Forms follow function

Load-balancing products typically assume one of three forms: software, dedicated hardware device or switch with built-in load-balancing features.

Software vendors generally deliver their products on CD-ROMs that you install on computer platforms and operating systems of your choice. The load balancer uses the memory, CPU and I/O resources of the platform.

It sounds pretty straightforward, yet within the software category, vendors manage to distinguish themselves by the way they've designed the scheduler, which is responsible for receiving and forwarding user requests to the appropriate server.

Through its Windows NT Load Balancing Service, Microsoft offers a completely distributed system wherein a scheduler is deployed on each content server and all traffic is sent to all servers. IBM's WebSphere Performance Pack is more traditional - software is centralized and simply loaded onto a hardware platform that becomes the site scheduler. All traffic passes through the single scheduler and is distributed to the content servers.

Resonate's Central Dispatch software partitions the scheduler function between a central scheduler and the content servers. This off-loads some functions from the scheduler, which improves performance. Central Dispatch also lets you turn on as many schedulers on as many servers as you need to meet site performance requirements.

Some vendors choose to prepackage their load balancing products in dedicated hardware devices. This allows the manufacturer to select peripherals and drivers, memory speeds and sizes, and operating system parameters tailored for each customer's performance and capacity needs.

To date, the preloaded device approach has been the simplest and fastest way to get new features to market. The hardware engineering effort required is small, and vendors can focus on software development. Unlike pure software vendors, dedicated hardware vendors don't have to support their products on many different operating systems. Many of the early load-balancing vendors, such as Cisco, F5 Networks and RadWare, have chosen the dedicated device path. Their products have helped define the market and add sophistication to feature sets. Newcomers IPivot and Coyote Point Systems have also chosen to use this model.

Load-balancing switches combine load-balancing functionality with Layer 2 and 3 routing features. These devices can reduce the number of hardware boxes at a site if you can consolidate individual routers, switches, bandwidth managers and load balancers, and implement all these functions with redundancy.

Load-balancing switches are often among the fastest performing devices. Application Specific Integrated Circuits (ASIC) built for server load balancers usually provide some kind of hardware assist for low-level packet processing because these are common functions needed by routers. After that it comes down to software processing, which is where the switch vendors really begin to distinguish themselves.

Foundry uses its internal management CPU for all load-balancing functions. Alteon, on the other hand, has integrated two Reduced Instruction Set Computing processor cores into each of their ASICs, one of which is used per physical port. Therefore, in a 10-port Alteon box, you have 20 processors working, performing all heavy-duty load-balancing tasks.

ArrowPoint's CS-100 and CS-800 switches use an internal MMC chipset that has been microcoded to provide distributed software-based processing at hardware speeds. A central processor handles HTTP processing and core management functions. HolonTech uses a chip for basic Layer 2 switching, but augments it with ASICs to accelerate some load-balancing functions.

While we recommend you consider a range of products rather than limit yourself to a particular type of load-balancing product, there are some advantages and disadvantages specific to each category of load balancers that can help you make up your mind:

Load-balancing software

Pros

( You don't have to add another box to your Web site infrastructure.

( You can increase capacity by adding memory to the base computer platform.

( Product performance scales immediately with the release of new server technology. You can upgrade to a new server to increase scheduler performance, for example, and reuse the software's former host as a content server to increase overall site scale.

( In general, you can quickly change site functionality and logical topology. Often you can do this remotely with no wiring or other physical changes.

Cons

( You need to ensure that the software is supported on your hardware and operating system.

( Installation can be time consuming, and customization may be necessary, which can result in extra deployment cost. You have to ensure that you haven't impacted another application or resource running on the same platform.

( Engineering guidelines are difficult to define. Throughput and capacity are dependent on the hardware platform, operating system, systems tuning and resident functions.

( Reliability is determined by the reliability of the computer platform and all resident software.

Dedicated load-balancing devices

Pros

( Performance and capacity parameters are well established.

( Units are easy to install and don't raise issues of operating systems, platform or driver compatibility.

( The devices don't affect the performance of existing applications on servers.

( Broad feature sets.

Cons

( Low to mid-range performance. A Pentium CPU running BSD Unix can only run so fast, even if the vendor pushes most of the functionality down into the kernel or driver space.

( Features can degrade performance. All functions on a dedicated load balancer share a common CPU, which must perform packet processing as well as all management functions, including monitoring, bandwidth management and packet filtering.

( Expensive.

Load-balancing switches

Pros

( A load-balancing switch can eliminate the need for separate router, switch, bandwidth management and load-balancing boxes.

( The cost for consolidation often doesn't add much to the cost of a state-of-the-art Layer 2 or 3 switch.

( High performance.

Cons

( It takes switch vendors longer to integrate and test new features that dedicated device vendors.

( Load-balancing switches typically have less capacity than dedicated hardware and software products.

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

Issues and trends
What to consider before you buy.

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.