SQL Server Magazine

Web sites that rely on sophisticated server applications have two main problems: reliability and scalability. Users need reliable access to Web applications, including their organization's intranet applications. The increase in traffic on both Internet and intranet sites has contributed to the problem of Web application scalability. I've seen the traffic on my company's Web site go from 1000 hits per day to 80,000 hits per day. Using one server to provide reliability presents a problem because if the server goes down, the applications go down. In addition, one server provides a finite amount of scalability.

Web masters and systems administrators based the first reliability and scalability solutions on hardware devices such as the round-robin DNS router. The router randomly handed requests off to different servers on the network to provide load balancing. This method worked fine with the original HTML sites and even some sites that Web masters built with early Web applications. However, the router solution began to fall apart with the introduction of Web applications with more sophisticated features such as Internet Information Server (IIS) session support, which stores information on the server between a user's sessions. The router solution was unable to provide accurate load balancing necessary for mature Web applications and their features.

To address reliability and scalability, vendors developed software such as Valence Research's Convoy Cluster Software for NT Server. In late 1998, Microsoft acquired Valence Research and renamed this product Windows NT Load Balancing Service (WLBS) 2.2. WLBS is free to users of NT Server 4.0, Enterprise Edition. (To obtain WLBS, go to http://www.microsoft.com/ntserver/all/ downloads.asp#windowsntserverfeatures.) WLBS also runs on NT Server 4.0, Standard Edition, but it isn't licensed for this OS. The README file states, "When planning upgrades to Windows 2000, please note that WLBS will only work on Windows 2000 Advanced Server and Windows 2000 Datacenter Server." You need to have the correct NT version and license to run WLBS, and you must upgrade to the correct version of Windows 2000 (Win2K).

Highlights and Caveats
WLBS solves server applications' reliability problems through clustering, and scalability problems through load balancing. Clustering provides failover support. If one server goes down, the program routes users to another server. Load balancing distributes the user load across the servers and lets you add new servers on the fly. For more information about how WLBS can improve application performance, see the Microsoft article "Windows NT Load Balancing Service (WLBS) Provides Higher Scalability and Improved Fault Tolerance" at http://www.microsoft.com/ntserver/ ntserverenterprise/exec/feature/wlbs/default.asp.

WLBS turns several servers into a group, or cluster, of servers that supports TCP/IP applications and services to provide reliability and scalability. The cluster can run applications and services that include Web servers (such as IIS), streaming media services (such as Microsoft NetShow Services), VPNs, and proxy services (such as Microsoft Proxy Server). Although these examples are Microsoft software, WLBS supports third-party applications and doesn't limit you to Microsoft applications. WLBS and its services are transparent to applications because the product sits between the incoming TCP/IP traffic and the application.

WLBS doesn't work with non-IP software such as a database server, mail server, or COM objects that run on the server. You can use Microsoft Cluster Server with non-IP applications, then use WLBS for the TCP/IP applications to provide total fault tolerance for your system. You can place COM objects on each server in the cluster to obtain COM object failover support with WLBS. In this case, if a server fails, then WLBS routes requests for the IP application accessing the COM objects to another server in the cluster.

Figure 1 illustrates a user accessing a WLBS cluster from a Web browser. WLBS installs as a network adapter on each NT server in the cluster. The WLBS cluster configuration has a distinct domain name (e.g., myco.com) and IP address (e.g., 199.200.101.20), which give the cluster a unique identity. You install the WLBS drivers and use the cluster name and IP address for each cluster-member server. Users can use the cluster name or address to access a TCP/IP application on the cluster. Each server's cluster software sees the request and dynamically sends the message to the correct server based on a load-balancing algorithm.

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