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POHow to refer to other ec2 instances? Is Elastic IP the only feasible solution?
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copied!<p>Initially my issue was "How do I RDP into an EC2 instance without having to first find its ip address". To solve that I wrote a script that executes periodically on each instance. The script reads a particular tag value and updates the corresponding entry in Route53 with the public dns name of the instance. </p> <p>This way I can always rdp into web-01.ec2.mydomain.com and be connected to the right instance. </p> <p>As I continued with setting up my instances, I realized to setup mongodb replication, I will need to somehow refer to three separated instances. I cannot use the internal private ip addresses as they keep changing (or are prone to change on instance stop/start & when the dhcp lease expires). </p> <p>Trying to access <code>web-01.ec2.mydomain.com</code> from within my EC2 instance returns the internal ip address of the instance. Which seems to be standard behaviour. Thus by mentioning the route53 cnames for my three instances, I can ensure that they can always be discovered by each other. I wouldn't be paying any extra data transfer charges, as the cnames will always resolve to internal ip. I would however be paying for all those route53 queries. </p> <p>I can run my script every 30 secs or even lesser to ensure that the dns entries are as uptodate as possible. </p> <p>At this point, I realized that what I have in place is very much an Elastic IP alternative. Maybe not completely, but surely for all my use cases. So I am wondering, whether to use Elastic IP or not. There is no charge involved as long as my instances are running. It does seem an easier option. </p> <p><strong>What do most people do?</strong> If someone with experience with this could reply, I would appreciate that. </p> <p>Secondly, what happens in those few seconds/minutes during which the instance loses its current private ip and gets a new internal ip. Am assuming all existing connections get dropped. Does that affect the ELB health checks (A ping every 30 secs)? Am assuming if I were using an Elastic IP, the dns name would immediately resolve to the new ip, as opposed to say after my script executes. Assuming my script runs every 30 secs, will there be only 30secs of downtime, or can there possibly be more? Will an Elastic ip always perform better than my scripted solution?</p>

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