In 1998, Microsoft acquired Windows NT Load Balancing Service (WLBS) from Valence Research. This product, which Valence marketed as Convoy Cluster, is a free add-on service to Windows NT Server, Enterprise Edition (NTS/E). Microsoft implemented this service in Windows 2000 Advanced Server (Win2K AS) and Windows 2000 Datacenter Server (Datacenter) as Network Load Balancing (NLB) service. Both services support 2 to 32 servers in the same cluster. Administrators most commonly use WLBS and NLB to distribute Web client requests among Web servers; however, both services support additional Internet applications such as FTP server load balancing.
You install WLBS or NLB on all servers in the same Web site or cluster, and a virtual IP (VIP) or cluster address represents the Web site or cluster. The software requires all servers to be on the same subnet, and both services use a media access control (MAC) multicast method to redirect client traffic. When the router that the server subnet connects to receives a client request, the router uses a MAC-layer multicast to multicast the request to the cluster. The load-balancing server uses its algorithm to choose the best available server for the client. That server responds and handles the client request. You can configure the service to evenly distribute requests to the servers or specify the percentage of the total cluster load that a server takes based on the server's capacity. Both WLBS and NLB can select a server and redirect traffic according to the client's IP address and port numbers, and the software can support a persistent connection based on the client's IP address or Class C network address. However, the software doesn't support delayed binding. Each server provides failover for every other server, thus the software provides load-balancing redundancy in an active-and-active implementation. Although you can install Microsoft's load-balancing services in multiple Web sites or cluster scenarios, the service doesn't support global load balancing. To distribute traffic to multiple sites, you must use the DNS round-robin feature, which doesn't provide failover or good load-balancing across sites.
WLBS and NLB are useful for traffic distribution among front-end Web servers. To support high availability of back-end applications such as Microsoft SQL Server, you can use Microsoft Cluster Server (MSCS) through a 2-node cluster in NTS/E and a 4-node cluster in Datacenter. In addition, Microsoft is developing a COM+ load-balancing service called Component Load Balancing (CLB). This service provides load balancing on the middle or business-logic tier of multitiered Windows applications. Microsoft originally planned to include CLB in Windows 2000 (Win2K), but decided to exclude this service from the final Win2K release. Microsoft will include CLB in the soon-to-be-released Application Center Server, which is a high-performance management solution for Win2K Web applications.
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