[Jakarta-jmeter Wiki] Update of "JMeterAndAmazon" by oberman

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The "JMeterAndAmazon" page has been changed by oberman.
http://wiki.apache.org/jakarta-jmeter/JMeterAndAmazon?action=diff&rev1=1&rev2=2

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  == Elastic Load Balancer (ELB) Issues ==
  
-  * The ELB is a name, not IP, and suffers from DNS caching.  Make sure you use "-Dsun.net.inetaddr.ttl=0" when starting JMeter
+  * The ELB is a name, not IP, and 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 [[http://www.shlomoswidler.com/2009/07/elastic-in-elastic-load-balancing-elb.html]] 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 backend instances, and all load in on one box
-    *On the JMeter box, I run "dig mydomain.com" and watch the TTL count down from 60 to 0
-    *If the ELB IP changes, all load moves to a different backend instance (and if the ELB IP stays the same, it stays in the same place
+  * For a 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 a small amount of load forever
+    * I checked all backend instances, and all the load was on one box
+    * On the JMeter box, I ran "dig mydomain.com" and watched the TTL count down from 60 to 0
+    * When the ELB IP changed, all load moved to a different backend instance (and if the ELB IP stayed the same, 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.
+