HAProxy is a free, very fast and reliable solution offering high availability, load balancing, and proxying for TCP and HTTP-based applications. It is particularly suited for web sites crawling under very high loads while needing persistence or Layer7 processing. Supporting tens of thousands of connections is clearly realistic with today's hardware. Its mode of operation makes its integration into existing architectures very easy and riskless, while still offering the possibility not to expose fragile web servers to the Net...
At the http/application level, I found a description of a simple but powerful tool called balance.
Balance is our surprisingly successful load balancing solution being a simple but powerful generic tcp proxy with round robin load balancing and failover mechanisms. Its behaviour can be controlled at runtime using a simple command line syntax.Another http load balancer that claims high performance and more features is XLB. It states
XLB is a high performance HTTP load balancer. connection management, caching, ssl, scripting. 300 mbit/sec / 4000 reqs/sec takes 30% cpu on a 2GhZ Xeon. connection pooling to backend servers reduces memory and cpu usage on backends.One problem with load balancers, is that if they fail, a potentially large number of modules would be out of action. The Ultra Monkey load balancer addresses this issue.
Ultra Monkey 3 makes use of The Linux Virtual Server (LVS) to provide fast load balancing. The Linux-HA framework is used to monitor the linux-directors - the hosts running LVS and doing the load balancing. This is combined with ldirectord which monitors real-server - the hosts that accept end-user's connections. These three core components allow Ultra Monkey 3 to provide highly available and/or load balanced network services.I haven't used any of these options, so I'm very interested to get recommendations, please comment if you have experience or alternatives to share, and I'll update this post.
In the array of modules scenario, I would dedicate a few modules to provide load balancing services. If the modules are all connected via Ethernet, then any module can be used. If we use the USB network then the central USB master that provides an Ethernet gateway is the natural place to install load balancer services.