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• The ELB is a name, not IP, and suffers can suffer from DNS caching. Make sure you use "-Dsun.net.inetaddr.ttl=0" when starting JMeter
  
• For a given ELB IP, there seems to be a static mapping of client IP <-> backend instance. This is a slightly complicated statement that assumes a some knowledge of how amazon in general, and ELBs in particular, work. If it's still up, this page description of how the ELB works, see http://www.shlomoswidler.com/2009/07/elastic-in-elastic-load-balancing-elb.html, but if the link is down or you just need a high level overview:
  • Because the ELB is a DNS name, Amazon can (and is) load balancing the load balancers. Example DNS lookup: www.mydomain.com -> loadbalancer123.amazon.com (this is controlled by you, and can be a long TTL), loadbalancer123.amazon.com -> 1.2.3.4 (this is controlled by amazon, and is a short-lived TTL, currently 60 seconds)
  • Thus, each ELB is backed by a pool of load balancer IPs (which amazon can scale up or down based on load)
  • The ELB can be associated with one to many availability zones, but each load balancer IP is only associated with a single zone
  • Each load balancer IP evenly distributes load among instances in its availability zone
  • Thus for normal web traffic, load will be distributed fairly evenly. But, if the traffic originates from a small number of clients (like during load testing), you can easily get unbalanced loads on a per availability zone basis. There are two solutions: make sure there are enough instances to handle 100% of the load in each availability zone, or only use one availability zone.
    
• The motivation for this page was I thought I had bad load balancer behavior given this scenario:
  • I had two availability zones (for redundancy) with auto-scaling for 1 -> N in each zone.
  • I started a test that generated
  has pretty much everything you need to know. But the basic idea is that the ELB is supposed to balance the inbound traffic to the currently known & healthy backend instances (e.g. the boxes you actually control). At any given time, the ELB DNS name resolves to a pool of ELB IP addresses (which grows or shrinks based on load). The TTL on an ELB name (which is owned & controlled by amazon, e.g. loadbalancer123.amazon.com) is 60 seconds. And again, in practice I've found the load balancing to be per client/ELB IP, rather than per request.
  *Specifically, the behavior I've seen in JMeter is:
  *I start a test that generates
  • a small amount of load forever
  *
  • I
  check
  • checked all backend instances, and all the load
  in
  • was on one box
  *
  • On the JMeter box, I
  run
  • ran "dig mydomain.com" and
  watch
  • watched the TTL count down from 60 to 0
  *If
  • When the ELB IP
  changes
  • changed, all load
  moves
  • moved to a different backend instance (and if the ELB IP
  stays
  • stayed the same,
  it stays
  • the load stayed in the same place)
• But, if I changed the setup to have one availability zone with auto-scaling for 2 -> N, then each instance had ~50% of the load.