Книга: Distributed operating systems

1.5.3. Reliability

1.5.3. Reliability

One of the original goals of building distributed systems was to make them more reliable than single-processor systems. The idea is that if a machine goes down, some other machine takes over the job. In other words, theoretically the overall system reliability could be the Boolean OR of the component reliabilities. For example, with four file servers, each with a 0.95 chance of being up at any instant, the probability of all four being down simultaneously is 0.054 = 0.000006, so the probability of at least one being available is 0.999994, far better than that of any individual server.

That is the theory. The practice is that to function at all, current distributed systems count on a number of specific servers being up. As a result, some of them have an availability more closely related to the Boolean and of the components than to the Boolean OR. In a widely-quoted remark, Leslie Lamport once defined a distributed system as "one on which I cannot get any work done because some machine I have never heard of has crashed." While this remark was (presumably) made somewhat tongue-in-cheek, there is clearly room for improvement here.

It is important to distinguish various aspects of reliability. Availability, as we have just seen, refers to the fraction of time that the system is usable. Lamport's system apparently did not score well in that regard. Availability can be enhanced by a design that does not require the simultaneous functioning of a substantial number of critical components. Another tool for improving availability is redundancy: key pieces of hardware and software should be replicated, so that if one of them fails the others will be able to take up the slack.

A highly reliable system must be highly available, but that is not enough. Data entrusted to the system must not be lost or garbled in any way, and if files are stored redundantly on multiple servers, all the copies must be kept consistent. In general, the more copies that are kept, the better the availability, but the greater the chance that they will be inconsistent, especially if updates are frequent. The designers of all distributed systems must keep this dilemma in mind all the time.

Another aspect of overall reliability is security. Files and other resources must be protected from unauthorized usage. Although the same issue occurs in single-processor systems, in distributed systems it is more severe. In a single-processor system, the user logs in and is authenticated. From then on, the system knows who the user is and can check whether each attempted access is legal. In a distributed system, when a message comes in to a server asking for something, the server has no simple way of determining who it is from. No name or identification field in the message can be trusted, since the sender may be lying. At the very least, considerable care is required here.

Still another issue relating to reliability is fault tolerance. Suppose that a server crashes and then quickly reboots. what happens? Does the server crash bring users down with it? If the server has tables containing important information about ongoing activities, recovery will be difficult at best.

In general, distributed systems can be designed to mask failures, that is, to hide them from the users. If a file service or other service is actually constructed from a group of closely cooperating servers, it should be possible to construct it in such a way that users do not notice the loss of one or two servers, other than some performance degradation. Of course, the trick is to arrange this cooperation so that it does not add substantial overhead to the system in the normal case, when everything is functioning correctly.

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