CloneSet

This controller provides advanced features to efficiently manage stateless applications that do not have instance order requirement during scaling and rollout. Analogously, CloneSet can be recognized as an enhanced version of upstream Deployment workload, but it does many more.

As name suggests, CloneSet is a Set -suffix controller which manages Pods directly. A sample CloneSet yaml looks like following:

apiVersion: apps.kruise.io/v1alpha1
kind: CloneSet
metadata:
  labels:
    app: sample
  name: sample
spec:
  replicas: 5
  selector:
    matchLabels:
      app: sample
  template:
    metadata:
      labels:
        app: sample
    spec:
      containers:
      - name: nginx
        image: nginx:alpine

Scale features

Support PVCs

CloneSet allows user to define PVC templates volumeClaimTemplates in CloneSetSpec, which can support PVCs per Pod. This cannot be done with Deployment. If not specified, CloneSet will only create Pods without PVCs.

A few reminders:

• Each PVC created by CloneSet has an owner reference. So when a CloneSet has been deleted, its PVCs will be cascading deleted.
• Each Pod and PVC created by CloneSet has a “apps.kruise.io/cloneset-instance-id“ label key. They use the same string as label value which is composed of a unique instance-id as suffix of the CloneSet name.
• When a Pod has been deleted by CloneSet controller, all PVCs related to it will be deleted together.
• When a Pod has been deleted manually, all PVCs related to the Pod are preserved, and CloneSet controller will create a new Pod with the same instance-id and reuse the PVCs.
• When a Pod is updated using recreate policy, all PVCs related to it will be deleted together.
• When a Pod is updated using in-place policy, all PVCs related to it are preserved.

The following shows a sample CloneSet yaml file which contains PVC templates.

apiVersion: apps.kruise.io/v1alpha1
kind: CloneSet
metadata:
  labels:
    app: sample
  name: sample-data
spec:
  replicas: 5
  selector:
    matchLabels:
      app: sample
  template:
    metadata:
      labels:
        app: sample
    spec:
      containers:
      - name: nginx
        image: nginx:alpine
        volumeMounts:
        - name: data-vol
          mountPath: /usr/share/nginx/html
  volumeClaimTemplates:
    - metadata:
        name: data-vol
      spec:
        accessModes: [ "ReadWriteOnce" ]
        resources:
          requests:
            storage: 20Gi

Selective Pod deletion

When a CloneSet is scaled down, sometimes user has preference to deleting specific Pods. This cannot be done using StatefulSet or Deployment, because StatefulSet always delete Pod in order and Deployment/ReplicaSet only delete Pod by its own sorted sequence.

CloneSet allows user to specify to-be-deleted Pod names when scaling down replicas. Take the following sample as an example,

apiVersion: apps.kruise.io/v1alpha1
kind: CloneSet
spec:
  # ...
  replicas: 4
  scaleStrategy:
    podsToDelete:
    - sample-9m4hp

when controller receives above update request, it ensures the number of replicas to be 4. If some Pods needs to be deleted, the Pods listed in podsToDelete will be deleted first. Controller will clear podsToDelete automatically once the listed Pods are deleted. Note that:

If one just adds a Pod name to podsToDelete and do not modify replicas, controller will delete this Pod, and create a new Pod. If one is unable to change CloneSet directly, an alternative way is to add a label apps.kruise.io/specified-delete: true onto the Pod waiting to delete.

Comparing to delete the Pod directly, using podsToDelete or apps.kruise.io/specified-delete: true will have CloneSet protection by maxUnavailable/maxSurge and lifecycle PreparingDelete triggering (See below).

Deletion Sequence

1. Node unassigned < assigned
2. PodPending < PodUnknown < PodRunning
3. Not ready < ready
4. Lower pod-deletion cost < higher pod-deletion-cost
5. Higher spread rank < lower spread rank
6. Been ready for empty time < less time < more time
7. Pods with containers with higher restart counts < lower restart counts
8. Empty creation time pods < newer pods < older pods

Pod deletion cost

FEATURE STATE: Kruise v0.9.0

The controller.kubernetes.io/pod-deletion-cost annotation is defined in Kubernetes since v1.21, Deployment/ReplicaSet will remove pods according to this cost when downscaling. And CloneSet has also supported it since Kruise v0.9.0.

The annotation should be set on the pod, the range is [-2147483647, 2147483647]. It represents the cost of deleting a pod compared to other pods belonging to the same CloneSet. Pods with lower deletion cost are preferred to be deleted before pods with higher deletion cost.

The implicit value for this annotation for pods that don’t set it is 0; negative values are permitted.

Deletion by Spread Constraints

FEATURE STATE: Kruise v0.10.0

The original proposal(design doc) is here.

Currently, it supports deletion by same node spread and deletion by pod topolocy spread constraints.

If there are Pod Topology Spread Constraints defined in CloneSet template, controller will choose pods according to spread constraints when the cloneset needs to scale down. Otherwise, controller will choose pods by same node spread by default when scaling down.

Short hash label

FEATURE STATE: Kruise v0.9.0

By default, CloneSet set the controller-revision-hash in Pod label to the full name of ControllerRevision, such as:

apiVersion: v1
kind: Pod
metadata:
  labels:
    controller-revision-hash: demo-cloneset-956df7994

It is joined by the name of CloneSet and the hash of the ControllerRevision. Length of the hash is usually 8~10 characters, and the label value in Kubernetes can not be more than 63 characters. So the name of CloneSet should be less than 52 characters.

A new feature-gate named CloneSetShortHash has been introduced. If it is enabled, CloneSet will only set the controller-revision-hash to the real hash, such as 956df7994. So there will be no limit to CloneSet name.

Don’t worry. Even if you enable the CloneSetShortHash, CloneSet will still recognize and manage the old Pods with full revision label.

Update features

In-place update

CloneSet provides three update policies just like AdvancedStatefulSet, defaults to ReCreate.

• ReCreate: controller will delete old Pods and PVCs and create new ones.
• InPlaceIfPossible: controller will try to in-place update Pod instead of recreating them if possible. Currently, only spec.template.metadata.* and spec.template.spec.containers[x].image field can be in-place updated.
• InPlaceOnly: controller will in-place update Pod instead of recreating them. With InPlaceOnly policy, user cannot modify any fields in spec.template other than spec.template.spec.containers[x].image.

When a Pod being in-place update, controller will firstly update Pod status to make it become not-ready using readinessGate, and then update images in Pod spec to trigger Kubelet recreate the container on Node. However, sometimes Kubelet recreate containers so fast that other controllers such as endpoints-controller in kcm have not noticed the Pod has turned to not-ready. This may lead to requests damaged.

So we bring graceful period into in-place update. CloneSet has supported gracePeriodSeconds, which is a period duration between controller update pod status and update pod images. So that endpoints-controller could have enough time to remove this Pod from endpoints.

apiVersion: apps.kruise.io/v1alpha1
kind: CloneSet
spec:
  # ...
  updateStrategy:
    type: InPlaceIfPossible
    inPlaceUpdateStrategy:
      gracePeriodSeconds: 10

Template and revision

spec.template defines the latest pod template in the CloneSet. Controller will calculate a revision hash for each version of spec.template when it has been initialized or modified. For example, when we create a sample CloneSet, controller will calculate the revision hash sample-744d4796cc and present the hash in CloneSet Status.

apiVersion: apps.kruise.io/v1alpha1
kind: CloneSet
metadata:
  generation: 1
  # ...
spec:
  replicas: 5
  # ...
status:
  observedGeneration: 1
  readyReplicas: 5
  replicas: 5
  currentRevision: sample-d4d4fb5bd
  updateRevision: sample-d4d4fb5bd
  updatedReadyReplicas: 5
  updatedReplicas: 5
  # ...

Here are the explanations for the counters presented in CloneSet status:

• status.replicas: Number of pods
• status.readyReplicas: Number of ready pods
• status.availableReplicas: Number of ready and available pods (satisfied with minReadySeconds)
• status.currentRevision: Latest revision hash that has used to be updated to all Pods
• status.updateRevision: Latest revision hash of this CloneSet
• status.updatedReplicas: Number of pods with the latest revision
• status.updatedReadyReplicas: Number of ready pods with the latest revision

Partition

Partition is the desired number or percent of Pods in old revisions, defaults to 0. This field does NOT imply any update order.

When partition is set during update:

• If it is a number: (replicas - partition) number of pods will be updated with the new version.
• If it is a percent: (replicas * (100% - partition)) number of pods will be updated with the new version.

For example, when we update sample CloneSet’s container image to nginx:mainline and set partition=3, after a while, the sample CloneSet yaml looks like the following:

apiVersion: apps.kruise.io/v1alpha1
kind: CloneSet
metadata:
  # ...
  generation: 2
spec:
  replicas: 5
  template:
    metadata:
      labels:
        app: sample
    spec:
      containers:
      - image: nginx:mainline
        imagePullPolicy: Always
        name: nginx
  updateStrategy:
    partition: 3
  # ...
status:
  observedGeneration: 2
  readyReplicas: 5
  replicas: 5
  currentRevision: sample-d4d4fb5bd
  updateRevision: sample-56dfb978d4
  updatedReadyReplicas: 2
  updatedReplicas: 2

Note that status.updateRevision has been updated to sample-56dfb978d4, a new hash. Since we set partition=3, controller only updates two Pods to the latest revision.

$ kubectl get pod -L controller-revision-hash
NAME           READY   STATUS    RESTARTS   AGE     CONTROLLER-REVISION-HASH
sample-chvnr   1/1     Running   0          6m46s   sample-d4d4fb5bd
sample-j6c4s   1/1     Running   0          6m46s   sample-d4d4fb5bd
sample-ns85c   1/1     Running   0          6m46s   sample-d4d4fb5bd
sample-jnjdp   1/1     Running   0          10s     sample-56dfb978d4
sample-qqglp   1/1     Running   0          18s     sample-56dfb978d4

Rollback by partition

FEATURE STATE: Kruise v0.9.0

By default, partition can only control Pods updating to the status.updateRevision. Which means for this CloneSet, when changes partition 5 -> 3, CloneSet will update 2 Pods to status.updateRevision. Then changes partition 3 -> 5 back, CloneSet will do nothing.

But if you have enabled CloneSetPartitionRollback feature-gate, in this case, CloneSet will update the 2 Pods in status.updateRevision back to status.currentRevision.

MaxUnavailable

MaxUnavailable is the maximum number of Pods that can be unavailable. Value can be an absolute number (e.g., 5) or a percentage of desired number of Pods (e.g., 10%). Default value is 20%.

apiVersion: apps.kruise.io/v1alpha1
kind: CloneSet
spec:
  # ...
  updateStrategy:
    maxUnavailable: 20%

Since Kruise v0.9.0, maxUnavailable not only controls Pods update, but also affect Pods specified deletion.

Which means if you declare to delete a Pod via podsToDelete or apps.kruise.io/specified-delete: true, CloneSet will delete it only if the number of unavailable Pods (comparing to the replicas number) is less than maxUnavailable.

MaxSurge

MaxSurge is the maximum number of pods that can be scheduled above the desired replicas. Value can be an absolute number (ex: 5) or a percentage of desired pods (ex: 10%). Defaults to 0.

If maxSurge is set somewhere, cloneset-controller will create maxSurge number of Pods above the replicas, when it finds multiple active revisions of Pods which means the CloneSet is in the update stage. After all Pods except partition number have been updated to the latest revision, maxSurge number Pods will be deleted, and the number of Pods will be equal to the replica number.

What’s more, maxSurge is forbidden to use with InPlaceOnly policy. When maxSurge is used with InPlaceIfPossible, controller will create additional Pods with latest revision first, and then update the rest Pods with old revisions,

apiVersion: apps.kruise.io/v1alpha1
kind: CloneSet
spec:
  # ...
  updateStrategy:
    maxSurge: 3

Since Kruise v0.9.0, maxSurge not only controls Pods update, but also affect Pods specified deletion.

Which means if you declare to delete a Pod via podsToDelete or apps.kruise.io/specified-delete: true, CloneSet may create new a Pod, wait it to be ready, and them delete the old one. It depends on maxUnavailable and the current number of unavailable Pods.

For example:

• For a CloneSet maxUnavailable=2, maxSurge=1 and currently only one unavailable Pods is pod-a, if you patch apps.kruise.io/specified-delete: true onto pod-b or put the Pod name into podsToDelete, CloneSet will delete it directly.
• For a CloneSet maxUnavailable=1, maxSurge=1 and currently only one unavailable Pods is pod-a, if you patch apps.kruise.io/specified-delete: true onto pod-b or put the Pod name into podsToDelete, CloneSet will create a new Pod, waiting it to be ready, and finally delete pod-b.
• For a CloneSet maxUnavailable=1, maxSurge=1 and currently only one unavailable Pods is pod-a, if you patch apps.kruise.io/specified-delete: true onto pod-a or put the Pod name into podsToDelete, CloneSet will delete it directly.
• …

Update sequence

When controller chooses Pods to update, it has default sort logic based on Pod phase and conditions: unscheduled < scheduled, pending < unknown < running, not-ready < ready. In addition, CloneSet also supports advanced priority and scatter strategies to allow users to specify the update order.

priority strategy

This strategy defines rules for calculating the priority of updating pods. All update candidates will be applied with the priority terms. priority can be calculated either by weight or by order.

• weight: Priority is determined by the sum of weights for terms that match selector. For example,

apiVersion: apps.kruise.io/v1alpha1
kind: CloneSet
spec:
  # ...
  updateStrategy:
    priorityStrategy:
      weightPriority:
      - weight: 50
        matchSelector:
          matchLabels:
            test-key: foo
      - weight: 30
        matchSelector:
          matchLabels:
            test-key: bar

• order: Priority will be determined by the value of the orderKey. The update candidates are sorted based on the “int” part of the value string. For example, 5 in string “5” and 10 in string “sts-10”.

apiVersion: apps.kruise.io/v1alpha1
kind: CloneSet
spec:
  # ...
  updateStrategy:
    priorityStrategy:
      orderPriority:
        - orderedKey: some-label-key

scatter strategy

This strategy defines rules to make certain Pods be scattered during update. For example, if a CloneSet has replica=10, and we add foo=bar label in 3 Pods and specify the following scatter rule. These 3 Pods will be the 1st, 6th and 10th updated Pods.

apiVersion: apps.kruise.io/v1alpha1
kind: CloneSet
spec:
  # ...
  updateStrategy:
    scatterStrategy:
    - key: foo
      value: bar

Note that:

• Although priority strategy and scatter strategy can be applied together, we strongly suggest to just use one of them to avoid confusion.
• If scatter strategy is used, we suggest to just use one term. Otherwise, the update order can be hard to understand.

Last but not the least, the above advanced update strategies require independent Pod labeling mechanisms, which are not provided by CloneSet.

Paused update

paused indicates that Pods updating is paused, controller will not update Pods but just maintain the number of replicas.

apiVersion: apps.kruise.io/v1alpha1
kind: CloneSet
spec:
  # ...
  updateStrategy:
    paused: true

Pre-download image for in-place update

FEATURE STATE: Kruise v0.9.0

If you have enabled the PreDownloadImageForInPlaceUpdate feature-gate during Kruise installation or upgrade, CloneSet controller will automatically pre-download the image you want to update to the nodes of all old Pods. It is quite useful to accelerate the progress of applications upgrade.

The parallelism of each new image pre-downloading by CloneSet is 1, which means the image is downloaded on nodes one by one. You can change the parallelism using the annotation on CloneSet according to the capability of image registry, for registries with more bandwidth and P2P image downloading ability, a larger parallelism can speed up the pre-download process.

apiVersion: apps.kruise.io/v1alpha1
kind: CloneSet
metadata:
  annotations:
    apps.kruise.io/image-predownload-parallelism: "5"

Note that to avoid most unnecessary image downloading, now controller will only pre-download images for CloneSet with replicas > 3.

Lifecycle hook

Each Pod managed by CloneSet has a clear state defined in lifecycle.apps.kruise.io/state label:

• Normal
• PreparingUpdate
• Updating
• Updated
• PreparingDelete

Lifecycle hook allows users to do something (for example remove pod from service endpoints) during Pod deleting and before/after in-place update.

type LifecycleStateType string
// Lifecycle contains the hooks for Pod lifecycle.
type Lifecycle struct {
    // PreDelete is the hook before Pod to be deleted.
    PreDelete *LifecycleHook `json:"preDelete,omitempty"`
    // InPlaceUpdate is the hook before Pod to update and after Pod has been updated.
    InPlaceUpdate *LifecycleHook `json:"inPlaceUpdate,omitempty"`
}
type LifecycleHook struct {
    LabelsHandler     map[string]string `json:"labelsHandler,omitempty"`
    FinalizersHandler []string          `json:"finalizersHandler,omitempty"`
}

Examples:

apiVersion: apps.kruise.io/v1alpha1
kind: CloneSet
spec:
  # define with finalizer
  lifecycle:
    preDelete:
      finalizersHandler:
      - example.io/unready-blocker
    inPlaceUpdate:
      finalizersHandler:
      - example.io/unready-blocker
  # or define with label
  lifecycle:
    inPlaceUpdate:
      labelsHandler:
        example.io/block-unready: "true"

State circulation

• When CloneSet delete a Pod (including scale in and recreate update):
  • Delete it directly if no lifecycle hook definition or Pod not matched preDelete hook
  • Otherwise, CloneSet will firstly update Pod to PreparingDelete state and wait for user controller to remove the label/finalizer and Pod not matched preDelete hook
  • Note that Pods in PreparingDelete state will not be updated
• When CloneSet update a Pod in-place:
  • If lifecycle hook defined and Pod matched inPlaceUpdate hook, CloneSet will update Pod to PreparingUpdate state
  • After user controller remove the label/finalizer (thus Pod not matched inPlaceUpdate hook), CloneSet will update it to Updating state and start updating
  • After in-place update completed, CloneSet will update Pod to Updated state if lifecycle hook defined and Pod not matched inPlaceUpdate hook
  • When user controller add label/finalizer into Pod and it matched inPlaceUpdate hook, CloneSet will finally update it to Normal state

Besides, although our design supports to change a Pod from PreparingDelete back to Normal (through cancel specified delete), but it is not recommended. Because Pods in PreparingDelete state will not be updated by CloneSet, it might be updating immediately if comes back to Normal. This case is hard for user controller to handle.

Example for user controller logic

Same as yaml example above, we can fisrtly define：

• example.io/unready-blocker finalizer as hook
• example.io/initialing annotation as identity for initializing

Add these fields into CloneSet template:

apiVersion: apps.kruise.io/v1alpha1
kind: CloneSet
spec:
  template:
    metadata:
      annotations:
        example.io/initialing: "true"
      finalizers:
      - example.io/unready-blocker
  # ...
  lifecycle:
    preDelete:
      finalizersHandler:
      - example.io/unready-blocker
    inPlaceUpdate:
      finalizersHandler:
      - example.io/unready-blocker

User controller logic:

• For Pod in Normal state, if there is example.io/initialing: true in annotation and ready condition in Pod status is True, then add it to endpoints and remove the annotation
• For Pod in PreparingDelete and PreparingUpdate states, delete it from endpoints and remove example.io/unready-blocker finalizer
• For Pod in Updated state, add it to endpoints and add example.io/unready-blocker finalizer