Invoke a script on EC2 termination

Question

I have to take certain actions during AWS autoscaling scale-in event.The ec2 instance should be able to save some logs and reports to S3 bucket. This can take anywhere between 5 to 15 mins.

I already have a script that gets called on termination:

ln -s /etc/ec2-termination /etc/rc0.d/S01ec2-termination

However the script ends abruptly within 5 mins. I am looking at leveraging AWS LifeCycle hooks to extend the EC2 lifetime. The documentation is not clear on invoking a script in a way similar to user-data script.

There are ways of using AWS lambda or SNS to receive notification. This can be potentially used to inform the ec2.

But, I would like to know if there is a simpler solution to this problem. Is there a way to register a script with Lifecycle hooks that gets called on a scale-in event.

Answer 1

No.

The fact that the instance is being terminated is managed within the AWS infrastructure. Auto Scaling does not have the ability to reach "into" the EC2 instance to trigger anything.

Instead, you would need to write some code on the instance that checks whether the instance is in the termination state and then takes appropriate action.

An example might be:

• The Lifecycle Hook sends a notification via Amazon SNS
• Amazon SNS triggers an AWS Lambda function
• The Lambda function could add a tag to the instance (eg Terminating = Yes)
• A script on the EC2 instance is triggered every 15 seconds to check the tags associated with the EC2 instance (on which it is running). If it finds the tag, it triggers the shutdown process.

(Be careful that the script doesn't trigger again during the shutdown process otherwise it might try performing the shutdown process every 15 seconds!)

Alternatively, store the shutdown information in the Systems Manager Parameter Store or a database, but using Tags seems nicely scalable!

Updated version:

Thanks to raevilman for the idea:

• The Lifecycle Hook sends a notification via Amazon SNS
• Amazon SNS triggers an AWS Lambda function
• The Lambda function calls the AWS Systems Manager Run Command to trigger code on the instance

Much simpler!