1.5.1.Transparency

Probably the single most important issue is how to achieve the single-system image. In other words, how do the system designers fool everyone into thinking that the collection of machines is simply an old-fashioned timesharing system? A system that realizes this goal is often said to be transparent.

Transparency can be achieved at two different levels. Easiest to do is to hide the distribution from the users. For example, when a UNIX user types make to recompile a large number of files in a directory, he need not be told that all the compilations are proceeding in parallel on different machines and are using a variety of file servers to do it. To him, the only thing that is unusual is that the performance of the system is halfway decent for a change. In terms of commands issued from the terminal and results displayed on the terminal, the distributed system can be made to look just like a single-processor system.

At a lower level, it is also possible, but harder, to make the system look transparent to programs. In other words, the system call interface can be designed so that the existence of multiple processors is not visible. Pulling the wool over the programmer's eyes is harder than pulling the wool over the terminal user's eyes, however.

What does transparency really mean? It is one of those slippery concepts that sounds reasonable but is more subtle than it at first appears. As an example, imagine a distributed system consisting of workstations each running some standard operating system. Normally, system services (e.g., reading files) are obtained by issuing a system call that traps to the kernel. In such a system, remote files should be accessed the same way. A system in which remote files are accessed by explicitly setting up a network connection to a remote server and then sending messages to it is not transparent because remote services are then being accessed differently than local ones. The programmer can tell that multiple machines are involved, and this is not allowed.

The concept of transparency can be applied to several aspects of a distributed system, as shown in Fig. 1-13. Location transparency refers to the fact that in a true distributed system, users cannot tell where hardware and software resources such as CPUs, printers, files, and data bases are located. The name of the resource must not secretly encode the location of the resource, so names like machine1:prog.c or /machine1/prog.c are not acceptable.

Fig. 1-13. Different kinds of transparency in a distributed system.

Migration transparency means that resources must be free to move from one location to another without having their names change. In the example of Fig. 1-10 we saw how server directories could be mounted in arbitrary places in the clients' directory hierarchy. Since a path like /work/news does not reveal the location of the server, it is location transparent. However, now suppose that the folks running the servers decide that reading network news really falls in the category "games" rather than in the category "work." Accordingly, they move news from server 2 to server 1. The next time client 1 boots and mounts the servers in his customary way, he will notice that /work/news no longer exists. Instead, there is a new entry, /games/news. Thus the mere fact that a file or directory has migrated from one server to another has forced it to acquire a new name because the system of remote mounts is not migration transparent.

If a distributed system has replication transparency, the operating system is free to make additional copies of files and other resources on its own without the users noticing. Clearly, in the previous example, automatic replication is impossible because the names and locations are so closely tied together. To see how replication transparency might be achievable, consider a collection of n servers logically connected to form a ring. Each server maintains the entire directory tree structure but holds only a subset of the files themselves. To read a file, a client sends a message containing the full path name to any of the servers. That server checks to see if it has the file. If so, it returns the data requested. If not, it forwards the request to the next server in the ring, which then repeats the algorithm. In this system, the servers can decide by themselves to replicate any file on any or all servers, without the users having to know about it. Such a scheme is replication transparent because it allows the system to make copies of heavily used files without the users even being aware that this is happening.

Distributed systems usually have multiple, independent users. What should the system do when two or more users try to access the same resource at the same time? For example, what happens if two users try to update the same file at the same time? If the system is concurrency transparent, the users will not notice the existence of other users. One mechanism for achieving this form of transparency would be for the system to lock a resource automatically once someone had started to use it, unlocking it only when the access was finished. In this manner, all resources would only be accessed sequentially, never concurrently.

Finally, we come to the hardest one, parallelism transparency. In principle, a distributed system is supposed to appear to the users as a traditional, uniprocessor timesharing system. What happens if a programmer knows that his distributed system has 1000 CPUs and he wants to use a substantial fraction of them for a chess program that evaluates boards in parallel? The theoretical answer is that together the compiler, runtime system, and operating system should be able to figure out how to take advantage of this potential parallelism without the programmer even knowing it. Unfortunately, the current state-of-the-art is nowhere near allowing this to happen. Programmers who actually want to use multiple CPUs for a single problem will have to program this explicitly, at least for the foreseeable future. Parallelism transparency can be regarded as the holy grail for distributed systems designers. When that has been achieved, the work will have been completed, and it will be time to move on to new fields.

All this notwithstanding, there are times when users do not want complete transparency. For example, when a user asks to print a document, he often prefers to have the output appear on the local printer, not one 1000 km away, even if the distant printer is fast, inexpensive, can handle color and smell, and is currently idle.