Load balancer for php application

Question

Questions about load balancers if you have time.

So I've been using AWS for some time now. Super basic instances, using them to do some tasks whenever I needed something done.

I have a task that needs to be load balanced now. It's not a public service though. It's pretty much a giant cron job that I don't want running on the same servers as my website.

I set up an AWS load balancer, but it doesn't do what I expected it to do. It get's stuck on one server, and doesn't load balance at all. I've read why it does this, and that's all fine and well, but I need it to be a serious round-robin load balancer.

edit:

I've set up the instances on different zones, but no matter how many instances I add to the ELB, it just uses one. If I take that instance down, it switches to a different one, so I know it's working. But I really would like it to always use a different one under every circumstance.

I know there are alternatives. Here's my question(s): Would a custom php load balancer be an ok option for now? IE: Have a list of servers, and have php randomly select a ec2 instance. Wouldn't be scalable at all, bu atleast I could set this up in 2 mins and it can work for now.

or

Should I take the time to learn how HAProxy works, and set that up in place of the AWS ELB?

or

Am I doing it wrong, and AWS's ELB does do round-robin. I just have something configured wrong?

edit:

Structure: 1) Web server finds a task to do.

2) If it's too large it sends it off to AWS (to load balancer).

3) Do the job on EC2

4) Report back via curl to an API

5) Rinse and repeat

Everything works great. But because the connection always comes from my server (one IP) it get's sticky'd to a single EC2 machine.

Answer 1

HAProxy is not hard to learn and is tremendously lightweight yet flexible. I actually use HAProxy behind ELB for the best of both worlds -- the hardened, managed, hands-off reliability of ELB facing the Internet and unwrapping SSL, and the flexible configuration of HAProxy to allow me to fine tune how things hit my servers. I've never lost an HAProxy instance yet, but it I do, ELB will just take that one out of rotation... as I have seen happen when the back-end servers have all become inaccessible, which (because of the way it's configured) makes ELB think the HAProxy is unhealthy, but that's by design in my setup.

Answer 2

This point comes as a surprise to many users using Amazon ELB. Amazon ELB behaves little strange when incoming traffic is originated from Single or Specific IP ranges, it does not efficiently do round robin and sticks the request. Amazon ELB starts favoring a single EC2 or EC2’s in Single Availability zones alone in Multi-AZ deployments during such conditions. For example: If you have application A(customer company) and Application B, and Application B is deployed inside AWS infrastructure with ELB front end. All the traffic generated from Application A(single host) is sent to Application B in AWS, in this case ELB of Application B will not efficiently Round Robin the traffic to Web/App EC2 instances deployed under it. This is because the entire incoming traffic from application A will be from a Single Firewall/ NAT or Specific IP range servers and ELB will start unevenly sticking the requests to Single EC2 or EC2’s in Single AZ. Note: Users encounter this usually during load test, so it is ideal to load test AWS Infra from multiple distributed agents.

More info at the Point 9 in the following article http://harish11g.blogspot.in/2012/07/aws-elastic-load-balancing-elb-amazon.html

Answer 3

ELB works well for sites whose loads increase gradually. If you are expecting an uncommon and sudden increase on the load, you can ask AWS to pre-warm it for you.

I can tell you I used ELB in different scenarios and it always worked well for me. As you didn't provide too much information about your architecture, I would bet that ELB works for you, and the case that all connections are hitting only one server, I would ask you:

1) Did you check the ELB to see how many instances are behind it? 2) The instances that you have behind the ELB, are all alive? 3) Are you accessing your application through the ELB DNS?

Anyway, I took an excerpt from the excellent article that does a very good comparison between ELB and HAProxy. http://harish11g.blogspot.com.br/2012/11/amazon-elb-vs-haproxy-ec2-analysis.html

ELB provides Round Robin and Session Sticky algorithms based on EC2 instance health status. HAProxy provides variety of algorithms like Round Robin, Static-RR, Least connection, source, uri, url_param etc.

Hope this helps.