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Reduce the ELB health check so that an instance is recognized as Healthy as quickly as possible. The Timeout must be more than most response times. Preferably more than the maximum response time. In this case 15 seconds may keep the instance healthy whilst allowing scaling to occur ASAP. Make the Interval 1 second longer at 16 seconds, the Unhealthy Threshold to the maximum of 10 and the Healthy Threshold to the minimum of 2. This takes health out of the equation as much as possible to allow the ASG to scale as it becomes necessary without instances taking too long to become healthy nor be terminated too soon when under load.
A load test was then run on this LANSA Stack with 30 virtual users for 43 minutes. This scaled out from 1 to 3 instances and then once the load finished, gradually reduced back to 1 instance.