Clever Geek Handbook

ELB Health Check - Health Threshold

The ELB health check configuration dialog states:

If the instance fails the health check, it is automatically removed from the load balancer. Customize the health check to your specific needs.

I get to delete the instance due to ELB when the health check fails. My question is related to the Healthy Threshold setting. When you hover over help, it says:

The number of successful health checks before EC2 is declared is a healthy example.

If the instance is declared healthy, is it rolled back to the load balancer group?

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It depends on how you set up your environment.

For example, if you manually registered your EC2 instance with your ELB ... it will display the status "InService" or "OutOfService" depending on the health check result. At the moment, no action is taken other than deciding whether to route traffic to the EC2 instance or not. If it is "OutOfService", it can automatically become "InService" again, passing a certain healthy threshold.

In this situation, your "OutOfService" instance is still associated with the load balancer, but the load balancer will not use it until it returns to a healthy state. The load balancer will continue to validate your instance until you manually remove the instance from the load balancer or fix the issue that caused the instance to fail.



There are other situations where the load balancer will not be able to continue validating your instance indefinitely, although:

  • If your unhealthy instance is a member of the autoscaler group that specified ELB health checks , it will be scheduled for replacement. This shuts down the existing instance and starts a new instance that will be linked to your ELB.

  • If your instance has been an OutOfService for a very long period of time, it may not be able to return to the service. A simple solution for this is to deindicate the instance from the load balancer and then re-register it.

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