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A distributed system is a collection of independent computers that appear to the users of the system as a single computer.. 1. An interprocess communication mechanism.. In the late 1980s, the world’s telephone companies finally began to realize that there was more to telecommunications than transmitting voice in 4 KHz analog channels. It is true that data networks, such as X.25, existed for years, but they were clearly stepchildren and frequently ran at 56 or 64 Kbps. Systems like the Internet were regarded as academic curiosities, akin to a two-headed cow in a circus sideshow. Analog voice was where the action (and money) was.. [Картинка: any2fbimgloader34]. 2.4.5. Implementation Issues. 1. How is an idle workstation found?. A distributed file system typically has two reasonably distinct components: the true file service and the directory service. The former is concerned with the operations on individual files, such as reading, writing, and appending, whereas the latter is concerned with creating and managing directories, adding and deleting files from directories, and so on. In this section we will discuss the true file service interface; in the next one we will discuss the directory service interface.. At the next level up we have a set of library procedures that provide a more convenient interface for user programs. Several flavors are provided. They do their job by calling the low-level interface procedures.. [Картинка: any2fbimgloader207]. Mach has a powerful, elaborate, and highly flexible memory management system based on paging, including features found in few other operating systems. In particular, it separates the machine-independent parts of the memory management system from the machine-dependent parts in an extremely clear and unusual way. This separation makes the memory management far more portable than in other systems. In addition, the memory management system interacts closely with the communication system, which we will discuss in the following section.. The goal of communication in Mach is to support a variety of styles of communication in a reliable and flexible way (Draves, 1990). It can handle asynchronous message passing, RPC, byte streams, and other forms as well. Mach’s interprocess communication mechanism is based on that of its ancestors, RIG and Accent. Due to this evolution, the mechanism used has been optimized for the local case (one node) rather than the remote case (distributed system).. 8.4.1. Ports. Fig. 10-3. A thread can be in one of four states.. DFS has a number of features that are worth mentioning. To start with, it has uniform naming, integrated with CDS, so file names are location independent. Administrators can move files from one file server to another one within the same cell without requiring any changes to user programs. Files can also be replicated to spread the load more evenly and maintain availability in the event of file server crashes. There is also a facility to automatically distribute new versions of binary programs and other heavily used read-only files to servers (including user workstations).. The directory service stores the names and locations of all kinds of resources and allows clients to look them up. The CDS holds local names (within the cell). The GDS holds global (out-of-cell) names. Both the DNS and X.500 naming systems are supported. Names form a hierarchy. The directory service is, in fact, a replicated, distributed data base system..