Книга: Distributed operating systems

5.3.3. Wide Area Networking

5.3.3. Wide Area Networking

Most current work on distributed systems focuses on LAN-based systems. In the future, many LAN-based distributed systems will be interconnected to form transparent distributed systems covering countries and continents. As an example, the French PTT is currently putting a small computer in every apartment and house in France. Although the initial goal is to eliminate the need for information operators and telephone books, at some point in time someone is going to ask if it is possible to connect 10 million or more computers spread over all of France into a single transparent system, for applications as yet undreamed of. What kind of file system would be needed to serve all of France? All of Europe? The entire world? At present, no one knows.

Although the French machines are all identical, in most wide-area networks, a large variety of equipment is encountered. This diversity is inevitable when multiple buyers with different-sized budgets and goals are involved, and the purchasing is spread over many years in an era of rapid technological change. Thus a wide-area distributed system must of necessity deal with heterogeneity. This raises issues such as how should you store a character file if not everyone uses ASCII, or what format one should use for files containing floating-point numbers if multiple representations are in use.

Also important is the expected change in applications. Most experimental distributed systems being built at universities focus on programming in a UNIX-like environment as the canonical application, because that is what the researchers themselves do all day (at least when they are not in committee meetings or writing grant proposals). Initial data suggest that not all 50 million French citizens are going to list C programming as their primary activity. As distributed systems become more widespread, we are likely to see a shift to electronic mail, electronic banking, accessing data bases, and recreational activities, which will change file usage, access patterns, and a great deal more in ways we as yet do not know.

An inherent problem with massive distributed systems is that the network bandwidth is extremely low. If the telephone line is the main connection, getting more than 64 Kbps out of it seems unlikely. Bringing fiber optics into everyone's house will take decades and cost billions. On the other hand, vast amounts of data can be stored cheaply on compact disks and videotapes. Instead of logging into the telephone company's computer to look up a telephone number, it may be cheaper for them to send everyone a disk or tape containing the entire data base. We may have to develop file systems in which a distinction is made between static, read-only information (e.g., the phone book), and dynamic information (e.g., electronic mail). This distinction may have to become the basis of the entire file system.

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