Application Load Balancer (ALB) now supports Automatic Target Weights (ATW), which uses a new routing algorithm to optimize the amount of traffic sent to each target based on information available to the load balancer. With this initial launch, ALB will adjust the amount of traffic sent to each target based on implicit health information, like 5XX errors and connection errors.
Before this launch, health checks were the primary mechanism to monitor health of backend targets. While health checks successfully caught most types of target failures, they sometimes missed partial failures where health checks continue to succeed even though an application was impaired, like a problematic application deployment. In these scenarios, the application is typically impaired because a small set of targets are anomalous and perform worse than their peers. ATW detects these anomalous targets. Then, depending on the settings you choose, ATW will either surface them using CloudWatch metrics or automatically mitigate impact. Impact is mitigated by iteratively reducing the volume of traffic that an anomalous target receives until the target recovers or the traffic volume is decreased to a negligible level. If the target recovers, the ALB will gradually increase the amount of traffic sent to the target until it reaches its original load.