Quiz questions and answers in distributed database MCQ 07

Distributed database quiz questions with answers, MCQ in distributed database, solved exam questions in distributed and parallel database, distributed query processing, data replication

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Multiple Choice Questions with Answers in Distributed Database

 

1. In distributed query processing, which of the following layer activities are performed by central control site?

a) Query decomposition

b) Data localization

c) Global optimization

d) Distributed execution

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Ans : (a), (b), and (c)

Answer: (a), (b), and (c) Query decomposition, data localization, global optimization, and distributed execution are the four layers of distributed query processing. The first three layer activities are performed at the central control site using the schema information stored in the global directory. The fourth layer performs distributed query execution by executing the plan and returns the answer to the query. This is performed at local sites.

 

2. Which of the following is NOT a step of query decomposition layer in distributed query processing?

a) Normalized query is analyzed semantically

b) Semantically correct query is simplified

c) Simplified calculus query is restructured as an algebraic query

d) The algebraic query is executed by the local sites

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Ans : (d)

Answer: (d) The algebraic query is executed by the local sites Query decomposition layer converts the calculus query into an algebraic query on global relations. This can be performed through four successive steps; The calculus query is rewritten in a normalized form Normalized query is analyzed semantically Semantically correct query is simplified Simplified calculus query is restructured as an algebraic query

 

3. Which of the following is/are NOT locking protocols used in distributed DBMS environment?

a) Centralized 2PL

b) Primary copy 2PL

c) Rigorous 2PL

d) Majority locking based 2PL

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Ans : (c)

Answer: (c) Rigorous 2PL Rigorous 2PL is a two phase locking protocol used in DBMS environment to ensure serializability. Centralized 2PL – a single site acts as the lock manager site and maintains all locking information. Primary copy 2PL – Unlike single site for all data items in centralized 2PL, primary copy 2PL chooses primary copy for each replicated data item. Each such primary copy is maintained in a site and that site acts as the lock manager for that particular data item alone. This way, lock managers are distributed here. Majority locking - When a transaction wishes to read or write a data item that is replicated at n sites, it must send a lock request to more than half of the n sites where the item is stored. The transaction cannot proceed until it obtains locks on a majority of the copies.

 

4. Let us assume that in 2PC protocol a transaction coordinator failed after a decision is taken (to abort/commit) and shared among the participating sites. What should the coordinator do during restart (recovery)?

a) Abort the transaction in any case

b) Commit the transaction in any case

c) Commit/abort only if received all acknowledgements from participating sites

d) Cannot be decided by the coordinator on what to be done

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Ans : (c)

Answer: (c) Commit/abort only if received all acknowledgements from participating sites During recovery, the transaction coordinator in 2PC can perform either commit or abort a transaction based on the decision taken and shared with the participating sites before failure. Commit or abort can be performed only if the coordinator received all the acknowledgements from the participating sites on the decision taken by it.

 

5. Which of the following is NOT an advantage of data replication in distributed database?

a) Fast access to shared data

b) High availability

c) Reduced network traffic

d) Easy updating of data items

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Ans : (d)

Answer: (d) Easy updating of data items Data replication is one of the strategies used in distributed database to distribute the database and its manipulation. As per this strategy, the data is replicated in several sites so as to improve the availability. As we have same data as several different locations, it is hard to update them. Also, during update those data items will not be available to other transactions in order to maintain consistency.

 

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