# A typical hard disk can transfer data at the rate of

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A typical hard disk can transfer data at the rate of 1 to 5 megabytes per second. In contrast, the bandwidth of a broadcast channel is typically only a few kilobytes per second. Total latency is likely to be of the order of seconds to hundreds or even thousands of seconds, compared to a few milliseconds for a hard disk. 25.8 Consider a database of documents in which all documents are kept in a central database. Copies of some documents are kept on mobile

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Practice Exercises 5 computers. Suppose that mobile computer A updates a copy of docu- ment 1 while it is disconnected, and, at the same time, mobile computer B updates a copy of document 2 while it is disconnected. Show how the version-vector scheme can ensure proper updating of the central database and mobile computers when a mobile computer reconnects. Answer: Let C be the computer onto which the central database is loaded. Each mobile computer (host) i stores, with its copy of each document d , a version-vector – that is a set of version numbers V d , i , j , with one entry for each other host j that stores a copy of the document d , which it could possibly update. Host A updates document 1 while it is disconnected from C. Thus, according to the version vector scheme, the version number V 1 , A , A is incremented by one. Now, suppose host A re-connects to C. This pair exchanges version- vectors and finds that the version number V 1 , A , A is greater than V 1 , C , A by 1, (assuming that the copy of document 1 stored host A was updated most recently only by host A). Following the version-vector scheme, the version of document 1 at C is updated and the change is reflected by an increment in the version number V 1 , C , A . Note that these are the only changes made by either host. Similarly, when host B connects to host C, they exchange version- vectors, and host B finds that V 1 , B , A is one less than V 1 , C , A . Thus, the version number V 1 , B , A is incremented by one, and the copy of docu- ment 1 at host B is updated. Thus, we see that the version-vector scheme ensures proper updating of the central database for the case just considered. This argument can be very easily generalized for the case where multiple off-line updates are made to copies of document 1 at host A as well as host B and host C. The argument for off-line updates to document 2 is similar.
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