If your pods have no requirements for how or where to run, you can let Karpenter choose nodes from the full range of available cloud provider resources. However, by taking advantage of Karpenter’s model of layered constraints, you can be sure that the precise type and amount of resources needed are available to your pods. Reasons for constraining where your pods run could include:
- Needing to run in zones where dependent applications or storage are available
- Requiring certain kinds of processors or other hardware
- Wanting to use techniques like topology spread to help insure high availability
Your Cloud Provider defines the first layer of constraints, including all instance types, architectures, zones, and purchase types available to its cloud. The cluster administrator adds the next layer of constraints by creating one or more provisioners. The final layer comes from you adding specifications to your Kubernetes pod deployments. Pod scheduling constraints must fall within a provisioner’s constraints or the pods will not deploy. For example, if the provisioner sets limits that allow only a particular zone to be used, and a pod asks for a different zone, it will not be scheduled.
Constraints you can request include:
- Resource requests: Request that certain amount of memory or CPU be available.
- Node selection: Choose to run on a node that is has a particular label (
- Node affinity: Draws a pod to run on nodes with particular attributes (affinity).
- Topology spread: Use topology spread to help insure availability of the application.
- Pod affinity/anti-affinity: Draws pods towards or away from topology domains based on the scheduling of other pods.
Karpenter supports standard Kubernetes scheduling constraints. This allows you to define a single set of rules that apply to both existing and provisioned capacity. Pod affinity is a key exception to this rule.
NoteKarpenter supports specific Well-Known Labels, Annotations and Taints that are useful for scheduling.
Within a Pod spec, you can both make requests and set limits on resources a pod needs, such as CPU and memory. For example:
apiVersion: v1 kind: Pod metadata: name: myapp spec: containers: - name: app image: myimage resources: requests: memory: "128Mi" cpu: "500m" limits: memory: "256Mi" cpu: "1000m"
In this example, the container is requesting 128MiB of memory and .5 CPU.
Its limits are set to 256MiB of memory and 1 CPU.
Instance type selection math only uses
limits may be configured to enable resource oversubscription.
See Managing Resources for Containers for details on resource types supported by Kubernetes, Specify a memory request and a memory limit for examples of memory requests, and Provisioning Configuration for a list of supported resources.
nodeSelector you can ask for a node that matches selected key-value pairs.
This can include well-known labels or custom labels you create yourself.
nodeSelector is like node affinity, it doesn’t have the same “and/or” matchExpressions that affinity has.
So all key-value pairs must match if you use
nodeSelector can do only do inclusions, while
affinity can do inclusions and exclusions (
Here is an example of a
nodeSelector for selecting nodes:
nodeSelector: topology.kubernetes.io/zone: us-west-2a karpenter.sh/capacity-type: spot
This example features a well-known label (
topology.kubernetes.io/zone) and a label that is well known to Karpenter (
If you want to create a custom label, you should do that at the provisioner level. Then the pod can declare that custom label.
See nodeSelector in the Kubernetes documentation for details.
Examples below illustrate how to use Node affinity to include (
In) and exclude (
See Node affinity for details.
When setting rules, the following Node affinity types define how hard or soft each rule is:
- requiredDuringSchedulingIgnoredDuringExecution: This is a hard rule that must be met.
- preferredDuringSchedulingIgnoredDuringExecution: This is a preference, but the pod can run on a node where it is not guaranteed.
IgnoredDuringExecution part of each tells the pod to keep running, even if conditions change on the node so the rules no longer matched.
You can think of these concepts as
preferred, since Kubernetes never implemented other variants of these rules.
All examples below assume that the provisioner doesn’t have constraints to prevent those zones from being used.
The first constraint says you could use
us-west-2b, the second constraint makes it so only
us-west-2b can be used.
affinity: nodeAffinity: requiredDuringSchedulingIgnoredDuringExecution: nodeSelectorTerms: - matchExpressions: - key: "topology.kubernetes.io/zone" operator: "In" values: ["us-west-2a, us-west-2b"] - key: "topology.kubernetes.io/zone" operator: "In" values: ["us-west-2b"]
Changing the second operator to
NotIn would allow the pod to run in
- key: "topology.kubernetes.io/zone" operator: "In" values: ["us-west-2a, us-west-2b"] - key: "topology.kubernetes.io/zone" operator: "NotIn" values: ["us-west-2b"]
Continuing to add to the example,
nodeAffinity lets you define terms so if one term doesn’t work it goes to the next one.
us-west-2a is not available, the second term will cause the pod to run on a spot instance in
affinity: nodeAffinity: requiredDuringSchedulingIgnoredDuringExecution: nodeSelectorTerms: - matchExpressions: # OR - key: "topology.kubernetes.io/zone" # AND operator: "In" values: ["us-west-2a, us-west-2b"] - key: "topology.kubernetes.io/zone" # AND operator: "NotIn" values: ["us-west-2b"] - matchExpressions: # OR - key: "karpenter.sh/capacity-type" # AND operator: "In" values: ["spot"] - key: "topology.kubernetes.io/zone" # AND operator: "In" values: ["us-west-2d"]
In general, Karpenter will go through each of the
nodeSelectorTerms in order and take the first one that works.
However, if Karpenter fails to provision on the first
nodeSelectorTerms, it will try again using the second one.
If they all fail, Karpenter will fail to provision the pod.
Karpenter will backoff and retry over time.
So if capacity becomes available, it will schedule the pod without user intervention.
Taints and tolerations
Taints are the opposite of affinity. Setting a taint on a node tells the scheduler to not run a pod on it unless the pod has explicitly said it can tolerate that taint. This example shows a Provisioner that was set up with a taint for only running pods that require a GPU, such as the following:
apiVersion: karpenter.sh/v1alpha5 kind: Provisioner metadata: name: gpu spec: requirements: - key: node.kubernetes.io/instance-type operator: In values: - p3.2xlarge - p3.8xlarge - p3.16xlarge taints: - key: nvidia.com/gpu value: true effect: “NoSchedule”
For a pod to request to run on a node that has provisioner, it could set a toleration as follows:
apiVersion: v1 kind: Pod metadata: name: mygpupod spec: containers: - name: gpuapp resources: requests: nvidia.com/gpu: 1 limits: nvidia.com/gpu: 1 image: mygpucontainer tolerations: - key: "nvidia.com/gpu" operator: "Exists" effect: "NoSchedule"
See Taints and Tolerations in the Kubernetes documentation for details.
By using the Kubernetes
topologySpreadConstraints you can ask the provisioner to have pods push away from each other to limit the blast radius of an outage.
Think of it as the Kubernetes evolution for pod affinity: it lets you relate pods with respect to nodes while still allowing spread.
spec: topologySpreadConstraints: - maxSkew: 1 topologyKey: "topology.kubernetes.io/zone" whenUnsatisfiable: ScheduleAnyway labelSelector: matchLabels: dev: jjones - maxSkew: 1 topologyKey: "kubernetes.io/hostname" whenUnsatisfiable: ScheduleAnyway labelSelector: matchLabels: dev: jjones - maxSkew: 1 topologyKey: "karpenter.sh/capacity-type" whenUnsatisfiable: ScheduleAnyway labelSelector: matchLabels: dev: jjones
Adding this to your podspec would result in:
- Pods being spread across zones, hosts, and capacity-type (
labelSelectorwill include all pods with the label of
dev=jjonesin topology calculations. It is recommended to use a selector to match all pods in a deployment.
- No more than one pod difference in the number of pods on each host (
maxSkew). For example, if there were three nodes and five pods the pods could be spread 1, 2, 2 or 2, 1, 2 and so on. If instead the spread were 5, pods could be 5, 0, 0 or 3, 2, 0, or 2, 1, 2 and so on.
The three supported
topologyKey values that Karpenter supports are:
See Pod Topology Spread Constraints for details.
By using the
podAntiAffinity configuration on a pod spec, you can inform the provisioner of your desire for pods to schedule together or apart with respect to different topology domains. For example:
spec: affinity: podAffinity: requiredDuringSchedulingIgnoredDuringExecution: - labelSelector: matchExpressions: - key: system operator: In values: - backend topologyKey: topology.kubernetes.io/zone podAntiAffinity: requiredDuringSchedulingIgnoredDuringExecution: - labelSelector: matchLabels: app: inflate topologyKey: kubernetes.io/hostname
The above pod affinity rule would cause the pod to only schedule in zones where a pod with the label
system=backend is already running.
The anti-affinity rule would cause it to avoid running on any node with a pod labeled
app=inflate. If this anti-affinity term was on a deployment pod spec along with a matching
app=inflate label, it would prevent more than one pod from the deployment from running on any single node.
See Inter-pod affinity and anti-affinity in the Kubernetes documentation for details.
Persistent Volume Topology
Karpenter automatically detects storage scheduling requirements and includes them in node launch decisions.
In the following example, the
StorageClass defines zonal topologies for
us-west-2b and binding mode
When the pod is created, Karpenter follows references from the
StorageClass and identifies that this pod requires storage in
It randomly selects
us-west-2a, provisions a node in that zone, and binds the pod to the node.
The CSI driver creates a
PersistentVolume according to the
PersistentVolumeClaim and gives it a node affinity rule for
Later on, the pod is deleted and a new pod is created that requests the same claim. This time, Karpenter identifies that a
PersistentVolume already exists for the
PersistentVolumeClaim, and includes its zone
us-west-2a in the pod’s scheduling requirements.
apiVersion: v1 kind: Pod metadata: name: app spec: containers: ... volumes: - name: storage persistentVolumeClaim: claimName: ebs-claim --- kind: StorageClass apiVersion: storage.k8s.io/v1 metadata: name: ebs provisioner: ebs.csi.aws.com volumeBindingMode: WaitForFirstConsumer allowedTopologies: - matchLabelExpressions: - key: topology.ebs.csi.aws.com/zone values: ["us-west-2a", "us-west-2b"] --- apiVersion: v1 kind: PersistentVolumeClaim metadata: name: ebs-claim spec: accessModes: - ReadWriteOnce storageClassName: ebs resources: requests: storage: 4Gi
☁️ AWS Specific
The EBS CSI driver uses
topology.ebs.csi.aws.com/zone instead of the standard
topology.kubernetes.io/zone label. Karpenter is aware of label aliasing and translates this label into
topology.kubernetes.io/zone in memory. When configuring a
StorageClass for the EBS CSI Driver, you must use
NoteThe topology key
topology.kubernetes.io/regionis not supported. Legacy in-tree CSI providers specify this label. Instead, install an out-of-tree CSI provider. Learn more about moving to CSI providers.