ELB Instance Out of service

Question

After implementing ELB and Autoscalling on Amazon AWS. There's a wierd issue happens and we don't know either the reason or the fix.

• We installed Auto Scalling configuration to add instance of AMI ID of Windows 2008 R2 x64, SQL Express and IIS.

• We also let the ELB to check health on "/", normal configuration and supposed that the IIS is running normally.

• But as soon as the instance started, it is shown as "Out Of Service" at ELB.

• Status turns into "In Service" suddenly after sometime.

• What's the reason for such issue?

• Is it possible to make it available in no time? "As for sure when instance is started it's needed to be load balanced istantly"

Solution

The health check in your ELB has five parameters:

Ping Target:          HTTP:80/
Timeout:              5 seconds 
Interval:             30 seconds
Unhealthy Threshold:  2
Healthy Threshold:    10

When a new instance is started it is assumed to be in an "unhealthy" state and must successfully respond to Healthy Threshold requests, each of which are separated by Interval before it will be included in the pool.

To reduce the time a new instance takes to be considered "healthy", lower the Interval and the Healthy Threshold.

Note that this increases the possibility of an instance "flapping" in and out of service when it is overloaded.