WorkloadSpread

FEATURE STATE: Kruise v0.10.0

WorkloadSpread can distribute Pods of workload to different types of Node according to some polices, which empowers single workload the abilities for multi-domain deployment and elastic deployment.

Some common policies include:

• fault toleration spread (for example, spread evenly among hosts, az, etc)
• spread according to the specified ratio (for example, deploy Pod to several specified az according to the proportion)
• subset management with priority, such as
  • deploy Pods to ecs first, and then deploy to eci when its resources are insufficient.
  • deploy a fixed number of Pods to ecs first, and the rest Pods are deployed to eci.
• subset management with customization, such as
  • control how many pods in a workload are deployed in different cpu arch
  • enable pods in different cpu arch to have different resource requirements

The feature of WorkloadSpread is similar with UnitedDeployment in OpenKruise community. Each WorkloadSpread defines multi-domain called subset. Each domain may provide the limit to run the replicas number of pods called maxReplicas. WorkloadSpread injects the domain configuration into the Pod by Webhook, and it also controls the order of scale in and scale out.

Currently, supported workload: CloneSet、Deployment、ReplicaSet.

WorkloadSpread Demo

apiVersion: apps.kruise.io/v1alpha1
kind: WorkloadSpread
metadata:
  name: workloadspread-demo
spec:
  targetRef:
    apiVersion: apps/v1 | apps.kruise.io/v1alpha1
    kind: Deployment | CloneSet
    name: workload-xxx
  subsets:
    - name: subset-a
      requiredNodeSelectorTerm:
        matchExpressions:
          - key: topology.kubernetes.io/zone
            operator: In
            values:
              - zone-a
    preferredNodeSelectorTerms:
      - weight: 1
        preference:
        matchExpressions:
          - key: another-node-label-key
            operator: In
            values:
              - another-node-label-value
      maxReplicas: 3
      tolertions: []
      patch:
        metadata:
          labels:
            xxx-specific-label: xxx
    - name: subset-b
      requiredNodeSelectorTerm:
        matchExpressions:
          - key: topology.kubernetes.io/zone
            operator: In
            values:
              - zone-b
  scheduleStrategy:
    type: Adaptive | Fixed
    adaptive:
      rescheduleCriticalSeconds: 30

targetRef: specify the target workload. Can not be mutated，and one workload can only correspond to one WorkloadSpread.

spec.subsets

subsets consists of multiple domain called subset, and each topology has different configuration.

subset sub-fields

• name: the name of subset, it is distinct in a WorkloadSpread, which represents a topology.

• maxReplicas：the replicas limit of subset, and must be Integer and >= 0. There is no replicas limit while the maxReplicas is nil.

Don’t support percentage type in current version.

• requiredNodeSelectorTerm: match zone hardly。

• preferredNodeSelectorTerms: match zone softly。

Caution：requiredNodeSelectorTerm corresponds the requiredDuringSchedulingIgnoredDuringExecution of nodeAffinity. preferredNodeSelectorTerms corresponds the preferredDuringSchedulingIgnoredDuringExecution of nodeAffinity.

• tolerations: the tolerations of Pod in subset.

tolerations:
- key: "key1"
  operator: "Equal"
  value: "value1"
  effect: "NoSchedule"

• patch: customize the Pod configuration of subset, such as Annotations, Labels, Env.

Example:

# patch pod with a topology label:
patch:
  metadata:
    labels:
      topology.application.deploy/zone: "zone-a"

# patch pod container resources:
patch:
  spec:
    containers:
    - name: main
      resources:
        limit:
          cpu: "2"
          memory: 800Mi

# patch pod container env with a zone name:
patch:
  spec:
    containers:
    - name: main
      env:
      - name: K8S_AZ_NAME
        value: zone-a

Schedule strategy

WorkloadSpread provides two kind strategies, the default strategy is Fixed.

Fixed:

Workload is strictly spread according to the definition of the subset.

Adaptive

Reschedule: Kruise will check the unschedulable Pods of subset. If it exceeds the defined duration, the failed Pods will be rescheduled to the other subset.

Requirements

WorkloadSpread defaults to be disabled. You have to configure the feature-gate WorkloadSpread when install or upgrade Kruise:

$ helm install kruise https://... --set featureGates="WorkloadSpread=true"

Pod Webhook

WorkloadSpread uses webhook to inject fault domain rules.

If the PodWebhook feature-gate is set to false, WorkloadSpread will also be disabled.

deletion-cost feature

CloneSet has supported deletion-cost feature in the latest versions.

The other native workload need kubernetes version >= 1.21. (In 1.21, users need to enable PodDeletionCost feature-gate, and since 1.22 it will be enabled by default)

Scale order:

The workload managed by WorkloadSpread will scale according to the defined order in spec.subsets.

The order of subset in spec.subsets can be changed, which can adjust the scale order of workload.

Scale out

• The Pods are scheduled in the subset order defined in the spec.subsets. It will be scheduled in the next subset while the replica number reaches the maxReplicas of subset

Scale in

• When the replica number of the subset is greater than the maxReplicas, the extra Pods will be removed in a high priority.
• According to the subset order in the spec.subsets, the Pods of the subset at the back are deleted before the Pods at the front.

#             subset-a   subset-b  subset-c
# maxReplicas    10          10        nil
# pods number    10          10        10
# deletion order: c -> b -> a
#             subset-a   subset-b  subset-c
# maxReplicas    10          10        nil
# pods number    20          20        20
# deletion order: b -> a -> c

feature-gates

WorkloadSpread feature is turned off by default, if you want to turn it on set feature-gates WorkloadSpread.

$ helm install kruise https://... --set featureGates="WorkloadSpread=true"

Example

Elastic deployment

zone-a(ACK) holds 100 Pods, zone-b(ECI) as an elastic zone holds additional Pods.

1. Create a WorkloadSpread instance.

apiVersion: apps.kruise.io/v1alpha1
kind: WorkloadSpread
metadta:
  name: ws-demo
  namespace: deploy
spec:
  targetRef: # workload in the same namespace
    apiVersion: apps.kruise.io/v1alpha1
    kind: CloneSet
    name: workload-xxx
  subsets:
  - name: ACK # zone ACK
    requiredNodeSelectorTerm:
      matchExpressions:
      - key: topology.kubernetes.io/zone
        operator: In
        values:
        - ack
    maxReplicas: 100
    patch: # inject label.
      metadata:
        labels:
          deploy/zone: ack
  - name: ECI # zone ECI
    requiredNodeSelectorTerm:
      matchExpressions:
      - key: topology.kubernetes.io/zone
        operator: In
        values:
        - eci
    patch:
      metadata:
        labels:
          deploy/zone: eci

1. Creat a corresponding workload, the number of replicas ca be adjusted freely.

Effect

• When the number of replicas <= 100, the Pods are scheduled in ACK zone.
• When the number of replicas > 100, the 100 Pods are in ACK zone, the extra Pods are scheduled in ECI zone.
• The Pods in ECI elastic zone are removed first when scaling in.

Multi-domain deployment

Deploy 100 Pods to two zone(zone-a, zone-b) separately.

1. Create a WorkloadSpread instance.

apiVersion: apps.kruise.io/v1alpha1
kind: WorkloadSpread
metadta:
  name: ws-demo
  namespace: deploy
spec:
  targetRef:
    apiVersion: apps.kruise.io/v1alpha1
    kind: CloneSet
    name: workload-xxx
  subsets:
  - name: subset-a
    requiredNodeSelectorTerm:
      matchExpressions:
      - key: topology.kubernetes.io/zone
        operator: In
        values:
        - zone-a
    maxReplicas: 100
    patch:
      metadata:
        labels:
          deploy/zone: zone-a
  - name: subset-b
    requiredNodeSelectorTerm:
      matchExpressions:
      - key: topology.kubernetes.io/zone
        operator: In
        values:
        - zone-b
    maxReplicas: 100
    patch:
      metadata:
        labels:
          deploy/zone: zone-b

1. Creat a corresponding workload with a 200 replicas, or perform a rolling update on an existing workload.

2. If the spread of zone needs to be changed, first adjust the maxReplicas of subset, and then change the replicas of workload.