Disruption

Control Flow

Karpenter sets a Kubernetes finalizer on each node and node claim it provisions. The finalizer blocks deletion of the node object while the Termination Controller taints and drains the node, before removing the underlying NodeClaim. Disruption is triggered by the Disruption Controller, by the user through manual disruption, or through an external system that sends a delete request to the node object.

Disruption Controller

Karpenter automatically discovers disruptable nodes and spins up replacements when needed. Karpenter disrupts nodes by executing one automated method at a time, first doing Drift then Consolidation. Each method varies slightly, but they all follow the standard disruption process. Karpenter uses disruption budgets to control the speed at which these disruptions begin.

1. Identify a list of prioritized candidates for the disruption method.
   • If there are pods that cannot be evicted on the node, Karpenter will ignore the node and try disrupting it later.
   • If there are no disruptable nodes, continue to the next disruption method.
2. For each disruptable node:
   1. Check if disrupting it would violate its NodePool’s disruption budget.
   2. Execute a scheduling simulation with the pods on the node to find if any replacement nodes are needed.
3. Add the karpenter.sh/disruption:NoSchedule taint to the node(s) to prevent pods from scheduling to it.
4. Pre-spin any replacement nodes needed as calculated in Step (2), and wait for them to become ready.
   • If a replacement node fails to initialize, un-taint the node(s), and restart from Step (1), starting at the first disruption method again.
5. Delete the node(s) and wait for the Termination Controller to gracefully shutdown the node(s).
6. Once the Termination Controller terminates the node, go back to Step (1), starting at the first disruption method again.

Termination Controller

When a Karpenter node is deleted, the Karpenter finalizer will block deletion and the APIServer will set the DeletionTimestamp on the node, allowing Karpenter to gracefully shutdown the node, modeled after Kubernetes Graceful Node Shutdown. Karpenter’s graceful shutdown process will:

1. Add the karpenter.sh/disruption=disrupting:NoSchedule taint to the node to prevent pods from scheduling to it.
2. Begin evicting the pods on the node with the Kubernetes Eviction API to respect PDBs, while ignoring all static pods, pods tolerating the karpenter.sh/disruption=disrupting:NoSchedule taint, and succeeded/failed pods. Wait for the node to be fully drained before proceeding to Step (3).
   • While waiting, if the underlying NodeClaim for the node no longer exists, remove the finalizer to allow the APIServer to delete the node, completing termination.
3. Terminate the NodeClaim in the Cloud Provider.
4. Remove the finalizer from the node to allow the APIServer to delete the node, completing termination.

Manual Methods

• Node Deletion: You can use kubectl to manually remove a single Karpenter node or nodeclaim. Since each Karpenter node is owned by a NodeClaim, deleting either the node or the nodeclaim will cause cascade deletion of the other:

  # Delete a specific nodeclaim
  kubectl delete nodeclaim $NODECLAIM_NAME
  
  # Delete a specific node
  kubectl delete node $NODE_NAME
  
  # Delete all nodeclaims
  kubectl delete nodeclaims --all
  
  # Delete all nodes owned by any nodepool
  kubectl delete nodes -l karpenter.sh/nodepool
  
  # Delete all nodeclaims owned by a specific nodepoolXS
  kubectl delete nodeclaims -l karpenter.sh/nodepool=$NODEPOOL_NAME
  

• NodePool Deletion: NodeClaims are owned by the NodePool through an owner reference that launched them. Karpenter will gracefully terminate nodes through cascading deletion when the owning NodePool is deleted.

Automated Graceful Methods

Automated graceful methods, can be rate limited through NodePool Disruption Budgets

• Consolidation: Karpenter works to actively reduce cluster cost by identifying when:
  • Nodes can be removed because the node is empty
  • Nodes can be removed as their workloads will run on other nodes in the cluster.
  • Nodes can be replaced with lower priced variants due to a change in the workloads.
• Drift: Karpenter will mark nodes as drifted and disrupt nodes that have drifted from their desired specification. See Drift to see which fields are considered.
• Interruption: Karpenter will watch for upcoming interruption events that could affect your nodes (health events, spot interruption, etc.) and will taint, drain, and terminate the node(s) ahead of the event to reduce workload disruption.

spec:
  disruption:
    consolidationPolicy: WhenUnderutilized
    expireAfter: 720h

Consolidation

Karpenter has two mechanisms for cluster consolidation:

1. Deletion - A node is eligible for deletion if all of its pods can run on free capacity of other nodes in the cluster.
2. Replace - A node can be replaced if all of its pods can run on a combination of free capacity of other nodes in the cluster and a single lower price replacement node.

Consolidation has three mechanisms that are performed in order to attempt to identify a consolidation action:

1. Empty Node Consolidation - Delete any entirely empty nodes in parallel
2. Multi Node Consolidation - Try to delete two or more nodes in parallel, possibly launching a single replacement whose price is lower than that of all nodes being removed
3. Single Node Consolidation - Try to delete any single node, possibly launching a single replacement whose price is lower than that of the node being removed

It’s impractical to examine all possible consolidation options for multi-node consolidation, so Karpenter uses a heuristic to identify a likely set of nodes that can be consolidated. For single-node consolidation we consider each node in the cluster individually.

When there are multiple nodes that could be potentially deleted or replaced, Karpenter chooses to consolidate the node that overall disrupts your workloads the least by preferring to terminate:

• Nodes running fewer pods
• Nodes that will expire soon
• Nodes with lower priority pods

If consolidation is enabled, Karpenter periodically reports events against nodes that indicate why the node can’t be consolidated. These events can be used to investigate nodes that you expect to have been consolidated, but still remain in your cluster.

Events:
  Type     Reason                   Age                From             Message
  ----     ------                   ----               ----             -------
  Normal   Unconsolidatable         66s                karpenter        pdb default/inflate-pdb prevents pod evictions
  Normal   Unconsolidatable         33s (x3 over 30m)  karpenter        can't replace with a lower-priced node

Spot consolidation

For spot nodes, Karpenter has deletion consolidation enabled by default. If you would like to enable replacement with spot consolidation, you need to enable the feature through the SpotToSpotConsolidation feature flag.

Lower priced spot instance types are selected with the price-capacity-optimized strategy. Sometimes, the lowest priced spot instance type is not launched due to the likelihood of interruption. As a result, Karpenter uses the number of available instance type options with a price lower than the currently launched spot instance as a heuristic for evaluating whether it should launch a replacement for the current spot node.

We refer to the number of instances that Karpenter has within its launch decision as a launch’s “instance type flexibility.” When Karpenter is considering performing a spot-to-spot consolidation replacement, it will check whether replacing the instance type will lead to enough instance type flexibility in the subsequent launch request. As a result, we get the following properties when evaluating for consolidation:

1. We shouldn’t continually consolidate down to the lowest priced spot instance which might have very high rates of interruption.
2. We launch with enough instance types that there’s high likelihood that our replacement instance has comparable availability to our current one.

Karpenter requires a minimum instance type flexibility of 15 instance types when performing single node spot-to-spot consolidations (1 node to 1 node). It does not have the same instance type flexibility requirement for multi-node spot-to-spot consolidations (many nodes to 1 node) since doing so without requiring flexibility won’t lead to “race to the bottom” scenarios.

Drift

Drift handles changes to the NodePool/EC2NodeClass. For Drift, values in the NodePool/EC2NodeClass are reflected in the NodeClaimTemplateSpec/EC2NodeClassSpec in the same way that they’re set. A NodeClaim will be detected as drifted if the values in its owning NodePool/EC2NodeClass do not match the values in the NodeClaim. Similar to the upstream deployment.spec.template relationship to pods, Karpenter will annotate the owning NodePool and EC2NodeClass with a hash of the NodeClaimTemplateSpec to check for drift. Some special cases will be discovered either from Karpenter or through the CloudProvider interface, triggered by NodeClaim/Instance/NodePool/EC2NodeClass changes.

Special Cases on Drift

In special cases, drift can correspond to multiple values and must be handled differently. Drift on resolved fields can create cases where drift occurs without changes to CRDs, or where CRD changes do not result in drift. For example, if a NodeClaim has node.kubernetes.io/instance-type: m5.large, and requirements change from node.kubernetes.io/instance-type In [m5.large] to node.kubernetes.io/instance-type In [m5.large, m5.2xlarge], the NodeClaim will not be drifted because its value is still compatible with the new requirements. Conversely, if a NodeClaim is using a NodeClaim image ami: ami-abc, but a new image is published, Karpenter’s EC2NodeClass.spec.amiSelectorTerms will discover that the new correct value is ami: ami-xyz, and detect the NodeClaim as drifted.

NodePool

EC2NodeClass

Behavioral Fields

Behavioral Fields are treated as over-arching settings on the NodePool to dictate how Karpenter behaves. These fields don’t correspond to settings on the NodeClaim or instance. They’re set by the user to control Karpenter’s Provisioning and disruption logic. Since these don’t map to a desired state of NodeClaims, behavioral fields are not considered for Drift.

NodePool

Read the Drift Design for more.

To enable the drift feature flag, refer to the Feature Gates.

Karpenter will add the Drifted status condition on NodeClaims if the NodeClaim is drifted from its owning NodePool. Karpenter will also remove the Drifted status condition if either:

1. The Drift feature gate is not enabled but the NodeClaim is drifted, Karpenter will remove the status condition.
2. The NodeClaim isn’t drifted, but has the status condition, Karpenter will remove it.

Automated Forceful Methods

Automated forceful methods will begin draining nodes as soon as the condition is met. Note that these methods blow past NodePool Disruption Budgets, and do not wait for a pre-spin replacement node to be healthy for the pods to reschedule, unlike the graceful methods mentioned above. Use Pod Disruption Budgets and do-not-disrupt on your nodes to rate-limit the speed at which your applications are disrupted.

Expiration

Karpenter will disrupt nodes as soon as they’re expired after they’ve lived for the duration of the NodePool’s spec.disruption.expireAfter. You can use expiration to periodically recycle nodes due to security concern.

Interruption

If interruption-handling is enabled, Karpenter will watch for upcoming involuntary interruption events that would cause disruption to your workloads. These interruption events include:

• Spot Interruption Warnings
• Scheduled Change Health Events (Maintenance Events)
• Instance Terminating Events
• Instance Stopping Events

When Karpenter detects one of these events will occur to your nodes, it automatically taints, drains, and terminates the node(s) ahead of the interruption event to give the maximum amount of time for workload cleanup prior to compute disruption. This enables scenarios where the terminationGracePeriod for your workloads may be long or cleanup for your workloads is critical, and you want enough time to be able to gracefully clean-up your pods.

For Spot interruptions, the NodePool will start a new node as soon as it sees the Spot interruption warning. Spot interruptions have a 2 minute notice before Amazon EC2 reclaims the instance. Karpenter’s average node startup time means that, generally, there is sufficient time for the new node to become ready and to move the pods to the new node before the NodeClaim is reclaimed.

Karpenter enables this feature by watching an SQS queue which receives critical events from AWS services which may affect your nodes. Karpenter requires that an SQS queue be provisioned and EventBridge rules and targets be added that forward interruption events from AWS services to the SQS queue. Karpenter provides details for provisioning this infrastructure in the CloudFormation template in the Getting Started Guide.

To enable interruption handling, configure the --interruption-queue CLI argument with the name of the interruption queue provisioned to handle interruption events.

Controls

Disruption Budgets

You can rate limit Karpenter’s disruption through the NodePool’s spec.disruption.budgets. If undefined, Karpenter will default to one budget with nodes: 10%. Budgets will consider nodes that are actively being deleted for any reason, and will only block Karpenter from disrupting nodes voluntarily through drift, emptiness, and consolidation. Note that NodePool Disruption Budgets do not prevent Karpenter from cleaning up expired or drifted nodes.

Reasons

Karpenter allows specifying if a budget applies to any of drifted, underutilized, or empty. When a budget has no reasons, it’s assumed that it applies to all reasons. When calculating allowed disruptions for a given reason, Karpenter will take the minimum of the budgets that have listed the reason or have left reasons undefined.

Nodes

When calculating if a budget will block nodes from disruption, Karpenter lists the total number of nodes owned by a NodePool, subtracting out the nodes owned by that NodePool that are currently being deleted and nodes that are NotReady. If the number of nodes being deleted by Karpenter or any other processes is greater than the number of allowed disruptions, disruption for this node will not proceed.

If the budget is configured with a percentage value, such as 20%, Karpenter will calculate the number of allowed disruptions as allowed_disruptions = roundup(total * percentage) - total_deleting - total_notready. If otherwise defined as a non-percentage value, Karpenter will simply subtract the number of nodes from the total (total - non_percentage_value) - total_deleting - total_notready. For multiple budgets in a NodePool, Karpenter will take the minimum value (most restrictive) of each of the budgets.

For example, the following NodePool with three budgets defines the following requirements:

• The first budget will only allow 20% of nodes owned by that NodePool to be disrupted if it’s empty or drifted. For instance, if there were 19 nodes owned by the NodePool, 4 empty or drifted nodes could be disrupted, rounding up from 19 * .2 = 3.8.
• The second budget acts as a ceiling to the previous budget, only allowing 5 disruptions when there are more than 25 nodes.
• The last budget only blocks disruptions during the first 10 minutes of the day, where 0 disruptions are allowed, only applying to underutilized nodes.

apiVersion: karpenter.sh/v1beta1
kind: NodePool
metadata:
  name: default
spec:
  disruption:
    consolidationPolicy: WhenUnderutilized
    expireAfter: 720h # 30 * 24h = 720h
    budgets:
    - nodes: "20%"
      reasons: 
      - "empty"
      - "drifted"
    - nodes: "5"
    - nodes: "0"
      schedule: "@daily"
      duration: 10m
      reasons: 
      - "underutilized"

Schedule

Schedule is a cronjob schedule. Generally, the cron syntax is five space-delimited values with options below, with additional special macros like @yearly, @monthly, @weekly, @daily, @hourly. Follow the Kubernetes documentation for more information on how to follow the cron syntax. Timezones are not currently supported. Schedules are always in UTC.

# ┌───────────── minute (0 - 59)
# │ ┌───────────── hour (0 - 23)
# │ │ ┌───────────── day of the month (1 - 31)
# │ │ │ ┌───────────── month (1 - 12)
# │ │ │ │ ┌───────────── day of the week (0 - 6) (Sunday to Saturday;
# │ │ │ │ │                                   7 is also Sunday on some systems)
# │ │ │ │ │                                   OR sun, mon, tue, wed, thu, fri, sat
# │ │ │ │ │
# * * * * *

Duration

Duration allows compound durations with minutes and hours values such as 10h5m or 30m or 160h. Since cron syntax does not accept denominations smaller than minutes, users can only define minutes or hours.

Pod-Level Controls

You can block Karpenter from voluntarily choosing to disrupt certain pods by setting the karpenter.sh/do-not-disrupt: "true" annotation on the pod. This is useful for pods that you want to run from start to finish without disruption. By opting pods out of this disruption, you are telling Karpenter that it should not voluntarily remove a node containing this pod.

Examples of pods that you might want to opt-out of disruption include an interactive game that you don’t want to interrupt or a long batch job (such as you might have with machine learning) that would need to start over if it were interrupted.

apiVersion: apps/v1
kind: Deployment
spec:
  template:
    metadata:
      annotations:
        karpenter.sh/do-not-disrupt: "true"

Examples of voluntary node removal that will be prevented by this annotation include:

• Consolidation
• Drift
• Expiration

Node-Level Controls

You can block Karpenter from voluntarily choosing to disrupt certain nodes by setting the karpenter.sh/do-not-disrupt: "true" annotation on the node. This will prevent disruption actions on the node.

apiVersion: v1
kind: Node
metadata:
  annotations:
    karpenter.sh/do-not-disrupt: "true"

Example: Disable Disruption on a NodePool

To disable disruption for all nodes launched by a NodePool, you can configure its .spec.disruption.budgets. Setting a budget of zero nodes will prevent any of those nodes from being considered for voluntary disruption.

apiVersion: karpenter.sh/v1beta1
kind: NodePool
metadata:
  name: default
spec:
  disruption:
    budgets:
      - nodes: "0"