Server groups are used to receive requests. Application Load Balancer (ALB) distributes requests from a client to one or more specified backend servers. When you create a forwarding rule for a listener, you can specify server groups for the listener and configure match conditions in the forwarding rule. Traffic that meets the match conditions in a forwarding rule is forwarded to the specified server groups.
ALB forms multiple ECS instances into a server group and sets a virtual IP address for the server group. These ECS instances are deployed in different zones of a region. This ensures the high performance and high availability of the server group. You can create server groups to receive different types of requests. Requests from a client are distributed to specified ports or backend servers based on the forwarding rules that are configured for the listeners.
You can configure health checks for each server group that is added to an ALB instance. By default, health checks are enabled for all server groups. After you specify a server group in the forwarding rule of a listener, ALB continuously monitors the conditions of all backend servers in the server group.