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Server load balancing: A practical guide

By Tony Bourke, Network World
April 24, 2007 09:04 AM ET

Network World - This piece was contributed by a reader. If you have expertise or an experience you would like to share, contact Network World Editor in Chief John Dix (

A few years ago server load balancing was an expensive luxury for sites with deep pockets. It wasn't uncommon to pony up $100,000 for a redundant pair of load balancers. But today load balancing is within reach of companies with far more modest means. Vendors old and new are offering full-featured load balancers for less than $5,000.

These sub-$5,000 devices are no blue light specials. They offer the features and performance small and midsize businesses require. In fact, many are as good or better than the highest end load balancers of five or six years ago. They come in redundant configurations, perform network address translation, health checking, and some even do cookie persistence, SSL acceleration/offloading and URL switching.

However, load balancing technology is foreign to many customers who, until recently, couldn’t afford it. In addition, it requires knowledge of networking, server administration and applications. Because of the novelty and divergent skills involved, shopping for a load balancer can be daunting. This story will help you make a more informed purchase decision.

To simplify the job of shopping for a load balancer, we’ll examine four primary concepts:

Feature set: What features you might need and what features the load balancing vendors offer.

Costs: This includes the price of the unit, plus support and maintenance contracts that can boost the price.

Performance: Everyone wants to make sure they get a box that will support their current traffic, as well as traffic for the foreseeable future.

Integration: Will a given product work with your network? Sometimes a feature here, and unsupported implementation there, will lead to a “gotcha” and derail a project.

Server load balancers can come with a dizzying array of features, from cookie persistence to on-the-fly HTTP header re-writes, so you should know what's important to you beyond the functions that all load balancers provide: distributing traffic from a virtual IP and port and checking to see if a server is responding.

Cookie persistence

Server persistence is one of the most important functions load balancers perform. Persistence (also called sticky and server affinity) is what keeps an individual user glued to a single Web server, rather than being sent to a different server for every request.

For interactive Web sites this is typically a must. Interactive session information for an individual (such as a logon credentials or a shopping cart) are often kept only on that server during the session and not shared with the other servers. So if a user were sent to a different Web server they might find their shopping cart is empty or be forced to log on again.

The two basic types of persistence methods for load balancers are source IP address and cookies. With source IP address, the load balancer looks at the source address of the incoming request to keep track of individual users. With cookie persistence, the load balancer looks at an HTTP cookie to differentiate users.

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