aws-batch: Support new default instance classes for EC2 compute environment

[beornf]

Describe the feature

Support for the new default_x86_64 and default_arm64 instance classes and deprecate the optimal instance class created by the useOptimalInstanceClasses property.

Use Case

I want to be able to select between default_x86_64 and default_arm64 as described in https://aws.amazon.com/blogs/hpc/introducing-default-instance-categories-for-aws-batch/.

We recommend updating your Batch environments to adopt the new categories. However, you are not required to update. Existing AWS Batch compute environments that use optimal remain valid. The CreateComputeEnvironment and UpdateComputeEnvironment API actions continue to accept optimal as a valid value. The behavior of optimal will stay the same until early November 2025. After that, optimal will behave the same as the default_x86_64 instance category.

Proposed Solution

No response

Other Information

There appears to be a regression of logic that warns whenever x86 and arm instance types are mixed.

aws-cdk/packages/aws-cdk-lib/aws-batch/lib/managed-compute-environment.ts

Lines 1162 to 1172 in 7e80cc9

	if (hasArmInstances && hasX86Instances) {
	Annotations.of(scope).addWarningV2('@aws-cdk/aws-batch:mixingARMAndx86InstancesNotSupported', 'Cannot mix ARM and x86 instance types or classes, deploying will cause an error');
	}
	if (hasArmInstances && useOptimalInstanceClasses) {
	Annotations.of(scope).addWarningV2('@aws-cdk/aws-batch:optimalNotSupportedWithARM', '\'optimal\' instance types are not supported with ARM instance types or classes. Deploying will cause an error, please set useOptimalInstanceClasses to false');
	}
	if (useOptimalInstanceClasses || (!hasArmInstances && useOptimalInstanceClasses === undefined)) {
	instances.push('optimal');
	}

Acknowledgements

• I may be able to implement this feature request
• This feature might incur a breaking change

AWS CDK Library version (aws-cdk-lib)

2.208.0

AWS CDK CLI version

2.1024.0

Environment details (OS name and version, etc.)

Debian GNU/Linux 12

[pahud]

Hi @beornf,

Thank you for bringing this important feature request to our attention! You're absolutely right that AWS CDK should support the new default_x86_64 and default_arm64 instance classes that AWS Batch introduced.

This is a valuable enhancement that aligns with AWS's recommendation to migrate away from the optimal instance class before the November 2025 deadline.

Official AWS Blog Post:
https://aws.amazon.com/blogs/hpc/introducing-default-instance-categories-for-aws-batch/

Official API Documentation:
• CreateComputeEnvironment API
• UpdateComputeEnvironment API

The key quote from the issue regarding the November 2025 deadline:

"We recommend updating your Batch environments to adopt the new categories. However, you are not required to update. Existing AWS Batch compute environments that use
optimal remain valid. The CreateComputeEnvironment and [UpdateComputeEnvironment]
(https://docs.aws.amazon.com/batch/latest/APIReference/API_UpdateComputeEnvironment.html) API actions continue to accept optimal as a valid value. The behavior of optimal will stay the same until early November 2025. After that, optimal will behave the same as the default_x86_64 instance category."

The current implementation in renderInstances only supports the legacy optimal class through the useOptimalInstanceClasses property.

Implementation Plan:

1. Add a new DefaultInstanceClass enum with DEFAULT_X86_64, DEFAULT_ARM64, and OPTIMAL values
2. Introduce a defaultInstanceClass property in ManagedEc2EcsComputeEnvironmentProps
3. Update the renderInstances function to handle the new instance classes
4. Deprecate useOptimalInstanceClasses with proper migration guidance
5. Restore the validation logic you mentioned for mixed x86/ARM instance types

This would provide a clean migration path while maintaining backward compatibility. Would you be interested in contributing a pull request for this feature? I'd be happy to provide guidance on the implementation details.

Related AWS Documentation:

• AWS Blog: Introducing default instance categories for AWS Batch
• CreateComputeEnvironment API Reference

As this is timeline urgency, I am making it a p1.

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