The first bottleneck of a Web app is the business logic behind the Web server, Prell says. Most dy namic content on Web sites has been developed using simple Common Gateway Interface programs, which don't scale well. "If you're developing a complex financial transaction system using CGI, who cares if you put complex load balancing on the front end? Everything's going to come to a screeching halt when it gets to the CGI back end," he says.

Distributed and object architectures help companies implement load balancing and fault tolerance within their back-end apps. Corba- and DCOM-compliant products, for example, let companies run multiple versions of their business logic as distributed objects on different machines behind Web servers.

Load-balancing products typically are not required until a system scales so high that the Web server becomes a bottleneck, Prell says. Once that happens, users have two choices: load balance their Web servers, or increase the bandwidth of their connections to the Web by using "burstable" T1 lines. Instead of paying for a T1 line around the clock, they only pay for the ba ndwidth they need.