ELBv2 — Elastic Load Balancing v2 (ALB/NLB)

AWS docs: https://docs.aws.amazon.com/elasticloadbalancing/latest/APIReference/Welcome.html

ELBv2 is served as an AWS Query API (Action=..., Version=2015-12-01). A load balancer is not metadata alone: a request addressed to its DNS name is forwarded to a target registered behind it, which is what makes a service deployed behind one reachable at the URL it was given.

Behavior Notes

• Service name in Overcast is elbv2.
• Version ownership is 2015-12-01.
• Requests are forwarded. A request whose Host is a load balancer’s DNSName is proxied round robin to a registered target of the target group its listener’s DefaultActions forward to, preserving the Host so an application behind it builds its own links correctly. A load balancer with nothing registered behind it answers 503, as ALB does.
• The DNS name does not resolve on its own — nothing listens on port 80. Reach a load balancer by sending its DNSName as the Host header to Overcast’s own port: curl -H "Host: <DNSName>" http://localhost:4566/. Setting OVERCAST_HOSTNAME to a resolvable base puts the name in the split-horizon zone, where it resolves like any other Overcast endpoint.
• An ECS service with a loadBalancers configuration registers and deregisters its tasks as it places and stops them, so scaling the service changes what the load balancer forwards to.
• DescribeTargetHealth reports every registered target healthy. A target group’s HealthCheck* properties (path, port, protocol, interval, timeout, thresholds, Matcher) are stored and echoed back by DescribeTargetGroups, but nothing evaluates them against a target’s actual state.
• Only forward listener actions have a data-plane effect. RedirectConfig and FixedResponseConfig round-trip through DescribeListeners, but a request against such a listener is still forwarded rather than redirected or answered with the fixed body. Weighted ForwardConfig, Certificates/SslPolicy/AlpnPolicy/MutualAuthentication, the Cognito/OIDC auth actions, and listener rules (CreateRule/DescribeRules) are not modelled at all.

Summary

Category	✅ Supported	❌ Unsupported
Load Balancers	3	1
Target Groups	6
Listeners	3
Targets	3
Listener Rules		2
Tags	3

---

Endpoints

Load Balancers

Operation	Status	Notes	AWS Docs
CreateLoadBalancer	✅ Supported	Threads Type, Scheme, IpAddressType, Subnets, SecurityGroups and Tags	docs
DescribeLoadBalancers	✅ Supported		docs
DeleteLoadBalancer	✅ Supported		docs
ModifyLoadBalancerAttributes	❌ Unsupported		docs

Target Groups

Operation	Status	Notes	AWS Docs
CreateTargetGroup	✅ Supported	Threads TargetType, ProtocolVersion, IpAddressType, the HealthCheck* family, Matcher and Tags; health checks are stored and echoed but not evaluated against targets	docs
DescribeTargetGroups	✅ Supported		docs
DeleteTargetGroup	✅ Supported		docs
ModifyTargetGroup	✅ Supported	Updates the HealthCheck* family and Matcher; TargetType/Protocol/Port/VpcId require replacement	docs
ModifyTargetGroupAttributes	✅ Supported	Stores and echoes attributes such as deregistration_delay.timeout_seconds; not enforced by the data plane	docs
DescribeTargetGroupAttributes	✅ Supported		docs

Listeners

Operation	Status	Notes	AWS Docs
CreateListener	✅ Supported	Forwards each DefaultActions member’s Type, TargetGroupArn, Order, RedirectConfig and FixedResponseConfig; weighted ForwardConfig, Certificates/SslPolicy/AlpnPolicy/MutualAuthentication and the Cognito/OIDC auth actions are not modelled	docs
DescribeListeners	✅ Supported		docs
DeleteListener	✅ Supported		docs

Targets

Operation	Status	Notes	AWS Docs
RegisterTargets	✅ Supported		docs
DeregisterTargets	✅ Supported		docs
DescribeTargetHealth	✅ Supported	Always reports “healthy”; the stored HealthCheck* block is not evaluated against targets	docs

Listener Rules

Operation	Status	Notes	AWS Docs
CreateRule	❌ Unsupported		docs
DescribeRules	❌ Unsupported		docs

Tags

Operation	Status	Notes	AWS Docs
AddTags	✅ Supported	Adds tags to load balancers and target groups	docs
RemoveTags	✅ Supported	Removes tags from load balancers and target groups	docs
DescribeTags	✅ Supported	Describes tags for load balancers and target groups	docs