Distributed Databases Questions and Answers

Q: Data replication is favored where most process requests are read-only and where the data are relatively static.

Correct answer: TrueReplication’s cost is keeping the copies in step, and that cost is proportional to how often the data changes. Where the data is largely static and mostly read, the copies rarely need refreshing and every read is answered locally - which is the case replication is designed for.

Q: The cost to perform a snapshot refresh may depend on whether the snapshot is simple or complex.

Correct answer: TrueA simple snapshot draws on a single table and can often be refreshed by sending only the rows that have changed. A complex snapshot involves joins or aggregation, so determining what has changed may require recomputing it, and a full refresh is frequently cheaper than working out the difference.

Q: Asynchronous technology can result in unsatisfactorily slow response time because the distributed DBMS is spending considerable time checking that an update is accurately and completely propagated across the network.

Correct answer: FalseIt is synchronous technology that spends the time, because the transaction cannot commit until every copy has been updated and confirmed. Asynchronous technology returns as soon as the local update is done and propagates afterwards, which is why it is faster and why it tolerates a site being unavailable.

Q: With failure transparency, all of the actions of a transaction are committed or none of them are committed.

Correct answer: TrueFailure transparency means a transaction remains atomic across the whole distributed system: either every site commits its part or none does. Achieving it requires a commit protocol such as two-phase commit, in which the sites first agree they can commit and only then do so.

Q: The problem of concurrency control is more complex in a distributed database.

Correct answer: TrueA centralised system controls concurrency with one lock manager holding the whole picture. Distributing it means locks held at several sites, messages that may be delayed or lost, and deadlocks whose cycle spans sites and which no single site can see. This is why timestamping, which avoids locks altogether, is attractive in a distributed setting.

Q: An increasingly popular option for data distribution as well as for fault tolerance of any database is to store a separate copy of the database at each of two or more sites.

Correct answer: TrueA full copy at each of two or more sites serves both purposes at once. It distributes the data so that each site can read locally, and it provides fault tolerance, since the failure of one site leaves a complete database elsewhere. Falling storage costs are what have made the approach increasingly attractive.

Q: The purpose of timestamping is to avoid the use of locks.

Correct answer: TrueTimestamping assigns each transaction an order in advance and resolves conflicts by that order, so no transaction has to wait for a lock. This removes deadlock entirely, which is a substantial gain in a distributed system where detecting a deadlock spanning several sites is difficult. The cost is that a transaction found to be out of order must be restarted.

Q: Databases that are stored on computers at multiple locations and are not interconnected by a network are known as distributed databases.

Correct answer: FalseA network connection is part of the definition. Databases on computers at several locations with no network between them are a decentralised system: nothing can be queried across them and there is no single logical database. Distribution requires the sites to be interconnected.

21.

A semijoin is which of the following?

A.Only the joining attributes are sent from one site to another and then all of the rows are returned.
B.All of the attributes are sent from one site to another and then only the required rows are returned.
C.Only the joining attributes are sent from one site to another and then only the required rows are returned.
D.All of the attributes are sent from one site to another and then only the required rows are returned.

Answer: C

Correct answer: Only the joining attributes are sent from one site to another and then only the required rows are returned.

The semijoin sends only the joining column values across the network, the remote site matches them against its own rows, and only the rows that actually participate come back. Both halves of the saving matter: fewer columns go out and fewer rows come back, which is why the technique is used where network cost dominates.

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22.

Data replication is favored where most process requests are read-only and where the data are relatively static.

A.True
B.False

Answer: A

Correct answer: True

Replication’s cost is keeping the copies in step, and that cost is proportional to how often the data changes. Where the data is largely static and mostly read, the copies rarely need refreshing and every read is answered locally - which is the case replication is designed for.

Discuss in Forum

23.

The cost to perform a snapshot refresh may depend on whether the snapshot is simple or complex.

A.True
B.False

Answer: A

Correct answer: True

A simple snapshot draws on a single table and can often be refreshed by sending only the rows that have changed. A complex snapshot involves joins or aggregation, so determining what has changed may require recomputing it, and a full refresh is frequently cheaper than working out the difference.

Discuss in Forum

24.

Asynchronous technology can result in unsatisfactorily slow response time because the distributed DBMS is spending considerable time checking that an update is accurately and completely propagated across the network.

A.True
B.False

Answer: B

Correct answer: False

It is synchronous technology that spends the time, because the transaction cannot commit until every copy has been updated and confirmed. Asynchronous technology returns as soon as the local update is done and propagates afterwards, which is why it is faster and why it tolerates a site being unavailable.

Discuss in Forum

25.

Which of the following is true concerning a global transaction?

A.The required data are at one local site and the distributed DBMS routes requests as necessary.
B.The required data are located in at least one nonlocal site and the distributed DBMS routes requests as necessary.
C.The required data are at one local site and the distributed DBMS passes the request to only the local DBMS.
D.The required data are located in at least one nonlocal site and the distributed DBMS passes the request to only the local DBMS.

Answer: B

Correct answer: The required data are located in at least one nonlocal site and the distributed DBMS routes requests as necessary.

A transaction is global when it needs data from beyond its own site, and the distributed DBMS then decomposes it and routes the pieces to wherever the data lives. A local transaction is satisfied entirely at one site and needs no such routing - the distinction that decides how much coordination the transaction costs.

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26.

With failure transparency, all of the actions of a transaction are committed or none of them are committed.

A.True
B.False

Answer: A

Correct answer: True

Failure transparency means a transaction remains atomic across the whole distributed system: either every site commits its part or none does. Achieving it requires a commit protocol such as two-phase commit, in which the sites first agree they can commit and only then do so.

Discuss in Forum

27.

The problem of concurrency control is more complex in a distributed database.

A.True
B.False

Answer: A

Correct answer: True

A centralised system controls concurrency with one lock manager holding the whole picture. Distributing it means locks held at several sites, messages that may be delayed or lost, and deadlocks whose cycle spans sites and which no single site can see. This is why timestamping, which avoids locks altogether, is attractive in a distributed setting.

Discuss in Forum

28.

An increasingly popular option for data distribution as well as for fault tolerance of any database is to store a separate copy of the database at each of two or more sites.

A.True
B.False

Answer: A

Correct answer: True

A full copy at each of two or more sites serves both purposes at once. It distributes the data so that each site can read locally, and it provides fault tolerance, since the failure of one site leaves a complete database elsewhere. Falling storage costs are what have made the approach increasingly attractive.

Discuss in Forum

29.

The purpose of timestamping is to avoid the use of locks.

A.True
B.False

Answer: A

Correct answer: True

Timestamping assigns each transaction an order in advance and resolves conflicts by that order, so no transaction has to wait for a lock. This removes deadlock entirely, which is a substantial gain in a distributed system where detecting a deadlock spanning several sites is difficult. The cost is that a transaction found to be out of order must be restarted.

Discuss in Forum

30.

Databases that are stored on computers at multiple locations and are not interconnected by a network are known as distributed databases.

A.True
B.False

Answer: B

Correct answer: False

A network connection is part of the definition. Databases on computers at several locations with no network between them are a decentralised system: nothing can be queried across them and there is no single logical database. Distribution requires the sites to be interconnected.

Discuss in Forum

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