Книга: Distributed operating systems

2.4. REMOTE PROCEDURE CALL

Although the client-server model provides a convenient way to structure a distributed operating system, it suffers from one incurable flaw: the basic paradigm around which all communication is built is input/output. The procedures send and receive are fundamentally engaged in doing I/O. Since I/O is not one of the key concepts of centralized systems, making it the basis for distributed computing has struck many workers in the field as a mistake. Their goal is to make distributed computing look like centralized computing. Building everything around I/O is not the way to do it.

This problem has long been known, but little was done about it until a paper by Birrell and Nelson (1984) introduced a completely different way of attacking the problem. Although the idea is refreshingly simple (once someone has thought of it), the implications are often subtle. In this section we will examine the concept, its implementation, its strengths, and its weaknesses.

In a nutshell, what Birrell and Nelson suggested was allowing programs to call procedures located on other machines. When a process on machine A calls a procedure on machine B, the calling process on A is suspended, and execution of the called procedure takes place on B. Information can be transported from the caller to the callee in the parameters and can come back in the procedure result. No message passing or I/O at all is visible to the programmer. This method is known as remote procedure call, or often just RPC.

While the basic idea sounds simple and elegant, subtle problems exist. To start with, because the calling and called procedures run on different machines, they execute in different address spaces, which causes complications. Parameters and results also have to be passed, which can be complicated, especially if the machines are not identical. Finally, both machines can crash, and each of the possible failures causes different problems. Still, most of these can be dealt with, and RPC is a widely-used technique that underlies many distributed operating systems.

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