docs: elaborate that crossZoneEnabled property cannot be changed for ALB in ELBv2.

[ashishdhingra]

Issue #29866

Closes #29866.

Reason for this change

Per below documentation:

• Cross-zone load balancing for target groups, it mentions that With Application Load Balancers, cross-zone load balancing is always turned on at the load balancer level, and cannot be turned off.. It also specifies, For target groups, the default is to use the load balancer setting, but you can override the default by explicitly turning cross-zone load balancing off at the target group level.
• AWS::ElasticLoadBalancingV2::LoadBalancer LoadBalancerAttribute also specifies for load_balancing.cross_zone.enabled that Indicates whether cross-zone load balancing is enabled. The possible values are true and false. The default for Network Load Balancers and Gateway Load Balancers is false. The default for Application Load Balancers is true, and cannot be changed..

So for Application Load Balancer, cross-zone load balancing is always turned on at the load balancer level, and cannot be turned off.

Description of changes

Elaborate API documentation for ELBv2 that crossZoneEnabled property cannot be changed for ALB in ELBv2.

Description of how you validated changes

N/A

Checklist

• My code adheres to the CONTRIBUTING GUIDE and DESIGN GUIDELINES

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

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• CodeBuild project: AutoBuildv2Project1C6BFA3F-wQm2hXv2jqQv
• Commit ID: c2766ec
• Result: SUCCEEDED
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[pahud]

I would suggest we fix here as well:

aws-cdk/packages/aws-cdk-lib/aws-elasticloadbalancingv2/lib/shared/base-load-balancer.ts

Line 246 in c2766ec

if (baseProps.crossZoneEnabled) { this.setAttribute('load_balancing.cross_zone.enabled', 'true'); }

Because the L2 crossZoneEnabled prop literally allows you to specify as false but not well handled here. For example, if users set crossZoneEnabled: false in the L2 props, this attribute will always be undefined, which is exactly the same behavior leaving the L2 prop undefined. This is weird to me. What do you think?

[aaythapa]

Need some clarification, I see here and here that there are way to disable crossZone for ALB. Why are we saying this cannot be changed here? If we can change it then this should be implement the change suggested here imo. Let me know if I'm understanding this wrong.

[ashishdhingra]

@aaythapa In #29866 (comment) and #29866 (comment), it is mentioned that the crossZone could be disabled at Target Group level per docs and it already works per example. However, it cannot be changed at ALB level per docs.

Both Target Group level and ALB level are different scopes. :)

This PR is only to make it clear in LoadBalancer docs that it cannot be changed at ALB level. @pahud is coming up with consolidated PR for some additional change (in addition to change in this PR).

NOTE: I'm unsure what will happen in CloudFormation deployment if we enforce crossZone to false, when service itself doesn't support disabling it at ALB level (will it fail or will it ignore it).

[ashishdhingra]

Closing this PR in favor of #29907.