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NonStop is a series of server computers introduced to market in 1976, beginning with the Tandem NonStop product line, which was followed by the HP Integrity NonStop product line extension. Because NonStop systems are based on an integrated hardware/software stack, HP also developed a special operating system for them: NonStop OS.
NonStop systems are, to an extent, self-healing. To circumvent single points of failure, they are equipped with some redundant components. When a mainline component fails, the system automatically falls back to the backup.
Originally introduced in 1976 by Tandem Computers Inc., the line was later owned by Compaq (from 1997) and Hewlett-Packard (since 2003). In 2005, the current product line of HP Integrity NonStop servers, based on Intel Itanium microprocessors, was introduced.
Early NonStop applications had to be specially coded to be fault-tolerant. That obstacle was removed in 1983 with the introduction of the Transaction Monitoring Facility (TMF), which handles the various aspects of fault tolerance on the system level, transparent to the application.
NonStop OS is a message-based operating system designed for software fault tolerance. It works with process pairs and ensures that backup processes in different CPUs take over in case of a process or CPU failure. Data integrity is maintained during those takeovers; no transactions or data are lost or corrupted.
The operating system as a whole is called NonStop OS and is broken down in the so-called Guardian layer, which is a lower level of the operating system and the so-called OSS personality which sits on top of Guardian and implements a Unix-like interface to the underlying Guardian layer. 
The HP Integrity NonStop computers are a line of fault-tolerant server computers based on the Intel Itanium processor platform, and optimized for transaction processing. Average availability levels of 99.999% have been observed. NonStop systems feature a massively parallel processing (MPP) architecture and provide linear scalability. Each CPU (systems can be expanded up to over 4000 CPUs) runs its own copy of the OS. This is a shared nothing architecture -- a "share nothing" arrangement also known as loosely coupled multiprocessing, and no "diminishing returns" occur as more processors are added.
Due to the integrated hardware/software stack and a single system image for even the largest configurations, system management requirements for NonStop systems are rather low. In most deployments there is just a single production server, not a complex server farm.
Most customers also have a backup server in a remote location for disaster recovery. There are standard products to keep the data of the production and the backup server in sync, hence there is fast takeover and no data loss also in a disaster situation with the production server being disabled or destroyed.
HP also developed a data warehouse and business intelligence server line, HP Neoview, based on the NonStop line. It acted as a database server, providing NonStop OS and NonStop SQL, but lacked the transaction processing functionality of the original NonStop systems. The line was retired, and no longer marketed, as of January 24, 2011.
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