Balancing act

A load balancer handles Web site traffic much like a maitre d' spreads customers around so the waitstaff doesn't burn out. No restaurant or Web site wants visitors to leave because service is too slow.

When a load balancer is in place, Web clients connect to a virtual Web site. The load balancer distributes user requests to multiple servers based on traffic volume and server availability. Many of today's load-balancing products also provide content-based balancing, which allows Web administrators to match traffic to server power by designating a large, fast disk array for image content and a fast and secure server for Secure Sockets Layer (SSL) and electronic commerce transactions. Some load balancers can perform trickier maneuvers, such as isolating Common Gateway Interface requests and sending them to a separate server or prioritizing SSL traffic over HTML traffic.

Sophisticated load balancers can operate globally, managing traffic among multiple, geographically dispersed sites. Global load balancing ensures that content and applications are quickly and cost-effectively made available to far-flung users.

Today, server load-balancing products are available from many sources. We polled 16 vendors and compiled specifications for more than 25 load-balancing products in our interactive Buyer's Guide (www.nwfusion.com, DocFinder: 3323). Browse our customizable database, and you'll find established companies and start-ups sharing the market.

A few of the big names in this market are Cisco, Microsoft and IBM. But we've found that a cadre of smaller, more focused start-ups - including Alteon WebSystems, ArrowPoint Communications, Coyote Point, F5 Networks, HydraWeb Technologies, IPivot, Radware and Resonate - tend to innovate a lot faster and often provide more comprehensive technical support because load balancing is their mainstay.

If you're in the market for a Web server load balancer, start by characterizing the nature of your site:

n First, determine your application requirements, which will determine the type of persistency policies you need. In the Web server world, persistence means a repeat user is connected to a Web server that is aware of that user's previous requests and account status. For example, if a user browses a site and places items in a shopping cart during one visit and then returns to the site to execute the purchase, the load balancer needs to connect that user to a server that remembers the items placed on hold.

n Determine the scale you need now and estimate the scale you expect to need in two years based on hits per second, number of simultaneous users and bandwidth requirements. Make sure you take into account the maximum capacity of other key elements of your system, such as WAN link size, database server capacity and application server capacity.

n Consider your application architecture. Is it two-tier or N-tier, with different levels of transaction processing at each tier? Do you plan to migrate over time? These answers will affect your monitoring requirements: The more complex your transactions are, the more refined your monitoring features should be to pinpoint bottlenecks.

A final design requires much more information. But addressing these factors will give you a good foundation and help narrow your list of alternatives. You can then use more detailed information to make good choices about the performance, scalability, redundancy and management features you desire.

RELATED LINKS

Load balancing forum
Discuss load balancing with Mark Hoover, author of our article on load balancing trends.

Form follows function
Detailed look at which vendors have adopted which architectural approaches. Network World, 6/14/99.

Review: load balancers
All nine products we tested improved Web server performance significantly, making it tough to select a single winner. But in the end, the fastest product under the greatest load, Resonate's Central Dispatch, won our Blue Ribbon Award. Network World, 6/14/99.

Interactive buyer's guide
Find a product that best matches your criteria, compare two or more load balancers in several categories or download a spreadsheet with all the product data.

User study: Dense traffic drives Web-server load balancing
With nearly 60 Web servers, the West Group needed load balancing quickly. See what they decided on and why. Network World Fusion, 6/14/99.

Hoover is the president of Acuitive, a network consultancy specializing in IP-related services and network management and operations. He can be reached at hoov@ acuitive.com.

Acuitive has published two research reports on server load balancing and traffic management: "Virtual Resource Management: Key Technologies, Tricks of the Trade and Application Requirements" and "Virtual Resource Management: Which Vendor is Right For You?" For information about these publications, visit www.acuitive.com.