1.1. Inherently concurrent systems
Systems which must respond to, or manage, simultaneous activities in their external environment are inherently concurrent and may broadly be classified as:
real-time and embedded systems (special-purpose operating systems);
operating systems (OSs);
distributed operating systems (the OS components are distributed);
database management and transaction processing systems (run above operating systems);
distributed, application-level services.
The implementation of a system may be centralized, running on a single computer (which may also be termed a host or node), or may be distributed over a number of computers. The user interface to a system may make its support for concurrent activities explicit.