spec:
  43.       containers:
  44.       - name: nginx
  45.         image: nginx:alpine

Implementation

PUB实现原理如下，详细设计请参考：Pub Proposal

PodUnavailableBudget

Comparison with Kubernetes native PDB

Kubernetes PDB是通过Eviction API接口来实现Pod安全防护，而Kruise PDB则是拦截了Pod Validating Request来实现诸多Voluntary Disruption场景的防护能力。 Kruise PUB包含了PDB的所有能力（防护Pod Eviction），业务可以根据需要两者同时使用，也可以单独使用Kruise PUB（推荐方式）。

feature-gates

PodUnavailableBudget Pod安全防护默认是关闭的，如果要开启请通过设置 feature-gates PodUnavailableBudgetDeleteGate 和 PodUnavailableBudgetUpdateGate.

  1. $ helm install kruise https://... --set featureGates="PodUnavailableBudgetDeleteGate=true\,PodUnavailableBudgetUpdateGate=true"

PodUnavailableBudget Status

  1. # kubectl describe podunavailablebudgets web-server-pub
  2. Name:         web-server-pub
  3. Kind:         PodUnavailableBudget
  4. Status:
  5.   unavailableAllowed:   3   # unavailableAllowed number of pod unavailable that are currently allowed
  6.   currentAvailable:     5   # currentAvailable current number of available pods
  7.   desiredAvailable:     2   # desiredAvailable minimum desired number of available pods
  8.   totalReplicas:        5   # totalReplicas total number of pods counted by this PUB