LOCUS is a distributed operating system which supports transparent access to data through a network wide filesystem, permits automatic replication of storage, supports transparent distributed process execution, supplies a number of high reliability functions such as nested transactions, and is upward compatible with Unix. Partitioned operation of subnet's and their dynamic merge is also supported.

Many distributed systems are now being developed to provide users with convenient access to data via some kind of communications network. In many cases it is desirable to keep the system functioning even when it is partitioned by network failures. A serious problem in this context is how one can support redundant copies of resources such as files (for the sake of reliability) while simultaneously monitoring their mutual consistency (the equality of multiple copies). This is difficult since network faiures can lead to inconsistency, and disrupt attempts at maintaining consistency. In fact, even the detection of inconsistent copies is a nontrivial problem. Naive methods either 1) compare the multiple copies entirely or 2) perform simple tests which will diagnose some consistent copies as inconsistent. Here a new approach, involving version vectors and origin points, is presented and shown to detect single file, multiple copy mutual inconsistency effectively. The approach has been used in the design of LOCUS, a local network operating system at UCLA.

LOCUS is a distributed operating system that provides a very high degree of network transparency while at the same time supporting high performance and automatic replication of storage. By network transparency we mean that at the system call interface there is no need to mention anything network related. Knowledge of the network and code to interact with foreign sites is below this interface and is thus hidden from both users and programs under normal conditions. LOCUS is application code compatible with Unix2, and performance compares favorably with standard, single system Unix. LOCUS runs on a high bandwidth, low delay local network. It is designed to permit both a significant degree of local autonomy for each site in the network while still providing a network-wide, location independent name structure. Atomic file operations and extensive synchronization are supported.