| Overview
Clusters of computer systems have been built and used for over a decade [1]. Pfister [2] defines a cluster as “a parallel or distributed system that consists of a collection of interconnected whole computers, that is utilized as a single, unified computing resource”. In general, the goal of a cluster is to make it possible to share a computing load over several systems without either the users or system administrators needing to know that more than one system is involved. If any component in the system, hardware or software fails the user may see degraded performance, but will not lose access to the service. Ideally, if more processing power is needed the user simply “plugs in a new component”, and presto, the performance of the system as a whole improves. Windows NT Clusters are, in general, shared nothing clusters. This means that while several systems in the cluster may have access to a device or resource, it is effectively owned and managed by a single system at a time. Currently the SCSI bus with multiple initiators is used as the storage connection. Fiber Channel will be supported in the near future.
To appreciate our design approach it is helpful to understand the business goals for the development team. The principal goal was to develop a product that addresses a very broad, high volume market, rather than specific market segments. Marketing studies showed a huge demand for higher availability in small businesses as data bases and electronic mail have become essential to their daily operation. These businesses cannot afford specialized computer operations staff, so ease of installation and management were defined as key product advantages. Providing improved availability for existing applications easily, as well as providing tools to enhance other applications to take advantage of the cluster features were also requirements. On the other hand we see Windows NT moving into larger, higher performance systems so the base operating system needed to be extended to provide a foundation to build large scalable clusters over several years.
Starting with these goals, we formed a plan to develop clusters in phases over several years. The first phase, now completed, installed the underpinnings into the base operating system, and built the foundation of the cluster components, providing enhanced availability to key applications using storage accessible by two nodes. Later phases will allow for much larger clusters, true distributed applications, higher performance interconnects, distributed storage, load balancing, etc. From a technical viewpoint, we needed to provide a way to know which systems were operating as a part of a cluster, what applications were running, and the current state of health of those applications. This information needed to be available in all nodes of the cluster so that if a single node failed we would know what it was doing, and what should be done about it. Advertising and locating services in the cluster is an especially complex issue and we also needed to develop tools to easily install and administer the cluster as a whole.
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