Main purpose of DDBMS:
DESCRIBE IN DETAIL WHAT IS THE MAIN PURPOSE OF DDBMS.
The main purpose of DDBMS software is to provide a method by which the distributed database appears to users as centralized database. This single centralized database appearance can be acquired by the means of several transparencies such as distributed transparencies, network transparencies and replication transparencies. The another main purpose of DDBMS Concurrency control. Concurrency control is the most important issue in the DDB (Distributed Database). In the multi user database management system, the method of synchronizing simultaneous accesses on the database is called concurrency control. Different techniques are using for obtain concurrency control such as two phase locking, wait-wound, wait-die, time out base scheme and optimistic concurrency control.
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Fundamental of transaction processing and concurrency control: (THE MAIN PURPOSE OF DDBMS)
Transaction: Transaction is a task or series of tasks performed on database. The main concern regarding the transaction is that database is consistent before and after the completion of transaction.
Properties of transactions:
Database reliability and consistency can be achieved by using ACID properties. In these properties A means Atomicity, C means Consistency, I means Isolation and D means Durability.
Atomicity: In the Atomicity transaction consider as unit of operation. This operation (Transaction) must be complete or must not take place.
Consistency is the correctness of transaction. If from one database to another database, transaction obeys all integrity constraints and does not violate integrity constraints then transaction is correct.
With respect to isolation when one transaction is changing or modifying the database, other transactions must not be able to modify or read the database.
Property related to database systems which assure that when it is committed then it stores data permanently, even the system crashes.
Several users in DDBMS are concurrently accessing the database. Concurrency control is the method of synchronizing concurrent access to the distributed database. The user feels that he is working on a single database.
Distributed transactions are those transactions which execute on multiple processes on different machines. These transactions execution require coordination between local resource manager and global transaction management system. Processing monitor and resource manager are two basic elements of distributed database. ACID properties are difficult to maintain in the distributed transactions because at any time any process can fail but these properties must also observed in the distributed transactions. We can preserve the ACID by using following two features in distributed transactions processing systems.
By recoverable processes log the activities and in the case of failure processes can be restored to their earlier states.
Commit protocol: In the commit protocol the several processes coordinate with each other for committing or aborting of transaction.
There are many business situations which encourage us for the use of distributed database.
Data communication costs: In the distributed database data is geographically dispersed on different sites so when application required that data, most of the data is available locally so the data communication cost reduce.
Database recovery: In the replication data is replicated on different sites so when data does not available from one site it also available on another sites and at the same time data can be restored at that site where data does not available.
Data Sharing: Many organizations located at different places and their units often need to communicate, share their data and resources. This demand can be fulfilled with the help of replication or common database.
Distribution and autonomy of business unit: Because many organizations located at different places and their divisions are geographically dispersed across the national boundaries and each division has power to build its own information. Due to this reason division want the control on local dat.
Performance Improvement: In the distributed database the data is stored on different sites and queries run in parallel. Due to this reason and using of several resources in parallel increase the performance of system.
Increase availability and reliability: In the centralized database system, database becomes unavailable when this system fails. On the other hand database also available in distributed database in case of any failure in its one component.
Faster response: In the distributed database data is located on many sites and data is available to the many users locally. In the case of centralized database there is rare possibility of data available locally for users. Due to the locally availability of data in distributed database increases the speed of query processing. For the faster response complex queries divided into sub queries and processed in parallel.
CONCLUSION: (THE MAIN PURPOSE OF DDBMS)
In this paper, we have compared some advantage of distributed database over centralized database. We also describe some algorithms of concurrency control in distributed database like 2PL, wound wait,
While concurrently accessing the database ACID properties are very important and it has to be maintained during accessing on the database.