How can I prevent EC2 instance termination by Auto Scaling?

Question

I would like to prevent EC2 instance termination by Auto Scaling feature if that instance is in the middle of some sort of processing.

Background:
Suppose I have

Answer 1

You can use Amazon CloudWatch to achieve this: http://aws.typepad.com/aws/2013/01/amazon-cloudwatch-alarm-actions.html. From the article:

You can use a similar strategy to get rid of instances that are tasked with handling compute-intensive batch processes. Once the CPU goes idle and the work is done, terminate the instance and save some money!

In this case, since you will be handling the termination, you will need to remove the scale-down policy. Also see another option: https://stackoverflow.com/a/19628453/432849.