Should Distributed Systems be Hidden?

EXisting multiprocessor systems can be divided in three classes according to the degree of coupling among the machines: shared memory, local network, or long-haul network. The lighter Lhe coupling, the deeper in lhe operating system wilt btl the optimal position of Lhe software for inlerprocess communications. Current designs atLempt either to ex Lend shared memory operating systems, or long-haul network protocols, to the local network. Neilher approach is opLin1iJ.1. We argue that the best position of Lhe communicalions layer is in Lhe middle levels of the opcrallng system-above Lhe vir"Lual memory and below the dircctor'y manager. With this position. the directory hierarchy can be extended to become a global name space for permanent objects jn the system. Eaeh of the higher levels is then easily converted Lo hide the remaining vestiges of the locations of objects it manuges. The resulting operating system can fit on machines as small as workstations. December 1982 CSD-TH-426 This is 11 preprint of u paper lo be presented al !..he Inlemationo.i \'forkshop on Computer SysLems Oreanb:lltion. March 29-31. 1983. 1 AutllOTB uddrcl."!lell: Peler J. Dennine, Cornputer Sciences DepllrLrnenL, l'urdue Universily, Wesl LulllyelLe, IN 47007 (net Ilddre:is: dcmlinllllPpurdue); llobcrL L. Brown, Computer Sciences Dcpurtmenl, Purdue University, Wesl LufuycLtc, IN 47007 (nel uddrc5's: rlb@purduc). INl'IlODUCTION 2 (12131/82) Distributed systems have been under active discussion for a decade. A few have been in operation for nearly that span, others are being: delivered, and many more are being designed. It is tltting to reflect on what has been aeeomplished. In this paper distributed system means a coordinated collection of computers connected by a communication network. 2 The computers and network are intended to form a sysLem with personalized working environments, easy communication among users, easy access to special services sueh as mail and printing. continued operation despite failures of individual computers, and the ability to extend the power of the system in modest steps by adding new computers. Are these goals being met by current designs? In many cases, no. Some distributed operating systems are nothing more than traditional designs with long-haul neLwork software added on to manage communicaLion with other machines. By allowing the machines and network to be visible in the programming environment, these systems force many users to learn about transport-level network protocols to carry out ordinary tasks. Their programming environments arc lower in level than those o[ many third generalion operating systems. Other distributed operating systems arc nothing more than a traditional design adapted by putting important functions on dedicated machines. These systems hide the machincs and the network but. because funcLions iU'C not replicated, they arc no mor'c ruult tolerant than the sinl:!C-ulill.:iline 2 SO!llclimes Lhe Lerm is used Lo denote lin nrrllY of microcomputers that work In pnrllllelloward lhe solulion ot (l common problem. (For example. lDrin7B, Hoar7B].) This cluss of dis. lribuled syslems is nol considered here.