Книга: Distributed operating systems

7.1.2. Research Goals

7.1.2. Research Goals

Many research projects in distributed operating systems have started with an existing system (e.g., UNIX) and added new features, such as networking and a shared file system, to make it more distributed. The Amoeba project took a different approach. It started with a clean slate and developed a new system from scratch. The idea was to make a fresh start and experiment with new ideas without having to worry about backward compatibility with any existing system. To avoid the chore of having to rewrite a huge amount of application software from scratch as well, a UNIX emulation package was added later.

The primary goal of the project was to build a transparent distributed operating system. To the average user, using Amoeba is like using a traditional timesharing system like UNIX. One logs in, edits and compiles programs, moves files around, and so on. The difference is that each of these actions makes use of multiple machines over the network. These include process servers, file servers, directory servers, compute servers, and other machines, but the user is not aware of any of this. At the terminal, it just looks like a timesharing system.

An important distinction between Amoeba and most other distributed systems is that Amoeba has no concept of a "home machine." When a user logs in, it is to the system as a whole, not to a specific machine. Machines do not have owners. The initial shell, started upon login, runs on some arbitrary machine, but as commands are started up, in general they do not run on the same machine as the shell. Instead, the system automatically looks around for the most lightly loaded machine to run each new command on. During the course of a long terminal session, the processes that run on behalf of any one user will be spread more-or-less uniformly spread over all the machines in the system, depending on the load, of course. In this respect, Amoeba is highly location transparent.

In other words, all resources belong to the system as a whole and are managed by it. They are not dedicated to specific users, except for short periods of time to run individual processes. This model attempts to provide the transparency that is the holy grail of all distributed systems designers.

A simple example is amake, the Amoeba replacement for the UNIX make program. When the user types amake, all the necessary compilations happen, as expected, except that the system (and not the user) determines whether they happen sequentially or in parallel, and on which machine or machines this occurs. None of this is visible to the user.

A secondary goal of Amoeba is to provide a testbed for doing distributed and parallel programming. While some users just use Amoeba the same way they would use any other timesharing system most users are specifically interested in experimenting with distributed and parallel algorithms, languages, tools, and applications. Amoeba supports these users by making the underlying parallelism available to people who want to take advantage of it. In practice, most of Amoeba's current user base consists of people who are specifically interested in distributed and parallel computing in its various forms. A language, Orca, has been specifically designed and implemented on Amoeba for this purpose. Orca and its applications are described in (Bal, 1991; Bal et al., 1990; and Tanenbaum et al., 1992). Amoeba itself, however, is written in C.

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