Nonstop is a series of server computers introduced to market in 1976 by Tandem Computers Inc., beginning with the NonStop product line, which was followed by the Hewlett-Packard Integrity NonStop product line extension. Because NonStop systems are based on an integrated hardware/software stack, HP also developed the NonStop OS operating system for them.
NonStop systems are, to an extent, self-healing. To circumvent single points of failure, they are equipped with almost only 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 i" (or TNS/E) servers, based on Intel Itanium microprocessors, was introduced. In 2014, the first systems "NonStop X" (or TNS/X) running on the Intel x86 chip were introduced.
Early NonStop applications had to be specifically coded for fault-tolerance. That requirement 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.
NonStop OS is a message-based operating system designed for fault tolerance. It works with process pairs and ensures that backup processes on redundant 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 branded NonStop OS and includes the Guardian layer, which is a low-level component of the operating system and the so-called OSS personality which runs atop this layer, which implements a Unix-like interface for other components of the OS to use.
The operating system and application are both designed to support the fault tolerant hardware. The operating system continually monitors the status of all components, switching control as necessary to maintain operations. There are also features designed into the software that allow programs to be written as continuously available programs. That is accomplished using a pair of processes where one process performs all the primary processing and the other serves as a "hot backup", receiving updates to data whenever the primary reaches a critical point in processing. Should the primary stop, the backup steps in to resume execution using the current transaction.
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 (see Amdahl's law).
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.