Книга: Distributed operating systems

9.7.6. Servers

9.7.6. Servers

Amoeba has a variety of servers for specific functions, including file management, directory management, object replication, and load balancing. All are based on objects and capabilities. Amoeba supports replicated objects via directories that contain capability sets. UNIX emulation is provided at the source code level, is based on POSIX, but is not 100 percent complete. The emulation is done by mapping UNIX concepts onto Amoeba ones and calling on the native Amoeba servers to do the work. For example, when a file is opened, the capability for the file is acquired and stored in the file descriptor table.

Mach has a single server that runs BSD UNIX as an application program. It provides 100 percent binary-compatible emulation, a great boon for running existing software for which the source code is not available. General object replication is not supported. Other servers also exist.

Chorus provides full binary compatibility with System V UNIX. The emulation is done by a collection of processes (like Amoeba) rather than by running UNIX as an application program (like Mach). However, some of these processes contain actual UNIX code, like Mach and unlike Amoeba. Like Amoeba, the native servers were designed from scratch with distributed computing in mind, so the emulation translates the UNIX constructs onto the native ones. Just as in Amoeba, for example, when a file is opened, a capability for it is fetched and stored in the file descriptor table.

A brief summary of some of the points discussed above is given in Fig. 9-28.

Fig. 9-28. A comparison of Amoeba, Mach, and Chorus.

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