To guide you through the decision-making process, let's explore the relevant issues pertaining to multiple-server Web sites, including DNS solutions, URL load-balancing and global load-balancing alternatives.

DNS Dilemma The structure of your Web site determines how you propagate content changes to your Web servers. Sites grow too large too fast for single-server solutions. The integration of additional servers into a single homogenous Web site is fundamental to a content-replication strategic design.

When a site exceeds single-server capacity, it still must maintain its single-server identity to the outside world. This is commonly accomplished with DNS records to bind a domain name, such as www.company.com, to multiple IP numbers. For example, a site with two servers would have a DNS record containing two IP addresses, a.b.c.d and a.b.c.e. Clients who visit your site resolve the URL to the site's DNS record. DNS servers return both IPs, while the client IP stack uses one of the two IPs (depending on implementation). Conversely, subsequent DNS queries may or may not return the same IP addresses in the same order. As a result, Web surfers are not guaranteed which IP their browser will access. To ensure proper-client request service, each server in the site must have identical content.

To make this possible, you can store identical content on each server or store content in one or more locations accessible over the network.

If you choose the former, you must take into account the number of available servers and the amount of content to be replicated, and carefully weigh your bandwidth requirements. A switched network combined with lightning-fast topologies, such as Gigabit Ethernet, is adequate for most large sites. Remember, though, that content discrepancies between servers may arise as files are being updated, network traffic will increase as content updates are replicated, and storage costs per server will escalate because each server will require hardware updates as the site expands.

Cutting Into Storage Costs The other option is to store content in one location using a file- or database-sharing system, such as NFS (Network File System) or AFS (Andrew File System). With content in a central location, storage costs decrease dramatically. Network use is limited to the Web servers retrieving content from the network file share.

A layer of network caching appliances inserted in front of your Web servers can reduce server load and traffic to the central store. AFS and NFS version 3 include caching conventions and boast advantages, such as simultaneous sitewide content updates and reduced storage costs. However, caching devices can produce failure points without redundant configurations. Research the effect of these appliances on your site before you invest in them.

Different Sites, Different Needs Not all Web pages are created equal. For instance, default pages and directory listings are downloaded and accessed more frequently than other site content. And sites may create multiple versions of Web content to better suit a variety of client-access speeds. If these alternatives apply to your site, you'll find they reduce server load, but also eat up storage space. The most server-intensive content is generated dynamically through scripts and CGI binaries. These scripts require Web-server processing and can easily gobble up a great deal of server resources.