Amazon ec2 user-data, how does it work?

Question

We are starting instances, and accessing the user-data we place. But does anybody understand the internals of this operation (from Amazon's side)? When we pass in the user-data, at what point is that data transmitted to the VM (is this a Xen feature) and where is it stored?

I first thought it was set as the USER_DATA env var, but we can also pass it as a file. Where is that file stored? Is it generic to all instances or varies depending on AMI?

This is not a problem per se, just wanted to know how Amazon does this.

Answer 1

The usage of a simple HTTP request at this particular URL will avail the user-data to the instance:

http://169.254.169.254/latest/user-data

Amazon EC2 does not put user-data directly on to the instance, although there are many AMIs have the code which instructs the instance to download and process user-data automatically.

Check out these links:

http://docs.aws.amazon.com/AWSEC2/latest/UserGuide/ec2-instance-metadata.html

http://docs.aws.amazon.com/AWSEC2/latest/UserGuide/user-data.html