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ADO.NET interview questions: - Show difference between “Optimistic” and “Pessimistic” locking and how many ways are there to implement optimistic locking in ADO.NET?
In optimistic locking there is no locking actually. It only checks if the old values are changed, in case they are changed it means somebody has changed the data, so it raises exception.
In pessimistic locking you actually lock the record, depending on the type of lock no other process can make modifications to the record.
Following are the ways to implement optimistic locking using ADO.NET:-
- When we call “Update” method of Data Adapter it handles locking internally. If the Dataset values are not matching with current data in Database, it raises concurrency exception error. We can easily trap this error using Try. Catch block and raise appropriate error message to the user.
- Define a Date time stamp field in the table. When actually you are firing the UPDATE SQL statements, compare the current timestamp with one existing in the database. Below is a sample SQL which checks for timestamp before updating and any mismatch in timestamp it will not update the records. This I the best practice used by industries for locking.
Update table1 set field1=@test
where Last Timestamp=@Current Timestamp
- Check for original values stored in SQL SERVER and actual changed values. In stored procedure check before updating that the old data is same as the current Example in the below shown SQL before updating field1 we check that is the old field1 value same. If not then some one else has updated and necessary action has to be taken.
Update table1 set field1=@test
where field1 = @oldfield1value
Locking can be handled at ADO.NET side or at SQL SERVER side i.e. in stored procedures. For more details of how to implementing locking in SQL SERVER read “What are different locks in SQL SERVER?” in SQL SERVER chapter.
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