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Geographic information system (GIS) application design has become Internet-centric; the enterprise has become spatially enabled; traditional databases have integrated islands of spatial and non-spatial data. These are some of the trends that are helping to promote the expansion of GIS into the information mainstream as a core technology, and more importantly into mission-critical applications. However, computer hardware can fail, and if mission-critical applications cannot be kept running effectively-that is, "available", such failures are costly to an organization. GIS applications are available only if they allow users to access the GIS server application(s) and the GIS data files. High availability environments are designed for computing installations that require critical systems to be automatically and seamlessly restarted in the event of a hardware failure. They can ensure that data remains accessible, and that applications be kept running, even during a prolonged hardware failure. This paper investigates the nature and architecture of a high availability GIS: the use of hardware and software common to most high performance computing implementations, to provide automatic failover and continuous operation in the event of system failure. It describes how high availability was implemented by two major GIS software platforms to minimize interruptions to applications and to keep file systems continuously available.