Betting the Farm

Web server farms can provide highly available infrastructures to prevent lost revenues from downtime

Redundancy. Redundancy is required at every level of the Web farm design, from the ISP connection at the front end to the back-end infrastructure that accesses the Web servers. In order to eliminate single points of failure, you must have a backup for every network device placed between the ISP and the Web farm. You must also provide fail-over strategies to transfer control to the "healthy" devices; the fail-over process should take less than a few seconds and should be transparent to the clients.

Security. A server farm is usually divided into secured and unsecured zones. Secured servers are placed behind a firewall. Web traffic initially goes through the unsecured servers and secure transactions go through the servers behind the firewall. Firewalls provide site security by monitoring Web traffic, network address translation, port-translation, and access-control lists. Secure socket layer (SSL) accelerators help speed up SSL transactions. SSL off-loading devices function as discrete network devices that store keys and certificates and perform all SSL processing - freeing secure servers from processing these functions.

Configuring a Web Server Farm

A Web farm requires some type of switch or load balancer (Web switch). The primary purpose of the Web switch is to load balance the incoming IP traffic to multiple Web servers. It provides high availability, improved performance, fault tolerance, easier manageability, and proxy functionality as well.

A Web switch must have two network interfaces. On one end, it must connect to the Internet via a router. On the other end, it must connect to the server network that connects the Web servers.

Let's look at an example to configure a Web server farm comprising four Web servers to the server www.myCompany.com This configuration uses two Web switches to make the Web server farm fault-tolerant to network failures (see Figure 2). This activity requires both a primary and a secondary Web switch. A special network cable connects the two switches together, which lets each switch check the status of the other. Both Web switches connect to the router LAN and to the server LAN. Each switch interface has a unique IP address. The IP address of the Web switch interface (connected to the router LAN) is in the same network as the Internet router.

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Availability Equals Revenue

E-business enterprises depend on their Web sites to generate revenue, satisfy customers, and optimize business processes. The enterprise Web site must respond quickly and be available 2437 to service customers, business partners, and employees. The Web infrastructure has to be designed, maintained, and scaled to achieve this high availability.

As we mentioned in our previous column, the lost revenues due to Web site unavailability outweigh the cost of providing redundancy in the Web infrastructure. Just how important is your Web site to your business?

Kamesh Namuduri [kamesh@mediaone.net] is an assistant professor in the computer and information science department at Clark Atlanta University in Atlanta.
Palani Ram [prr@webahn.com] is an IT consultant. He currently designs Web-based enterprise architectures and B2B systems.

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