Two mark questions in Distributed Database







Two mark questions with answers in Distributed database / Important two mark questions and answers in distributed database


Two mark questions (To know more, please click the links)

 

  • List the fragmentation types.
Vertical fragmentation
Hybrid fragmentation
  • Give the different strategies used for placement of data in distributed database.
What are the advantages of allocation of fragments?
Locality of reference – data are close where they are required
Improved reliability and availability – data are highly available
Balanced storage and cost – simple hard disks are fine. Cost effective.
Minimal communication cost - Communication cost is less
Completeness
Reconstruction
Dis-jointness
Loss of message
Communication link failure
Site failure.
  • Who is transaction coordinator?
In distributed database, the site in which a transaction is initiated is completely responsible for finishing the transaction. This is done by the Transaction Manager of that particular site. That site’s Transaction Manager is called as transaction coordinator.
  • Define replication
Creating a copy of existing data or table is called replication. It increases availability at the cost of redundancy.
  • List down some advantages of DDBMSs.
Reflects organizational structure
Improved ability to share and local autonomy
Improved availability
Improved reliability
Improved performance
Economy in deploying software and hardware resources
Modular growth of the system
Integration
  • Name some disadvantages of fragmentation.
Performance degradation in case of larger applications which handles required data from several sites.
Integrity control may be more difficult as the data are stored at multiple sites.

Primary Horizontal fragmentation is a fragmentation technique which fragments (partitions) a single relation (table) using a set of min-term fragments.
  • List down the steps involved in Primary horizontal fragmentation.
Identify set of simple predicates.
Construct set of min-term predicates using simple predicates.
Eliminate redundant or unnecessary min-term predicates.
Use all valid min-term predicates to create fragments.
It is a horizontal fragmentation process where a table is fragmented based on the fragments that are created by horizontal fragmentation process of another table (parent table – think about primary key foreign key relationship).
  • Define fragmentation transparency.
The end users of a distributed database system need not know about how the database is fragmented. Hiding the information about how a table is fragmented from the end user is called fragmentation transparency.
  • Who is the transaction coordinator in distributed transactions?
The module of a distributed site which coordinates the transactions that are initiated at that site is called transaction coordinator. Let us assume that you have configured a distributed database system where we have sites A, B, C, and D containing different or same data. Any transaction requested at one of the sites say A, and in need of data available at sites B, and C, the transaction coordinator of site A is responsible to process the requested data at other sites.
  • What are the roles of transaction manager?
Maintaining a log for recovery purposes
Coordinating the concurrent transactions that are executed at that site.
  • List down the roles of transaction coordinator.
Starting the execution of a transaction initiated at that site.
Breaking the transaction into a set of sub-transactions if needed.
Distributing the sub-transactions to different sites.
Coordinating the completion of the transaction.
  • What are the messages used by 2 Phase Commit protocol?
<prepare T>
<no T>
<ready T>
<abort T>
<commit T>
  • What are the two phases of 2PC protocol?
Phase 1: Obtaining the decision – whether to commit or abort a transaction T.
Phase 2: Recording the decision – implement the decision taken in phase 1.
  • What is the major disadvantage of 2PC protocol?
Blocking problem – In case of coordinator failure, 2PC uses various ways to complete the transaction (to abort or commit). It can be done using the messages found in the log files of participating sites. If all the participating sites have <ready T> message and no other control messages (such as <abort T> or <commit T>) then all the active sites which are participating in that transaction have to wait for their transaction coordinator to recover. This is called blocking problem.







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