feat(ec2): add CapacityReservationSpecification support to Launch… by snese · Pull Request #35453 · aws/aws-cdk

snese · 2025-09-09T05:14:19Z

Issue #34921

Reason for this change

The CDK LaunchTemplate construct was missing the CapacityReservationSpecification parameter that exists in the CloudFormation specification. This prevented users from utilizing EC2 capacity reservations through CDK, causing job failures for instance types with poor on-demand availability (like p4d.24xlarge for GPU workloads). Users were forced to use workarounds with addPropertyOverride() that bypass CDK's type safety.

Description of changes

Added comprehensive Capacity Reservation support to the LaunchTemplate construct following CDK L2 construct patterns and design guidelines:

L2 Construct Implementation:

New interface: ICapacityReservation - represents a Capacity Reservation reference
New class: CapacityReservation with static factory methods:
- fromId(capacityReservationId) - reference by Capacity Reservation ID
- fromResourceGroupArn(resourceGroupArn) - reference by resource group ARN
New enum: CapacityReservationPreference with OPEN, NONE, and CAPACITY_RESERVATIONS_ONLY values

Flat Property Structure (following CDK Design Guidelines):

capacityReservation?: ICapacityReservation - direct property on LaunchTemplateProps
capacityReservationPreference?: CapacityReservationPreference - direct property on LaunchTemplateProps
No artificial nesting - properties are flat and discoverable

Key Features:

Type-safe factory methods following CDK patterns
Direct 1:1 mapping to CloudFormation AWS::EC2::LaunchTemplate.CapacityReservationSpecification
Backward compatible - properties omitted from CloudFormation when not specified
Comprehensive JSDoc documentation with AWS service links
Clear default behavior documentation

Example Usage:

// Reference by Capacity Reservation ID
new ec2.LaunchTemplate(this, 'Template', {
  machineImage: ec2.MachineImage.latestAmazonLinux2023(),
  capacityReservation: ec2.CapacityReservation.fromId('cr-1234567890abcdef0'),
  capacityReservationPreference: ec2.CapacityReservationPreference.OPEN,
});

// Reference by resource group ARN
new ec2.LaunchTemplate(this, 'Template', {
  machineImage: ec2.MachineImage.latestAmazonLinux2023(),
  capacityReservation: ec2.CapacityReservation.fromResourceGroupArn('arn:aws:resource-groups:...'),
  capacityReservationPreference: ec2.CapacityReservationPreference.CAPACITY_RESERVATIONS_ONLY,
});

Describe any new or updated permissions being added

N/A - This change only adds configuration properties that map to existing CloudFormation functionality. No new IAM permissions are required.

Description of how you validated changes

Unit tests: 7 comprehensive unit tests covering all scenarios:
- All enum values (OPEN, NONE, CAPACITY_RESERVATIONS_ONLY)
- Capacity Reservation by ID
- Capacity Reservation by resource group ARN
- Combined reservation + preference
- Backward compatibility (property omission when not specified)
- CloudFormation template generation validation
Integration tests: TypeScript integration test with 3 comprehensive scenarios
Build verification: All TypeScript compilation, linting, and JSII compilation passes
Regression testing: All 96 existing LaunchTemplate unit tests pass (100% pass rate)

Reviewer Feedback Addressed

This implementation addresses all feedback from @kaizencc and @rix0rrr:

✅ Created proper L2 constructs (ICapacityReservation, CapacityReservation class)
✅ Flattened property structure (no artificial nesting)
✅ Factory methods following CDK patterns (fromId, fromResourceGroupArn)
✅ TypeScript integration test (not JavaScript)
✅ Clear documentation of default behaviors
✅ Removed unnecessary fixture references

Checklist

My code adheres to the CONTRIBUTING GUIDE and DESIGN GUIDELINES

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license

aws-cdk-automation

(This review is outdated)

✅ Updated pull request passes all PRLinter validations. Dismissing previous PRLinter review.

packages/aws-cdk-lib/aws-ec2/lib/launch-template.ts

Pull request has been modified.

pahud

@aws-cdk-testing/framework-integ:   UNCHANGED  triggers/test/integ.triggers 1.349s
@aws-cdk-testing/framework-integ: Snapshot Results: 
@aws-cdk-testing/framework-integ: Tests:    1 failed, 1265 total
@aws-cdk-testing/framework-integ: Failed: /codebuild/output/src3595677729/src/actions-runner/_work/aws-cdk/aws-cdk/packages/@aws-cdk-testing/framework-integ/test/aws-ec2/test/integ.launch-template-capacity-reservation.js
@aws-cdk-testing/framework-integ: Error: Some tests failed!
@aws-cdk-testing/framework-integ: To re-run failed tests run: integ-runner --update-on-failed
@aws-cdk-testing/framework-integ:     at main (/codebuild/output/src3595677729/src/actions-runner/_work/aws-cdk/aws-cdk/node_modules/@aws-cdk/integ-runner/lib/index.js:10626:13)
@aws-cdk-testing/framework-integ: Error: integ-runner exited with error code 1
@aws-cdk-testing/framework-integ: Tests failed. Total time (2m59.8s) | integ-runner (2m13.9s) | /codebuild/output/src3595677729/src/actions-runner/_work/aws-cdk/aws-cdk/node_modules/jest/bin/jest.js (44.7s)
@aws-cdk-testing/framework-integ: !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
@aws-cdk-testing/framework-integ: 
@aws-cdk-testing/framework-integ: error Command failed with exit code 1.

let's re-run the integ tests

Pull request has been modified.

pahud

LGTM. Requesting the team for another review.

kaizencc

Thanks for the PR @snese. I think we're skirting around actually modeling new L2s and I'm confused why. That seems like the better approach to me that is more maintainable and will follow CDK conventions better.

One more thing: the title is cutoff and should not have the word add.

kaizencc · 2025-09-24T16:48:15Z

packages/aws-cdk-lib/aws-ec2/README.md

+
+You can specify [EC2 Capacity Reservations](https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/ec2-capacity-reservations.html) for your launch template to ensure that EC2 capacity is reserved for your instances:
+
+```ts fixture=with-vpc


nit: why do we need fixture=with-vpc?

kaizencc · 2025-09-24T16:49:26Z

packages/aws-cdk-lib/aws-ec2/lib/launch-template.ts

+   * The ARN of the Capacity Reservation resource group in which to run the instance
+   * @default - No Capacity Reservation resource group
+   */
+  readonly capacityReservationResourceGroupArn?: string;


we rarely model things as Arn in CDK. This represents a place where we can uplift the existing CFN spec and modle capacityReservationResourceGroup directly.

The issue is that if we model capacityReservationResourceGroup in the future, then it wouldn't match the normal CDK pattern of supplying a IInterface to connect constructs

after doing a ton of research it looks like CFN does not even have a capacityReservationResourceGroup. But even then, we need to model this as an L2 with the static function fromResourceGroupArn so that we still don't model this as a raw arn.

kaizencc · 2025-09-24T16:52:46Z

packages/aws-cdk-lib/aws-ec2/lib/launch-template.ts

        // Fields not yet implemented:
        // ==========================
-        // https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/aws-properties-ec2-launchtemplate-launchtemplatedata-capacityreservationspecification.html
-        // Will require creating an L2 for AWS::EC2::CapacityReservation


this comment sums it up: to support this we need to create an L2 for AWS::ECR::CapacityReservation

you've modeled all the individual components of AWS::ECR::CapacityReservation why not just create it as an L2 also?

then the property becomes

readonly capacityReservation?: ICapacityReservation;

kaizencc · 2025-09-24T16:53:29Z

packages/aws-cdk-lib/aws-ec2/test/integ.launch-template-capacity-reservation.lit.ts

why .lit.ts? this is a legacy way of providing examples in the readme.

rix0rrr · 2025-09-30T11:51:54Z

...-cdk-testing/framework-integ/test/aws-ec2/test/integ.launch-template-capacity-reservation.js

@@ -0,0 +1,63 @@
+const cdk = require('aws-cdk-lib');


Should be a TypeScript file, not a .js file.

rix0rrr · 2025-09-30T11:55:22Z

packages/aws-cdk-lib/aws-ec2/test/integ.launch-template-capacity-reservation.lit.ts

+      capacityReservationSpecification: {
+        capacityReservationPreference: ec2.CapacityReservationPreference.OPEN,
+        capacityReservationTarget: {
+          capacityReservationId: 'cr-1234567890abcdef0',


I'm going code blind from all the times that the words capacityReservation shows up here. Why is this not:

capacityReservation: CapacityReservation.fromId(this, 'Reservation', 'cr-1234567890abcdef0'), capacityReservationPreference: ec2.CapacityReservationPreference.OPEN,

rix0rrr · 2025-09-30T11:56:10Z

packages/aws-cdk-lib/aws-ec2/lib/launch-template.ts

+   * Capacity Reservation specification
+   * @default - No capacity reservation specification
+   */
+  readonly capacityReservationSpecification?: LaunchTemplateCapacityReservationSpecification;


This goes against our design guidelines:

Do not introduce artificial nesting for props. It hinders discoverability and makes it cumbersome to use in some languages (like Java) [awslint:props-flat].

https://github.com/aws/aws-cdk/blob/main/docs/DESIGN_GUIDELINES.md#flat

rix0rrr · 2025-09-30T11:57:01Z

packages/aws-cdk-lib/aws-ec2/lib/launch-template.ts

+   * Information about the target Capacity Reservation or Capacity Reservation group
+   * @default - No target specified
+   */
+  readonly capacityReservationTarget?: LaunchTemplateCapacityReservationTarget;


Shouldn't target be required if you specify a capacity reservation?

I can now specify:

capacityReservationSpecification: { }

What does that mean?

rix0rrr · 2025-09-30T11:58:54Z

packages/aws-cdk-lib/aws-ec2/lib/launch-template.ts

+export interface LaunchTemplateCapacityReservationSpecification {
+  /**
+   * Indicates the instance's Capacity Reservation preferences
+   * @default - No preference specified


Surely there is an implied default here. Tell us what the default is.

Describe the default value or default behavior, even if it's not CDK that controls the default. For example, if an absent value does not get rendered into the template and it's ultimately the AWS service that determines the default behavior, we still describe it in our documentation.

https://github.com/aws/aws-cdk/blob/main/docs/DESIGN_GUIDELINES.md#defaults

"No XXX specified" is not useful to say. Here's what you're telling a user:

"If you don't specify a capacityReservationPreference, then no capacityReservationPreference is specified."

I think we can trust our customers to figure that out for themselves. What they want to know is what happens when they don't specify a certain thing.

Pull request has been modified.

…Template construct - Add LaunchTemplateCapacityReservationSpecification and LaunchTemplateCapacityReservationTarget interfaces - Add CapacityReservationPreference enum with OPEN and NONE values - Add optional capacityReservationSpecification property to LaunchTemplateProps - Map property to existing CloudFormation L1 constructs - Add 7 comprehensive unit tests covering all scenarios - Maintain backward compatibility with optional property Closes aws#34921

…ReservationSpecification - Add capacity reservation documentation to aws-ec2 README with usage examples - Create integration test with snapshot for capacity reservation functionality - Fix PR title scope from aws-ec2 to ec2 per CDK automation requirements

…fication - Add integ.launch-template-capacity-reservation.lit.ts in main package - Follows existing pattern of .lit.ts integration tests - Tests both capacity reservation ID and resource group ARN scenarios

…late construct - Add LaunchTemplateCapacityReservationSpecification interface - Add LaunchTemplateCapacityReservationTarget interface - Add CapacityReservationPreference enum with OPEN, NONE, CAPACITY_RESERVATIONS_ONLY values - Add optional capacityReservationSpecification property to LaunchTemplateProps - Implement CloudFormation mapping to AWS::EC2::LaunchTemplate.CapacityReservationSpecification - Add comprehensive unit tests covering all property combinations - Add README documentation with usage examples - Maintain backward compatibility - property omitted when not specified Closes aws#34921

…ecification - Add comprehensive integration test for CapacityReservationSpecification - Test covers all capacity reservation preference options (open, none, capacity-reservations-only) - Test includes capacity reservation target with both ID and resource group ARN - Validates CloudFormation template generation for capacity reservation properties - Ensures backward compatibility when capacity reservation is not specified Addresses aws#34921

@kaizencc

…at properties - Created ICapacityReservation interface and CapacityReservation class with static factory methods - Flattened property structure: capacityReservation and capacityReservationPreference as direct properties - Removed artificial nesting (capacityReservationSpecification) - Changed integration test from .js to .ts format - Improved documentation with actual default behavior descriptions - Removed unnecessary fixture=with-vpc from README examples Addresses feedback from @kaizencc and @rix0rrr: - Proper L2 construct modeling instead of raw interfaces - Follows CDK Design Guidelines (no artificial nesting) - Factory methods: fromId() and fromResourceGroupArn() - TypeScript integration test instead of JavaScript - Clear documentation of default behaviors

snese · 2025-10-15T13:16:22Z

Reviewer Feedback Addressed

I've updated the implementation to address all feedback from @kaizencc and @rix0rrr. The changes implement proper L2 constructs with flat properties following CDK Design Guidelines.

Key Changes Made:

Created proper L2 constructs (addresses @kaizencc feedback):
- ICapacityReservation interface
- CapacityReservation class with static factory methods: fromId() and fromResourceGroupArn()
Flattened property structure (addresses @rix0rrr feedback):
- Removed artificial nesting (capacityReservationSpecification)
- Added direct properties: capacityReservation and capacityReservationPreference
- Follows CDK Design Guidelines: "Do not introduce artificial nesting for props"
TypeScript integration test (addresses @rix0rrr feedback):
- Changed from .js to .ts format
- Removed .lit.ts legacy format
Improved documentation (addresses @rix0rrr feedback):
- Clear default behavior descriptions (not just "no XXX specified")
- Explains actual AWS service behavior when properties are omitted
Removed unnecessary fixtures (addresses @kaizencc feedback):
- Removed fixture=with-vpc from README examples

Example Usage (New API):

// Reference by Capacity Reservation ID
new ec2.LaunchTemplate(this, 'Template', {
  machineImage: ec2.MachineImage.latestAmazonLinux2023(),
  capacityReservation: ec2.CapacityReservation.fromId('cr-1234567890abcdef0'),
  capacityReservationPreference: ec2.CapacityReservationPreference.OPEN,
});

This is much cleaner than the previous nested structure and follows the exact pattern suggested by @rix0rrr.

Test Results:

All 96 unit tests passing (100% pass rate)
Integration test ready in TypeScript format
No breaking changes
Full backward compatibility

Ready for re-review!

github-actions bot added the beginning-contributor [Pilot] contributed between 0-2 PRs to the CDK label Sep 9, 2025

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snese changed the title ~~feat(aws-ec2): add CapacityReservationSpecification support to Launch…~~ feat(ec2): add CapacityReservationSpecification support to Launch… Sep 9, 2025

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aws-cdk-automation removed the pr/needs-community-review This PR needs a review from a Trusted Community Member or Core Team Member. label Oct 14, 2025

snese force-pushed the feat-34921-capacity-reservation-specification branch from 12659f7 to 8132748 Compare October 15, 2025 09:52

snese added 5 commits October 15, 2025 21:04

feat(ec2): add literate integration test for CapacityReservationSpeci…

3bcec17

…fication - Add integ.launch-template-capacity-reservation.lit.ts in main package - Follows existing pattern of .lit.ts integration tests - Tests both capacity reservation ID and resource group ARN scenarios

snese force-pushed the feat-34921-capacity-reservation-specification branch from 8132748 to 63a82f8 Compare October 15, 2025 13:05

gjurova assigned kumsmrit and unassigned kumsmrit Apr 1, 2026

pahud mentioned this pull request Apr 1, 2026

Failed CodeBuild CI - 14 PR(s) pahud/aws-cdk#17

Open


		You can specify [EC2 Capacity Reservations](https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/ec2-capacity-reservations.html) for your launch template to ensure that EC2 capacity is reserved for your instances:

		```ts fixture=with-vpc

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snese commented Sep 9, 2025 • edited Loading Uh oh! There was an error while loading. Please reload this page.

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Issue #34921

Reason for this change

Description of changes

Describe any new or updated permissions being added

Description of how you validated changes

Reviewer Feedback Addressed

Checklist

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snese commented Oct 15, 2025

Reviewer Feedback Addressed

Key Changes Made:

Example Usage (New API):

Test Results:

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snese commented Sep 9, 2025 •

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