Distributed Databases Questions and Answers

Q: Synchronization for pull replication is less disruptive and occurs only when needed by each site, not when a central master site thinks it is best to update.

Correct answer: TrueWith pull replication each site requests a refresh when it wants one, so the update happens when the receiving site is ready rather than when a master decides. This spreads the load and avoids interrupting a site at an inconvenient moment, at the cost of the sites being less predictably in step.

Q: A distributed database has which of the following advantages over a centralized database?

Correct answer: Modular growthA distributed system can be extended by adding a site, without replacing or resizing a central machine, so capacity grows in steps matched to demand. The other three options are the recognised disadvantages: the software costs more, is more complex, and can respond more slowly where a query has to reach several sites.

Q: With asynchronous technology, if any copy of a data item is updated anywhere on the network, the same update is immediately applied to all other copies or it is aborted.

Correct answer: FalseImmediate application of every update to every copy, or abort, describes synchronous technology. Asynchronous replication is defined by the opposite: the update is applied locally and propagated later, so the copies are temporarily inconsistent. The statement has given the definition of one under the name of the other.

Q: A distributed unit of work allows various statements within a unit of work to refer to multiple remote DBMS locations.

Correct answer: TrueA distributed unit of work allows the statements within one transaction to address different remote sites, though each individual statement is still directed at a single site. Lifting that last restriction - letting one statement span several sites - is the next stage, the distributed request.

Q: Each site (or node) in a distributed system is subject to the same types of failure as in a centralized system.

Correct answer: TrueEvery site still has its own disks, processors, power and software, and can fail in all the ways a centralised system can. What distribution adds is failures of its own - a lost network link, a partitioned network, a site unreachable but still running - so the total number of ways to fail is greater, not smaller.

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.

11.

Synchronization for pull replication is less disruptive and occurs only when needed by each site, not when a central master site thinks it is best to update.

A.True
B.False

Answer: A

Correct answer: True

With pull replication each site requests a refresh when it wants one, so the update happens when the receiving site is ready rather than when a master decides. This spreads the load and avoids interrupting a site at an inconvenient moment, at the cost of the sites being less predictably in step.

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12.

A homogenous distributed database is which of the following?

A.The same DBMS is used at each location and data are not distributed across all nodes.
B.The same DBMS is used at each location and data are distributed across all nodes.
C.A different DBMS is used at each location and data are not distributed across all nodes.
D.A different DBMS is used at each location and data are distributed across all nodes.

Answer: B

Correct answer: The same DBMS is used at each location and data are distributed across all nodes.

Homogeneous means one product throughout, which removes the translation problems of a heterogeneous system: the same dialect, the same data types, the same transaction semantics. It is the easier case to build and administer, and it is realistic only where the organisation controls every site.

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13.

A distributed database has which of the following advantages over a centralized database?

A.Software cost
B.Software complexity
C.Slow Response
D.Modular growth

Answer: D

Correct answer: Modular growth

A distributed system can be extended by adding a site, without replacing or resizing a central machine, so capacity grows in steps matched to demand. The other three options are the recognised disadvantages: the software costs more, is more complex, and can respond more slowly where a query has to reach several sites.

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14.

With asynchronous technology, if any copy of a data item is updated anywhere on the network, the same update is immediately applied to all other copies or it is aborted.

A.True
B.False

Answer: B

Correct answer: False

Immediate application of every update to every copy, or abort, describes synchronous technology. Asynchronous replication is defined by the opposite: the update is applied locally and propagated later, so the copies are temporarily inconsistent. The statement has given the definition of one under the name of the other.

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15.

A distributed unit of work allows various statements within a unit of work to refer to multiple remote DBMS locations.

A.True
B.False

Answer: A

Correct answer: True

A distributed unit of work allows the statements within one transaction to address different remote sites, though each individual statement is still directed at a single site. Lifting that last restriction - letting one statement span several sites - is the next stage, the distributed request.

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16.

A distributed database is which of the following?

A.A single logical database that is spread to multiple locations and is interconnected by a network
B.A loose collection of file that is spread to multiple locations and is interconnected by a network
C.A single logical database that is limited to one location.
D.A loose collection of file that is limited to one location.

Answer: A

Correct answer: A single logical database that is spread to multiple locations and is interconnected by a network

Two conditions must both hold. It must be a single logical database, so users see one coherent whole rather than several separate ones; and the sites must be connected by a network, or nothing can be combined. Separate databases on separate machines are a decentralised system, not a distributed database.

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17.

Each site (or node) in a distributed system is subject to the same types of failure as in a centralized system.

A.True
B.False

Answer: A

Correct answer: True

Every site still has its own disks, processors, power and software, and can fail in all the ways a centralised system can. What distribution adds is failures of its own - a lost network link, a partitioned network, a site unreachable but still running - so the total number of ways to fail is greater, not smaller.

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18.

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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19.

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.

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20.

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

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