AWS loadbalancing between EC2 instances

Question

I am experiencing quite a strange behaviour with AWS ELB and AWS ASG.

I gave some stress for my server and AWS ASG created 2 EC2 instances to handle the load. Both instances were running on 100% CPU utilisation so AWS ASG created two more - a 3rd and a 4th instance. The thing is that now the 3rd and 4th instances run at 0% CPU utilisation and 1st and 2nd run at ~90% CPU utilisation.

Why the load between instances is not shared so all of them would be loaded with ~45% CPU utilisation ? Why some instances are maxed and some stay idle?

P.S. My AWS ASG scaling policy :

Execute policy when: As required to maintain Average CPU Utilization at 80

Take the action: Add or remove instances as required

Answer 1

Some quick thoughts:

• Are your two new instances flagged as Healthy or Unhealthy?
• Are your two new instances within availability zones that the ALB can route to? Your ALB might be set to span two AZs whereas your ASG might span 3.
• Are your two new instances in the correct subnets to receive traffic from the ALB?
• Are your EC2s configured with the same security groups as the two live ones?
• Is whatever service that should be running on the new instances actually running on it?