K8s工作负载-StatefulSet

基于1.25

什么是StatefulSet

StatefulSet负责管理有状态应用，缩写sts

  • 要求管理的Pod具有稳定的网络标识符和存储卷，实现又状态应用的数据持久化和数据访问
  • 管理Pod，会对Pod有一个按照顺序增大的ID

使用场景

  • 具有稳定、唯一的网络标识符（DNS Name）
  • 每个Pod是中对应各自的存储路径（PersistVolumeClaimTempleate）
  • 按照顺序增加副本、减少副本，并且在减少副本的时执行清理
  • 按照顺序执行滚动更新

限制

  • Pod存储的要么由Storage Class的PVC提供，要么事先创建
  • 删除或缩容一个StatefulSet不会删除对应的数据卷，确保数据安全
  • 在删除StatefulSet，无法确保Pod的终止的正常的
    • 如果需要，需要使用优雅的终止，需要先Scale Down到0
  • 在使用默认的Pod Management Policy（OraderReady）进行滚动更新，可能会进入到错误状态，需要人工介入

⚠️：不要时强制删除StatefulSet管理的Pod，本身机制，只会最多提供一个对外访问。

强制删除，可能出现超过一个Pod对外提供

StatefulSetSpec

// A StatefulSetSpec is the specification of a StatefulSet.
type StatefulSetSpec struct {
	// replicas is the desired number of replicas of the given Template.
	// These are replicas in the sense that they are instantiations of the
	// same Template, but individual replicas also have a consistent identity.
	// If unspecified, defaults to 1.
	// TODO: Consider a rename of this field.
	// +optional
  // 期望的Pod数量，默认1
	Replicas *int32 `json:"replicas,omitempty" protobuf:"varint,1,opt,name=replicas"`

	// selector is a label query over pods that should match the replica count.
	// It must match the pod template's labels.
	// More info: https://kubernetes.io/docs/concepts/overview/working-with-objects/labels/#label-selectors
  // 标签选择器，必须和PodTemplate匹配
	Selector *metav1.LabelSelector `json:"selector" protobuf:"bytes,2,opt,name=selector"`

	// template is the object that describes the pod that will be created if
	// insufficient replicas are detected. Each pod stamped out by the StatefulSet
	// will fulfill this Template, but have a unique identity from the rest
	// of the StatefulSet. Each pod will be named with the format
	// <statefulsetname>-<podindex>. For example, a pod in a StatefulSet named
	// "web" with index number "3" would be named "web-3".
	// The only allowed template.spec.restartPolicy value is "Always".
  // 描述Pod 的Template
	Template v1.PodTemplateSpec `json:"template" protobuf:"bytes,3,opt,name=template"`

	// volumeClaimTemplates is a list of claims that pods are allowed to reference.
	// The StatefulSet controller is responsible for mapping network identities to
	// claims in a way that maintains the identity of a pod. Every claim in
	// this list must have at least one matching (by name) volumeMount in one
	// container in the template. A claim in this list takes precedence over
	// any volumes in the template, with the same name.
	// TODO: Define the behavior if a claim already exists with the same name.
	// +optional
	// +listType=atomic
  // 允许Pod引用的PVC列表
	VolumeClaimTemplates []v1.PersistentVolumeClaim `json:"volumeClaimTemplates,omitempty" protobuf:"bytes,4,rep,name=volumeClaimTemplates"`

	// serviceName is the name of the service that governs this StatefulSet.
	// This service must exist before the StatefulSet, and is responsible for
	// the network identity of the set. Pods get DNS/hostnames that follow the
	// pattern: pod-specific-string.serviceName.default.svc.cluster.local
	// where "pod-specific-string" is managed by the StatefulSet controller.
  // 管理此StatefulSet的Service Name
	ServiceName string `json:"serviceName" protobuf:"bytes,5,opt,name=serviceName"`

	// podManagementPolicy controls how pods are created during initial scale up,
	// when replacing pods on nodes, or when scaling down. The default policy is
	// `OrderedReady`, where pods are created in increasing order (pod-0, then
	// pod-1, etc) and the controller will wait until each pod is ready before
	// continuing. When scaling down, the pods are removed in the opposite order.
	// The alternative policy is `Parallel` which will create pods in parallel
	// to match the desired scale without waiting, and on scale down will delete
	// all pods at once.
	// +optional
  // StatefulSet的管理模式，可选为OrderedReady和Parallel ，默认OrderedReady
	PodManagementPolicy PodManagementPolicyType `json:"podManagementPolicy,omitempty" protobuf:"bytes,6,opt,name=podManagementPolicy,casttype=PodManagementPolicyType"`

	// updateStrategy indicates the StatefulSetUpdateStrategy that will be
	// employed to update Pods in the StatefulSet when a revision is made to
	// Template.
  // 对于Template进行修改时 更新StatefulSet中Pod的策略
	UpdateStrategy StatefulSetUpdateStrategy `json:"updateStrategy,omitempty" protobuf:"bytes,7,opt,name=updateStrategy"`

	// revisionHistoryLimit is the maximum number of revisions that will
	// be maintained in the StatefulSet's revision history. The revision history
	// consists of all revisions not represented by a currently applied
	// StatefulSetSpec version. The default value is 10.
  // 限制StaetfulSet修订历史的数量
	RevisionHistoryLimit *int32 `json:"revisionHistoryLimit,omitempty" protobuf:"varint,8,opt,name=revisionHistoryLimit"`

	// Minimum number of seconds for which a newly created pod should be ready
	// without any of its container crashing for it to be considered available.
	// Defaults to 0 (pod will be considered available as soon as it is ready)
	// +optional
  // 最小就绪准备时间
	MinReadySeconds int32 `json:"minReadySeconds,omitempty" protobuf:"varint,9,opt,name=minReadySeconds"`

	// persistentVolumeClaimRetentionPolicy describes the lifecycle of persistent
	// volume claims created from volumeClaimTemplates. By default, all persistent
	// volume claims are created as needed and retained until manually deleted. This
	// policy allows the lifecycle to be altered, for example by deleting persistent
	// volume claims when their stateful set is deleted, or when their pod is scaled
	// down. This requires the StatefulSetAutoDeletePVC feature gate to be enabled,
	// which is beta.
	// +optional
  // 创建VolumeClaimTemplates 的持久卷生命周期
  // 默认情况下持久卷生命都保留，此选项，可以设置删除声明
  // 处于alpha 需要启用StatefulSetAutoDeletePVC门控
	PersistentVolumeClaimRetentionPolicy *StatefulSetPersistentVolumeClaimRetentionPolicy `json:"persistentVolumeClaimRetentionPolicy,omitempty" protobuf:"bytes,10,opt,name=persistentVolumeClaimRetentionPolicy"`

	// ordinals controls the numbering of replica indices in a StatefulSet. The
	// default ordinals behavior assigns a "0" index to the first replica and
	// increments the index by one for each additional replica requested.
	// +optional
	Ordinals *StatefulSetOrdinals `json:"ordinals,omitempty" protobuf:"bytes,11,opt,name=ordinals"`
}

Pod管理策略

  • OrderedReady：默认值
    • 创建副本数为N的StatefulSet，Pod按照（0，1…N-1)顺序依次创建
    • 删除副本数为N的StatefulSet，Pod按照（N-1…0）顺序依次删除
    • 在对StatefulSet进行扩容操作，新增Pod所有前序Pod必须是Running或者Ready状态
    • 终止或者删除Pod，后序所有Pod必须终止
  • Paralled：创建或者终止所有Pod

Pod更新策略

提供了俩种更新策略

  • OnDelete：更新，不会销毁任何Pod，只有手动删除一个Pod或者STS对象的时候，才会按照新的定义创建Pod
    • 会更新所有的Pod
    • 网络标识符不会变
  • RollingUpdate：逐步更新Pod，默认更新策略（更新速度慢一点）
    • 从次序大Pod，逐个更新Pod，至到最小的Pod被更新
    • 正在更新的Pod进入Running或者Ready，才继续更新前面一个Pod
    • 每一个Pod网络标识符可能会变化

⚠️使用默认管理策略OrderedReady，可能进入卡住

  • 更新Pod的Template，导致某一个Pod一致无法启动
  • STS停止滚动更新
  • 需要删除所有的使用有问题的tamplate的Pod

RollingUpdate参数

// RollingUpdateStatefulSetStrategy is used to communicate parameter for RollingUpdateStatefulSetStrategyType.
type RollingUpdateStatefulSetStrategy struct {
	// Partition indicates the ordinal at which the StatefulSet should be partitioned
	// for updates. During a rolling update, all pods from ordinal Replicas-1 to
	// Partition are updated. All pods from ordinal Partition-1 to 0 remain untouched.
	// This is helpful in being able to do a canary based deployment. The default value is 0.
	// +optional
  // 指示StatefulSet应该被分区进行更新的序号
	Partition *int32 `json:"partition,omitempty" protobuf:"varint,1,opt,name=partition"`
	// The maximum number of pods that can be unavailable during the update.
	// Value can be an absolute number (ex: 5) or a percentage of desired pods (ex: 10%).
	// Absolute number is calculated from percentage by rounding up. This can not be 0.
	// Defaults to 1. This field is alpha-level and is only honored by servers that enable the
	// MaxUnavailableStatefulSet feature. The field applies to all pods in the range 0 to
	// Replicas-1. That means if there is any unavailable pod in the range 0 to Replicas-1, it
	// will be counted towards MaxUnavailable.
	// +optional
  // 更新期间最大不可用Pod数量  默认1
  // 属于alpha级别，需要启用MaxUnavaileableStatefulSet
	MaxUnavailable *intstr.IntOrString `json:"maxUnavailable,omitempty" protobuf:"varint,2,opt,name=maxUnavailable"`
}