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Database Access Layer Module &

Database Transaction

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Data Access Layer

• A data access layer follows the idea of "separation of concerns“.

• DAL consist of the code used to pull data from a db.

• Its isolated from business logic layer and presentation layer.

mamanages database

Why???? DAL

• DAL allow us to change more easily the backend physical data storage technology without having large impact on business logic layer.

• Data access layers allow us to access data the same way for different application in our program.

DB Transaction

• A transaction is a discrete unit of work that must be completely processed or not processed at all.

Transaction states• Active, the initial state; the transaction stays

in this state until while it is still executing.

• A transition is terminated only if it has either been committed or aborted.

Transaction States

• Partially committed, After the final statement has been executed

• At this point failure is still possible since changes may have been only done in main memory, a hardware failure could still occur.

Transaction States

• Committed- after successful completion.• Once committed, the transaction can no

longer be undone by aborting it. • Its effect could be undone only by a

compensating transaction.

Transaction States

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

• Once a transaction can not be completed, any changes that it made must be undone rolling it back.

Transaction States

• Aborted, after the transaction has been rolled back the database has been restored to its state prior to the start of the transaction.

• The DBMS could either kill the transaction or restart the transaction.

Create procedure DeleteDepartment ( @DepartmentID int ) AS BEGIN TRANSACTION DELETE FROM Employees WHERE DepartmentID = @DepartmentIDIF @@ERROR <> 0 BEGIN ROLLBACK RAISERROR ('Error in deleting employees in DeleteDepartment.', 16, 1) RETURN END DELETE FROM Departments WHERE DepartmentID = @DepartmentID IF @@ERROR <> 0 BEGIN ROLLBACK RAISERROR ('Error in deleting department in DeleteDepartment.', 16, 1) RETURN END COMMIT

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