Sunday, May 3, 2020

Distributed Database Question Bank with Answers 11

Distributed Database Question Bank with Answers

1. What is a transaction? What about a distributed transaction?
A transaction is a logical unit (which must obey the ACID properties) which consists of read/write operations on the database. A distributed transaction is a transaction that operates at two or more different geographical sites (servers).

2. What is the use of location transparency?
Location transparency means that the user and/or the application need not know at what site each relation is stored at. The user can behave as if the site were stored at his/her site. This not only simplifies application programming, but as new ways of using DB come up, data movement is made easier.

3. Why can fragmenting a relation be often more useful than duplicating (replicating) the whole relation?

Fragmenting a relation is often more useful than replicating the whole relation because the queries related to the fragmented relation can be efficiently processed in parallel (assuming several processors). Fragmentation can also reduce the traffic load over the network and help to make many operations local.
Replicating a relation involves additional overhead in case of all data manipulation.

4. A global lock table stored at the Server contains what information?
The global lock table stores at the Server the locks for each client as the tuple (c, x, m, d) where x is the name of the lock, m is the lock-type (e.g write or read lock), d its duration such as commit-duration, short-duration(=kestoaika) and c is an identifier for the client.

5. What are the two main tasks of buffer coherency control in shared-disk system?
i) It must detect when a page on disk becomes invalid (i.e. is no longer clean as a result of being updated in the buffer of one node)
ii) It is responsible for providing the node with the most-up-to-date version of the page, meaning when a node needs a new version of a given page, the buffer coherency control must know from where to get it. The owner of the page is the only node authorized to write a dirty page back to the disk and if requested, must provide other nodes with the up-to-date version of that page.


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