Книга: Distributed operating systems

2.6. SUMMARY

2.6. SUMMARY

The key difference between a centralized operating system and a distributed one is the importance of communication in the latter. Various approaches to communication in distributed systems have been proposed and implemented. For relatively slow, wide-area distributed systems, connection-oriented layered protocols such as OSI and TCP/IP are sometimes used because the main problem to be overcome is how to transport the bits reliably over poor physical lines.

For LAN-based distributed systems, layered protocols are rarely used. Instead, a much simpler model is usually adopted, in which the client sends a message to the server and the server sends back a reply to the client. By eliminating most of the layers, much higher performance can be achieved. Many of the design issues in these message-passing systems concern the communication primitives: blocking versus nonblocking, buffered versus unbuffered, reliable versus unreliable, and so on.

The problem with the basic client-server model is that conceptually interprocess communication is handled as I/O. To present a better abstraction, remote procedure call is widely used. With RPC, a client running on one machine calls a procedure running on another machine. The runtime system, embodied in stub procedures, handles collecting parameters, building messages, and the interface with the kernel to actually move the bits.

Although RPC is a step forward above raw message passing, it has its own problems. The correct server has to be located. Pointers and complex data structures are hard to pass. Global variables are difficult to use. The exact semantics of RPC are tricky because clients and servers can fail independently of one another. Finally, implementing RPC efficiently is not straightforward and requires careful thought.

RPC is limited to those situations where a single client wants to talk to a single server. When a collection of processes, for example, replicated file servers, need to communicate with each other as a group, something else is needed. Systems such as ISIS provide a new abstraction for this purpose: group communication. ISIS offers a variety of primitives, the most important of which is CBCAST. CBCAST offers weakened communication semantics based on causality and implemented by including sequence number vectors in each message to allow the receiver to see whether the message should be delivered immediately or delayed until some prior messages have arrived.

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