How do unhealthy instances get onto my AWS load balancer?

Question

We are using CodeDeploy to load code onto our instances as they boot up. Our intention was that they would not be added to the LB prior to the code being loaded. To do this, we set a health check which looked for one of the files being deployed. What we have found is that some times instances without code are created (I assume code deploy failed) and these instances are staying in the LB even when marked unhealthy? How is this possible? Is this related to the grace period? Shouldn't instances that are unhealthy be removed automatically?

Answer 1

I believe I have found a large part of my problem: My Auto-scale group was set to use EC2 health checks and not my ELB health check. This resulted in the instance not being terminated. The traffic may have continued to flow longer to this crippled instance due to the need the need for a very long unhealthy state before having traffic completely stopped.