Software, appliance, or switch

Load balancers can be divided into two general categories: software- or appliance-based systems (running on Linux or Windows and Intel- or AMD-based hardware), and switch-based systems that make use of proprietary operating systems and hardware.

The software- or appliance-based systems are easy to add functionality to, but they are limited in throughput, since they rely on the two to four network interfaces installed in the appliance. Switch-based load balancers often have 24 or more ports, and offer gigabit or higher performance on every port. In the last few years, switch-based products have come to offer the same kinds of extended functionality as appliances, including support for e-commerce, additional security features, network acceleration, geographic load balancing, and more.

Once owned by software-based solutions, the low end of the market is in flux. First, the added cost of hardware on Linux- or Windows-based appliances is offset by the dramatically reduced time to configure the system. Second, the entire "commodity" category is getting smaller as time goes by - the ready availability of ASICS (Application-Specific Integrated Circuits) means that switch-based load balancers can be inexpensively made. As prices of switch-based load balancers have dropped, and more organizations have started delivering Web-based applications both internally over high-speed connections and externally over a slow Internet links, switch-based products are coming to dominate the market. However, appliances offer a low starting cost, are easily expanded with SSL acceleration boards and other add-ons, and still have a place, especially for organizations with no need for multiple gigabit ports.

If you want to spend the time, you can use an existing Windows server and WSLB (Windows Server Load Balancing) software, or with Linux installed and add IPVS (IP Virtual Server) or BalanceNG load balancing software to create your own load balancer. However, unless you're paid a lot less than the average admin, the time you spend configuring the Linux server and the IPVS software will more than offset the cost of a small appliance, some of which are available for less than US$2,000 including the hardware.

The product offerings

Appliance vendors include Barracuda Networks, Kemp Technologies, and Zeus Technology. Coyote Point Systems, which was on the appliance side for a long time, now sells switch-based systems and offers some of the least expensive switch-based products available, still with good functionality.

Switch-based vendors include Array Networks, Cisco, Citrix (NetScaler), Foundry, F5 Networks, and Juniper Networks, though Juniper announced in January that it is discontinuing its product line (DX systems will no longer be available after July 24, 2008). These vendors provide high levels of functionality aimed at serving Web sites that are scalable and highly available.

Beyond the basic parceling out of requests to groups of servers, some load balancers also use several different methods to speed up responses to requests or to compress traffic, lessening utilization of bandwidth. One of the main optimization techniques is to group what would otherwise be anywhere from a few to more than 100 TCP sessions into one, using TCP session multiplexing or TCP session consolidation, depending on the vendor. Because the overhead of starting a session is fairly large, and some Web browsers can start dozens or even hundreds of TCP sessions to display a single page, multiplexing and session consolidation can improve performance substantially.