Книга: Distributed operating systems

10.8. SUMMARY

10.8. SUMMARY

DCE is a different approach to building a distributed system than that taken by Amoeba, Mach, and Chorus. Instead of starting from scratch with a new operating system running on the bare metal, DCE provides a layer on top of the native operating system that hides the differences among the individual machines, and provides common services and facilities that unify a collection of machines into a single system that is transparent in some (but not all) respects. DCE runs on top of UNIX and other operating systems.

DCE supports two facilities that are used heavily, both within DCE itself and by user programs — threads and RPC. Threads allow multiple control streams to exist within one process. Each has its own program counter, stack, and registers, but all the threads in a process share the same address space, file descriptors, and other process resources.

RPC is the basic communication mechanism used throughout DCE. It allows a client process to call a procedure on a remote machine. DCE provides a variety of options for a client to select and bind to a server.

DCE supports four major services (and several minor ones) that can be accessed by clients. These are the time, directory, security, and file services. The time service attempts to keep all the clocks with a DCE system synchronized within known limits. An interesting feature of the time service is that it represents times not as single values, but as intervals. As a result, it is possible that when comparing two times it is not possible to say unambiguously which came first.

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.

The security service allows clients and servers to authenticate each other and perform authenticated RPC. The heart of the security system is a way for clients to be authenticated and receive PACs without having their passwords appear on the network, not even in encrypted form. PACs allow clients to prove who they are in a convenient and foolproof way.

Finally, the distributed file system provides a single, system-wide name space for all files. A global file name consists of a cell name followed by a local name. The DCE file system consists (optionally) of the DCE local file system, Episode, plus the file exporter, which makes all the local file systems visible throughout the system. Files are cached using a token scheme that maintains the traditional single-system file semantics.

Although DCE provides many facilities and tools, it is not complete and probably never will be. Some areas in which more work is needed are specification and design techniques and tools, debugging aids, runtime management, object orientation, atomic transactions, and multimedia support.

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