Optimistic concurrency control is a technique used in database management to handle concurrent modifications to the same data. It assumes that conflicts are rare, so it allows multiple transactions to proceed without locking resources. Only when the data needs to be committed does it check for conflicts. If a conflict is detected, the transaction is rolled back or retried.
In SQL Server, you can implement optimistic concurrency control using timestamps or row versioning. Here’s how you can do it:
Step 1: Use Row Versioning
1. Modify your table to include a ROWVERSION or TIMESTAMP column:
CREATE TABLE ExampleTable (
Id INT PRIMARY KEY,
Data NVARCHAR(100),
RowVersionColumn ROWVERSION
);
The ROWVERSION column will automatically generate a unique binary number every time a row is modified.
Step 2: Retrieve Data with the Row Version
2. Retrieve the data along with the Row Version when you perform a read operation:
SELECT Id, Data, RowVersionColumn
FROM ExampleTable
WHERE Id = @Id;
3. Store the Row Version in your application to check against during update operations.
Step 3: Update the Data Using the Row Version
4. Perform the update using a WHERE clause that includes the original RowVersionColumn value:
UPDATE ExampleTable
SET Data = @NewData
WHERE Id = @Id AND RowVersionColumn = @OriginalRowVersion;
In this SQL statement, @OriginalRowVersion is the value of the RowVersionColumn you retrieved earlier.
Step 4: Check for Conflicts
5. Check if the update was successful by examining the number of affected rows:
IF @@ROWCOUNT = 0
BEGIN
-- The update failed because the row has changed since it was read.
-- Handle the concurrency conflict here (e.g., notify the user, retry, etc.)
PRINT 'Concurrency conflict detected. The record has been modified by another transaction.';
END
ELSE
BEGIN
-- The update was successful
PRINT 'Record updated successfully.';
END
Considerations
- Error Handling: Always handle exceptions and errors that may occur during the transaction.
- Transaction Control: Depending on the complexity of your business logic, consider wrapping these operations within a transaction to maintain atomicity.
- User Experience: When dealing with web applications, inform users if their update attempt has failed due to concurrent modifications and give them an option to review the changes before retrying.
Conclusion
Optimistic concurrency control is an effective way to manage concurrent transactions in SQL Server when conflicts are considered infrequent. By using a ROWVERSION column, you can reliably detect modifications made by other transactions, thereby ensuring data integrity and consistency.