FAQ: Distributing HTTP requests with hardware load balancers
Hardware load balancers are very common these days. A hardware load-balancing solution is typically much smarter than the Round-Robin DNS solution discussed in the previous section. A hardware load-balancing device can implement various ways to monitor each of the Web serverís network load, performance, and availability by using response time, by number of requests sent, and by making self-generated HTTP test requests. Consequently, these devices offer a greater control for your load-balancing scheme.
Some of the load-balancing devices also enable you to create server pools in which some servers have higher priority over others. For example, if you have a Pentium 4 1.3 GHz system with 2GB of RAM and a Pentium III 550 MHz system with 512GB of RAM, you can give higher priority to the Pentium 4 system because itís more powerful and more likely to be capable of servicing many more requests than the Pentium III system. Local Director (CISCO) and Web Director (Radware) are hardware loadbalancing solutions that have worked well for me.
Figure 23-1 shows a simple load-balanced solution in which each client request comes to the load-balancing hardware.
The load balancer decides which Web server node to use to service the request and passes the request to the most suitable Web server, which responds to the request as usual. The selection criteria for the Web server can be dependent on priority, availability, and reliability. Avid readers will notice that this solution has a single point of entry into the Web network. For example, requests for http://www.domain.com must come to the load balancer, which internally translates this request to a request for service by a Web server node on the network. Figure 23-2 shows an example load-balanced Web network.