Basic of Data Transaction

Basic of Data Transaction

1. Definition

   • A unit of work(collection of database operations) that should be processed reliably
     • Supports daily operations of an organization
     • DBMSs provide recovery and concurrency control service to process transactions efficiently and reliably so that there is no loss of data due to multiple users and system failures

2. Propertise of transaction

   • Atomic(A)
     • either all the work in the transaction is completed or nothing is done
   • Consistent(C)
     • database must be consistent before and after a transaction
   • Isolated(I)
     • no unwanted interference from other users.A transaction should never overwrite changes made by another transaction
   • Durable(D)
     • database changes are permanent after the transaction completes

3. Transaction SQL statements

   • COMMIT
     • A COMMIT statement is reached,in which case all changes are permanently recorded within the database.The COMMIT statement statement automatically ends the SQL transaction
   • ROLLBACK
     • A ROLLBACK statement is reached in which case all changes are aborted and the database is rolled back to its previous consistent state.

4. Transaction Processing Services

   • Concurrency Transparency
     • user perceive the database as a single-user system even though there may be many simultaneous users
   • Recovery Transparency
     • DBMS automatically restore a database to a consistent state after a failure