Distributed Databases Questions and Answers

Answer: C

Correct answer: Each site must have the same storage capacity.

A full copy of the database at every site means every site must be able to hold the whole thing, which can be a heavy requirement at a small location. The other three options are advantages of replication rather than disadvantages - less network traffic, resilience to the failure of one site, and local processing without network coordination.

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.

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.

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.

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.

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.

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.

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.

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.

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.