Transaction
Introduction
Transaction is the action or series of actions which is carried out by a single user or an application program (Newcomer, 2009).This results to reading or updating the content of a database. An example of a transaction is :
Begin T
Read (x)
X=x+30
Write(x)
End T
Where T is the transaction
Transaction processing on the other hand is the means of facilitating a series of actions which consist of reading and writing of the database. A system or software that is developed to ensure that transactions being carried out are successful is called transaction processing system. A transaction processing system facilitates various techniques to ensure that there is no conflict and concurrency is enhanced .Since a transaction aims at transforming a database from one state to another, at any one a transaction can either be active, committed or aborted. An active transaction means that the transaction is already in progress. A committed transaction is the one that has already executed successfully while an aborted transaction is the one that its execution is not successful. A committed transaction makes the database reach a new consistency stage. If on the other hand the transaction was not success, in other words the transaction was aborted; the database must be restored to the previous state the database was before the transaction. This is called rolling back which means undoing the transaction. Apart from the obvious states of a transaction there exist partially committed transaction and also fail state .The former means that the final statement has already been executed but has violated an integrity constraint and therefore has to be aborted, while the latter means that an active transaction has been terminated. A deadlock occurs when two or more processes are waiting for a resource which is being held by another process (Emmerich, 2000).
Discussion
Solution to question 1
|
Transaction ID |
Affected table |
Modification |
|
1 |
ServiveRequest |
added |
|
2 |
Marina and marinaslip |
Changed(both) |
|
3 |
Owner and MarinaSlip |
Changed(both) |
|
4 |
MarinaSlip and Owner |
Changed(both) |
Solution to question 2
Transactions 1 and 3 would be used to restore the database because they had already committed and permanent changes had been made before the failure.
Transactions 2 and 4 would have to be re-entered by the user because changes had not been made to the database before the failure.
Solution question 3
Transactions 3 and 4 would cause deadlock because they are working with the same values. It is therefore important to make sure that they both start at different times. This means that these two transactions will never commit.
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