How We Tested Web Load Balancers
To quantify Web load-balancer performance, we created test environments similar to both a corporate intranet and the Internet. To accomplish this, we placed an ATM-based network between our client and server systems. We chose ATM for its ability to limit bandwidth during portions of our testing. To emulate Internet traffic, bandwidth was constrained to 45 Mbps, the speed of a T3 connection. For intranet testing, we bypassed our ATM network and used Fast Ethernet to access our servers.

Our ATM network consisted of two FORE Systems ES-3810 edge switches and one ASX-200BX ATM switch. On the server side, a 3Com Corp. SuperStack II Switch 3300 Fast Ethernet switch was attached to the ES-3810 via a Fast Ethernet port. Each server was connected to the SuperStack. On the client side, a Lucent Technologies' P550 Cajun Fast Ethernet Switch was placed between the ES-3810 and the client cluster.

Our server farm consisted of eight 200-MHz Pentium Pro systems configured with 64 MB of RAM. A 300-MHz Pentium II was used to test Resonate Central Dispatch. This system served as a non-content-bearing server and worked as a part of the load-balancing system rather than a back-end server. Our client systems consisted of eight rack-mounted Cubix ERS-Fault Tolerant II units, each of which contained seven 200-MHz Pentium MMX PCs with Fast Ethernet NICs. Each client ran multiple instances of the testing scripts in order to emulate a total of 100 client machines.

We ran performance tests in two scenarios. Using custom client scripts developed in-house, each client randomly retrieved Web content from a pool of URLs for 10 minutes. The first test was performed with Fast Ethernet bandwidth. The same scripts were then run again using the new network configuration limiting the bandwidth to T3 speeds. Numerous failures were simulated on the primary load balancer, including removed network cables, power loss to the unit and removed failover cables.

Performance presented itself as a tricky issue; products were all over the map in terms of throughput and efficiency. In evaluating performance, saturating the 45-Mbps bandwidth test was good for a passing grade, while acing our Fast Ethernet testing meant an above-average grade. Server failover worked flawlessly for each product tested. Unless otherwise specified, we were satisfied with the failover mechanism in each product.