Advanced StatefulSet

This controller enhances the rolling update workflow of Kubernetes StatefulSet controller in large-scale scenarios, such as adding maxUnavailable and introducing in-place update strategy.

If you don’t know much about the Kubernetes StatefulSet, we strongly recommend you read its documents before learning Advanced StatefulSet.

Note that Advanced StatefulSet extends the same CRD schema of default StatefulSet with newly added fields. The CRD kind name is still StatefulSet. This is done on purpose so that user can easily migrate workload to the Advanced StatefulSet from the default StatefulSet. For example, one may simply replace the value of apiVersion in the StatefulSet yaml file from apps/v1 to apps.kruise.io/v1beta1 after installing Kruise manager.

  1. -  apiVersion: apps/v1
  2. +  apiVersion: apps.kruise.io/v1beta1
  3.    kind: StatefulSet
  4.    metadata:
  5.       name: sample
  6.    spec:
  7.    #...

Note that since Kruise v0.7.0, Advanced StatefulSet has been promoted to v1beta1, which is compatible with v1alpha1. And for Kruise version lower than v0.7.0, you can only use v1alpha1.

Pod Identity

StatefulSet Pods have a unique identity that consists of an ordinal, a stable network identity, and stable storage. The identity sticks to the Pod, regardless of which node it’s (re)scheduled on.

Ordinal Index

FEATURE STATE: Kruise v1.7.0

For a StatefulSet with N replicas, each Pod in the StatefulSet will be assigned an integer ordinal, that is unique over the Set. By default, pods will be assigned ordinals from 0 up through N-1. The StatefulSet controller will also add a pod label with this index: apps.kubernetes.io/pod-index, as follows:

  1. apiVersion: v1
  2. kind: Pod
  3. metadata:
  4.   labels:
  5.     app: sample
  6.     apps.kubernetes.io/pod-index: "0"
  7.   name: sample-0

Start ordinal

FEATURE STATE: Kruise v1.7.0

Pod start ordinal numbers start at 0 by default, and you can also set the pod start ordinal number by setting the .spec.ordinals.start field. To use this capability, you need to enable FeatureGate StatefulSetStartOrdinal=true.

  • .spec.ordinals.start: If the .spec.ordinals.start field is set, Pods will be assigned ordinals from .spec.ordinals.start up through .spec.ordinals.start + .spec.replicas - 1. For example: replicas=5, ordinals.start=3, Pod Range = [3, 7].
  1. apiVersion: apps.kruise.io/v1beta1
  2. kind: StatefulSet
  3. metadata:
  4.   name: sample
  5. spec:
  6.   replicas: 5
  7.   ordinals:
  8.     start: 3
  9.   serviceName: fake-service
  10.   selector:
  11.     matchLabels:
  12.       app: sample
  13.   template:
  14.     metadata:
  15.       labels:
  16.         app: sample
  17.     spec:
  18.       containers:
  19.       - name: main
  20.         image: nginx:alpine

User Stories

The main motivation of this feature is to support a more flexible StatefulSet, a building block in an ecosystem where Stateful applications can be migrated across Kubernetes clusters with more automation. As follows:

Story 1

Migrating across namespaces: Many organizations use namespaces for team isolation. Consider a team that is migrating a StatefulSet to a new namespace in a cluster. Migration could be motivated by a branding change, or a requirement to move out of a shared namespace. Consider the StatefulSet my-app with replicas: 5, running in a shared namespace.

  1. name: my-app
  2. namespace: shared
  3. replicas: 5
  4. -----------------------------------------------
  5. [ nginx-0, nginx-1, nginx-2, nginx-3, nginx-4 ]

To move two pods, the my-app StatefulSet in the shared namespace can be scaled down to replicas: 3, ordinals.start: 0, and an analogous StatefulSet in the app-team namespace scaled up to replicas: 2, ordinals.start: 3. This allows for pod ordinals to be managed during migration. The application operator should manage network connectivity, volumes and slice orchestration (when to migrate and by how many replicas).

  1. name: my-app                        name: my-app
  2. namespace: shared                   namespace: app-team
  3. replicas: 3                         replicas: 2
  4. ordinals.start: 0                   ordinals.start: 3
  5. ------------------------------      ---------------------
  6. [ nginx-0, nginx-1, nginx-2 ]       [ nginx-3, nginx-4 ]

The replicasStatefulSet and replicas fields should be updated jointly, depending on the requirements of the migration.

Story 2

Migrating across clusters: Organizations taking a multi cluster approach may need to move workloads across clusters due to capacity constraints, infrastructure constraints, or for better application isolation. Similar to namespace migration, the application operator should manage network connectivity, volumes and slice orchestration.

Story 3

Non-Zero Based Indexing: A user may want to number their StatefulSet starting from ordinal 1, rather than ordinal 0. Using 1 based numbering may be easier to reason about and conceptualize (eg: ordinal k is the k‘th replica, not the k+1‘th replica).

Scale features

PersistentVolumeClaim retention

FEATURE STATE: Kruise v1.1.0

If you have enabled the StatefulSetAutoDeletePVC feature-gate during Kruise installation or upgrade, you can use .spec.persistentVolumeClaimRetentionPolicy field to control if and how PVCs are deleted during the lifecycle of a StatefulSet.

This is same to the upstream StatefulSet (K8s >= 1.23 [alpha]), please refer to the upstream document for it.

Scaling with rate limiting

FEATURE STATE: Kruise v0.10.0

To avoid creating all failure pods at once when a new CloneSet applied, a maxUnavailable field for scale strategy has been added since Kruise v0.10.0.

  1. apiVersion: apps.kruise.io/v1beta1
  2. kind: StatefulSet
  3. spec:
  4.   # ...
  5.   replicas: 100
  6.   scaleStrategy:
  7.     maxUnavailable: 10% # percentage or absolute number

When this field has been set, Advanced StatefulSet will create pods with the guarantee that the number of unavailable pods during the update cannot exceed this value.

For example, the StatefulSet will firstly create 10 pods. After that, it will create one more pod only if one pod created has been running and ready.

Note that it can just be allowed to work with Parallel podManagementPolicy.

Ordinals reserve(skip)

Since Advanced StatefulSet v1beta1 (Kruise >= v0.7.0), it supports ordinals reserve.

By adding the ordinals to reserve into reserveOrdinals fields, Advanced StatefulSet will skip to create Pods with those ordinals. If these Pods have already existed, controller will delete them. Note that spec.replicas is the expectation number of running Pods and spec.reserveOrdinals is the ordinals that should be skipped.

  1. apiVersion: apps.kruise.io/v1beta1
  2. kind: StatefulSet
  3. spec:
  4.   # ...
  5.   replicas: 4
  6.   reserveOrdinals:
  7.   - 1

For an Advanced StatefulSet with replicas=4, reserveOrdinals=[1], the ordinals of running Pods will be [0,2,3,4].

  • If you want to migrate Pod-3 and reserve this ordinal, just append 3 into reserveOrdinals list. Then controller will delete Pod-3 and create Pod-5 (existing Pods will be [0,2,4,5]).
  • If you just want to delete Pod-3, you should append 3 into reserveOrdinals list and set replicas to 3. Then controller will delete Pod-3 (existing Pods will be [0,2,4]).

Specified Pod Deletion

FEATURE STATE: Kruise v1.5.5, Kruise v1.6.4, Kruise v1.7.2+

Compared to manually deleting a Pod directly, pod deletion by labeling pod with apps.kruise.io/specified-delete: true will be protected by the maxUnavailable of the Advanced StatefulSet during deletion, and it will trigger the PreparingDelete lifecycle hook (see below).

  1. apiVersion: v1
  2. kind: Pod
  3. metadata:
  4.   labels:
  5.     # ...
  6.     apps.kruise.io/specified-delete: true
  7. spec:
  8.   containers:
  9.   - name: main
  10.   # ...

When the controller receives the above Pod update, it will trigger the deletion process of the pod with specified deletion label and ensure that the maxUnavailable limit is not exceeded. The pod will be re-built by the workload if the ordinal is not reserved.

Update features

In-Place Update

Advanced StatefulSet adds a podUpdatePolicy field in spec.updateStrategy.rollingUpdate which controls recreate or in-place update for Pods.

  • ReCreate controller will delete old Pods and create new ones. This is the same behavior as default StatefulSet.
  • InPlaceIfPossible controller will try to in-place update Pod instead of recreating them if possible. Please ready the concept doc below.
  • InPlaceOnly controller will in-place update Pod instead of recreating them. With InPlaceOnly policy, user cannot modify any fields other than the fields that supported to in-place update.

You may need to read the concept doc for more details of in-place update.

We also bring graceful period into in-place update. Advanced StatefulSet 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.

  1. apiVersion: apps.kruise.io/v1beta1
  2. kind: StatefulSet
  3. spec:
  4.   # ...
  5.   podManagementPolicy: Parallel
  6.   updateStrategy:
  7.     type: RollingUpdate
  8.     rollingUpdate:
  9.       podUpdatePolicy: InPlaceIfPossible
  10.       inPlaceUpdateStrategy:
  11.         gracePeriodSeconds: 10

More importantly, a readiness-gate named InPlaceUpdateReady must be added into template.spec.readinessGates when using InPlaceIfPossible or InPlaceOnly. The condition InPlaceUpdateReady in podStatus will be updated to False before in-place update and updated to True after the update is finished. This ensures that pod remain at NotReady state while the in-place update is happening.

An example for StatefulSet using in-place update:

  1. apiVersion: apps.kruise.io/v1beta1
  2. kind: StatefulSet
  3. metadata:
  4.   name: sample
  5. spec:
  6.   replicas: 3
  7.   serviceName: fake-service
  8.   selector:
  9.     matchLabels:
  10.       app: sample
  11.   template:
  12.     metadata:
  13.       labels:
  14.         app: sample
  15.     spec:
  16.       readinessGates:
  17.          # A new condition that ensures the pod remains at NotReady state while the in-place update is happening
  18.       - conditionType: InPlaceUpdateReady
  19.       containers:
  20.       - name: main
  21.         image: nginx:alpine
  22.   podManagementPolicy: Parallel # allow parallel updates, works together with maxUnavailable
  23.   updateStrategy:
  24.     type: RollingUpdate
  25.     rollingUpdate:
  26.       # Do in-place update if possible, currently only image update is supported for in-place update
  27.       podUpdatePolicy: InPlaceIfPossible
  28.       # Allow parallel updates with max number of unavailable instances equals to 2
  29.       maxUnavailable: 2

Pre-download image for in-place update

FEATURE STATE: Kruise v0.10.0

If you have enabled the PreDownloadImageForInPlaceUpdate feature-gate during Kruise installation or upgrade, Advanced StatefulSet 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 Advanced StatefulSet is 1, which means the image is downloaded on nodes one by one. You can change the parallelism using apps.kruise.io/image-predownload-parallelism annotation on Advanced StatefulSet 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.

Since Kruise v1.1.0, you can use apps.kruise.io/image-predownload-min-updated-ready-pods to make sure the new image starting pre-download after a few new Pods have been updated ready. Its value can be absolute number or percentage.

  1. apiVersion: apps.kruise.io/v1beta1
  2. kind: StatefulSet
  3. metadata:
  4.   annotations:
  5.     apps.kruise.io/image-predownload-parallelism: "10"
  6.     apps.kruise.io/image-predownload-min-updated-ready-pods: "3"

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

Update sequence

Advanced StatefulSet adds a unorderedUpdate field in spec.updateStrategy.rollingUpdate, which contains strategies for non-ordered update. If unorderedUpdate is not nil, pods will be updated with non-ordered sequence. Noted that UnorderedUpdate can only be allowed to work with Parallel podManagementPolicy.

Currently unorderedUpdate only contains one field: priorityStrategy.

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,
  1. apiVersion: apps.kruise.io/v1beta1
  2. kind: StatefulSet
  3. spec:
  4.   # ...
  5.   updateStrategy:
  6.     rollingUpdate:
  7.       unorderedUpdate:
  8.         priorityStrategy:
  9.           weightPriority:
  10.           - weight: 50
  11.             matchSelector:
  12.               matchLabels:
  13.                 test-key: foo
  14.           - weight: 30
  15.             matchSelector:
  16.               matchLabels:
  17.                 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”.
  1. apiVersion: apps.kruise.io/v1beta1
  2. kind: StatefulSet
  3. spec:
  4.   # ...
  5.   updateStrategy:
  6.     rollingUpdate:
  7.       unorderedUpdate:
  8.         priorityStrategy:
  9.           orderPriority:
  10.           - orderedKey: some-label-key

MaxUnavailable

Advanced StatefulSet adds a maxUnavailable capability in the RollingUpdateStatefulSetStrategy to allow parallel Pod updates with the guarantee that the number of unavailable pods during the update cannot exceed this value. It is only allowed to use when the podManagementPolicy is Parallel.

This feature achieves similar update efficiency like Deployment for cases where the order of update is not critical to the workload. Without this feature, the native StatefulSet controller can only update Pods one by one even if the podManagementPolicy is Parallel.

  1. apiVersion: apps.kruise.io/v1beta1
  2. kind: StatefulSet
  3. spec:
  4.   # ...
  5.   podManagementPolicy: Parallel
  6.   updateStrategy:
  7.     type: RollingUpdate
  8.     rollingUpdate:
  9.       maxUnavailable: 20%

For example, assuming an Advanced StatefulSet has five Pods named P0 to P4, and the application can tolerate losing three replicas temporally. If we want to update the StatefulSet Pod spec from v1 to v2, we can perform the following steps using the MaxUnavailable feature for fast update.

  1. Set MaxUnavailable to 3 to allow three unavailable Pods maximally.
  2. Optionally, Set Partition to 4 in case canary update is needed. Partition means all Pods with an ordinal that is greater than or equal to the partition will be updated. In this case P4 will be updated even though MaxUnavailable is 3.
  3. After P4 finish update, change Partition to 0. The controller will update P1,P2 and P3 concurrently. Note that with default StatefulSet, the Pods will be updated sequentially in the order of P3, P2, P1.
  4. Once one of P1, P2 and P3 finishes update, P0 will be updated immediately.

Paused update

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

  1. apiVersion: apps.kruise.io/v1beta1
  2. kind: StatefulSet
  3. spec:
  4.   # ...
  5.   updateStrategy:
  6.     rollingUpdate:
  7.       paused: true

Lifecycle hook

FEATURE STATE: Kruise v0.8.0

This is similar to Lifecycle hook of CloneSet.