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COMPUTER SOFTWARE APPLICATION - CITS




           5  Failed:  The  failed  state  is  different  from  the  aborted  state. A  transaction  enters  the  failed  state  when  it
              encounters a critical error that prevents it from continuing. In this case, the DBMS automatically rolls back the
              transaction to maintain data integrity and consistency.

           6  Terminated (or Completed):  After a transaction has reached a terminal state (committed, aborted, or failed),
              it is considered terminated. The transaction can no longer be modified or interact with the database. It may still
              be examined for auditing or debugging purposes.

           7  Pending: In some systems, a transaction may enter a pending state temporarily. This typically occurs when
              a transaction is waiting for a resource or a lock that is held by another transaction. While in the pending state,
              the transaction is not actively executing but is waiting for its turn to proceed.
              A transaction is a single logical unit of work that accesses and possibly modifies the contents of a database.
              Transactions access data using read and write operations.

              In order to maintain consistency in a database, before and after the transaction, certain properties are followed.
              These are called ACID properties.








































           Atomicity
           By this, we mean that either the entire transaction takes place at once or doesn’t happen at all. There is no
           midway i.e. transactions do not occur partially. Each transaction is considered as one unit and either runs to
           completion or is not executed at all. It involves the following two operations.
           -  Abort: If a transaction aborts, changes made to the database are not visible.
           -  Commit: If a transaction commits, changes made are visible.
           Atomicity is also known as the ‘All or nothing rule’.

           Consider the following transaction T consisting of T1 and T2: Transfer of 100 from account X to account Y.
           If the transaction fails after completion of T1 but before completion of T2. (say, after write(X) but before write(Y)),
           then the amount has been deducted from X but not added to Y. This results in an inconsistent database state.
           Therefore, the transaction must be executed in its entirety in order to ensure the correctness of the database
           state.




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                              CITS : IT&ITES - Computer software application - Lesson 18 - 36
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