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dbms transaction processing concepts

TRANSACTION PROCESSING CONCEPTS

Transaction Concept

Ø  A transaction is a unit of program execution that accesses and  possibly updates one or more data items in  the database.

 

Ø  A group of tasks where task is a minimum processing unit which cannot be divided further.

 

Ø  Two main issues to deal with:

·         Failures of various kinds, such as hardware failures and system crashes

·         Concurrent execution of multiple transactions

 

ACID PROPERTIES:

To preserve integrity of data, the database system must ensure:

·         Atomicity.  Either all operations of the transaction are properly reflected in the database or none are.

 

·         Consistency.  Execution of a transaction in isolation preserves the consistency of the database.

 

·         Isolation.  Although multiple transactions may execute concurrently, each transaction must be unaware of other concurrently executing transactions.  Intermediate transaction results must be hidden from other concurrently executed transactions. 

 

That is, for every pair of transactions Ti and Tj, it appears to Ti that either Tj, finished execution before Ti started, or Tj started execution after Ti finished.

 

·         Durability.  After a transaction completes successfully, the changes it has made to the database persist, even if there are system failures.

 

Example of Fund Transfer :

Ø  Transaction to transfer $50 from account A to account B:

1.         read(A)

2.         A := A – 50

3.         write(A)

4.         read(B)

5.         B := B + 50

6.         write(B)

 

Ø  Consistency requirement – the sum of A and B is unchanged by the execution of the transaction.

 

Ø  Atomicity requirement — if the transaction fails after step 3 and before step 6, the system should ensure that its updates are not reflected in the database, else an inconsistency will result.

 

Ø  Durability requirement — once the user has been notified that the transaction has completed (i.e., the transfer of the $50 has taken place), the updates to the database by the transaction must persist despite failures.

 

Ø  Isolation requirement — if between steps 3 and 6, another transaction is allowed to access the partially updated database, it will see an inconsistent database (the sum A + B will be less than it should be).
Can be ensured trivially by running transactions serially, that is one after the other. 

 

TRANSACTION STATES :

Ø  Active, the initial state; the transaction stays in this state while it is executing

 

Ø  Partially committed, after the final statement has been executed.

 

Ø  Failed, after the discovery that normal execution can no longer proceed.

 

Ø  Aborted, after the transaction has been rolled back and the database restored to its state prior to the start of the transaction.  Two options after it has been aborted:

·         restart the transaction – only if no internal logical error

·         kill the transaction

 

Ø  Committed, after successful completion.

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