ELB - Uneven Availability Zone Traffic Distribution

Question

Up until a few days ago I was using ELB to distribute traffic for multiple instance in a single availability zone.

I then decided to raise a few more servers on another AZ to see how it affects our performance.

What I am seeing is that about 85% of our traffic is directed to instances in the new AZ I added. The old one is getting about 15% of the traffic.

The number/type of instances is exactly the same in each of the AZs.

All of the instances are marked as healthy in the ELB dashboard and looking at cloudwatch monitoring shows it has been like that for the past few days.

Does anyone has a clue what is going on here?

Thanks for the help.

Answer 1

Have you enabled cross-zone load balancing on your ELB? See Request routing and the procedure to enable or disable cross-zone load balancing in the AWS docs.

Answer 2

I can't find where I read this but it has been very helpful to know. Even if you have stickiness turned off (you did check to make sure stickiness is not turned on?) for a load balancer it still has some stickiness. Apparently all of the traffic coming form the same IP will go to the same destination. You normally see this when you doing load testing and it does not distribute the load.

If you have a situation where most of your traffic comes from just a few IP addresses you will need to use an EIP and HAproxy (or a similar proxy) to do the load balancing yourself.