这篇文章主要介绍“Kubernetes的PDB怎么应用”，在日常操作中，相信很多人在Kubernetes的PDB怎么应用问题上存在疑惑，小编查阅了各式资料，整理出简单好用的操作方法，希望对大家解答”Kubernetes的PDB怎么应用”的疑惑有所帮助！接下来，请跟着小编一起来学习吧！
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PDB的应用场景
大概在Kubernetes 1.4新增了PodDisruptionBudget Object（后面简称PDB），在1.5的时候升级到Beta，但是直到1.9 Released还是Beta。不过没关系，我们抛开这些，先来想想PDB是为了解决什么问题的。PDB Feature已经一年多了，以前没有研究过它，主要是没场景。最近在做基于Kubernetes的ElasticSearch as a Service(简称ESaaS)项目方案，要尽量保证任何ElasticSearch Cluster中始终至少要有一个健康可用的ES client pod, ES master pod和ES data pod。很多同学都学想到Deployment中可以设置maxUnavailable，那不就行了吗？再说了，还会有RS Controller在做副本控制呢？
等下！Deployment中的maxUnavailable是什么时候用的？—— 是用来对使用Deployment部署的应用进行滚动更新时保障最少可服务副本数的！RS Controller呢？—— 那只是副本控制器之一，它并不能给你保证集群中始终有几个副本的，它是负责尽快的让实际副本数跟你的期望副本数相同的，它才不管中间某些时刻的实际副本数呢。这个时候，你就可以考虑使用Kubernetes PDB了，它是用来保证应用的高可用的，对那些Voluntary（自愿的）Disruption做好Budgets(预算方案)。
前面提到了Voluntary Disruption，我们来捋一下，什么是Voluntary Disruption？什么又是Involuntary Disruption？
Involuntary Disruption及其应对措施
Involuntary Disruption指的是那些不可控的（或者目前来说难于控制的）外界因素导致的Disruption，比如：
服务器的硬件故障或者内核崩溃导致节点Down了。
如果容器部署在VM，VM被误删了或者Hyperwisor出问题了。
集群出现了网络脑裂。（Kubernetes通过NodeController来处理网络脑裂情况，但是evict pods时仍然没有考虑到保证应用的高可用）关于NodeController深度解析，请参考我的下面博文：
Kubernetes Node Controller源码分析之执行篇
Kubernetes Node Controller源码分析之创建篇
Kubernetes Node Controller源码分析之配置篇
Kubernetes Node Controller源码分析之Taint Controller
某个节点因为不合理的超配导致出现计算资源不足时，触发kubelet eviction时也没有考虑到保证应用的高可用。关于kubelet eviction深度解析，请参考我的下面博文：
PDB不是解决Involuntary Disruption的，我们如何在使用Kubernetes时尽量减轻或者缓解Involuntary Disruption对应用高可用的影响呢？
一个应用尽量使用Deployment,RS,StatefulSet等副本控制器部署，并且replicas大于1。
设置应用container的request值，使得即使在资源非常紧张的情况下，也能有足够的资源供它使用。
另外，尽量考虑物理设备上的HA，比如一个应用的不同副本要跨服务器部署，跨机柜跨机架部署，跨交换机部署等。
PDB是为了Voluntary Disruption时保障应用的高可用
Involuntary Disruption对立的场景，自然就是Voluntary Disruption了，指的是用户或者集群管理员触发的，Kubernetes可控的Disruption场景，比如：
PDB就是针对Voluntary Disruption场景设计的，属于Kubernetes可控的范畴之一，而不是为Involuntary Disruption设计的。
Kube-Node项目上线后，可以支持对接Openstack，AWS，GCE等cloud provider实现Node的自动管理，因此可能会经常有HNA(Horizontal Node Autoscaleer)事件,工作流就有类似drain a node的逻辑，因此需要使用PDB来保障应用的HA。
PDB的使用方法及注意事项
使用说明及注意点
部署在Kubernetes的每个App都可以创建一个对应PDB Object，用来限制Voluntary Disruptions时最大可以down的副本数或者最少应该保持Available的副本数，以此来保证应用的高可用。
PDB可以用来保护由Kubernetes内置控制器管理的应用，这种情况下要求DPB selector等同于这些Controller Object的Selector：
Deployment
ReplicationController
ReplicaSet
StatefulSet
也可以用来保护那些仅仅由PDB Selector自己选择的Pods Set，但是有两个使用限制：
因此，不管怎么说，PDB影响的Pods Set都是通过自己的Selector来选择的，使用时要注意同一个namespace下不同的PDB Object不要使用有重叠的Selectors。
在使用PDB时，你需要弄清楚你的应用类型以及你想要的应对措施：
无状态应用：比如想至少有60%的副本Available。
单实例的有状态应用：终止这个实例之前必须提前通知客户并取得同意。
多实例的有状态应用：最少可用的实例数不能少于某个数N（比如受限于raft协议类应用的选举机制）
批处理Job：Job需要最终有一个Pod成功完成任务。
定义PDB Object
进行了以上思考后，确定了要创建PDB，接下来就看看PodDisruptionBudget怎么定义的，下面是个Sample：
apiVersion: policy/v1beta1
kind: PodDisruptionBudget
metadata:
name: zk-pdb
spec:
minAvailable: 2
selector:
matchLabels:
app: zookeeper
PDB的定义，其实就三项关键内容：
.spec.selector用来选择后端Pods Set，最佳实践是与应用对应的Deployment,StatefulSet的Selector一致；
.spec.minAvailable表示发生voluntary disruptions的过程中，要保证至少可用的Pods数或者比例；
.spec.maxUnavailable表示发生voluntary disruptions的过程中，要保证最大不可用的Pods数或者比例，要求Kubernetes version >= 1.7；这个配置只能用来对应Deployment，RS，RC，StatefulSet的Pods，推荐优先使用.spec.maxUnavailable。
注意:
同一个PDB Object中不能同时定义.spec.minAvailable和.spec.maxUnavailable。
前面提到，应用滚动更新时Pod的delete和unavailable虽然也属于voluntary disruption，但是实际上滚动更新有自己的策略控制（marSurge和maxUnavailable），因此PDB不会干预这个过程。
PDB只能保证voluntary disruptions时的副本数，比如evict pod过程中刚好满足.spec.minAvailable或.spec.maxUnavailable，这时某个本来正常的Pod突然因为Node Down(Involuntary Disruption)了挂了，那么这个时候实际Pods数就比PDB中要求的少了，因此PDB不是万能的！
使用上，如果设置.spec.minAvailable为100%或者.spec.maxUnavailable为0%，意味着会完全阻止evict pods的过程（Deployment和StatefulSet的滚动更新除外）。
创建PDB Object
$ kubectl get poddisruptionbudgets
NAME MIN-AVAILABLE ALLOWED-DISRUPTIONS AGE
zk-pdb 2 1 7s
$ kubectl get poddisruptionbudgets zk-pdb -o yaml
apiVersion: policy/v1beta1
kind: PodDisruptionBudget
metadata:
creationTimestamp: 2017-08-28T02:38:26Z
generation: 1
name: zk-pdb
...
status:
currentHealthy: 3
desiredHealthy: 3
disruptedPods: null
disruptionsAllowed: 1
expectedPods: 3
observedGeneration: 1
PDB的工作原理及源码分析
PDB Object定义是遇到voluntary disruption时用户的期望状态，真正去维护这个期望状态的也是一个由kube-controller-manager管理的Controller，那便是Disruption Controller。
Disruption Controller主要watch Pods和PDBs，当监听到pod/pdb的Add/Del/Update事件后，并会将对应的pdb object放到rate limit queue中等待worker处理，worker的主要逻辑就是计算PodDisruptionBudgetStatus的currentHealthy, desiredHealthy, expectedCount, disruptedPods,然后调用api更新PDB Status。
pkg/controller/disruption/disruption.go:498
func (dc *DisruptionController) trySync(pdb *policy.PodDisruptionBudget) error {
pods, err := dc.getPodsForPdb(pdb)
if err != nil {
dc.recorder.Eventf(pdb, v1.EventTypeWarning, "NoPods", "Failed to get pods: %v", err)
return err
}
if len(pods) == 0 {
dc.recorder.Eventf(pdb, v1.EventTypeNormal, "NoPods", "No matching pods found")
}
expectedCount, desiredHealthy, err := dc.getExpectedPodCount(pdb, pods)
if err != nil {
dc.recorder.Eventf(pdb, v1.EventTypeWarning, "CalculateExpectedPodCountFailed", "Failed to calculate the number of expected pods: %v", err)
return err
}
currentTime := time.Now()
disruptedPods, recheckTime := dc.buildDisruptedPodMap(pods, pdb, currentTime)
currentHealthy := countHealthyPods(pods, disruptedPods, currentTime)
err = dc.updatePdbStatus(pdb, currentHealthy, desiredHealthy, expectedCount, disruptedPods)
if err == nil && recheckTime != nil {
// There is always at most one PDB waiting with a particular name in the queue,
// and each PDB in the queue is associated with the lowest timestamp
// that was supplied when a PDB with that name was added.
dc.enqueuePdbForRecheck(pdb, recheckTime.Sub(currentTime))
}
return err
}下面是PodDisruptionBudgetStatus的定义：
pkg/apis/policy/types.go:48
type PodDisruptionBudgetStatus struct {
// Most recent generation observed when updating this PDB status. PodDisruptionsAllowed and other
// status informatio is valid only if observedGeneration equals to PDB's object generation.
// +optional
ObservedGeneration int64 `json:"observedGeneration,omitempty" protobuf:"varint,1,opt,name=observedGeneration"`
// DisruptedPods contains information about pods whose eviction was
// processed by the API server eviction subresource handler but has not
// yet been observed by the PodDisruptionBudget controller.
// A pod will be in this map from the time when the API server processed the
// eviction request to the time when the pod is seen by PDB controller
// as having been marked for deletion (or after a timeout). The key in the map is the name of the pod
// and the value is the time when the API server processed the eviction request. If
// the deletion didn't occur and a pod is still there it will be removed from
// the list automatically by PodDisruptionBudget controller after some time.
// If everything goes smooth this map should be empty for the most of the time.
// Large number of entries in the map may indicate problems with pod deletions.
DisruptedPods map[string]metav1.Time `json:"disruptedPods" protobuf:"bytes,2,rep,name=disruptedPods"`
// Number of pod disruptions that are currently allowed.
PodDisruptionsAllowed int32 `json:"disruptionsAllowed" protobuf:"varint,3,opt,name=disruptionsAllowed"`
// current number of healthy pods
CurrentHealthy int32 `json:"currentHealthy" protobuf:"varint,4,opt,name=currentHealthy"`
// minimum desired number of healthy pods
DesiredHealthy int32 `json:"desiredHealthy" protobuf:"varint,5,opt,name=desiredHealthy"`
// total number of pods counted by this disruption budget
ExpectedPods int32 `json:"expectedPods" protobuf:"varint,6,opt,name=expectedPods"`
}PodDisruptionBudgetStatus最重要的元素就是**DisruptedPods和PodDisruptionsAllowed**：
DisruptedPods：用来保存那些已经通过apiserver pod eviction subresource处理的pods，但是还没被PDB Controller发现处理的Pods，是Map类型，key为Pod Name，value是apiserver接受eviction subresource请求的时间。加入里面的Pod有2min的超时时间，如果2min后Pod仍然没有被删除，则会将该Pod从队列中剔除。
PodDisruptionsAllowed：表示当前允许Disruption的Pods数。
Disruption Controller的主要逻辑就是更新PDB.Status，那么问题来了，到底是谁去控制voluntary distribution时eviction的maxUnavailable或者minAvailable的呢？
要再次提醒的是，PDB Controller只处理那些通过pod eviction subresource请求对应的pods，因此上面的这个问题就要到对应的Pod的evictionRest中去找了。
pkg/registry/core/pod/storage/eviction.go:81
// Create attempts to create a new eviction. That is, it tries to evict a pod.
func (r *EvictionREST) Create(ctx genericapirequest.Context, obj runtime.Object, createValidation rest.ValidateObjectFunc, includeUninitialized bool) (runtime.Object, error) {
eviction := obj.(*policy.Eviction)
obj, err := r.store.Get(ctx, eviction.Name, &metav1.GetOptions{})
if err != nil {
return nil, err
}
pod := obj.(*api.Pod)
var rtStatus *metav1.Status
var pdbName string
err = retry.RetryOnConflict(EvictionsRetry, func() error {
pdbs, err := r.getPodDisruptionBudgets(ctx, pod)
if err != nil {
return err
}
if len(pdbs) > 1 {
rtStatus = &metav1.Status{
Status: metav1.StatusFailure,
Message: "This pod has more than one PodDisruptionBudget, which the eviction subresource does not support.",
Code: 500,
}
return nil
} else if len(pdbs) == 1 {
pdb := pdbs[0]
pdbName = pdb.Name
// Try to verify-and-decrement
// If it was false already, or if it becomes false during the course of our retries,
// raise an error marked as a 429.
if err := r.checkAndDecrement(pod.Namespace, pod.Name, pdb); err != nil {
return err
}
}
return nil
})
if err == wait.ErrWaitTimeout {
err = errors.NewTimeoutError(fmt.Sprintf("couldn't update PodDisruptionBudget %q due to conflicts", pdbName), 10)
}
if err != nil {
return nil, err
}
if rtStatus != nil {
return rtStatus, nil
}
// At this point there was either no PDB or we succeded in decrementing
// Try the delete
_, _, err = r.store.Delete(ctx, eviction.Name, eviction.DeleteOptions)
if err != nil {
return nil, err
}
// Success!
return &metav1.Status{Status: metav1.StatusSuccess}, nil
}{
"apiVersion": "policy/v1beta1",
"kind": "Eviction",
"metadata": {
"name": "quux",
"namespace": "default"
}
}
$ curl -v -H 'Content-type: application/json' http://127.0.0.1:8080/api/v1/namespaces/default/pods/quux/eviction -d @eviction.json// checkAndDecrement checks if the provided PodDisruptionBudget allows any disruption.
func (r *EvictionREST) checkAndDecrement(namespace string, podName string, pdb policy.PodDisruptionBudget) error {
if pdb.Status.ObservedGeneration < pdb.Generation {
// TODO(mml): Add a Retry-After header. Once there are time-based
// budgets, we can sometimes compute a sensible suggested value. But
// even without that, we can give a suggestion (10 minutes?) that
// prevents well-behaved clients from hammering us.
err := errors.NewTooManyRequests("Cannot evict pod as it would violate the pod's disruption budget.", 0)
err.ErrStatus.Details.Causes = append(err.ErrStatus.Details.Causes, metav1.StatusCause{Type: "DisruptionBudget", Message: fmt.Sprintf("The disruption budget %s is still being processed by the server.", pdb.Name)})
return err
}
if pdb.Status.PodDisruptionsAllowed < 0 {
return errors.NewForbidden(policy.Resource("poddisruptionbudget"), pdb.Name, fmt.Errorf("pdb disruptions allowed is negative"))
}
if len(pdb.Status.DisruptedPods) > MaxDisruptedPodSize {
return errors.NewForbidden(policy.Resource("poddisruptionbudget"), pdb.Name, fmt.Errorf("DisruptedPods map too big - too many evictions not confirmed by PDB controller"))
}
if pdb.Status.PodDisruptionsAllowed == 0 {
err := errors.NewTooManyRequests("Cannot evict pod as it would violate the pod's disruption budget.", 0)
err.ErrStatus.Details.Causes = append(err.ErrStatus.Details.Causes, metav1.StatusCause{Type: "DisruptionBudget", Message: fmt.Sprintf("The disruption budget %s needs %d healthy pods and has %d currently", pdb.Name, pdb.Status.DesiredHealthy, pdb.Status.CurrentHealthy)})
return err
}
pdb.Status.PodDisruptionsAllowed--
if pdb.Status.DisruptedPods == nil {
pdb.Status.DisruptedPods = make(map[string]metav1.Time)
}
// Eviction handler needs to inform the PDB controller that it is about to delete a pod
// so it should not consider it as available in calculations when updating PodDisruptions allowed.
// If the pod is not deleted within a reasonable time limit PDB controller will assume that it won't
// be deleted at all and remove it from DisruptedPod map.
pdb.Status.DisruptedPods[podName] = metav1.Time{Time: time.Now()}
if _, err := r.podDisruptionBudgetClient.PodDisruptionBudgets(namespace).UpdateStatus(&pdb); err != nil {
return err
}
return nil
}checkAndDecrement主要检查pdb.Status.PodDisruptionsAllowed是否大于0，并且DisruptedPods包含的Pods数不能超过2000（Disruption Controller性能可能不足以支撑这么多）。
检查通过，就对pdb.Status.PodDisruptionsAllowed减1，然后将该Pod加到DisruptedPods这个Map中，map的value就是当前时间（apiserver接受该eviction request的时间）。
更新PDB，PDB Controller因为监听了PDB的Update Event，接着就会触发PDB Controller的逻辑，再次去维护PDB Status。
Note：PDB在scheduler中也有用到。基于Pod Priority进行抢占式调度时，generic_scheduler进行preempte pod时会对Node上所有Pod进行PDB验证，统计违背PDB的Pods数量，Select Node时尽量选择违背PDB Pods数更少的node。
到此，关于“Kubernetes的PDB怎么应用”的学习就结束了，希望能够解决大家的疑惑。理论与实践的搭配能更好的帮助大家学习，快去试试吧！若想继续学习更多相关知识，请继续关注创新互联网站，小编会继续努力为大家带来更多实用的文章！
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