A target group is a collection of backend targets that a load balancer uses to route requests. An Application Load Balancer listener forwards requests to a default target group, while listener rules can select another group using the request path or host. section 2.2.2 section 14.1.1
A target group coordinates routing and backend availability:
- Performs periodic health checks and routes requests only to healthy targets. section 2.2.2
- Usually contains EC2 instances, but can also contain ECS or EKS containers, Lambda functions, or machines connected to the VPC. section 14.1.1
- Receives automatic registrations and deregistrations when an Auto Scaling group adds or removes EC2 instances. section 14.1.1
An HTTP target group
A target group can use port 80 and HTTP, check /index.html every 10 seconds, and treat response codes 200–299 as healthy. A listener forwards unmatched requests to it; an Auto Scaling group keeps its EC2 membership current.
