PUTTASWAMY B S

  Transaction States and Additional Operations:                                                  A transaction is an atomic unit (or logical unit) of work that should either be completed in its entirety or not done at all. For recovery purposes, the system needs to keep track of when each transaction starts, terminates, and commits or aborts. Therefore, the recovery manager of the DBMS needs to keep track of the following operations: BEGIN_TRANSACTION : This marks the beginning of transaction execution. READ or WRITE : These specify read or write operations on the database items that are executed as part of a transaction. END_TRANSACTION : This specifies that READ and WRITE transaction operations have ended and marks the end of transact...

  Why Recovery Is Needed:                                                     Whenever a transaction is submitted to a DBMS for execution, the system is responsible for making sure that either all the operations in the transaction are completed successfully and their effect is recorded permanently in the database, or that the transaction does not have any effect on the database or any other transactions. In the first case, the transaction is said to be committed , whereas in the second case, the transaction is aborted . The DBMS must not permit some operations of a transaction T to be applied to the database while other operations of T are not, because the whole transaction is a logical unit of database processing. If a transac...

  Why Concurrency Control Is Needed:        Concurrency control  is the process of managing simultaneous execution of transactions (managing simultaneous operations on the database) such as queries, updates, inserts, deletes and so on in a multiprocessing database system without having them interfere with one another. [• Prevents interference when two or more users are accessing database simultaneously and at least one is updating data. • Although two transactions may be correct in themselves, interleaving of operations may produce an incorrect result.] When multiple transactions execute concurrently in an uncontrolled or unrestricted manner, then it might lead to several problems. Such problems are called as  concurrency problems . For example: In airline reservations database in which a record is stored for each airline flight. Each record includes the number of reserved seats on that flight as a named (uniquely identifiable) dat...

  UNIT - V Introduction to Transaction Processing Concepts and Theory Topics Covered: Ø   Introduction Ø   Introduction to Transaction processing     Single user versus Multiuser systems     Transactions, Database Items, Read and Write Operations and DBMS Buffers.     Why concurrency control is needed     Why Recovery is needed Ø   Transactions and System concepts     Transaction states and Additional operations     System log     Ø   Desirable properties of transactions Ø      Characterizing Schedules based on Serializability. Introduction:                      One of the main advantages of storing data in a database is to allow sharing of it among multiple users. Several users access the database or perform operations at the same time. What if a user tries to access a d...