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feat: capacity block support #8011
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| Original file line number | Diff line number | Diff line change |
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| # Capacity Block Support | ||
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| ## Overview | ||
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| In v1.3.0 Karpenter introduced formal support for on-demand capacity reservations. | ||
| However, this did not include a subset of ODCRs: Capacity Blocks. | ||
| Capacity Blocks enable users to “reserve highly sought-after GPU instances on a future date to support short duration ML workloads”. | ||
| This doc will focus on the extension to Karpenter’s existing ODCR feature to support Capacity Blocks. | ||
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| ## Goals | ||
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| - Karpenter should enable users to select against Capacity Blocks when scheduling workloads | ||
| - Karpenter should discover Capacity Blocks through `ec2nodeclass.spec.capacityReservationSelectorTerms` | ||
| - Karpenter should gracefully handle Capacity Block expiration | ||
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| ## API Updates | ||
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| We will add the `karpenter.k8s.aws/capacity-reservation-type` label, which can take on the values `default` and `capacity-block`. | ||
| This mirrors the `reservationType` field in the `ec2:DescribeCapacityReservation` response and will enable users to select on capacity block nodes via NodePool requirements or node selector terms. | ||
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| ```yaml | ||
| # Configure a NodePool to only be compatible with instance types with active | ||
| # capacity block reservations | ||
| kind: NodePool | ||
| apiVersion: karpenter.sh/v1 | ||
| spec: | ||
| template: | ||
| spec: | ||
| requirements: | ||
| - key: karpenter.k8s.aws/capacity-reservation-type | ||
| operator: In | ||
| values: ['capacity-block'] | ||
| --- | ||
| # Configure a pod to only schedule against nodes backed by capacity blocks | ||
| kind: Pod | ||
| apiVersion: v1 | ||
| spec: | ||
| nodeSelector: | ||
| karpenter.k8s.aws/capacity-reservation-type: capacity-block | ||
| ``` | ||
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| Additionally, we will update the NodeClass status to reflect the reservation type and state for a given capacity reservation: | ||
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| ```yaml | ||
| kind: EC2NodeClass | ||
| apiVersion: karpenter.k8s.aws/v1 | ||
| status: | ||
| capacityReservations: | ||
| - # ... | ||
| reservationType: Enum (default | capacity-block) | ||
| state: Enum (active | expiring) | ||
| ``` | ||
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| No changes are required for `ec2nodeclass.spec.capacityReservationSelectorTerms`. | ||
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| ## Launch Behavior | ||
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| Today, when Karpenter creates a NodeClaim targeting reserved capacity, it ensures it is launched into one of the correct reservations by injecting a `karpenter.k8s.aws/capacity-reservation-id` requirement into the NodeClaim. | ||
| By injecting this requirement, we ensure Karpenter can maximize flexibility sent to CreateFleet (minimizing risk of ReservedCapacityExceeded errors) while also ensuring Karpenter doesn’t overlaunch into any given reservation. | ||
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| ```yaml | ||
| kind: NodeClaim | ||
| apiVersion: karpenter.sh/v1 | ||
| spec: | ||
| requirements: | ||
| - key: karpenter.k8s.aws/capacity-reservation-id | ||
| operator: In | ||
| values: ['cr-foo', 'cr-bar'] | ||
| # ... | ||
| ``` | ||
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| Given the NodeClaim spec above, Karpenter will create launch templates for both `cr-foo` and `cr-bar`, providing both in the CreateFleet request. | ||
| However, this breaks down when we begin to mix default and capacity-block ODCRs (e.g. `cr-foo` is a default capacity reservation, and `cr-bar` is a capacity-block). | ||
| This is because the `TargetCapacitySpecificationRequest.DefaultTargetCapacityType` field in the CreateFleet request needs to be set to on-demand or capacity-block, preventing us from mixing them in a single request. | ||
| Instead, if a NodeClaim is compatible with both types of ODCRs, we must choose a subset of those ODCRs to include in the CreateFleet request. | ||
| We have the following options for prioritization when making this selection: | ||
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| - Prioritize price (the subset with the “cheapest” offering) | ||
| - Prioritize flexibility (the subset with the greatest number of offerings) | ||
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| Although prioritizing flexibility is desireable to reduce the risk of ReservedCapacityExceeded errors, it won’t interact well with consolidation and result in additional node churn. | ||
| For that reason, we should prioritize the set of ODCRs with the “cheapest” offering when generating the CreateFleet request. | ||
| If there is a tie between a default and capacity-block offering, we will prioritize the capacity-block offering. | ||
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| ## Interruption | ||
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| Although capacity blocks are modeled as ODCRs, their expiration behavior differs. | ||
| Any capacity still in use when a default ODCR expires falls back to a standard on-demand instance. | ||
| On the other hand, instances in use from a capacity block reservation are terminated ahead of their end date. | ||
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| From the [EC2 documentation](https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/ec2-capacity-blocks.html): | ||
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| > You can use all the instances you reserved until 30 minutes before the end time of the Capacity Block. | ||
| > With 30 minutes left in your Capacity Block reservation, we begin terminating any instances that are running in the Capacity Block. | ||
| > We use this time to clean up your instances before delivering the Capacity Block to the next customer. | ||
| > We emit an event through EventBridge 10 minutes before the termination process begins. | ||
| > For more information, see Monitor Capacity Blocks using EventBridge. | ||
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| Karpenter should gracefully handle this interruption by draining the nodes ahead of termination. | ||
| While we could integrate with the EventBridge event referenced above, this introduces complications when rehydrating state after a controller restart. | ||
| Instead, we will rely on the fact that interruption occurs at a fixed time relative to the end date of the capacity reservation, which is already discovered via `ec2:DescribeCapacityReservation`. | ||
| Matching the time the expiration warning event is emmitted, Karpenter will begin to drain the node 10 minutes before EC2 begins reclaiming the capacity (40 minutes before the end date). | ||
| Once the reclaimation period begins, Karpenter will mark the capacity reservation as expiring in the EC2NodeClass' status. | ||
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