Книга: Distributed operating systems

10.4. TIME SERVICE

Time is an important concept in most distributed systems. To see why, consider a research program in radio astronomy. A number of radio telescopes spread all over the world observe the same celestial radio source simultaneously, accurately recording the data and the observation time. The data are sent over a network to a central computer for processing. For some kinds of experiments (e.g., long-baseline interferometry), it is essential for the analysis that the various data streams be synchronized exactly. Thus the experiment succeeds or fails with the ability to synchronize remote clocks accurately.

As another example, in a computerized stock trading system, it might matter who bid first on a block of stock offered for sale. If the bidders were located in New York, San Francisco, London, and Tokyo, timestamping the bids might be one way to achieve fairness. However, if the clocks at all these locations were not properly synchronized, the whole scheme would collapse and result in numerous lawsuits featuring expert witnesses trying to explain the concept of the speed of light to bewildered juries.

To try to prevent problems like these, DCE has a service called DTS (Distributed Time Service). The goal of DTS is to keep clocks on separate machines synchronized. Getting them synchronized once is not enough, because the crystals in different clocks tick at a slightly different rates, so the clocks gradually drift apart. For example, a clock might be rated to have a relative error of one part in a million. This means that even if it is set perfectly, after 1 hour, the clock could be off by as much as 3.6 msec either way. After 1 day it could be off by as much as 86 msec either way. After a month, two clocks that were synchronized precisely might now differ by 5 seconds.

DTS manages the clocks in DCE. It consists of time servers that keep asking each other: "What time is it?" as well as other components. If DTS knows the maximum drift rate of each clock (which it does because it measures the drift rate), it can run the time calculations often enough to achieve the desired synchronization. For example, with clocks that are accurate to one part in a million, if no clock is to ever to be off by more than 10 msec, resynchronization must take place at least every three hours.

Actually, DTS must deal with two separate issues:

1. Keeping the clocks mutually consistent.

2. Keeping the clocks in touch with reality.

The first point has to do with making sure that all clocks return the same value when queried about the time (corrected for time zones, of course). The second has to do with ensuring that even if all the clocks return the same time, this time agrees with clocks in the real world. Having all the clocks agree that it is 12:04:00.000 is little consolation if it is actually 12:05:30.000. How DTS achieves these goals will be described below, but first we will explain what the DTS time model is and how programs interface to it.

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