Locking vs Optimistic Concurrency Control: Strategies for Concurrent Edits in SQL Server
- Collision: If two users try to update the same record simultaneously, their changes might conflict.
Solutions:
Additional Techniques:
- Timestamps: Adding a timestamp column to tables allows tracking when a record was last updated. This helps identify conflicts during OCC.
- Audit Trails: Logging changes made to records helps trace edits and identify potential conflicts.
Important Considerations:
- Security: Granting appropriate permissions to users ensures they can only edit authorized data.
- Application Logic: The application interacting with the database should handle potential conflicts gracefully, informing users and allowing them to resolve issues.
SQL Server Locking Example with Transactions
CREATE TABLE Users (
ID int PRIMARY KEY,
Username nvarchar(50) NOT NULL UNIQUE,
Email nvarchar(100) NOT NULL UNIQUE
);
GO
-- Simulate user A updating a record
BEGIN TRANSACTION;
UPDATE Users SET Email = '[email protected]' WHERE Username = 'userA';
-- Simulate user B trying to update the same record concurrently (causing a lock wait)
SELECT * FROM Users WITH (XLOCK, ROWLOCK) WHERE Username = 'userA';
-- User A commits their update, releasing the lock
COMMIT TRANSACTION;
-- Now user B's update can proceed
UPDATE Users SET Username = 'updatedUsername' WHERE Username = 'userA';
GO
Explanation:
- We create a simple
Users
table. - User A starts a transaction and updates the email for 'userA'.
- User B tries to select the same user with an
XLOCK
(exclusive lock) andROWLOCK
(locks specific row) hint, causing them to wait until User A finishes. - User A commits their update, releasing the lock.
- User B's update can now proceed successfully.
This is a basic example. In practice, error handling and conflict resolution logic would be implemented in the application layer.
CREATE TABLE Products (
ID int PRIMARY KEY,
Name nvarchar(50) NOT NULL UNIQUE,
Price decimal(10,2) NOT NULL,
LastUpdated datetime NOT NULL DEFAULT(GETUTCDATE()) -- Timestamp for OCC
);
GO
-- User A retrieves product data with timestamp
DECLARE @productID int = 1;
DECLARE @initialTimestamp datetime;
SELECT @productID, Name, Price, LastUpdated
INTO @productID, @initialTimestamp
FROM Products WITH (NOLOCK) WHERE ID = @productID;
-- Simulate user A modifying the price
UPDATE Products
SET Price = Price + 5.00
WHERE ID = @productID AND LastUpdated = @initialTimestamp;
-- Simulate user B updating the same product concurrently
UPDATE Products
SET Name = 'New Name'
WHERE ID = @productID;
GO
-- User A might encounter an update conflict
IF @@ERROR <> 0
BEGIN
-- Handle the conflict (e.g., inform user and prompt to retry)
DECLARE @errorMessage nvarchar(max) = 'Update conflict detected!';
RAISERROR (@errorMessage, 10, 1);
END;
- We create a
Products
table with aLastUpdated
timestamp column. - User A retrieves product details with
NOLOCK
hint (avoiding unnecessary locks) and stores the timestamp. - User A updates the price, but includes a
WHERE
clause checking if theLastUpdated
timestamp matches the retrieved value. - User B concurrently updates the product name.
- If User B's update happens first, User A's update will fail due to a mismatch in the
LastUpdated
timestamp, indicating a conflict. - The application logic (IF statement) can handle the error by informing the user and potentially allowing them to retry the update.
Benefits of OCC:
- Reduced locking overhead compared to traditional locking methods.
- Improved concurrency and performance in scenarios with low conflict rates.
Drawbacks of OCC:
- Requires additional logic in the application layer to handle conflicts.
- Might lead to lost updates if conflicts are frequent.
sql-server database