Kubernetes StatefulSet 与 Deployment 在 Node NotReady代码逻辑区别

节点离线后的 pod 状态

在 kubernetes 使用过程中，根据集群的配置不同，往往会因为如下情况的一种或几种导致节点 NotReady:

1. kubelet 进程停止
2. apiserver 进程停止
3. etcd 进程停止
4. kubernetes 管理网络 Down

当出现这种情况的时候，会出现节点 NotReady，进而当kube-controller-manager 中的 --pod-eviction-timeout 定义的值，默认 5 分钟后，将触发 Pod eviction 动作。

对于不同类型的 workloads，其对应的 pod 处理方式因为 controller-manager 中各个控制器的逻辑不通而不同。总结如下:

deployment
statefulset
daemonSet

这里说到，对于 deployment 和 statefulSet 类型资源，当节点 NotReady 后显示的 pod 状态为 Unknown。 这里实际上 etcd 保存的状态为 NodeLost，只是显示时做了处理，与 daemonSet 做了区分。对应代码中的逻辑为:

### node controller

// 触发 NodeEviction 操作时会 DeletePods，这个删除为 GracefulDelete，
// apiserver rest 接口对 PodObj 添加了 DeletionTimestamp
func DeletePods(kubeClient clientset.Interface, recorder record.EventRecorder, nodeName, nodeUID string, daemonStore extensionslisters.DaemonSetLister) (bool, error) {
...
        for _, pod := range pods.Items {
...
        // Set reason and message in the pod object.
        if _, err = SetPodTerminationReason(kubeClient, &pod, nodeName); err != nil {
            if apierrors.IsConflict(err) {
                updateErrList = append(updateErrList,
                    fmt.Errorf("update status failed for pod %q: %v", format.Pod(&pod), err))
                continue
            }
        }
        // if the pod has already been marked for deletion, we still return true that there are remaining pods.
        if pod.DeletionGracePeriodSeconds != nil {
            remaining = true
            continue
        }
        // if the pod is managed by a daemonset, ignore it
        _, err := daemonStore.GetPodDaemonSets(&pod)
        if err == nil { // No error means at least one daemonset was found
            continue
        }

        glog.V(2).Infof("Starting deletion of pod %v/%v", pod.Namespace, pod.Name)
        recorder.Eventf(&pod, v1.EventTypeNormal, "NodeControllerEviction", "Marking for deletion Pod %s from Node %s", pod.Name, nodeName)
        if err := kubeClient.CoreV1().Pods(pod.Namespace).Delete(pod.Name, nil); err != nil {
            return false, err
        }
        remaining = true
    }
...
}

### staging apiserver REST 接口

// 对于优雅删除，到这里其实已经停止，不再进一步删除，剩下的交给 kubelet watch 到变化后去做 delete
func (e *Store) Delete(ctx genericapirequest.Context, name string, options *metav1.DeleteOptions) (runtime.Object, bool, error) {
...
    if graceful || pendingFinalizers || shouldUpdateFinalizers {
            err, ignoreNotFound, deleteImmediately, out, lastExisting = e.updateForGracefulDeletionAndFinalizers(ctx, name, key, options, preconditions, obj)
        }
    // !deleteImmediately covers all cases where err != nil. We keep both to be future-proof.
    if !deleteImmediately || err != nil {
        return out, false, err
    }
...
}
        
// stagging/apiserver中的 rest 接口调用，设置了 DeletionTimestamp 和 DeletionGracePeriodSeconds
func (e *Store) updateForGracefulDeletionAndFinalizers(ctx genericapirequest.Context, name, key string, options *metav1.DeleteOptions, preconditions storage.Preconditions, in runtime.Object) (err error, ignoreNotFound, deleteImmediately bool, out, lastExisting runtime.Object) {
...

        if options.GracePeriodSeconds != nil {
            period := int64(*options.GracePeriodSeconds)
            if period >= *objectMeta.GetDeletionGracePeriodSeconds() {
                return false, true, nil
            }
            newDeletionTimestamp := metav1.NewTime(
                objectMeta.GetDeletionTimestamp().Add(-time.Second * time.Duration(*objectMeta.GetDeletionGracePeriodSeconds())).
                    Add(time.Second * time.Duration(*options.GracePeriodSeconds)))
            objectMeta.SetDeletionTimestamp(&newDeletionTimestamp)
            objectMeta.SetDeletionGracePeriodSeconds(&period)
            return true, false, nil
        }
...
}

### node controller 

// SetPodTerminationReason 尝试设置 Pod状态和原因到 Pod 对象中
func SetPodTerminationReason(kubeClient clientset.Interface, pod *v1.Pod, nodeName string) (*v1.Pod, error) {
    if pod.Status.Reason == nodepkg.NodeUnreachablePodReason {
        return pod, nil
    }

    pod.Status.Reason = nodepkg.NodeUnreachablePodReason
    pod.Status.Message = fmt.Sprintf(nodepkg.NodeUnreachablePodMessage, nodeName, pod.Name)

    var updatedPod *v1.Pod
    var err error
    if updatedPod, err = kubeClient.CoreV1().Pods(pod.Namespace).UpdateStatus(pod); err != nil {
        return nil, err
    }
    return updatedPod, nil
}

### 命令行输出

// 打印输出时状态的切换，如果 "DeletionTimestamp 不为空" 且 "podStatus 为 NodeLost 状态"时，
// 显示的状态为 Unknown
func printPod(pod *api.Pod, options printers.PrintOptions) ([]metav1alpha1.TableRow, error) {
...
    if pod.DeletionTimestamp != nil && pod.Status.Reason == node.NodeUnreachablePodReason {
        reason = "Unknown"
    } else if pod.DeletionTimestamp != nil {
        reason = "Terminating"
    }
...
}

节点恢复 Ready 后 pod 状态

当节点恢复后，不同的 workload 对应的 pod 状态变化也是不同的。

deployment : 根据上一节描述，此时 pod 已经有正确的 pod 在其他节点 running，此时故障节点恢复后，kubelet 执行优雅删除，删除旧的 PodObj。

statefulset : statefulset 会从Unknown 状态变为 Terminating 状态，执行优雅删除，detach PV，然后执行重新调度与重建操作。

daemonset : daemonset 会从 NodeLost 状态直接变成 Running 状态，不涉及重建。

Statefulset 为什么没有重建?

我们往往会考虑下面两个问题，statefulset 为什么没有重建？ 如何保持单副本 statefulset 的高可用呢？

关于为什么没重建，首先简单介绍下 statefulset 控制器的逻辑。

Statefulset 控制器通过 StatefulSetControl 以及 StatefulPodControl 2个模块协调完成对 statefulSet 类型 workload 的状态管理（StatefulSetStatusUpdater）和扩缩控制（StatefulPodControl）。实际上，StatefulsetControl是对 StatefulPodControl 的调用来增删改 Pod。

StatefulSet 在 podManagementPolicy 为默认值 OrderedReady 时，会按照整数顺序单调递增的依次创建 Pod，否则在 Parallel 时，虽然是按整数，但是 Pod 是同时调度与创建。

具体的逻辑在核心方法 UpdateStatefulSet 中，见图:

我们看到的 Stateful Pod 一直处于 Unknown 状态的原因就是因为这个控制器屏蔽了对该 Pod 的操作。因为在第一节介绍了，NodeController 的 Pod Eviction 机制已经把 Pod 标记删除，PodObj 中包含的 DeletionTimestamp 被设置，StatefulSet Controller 代码检查 IsTerminating 符合条件，便直接 return 了。

// updateStatefulSet performs the update function for a StatefulSet. This method creates, updates, and deletes Pods in
// the set in order to conform the system to the target state for the set. The target state always contains
// set.Spec.Replicas Pods with a Ready Condition. If the UpdateStrategy.Type for the set is
// RollingUpdateStatefulSetStrategyType then all Pods in the set must be at set.Status.CurrentRevision.
// If the UpdateStrategy.Type for the set is OnDeleteStatefulSetStrategyType, the target state implies nothing about
// the revisions of Pods in the set. If the UpdateStrategy.Type for the set is PartitionStatefulSetStrategyType, then
// all Pods with ordinal less than UpdateStrategy.Partition.Ordinal must be at Status.CurrentRevision and all other
// Pods must be at Status.UpdateRevision. If the returned error is nil, the returned StatefulSetStatus is valid and the
// update must be recorded. If the error is not nil, the method should be retried until successful.
func (ssc *defaultStatefulSetControl) updateStatefulSet(
    ...
    for i := range replicas {
        ...
        // If we find a Pod that is currently terminating, we must wait until graceful deletion
        // completes before we continue to make progress.
        if isTerminating(replicas[i]) && monotonic {
            glog.V(4).Infof(
                "StatefulSet %s/%s is waiting for Pod %s to Terminate",
                set.Namespace,
                set.Name,
                replicas[i].Name)
            return &status, nil
        }
    ...
    }
}


// isTerminating returns true if pod's DeletionTimestamp has been set
func isTerminating(pod *v1.Pod) bool {
    return pod.DeletionTimestamp != nil
}