Can two EC2 instances have the same $_SERVER['SERVER_ADDR']? Or how to uniquely identify each, in EC2?

Question

I need a unique identifier for each EC2 instance of my scaling group on AWS for a prefix for uniqid() .

Can I use $_SERVER['SERVER_ADDR'] for this?

Answer 1

The following code can fetch the instance id of your ec2 instance:

#!/usr/bin/php
<?php
$instance_id = file_get_contents("http://instance-data/latest/meta-data/instance-id");
echo $instance_id;
?>

Answer 2

The instance ID from the ec2 metadata is a good answer, but stashing its value in the environment seems like a cleaner solution.

Wherever you set environment variables in your setup -- in the startup script for the web server itself (one example, not universally applicable, would be /etc/init.d/apache2 ), fetch the value and store it in the environment.

export EC2_INSTANCE_ID=$(ec2metadata --instance-id)

$_ENV["EC2_INSTANCE_ID"] should then contain the value, much less tedious than caching it in a file, and not subject to rate limiting (which is documented as being imposed, but without specific thresholds being mentioned).

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Note that the following three examples for fetching the instance ID are functionally equivalent:

ec2metadata --instance-id
curl http://instance-data/latest/meta-data/instance-id
curl http://169.254.169.254/latest/meta-data/instance-id