Tuesday, November 24, 2009

Timestamp (rowversion) Data Type

A table can have only one timestamp column. The value in the timestamp column is updated every time a row containing a timestamp column is inserted or updated. Of course this means we should not use it as a primary key, because we can get many orphans quickly if updates are performed on other columns. As a row is modified in a table, the timestamp is updated with the current database timestamp value obtained from the @@DBTS function.

Timestamp is the wrong name, quite confusing in fact. It has nothing to do with time. Microsoft will rename it rowversion in the future. Rowversion is the synonym for timestamp in SQL Server 2005 and SQL Server 2008.

It is an 8 bytes unique binary key within the database.

Here is how it looks like: 0x0000000000000743. After an update to the row: 0x0000000000000745.
The rowversion(timestamp) starts changing as soon as the transaction begins. If the transaction is rolled back, it returns to the original value.

So how can we use it?

The main purpose is row versioning in multi user environment, in other words concurrency checking.
Assume you are a developer and developing a program in Visual Basic to update the name and address table of customers. There will be 100 staff member who can perform this application function. How can you be sure that while staff A typing in the change, staff X is not changing the same row?

Here is what you do:

1. Read the name and address table including the timestamp. You display the info to the user for update and save the timestamp.
2. Certain amount of time later, like 2 minutes, the user presses the submit button after changes were typed in.
3. You open a transaction with Begin Transaction
4. You read the timestamp of the name and address row
5. You compare the current timestamp to the saved timestamp.
6. If the timestamps are same, you update the row and commit the transaction
7. If timestamps are different, you roll back the transaction and notify the user about the fact that the data was changed by someone else. You can let the user decide what to do or follow the appropriate company business rule for data entry conflict resolution.
This is pretty common practice in multi user environment. The alternate would be to examine a datetime column, or the entire row which is more processing intensive.

The following example shows timestamp (rowversion in SQL Server 2008) in action:
-- SQL Server 2008 T-SQL Code
USE tempdb;

-- SQL create table for Concurrency Checking demo
CREATE TABLE Celebrity (
CelebrityID INT     IDENTITY     PRIMARY KEY,
FirstName   VARCHAR(25),
LastName     VARCHAR(30),
VERSIONSTAMP  ROWVERSION)
GO

-- SQL insert - populate table
INSERT Celebrity (FirstName, LastName)
VALUES
('Jessica', 'Simpson'),
('Nick', 'Carter'),
('Stevie', 'Brock'),
('Christina', 'Aguilera'),
('Frank','Sinatra'),
('Doris','Day'),
('Elvis', 'Presley')
GO

SELECT * FROM Celebrity
GO

/* Results
CelebrityID FirstName  VERSIONSTAMP
1             Jessica      Simpson      0x0000000000000876
2             Nick         Carter       0x0000000000000877
3             Stevie       Brock        0x0000000000000878
4             Christina   Aguilera     0x0000000000000879
5             Frank        Sinatra      0x000000000000087A
6             Doris        Day          0x000000000000087B
7             Elvis        Presley      0x000000000000087C
*/

-- SQL update demo: SOMEONE UPDATED RECORD since it was read
CREATE TABLE #Semaphore (ID int identity(1,1) primary key,
StartVersion bigint,
PK int)
DECLARE @MyKey int
INSERT INTO #Semaphore (StartVersion, PK)
SELECT  VERSIONSTAMP, 1
FROM Celebrity WHERE CelebrityID=1
SELECT @MyKey = SCOPE_IDENTITY()

-- SIMULATION: somebody else updating the same record
UPDATE Celebrity
SET     FirstName = 'Celine',
LastName = 'Dion'
WHERE  CelebrityID = 1

-- We are attempting to update.
BEGIN TRANSACTION
IF (SELECT StartVersion
FROM   #Semaphore
WHERE  ID = @MyKey) = (SELECT VERSIONSTAMP
FROM   Celebrity
WHERE  CelebrityID = 1)
BEGIN
UPDATE Celebrity
SET     FirstName = 'Lindsay',
LastName = 'Lohan'
WHERE  CelebrityID = 1

COMMIT TRANSACTION
END
ELSE
BEGIN
ROLLBACK TRANSACTION
PRINT 'ROLLBACK - UPDATE CONFLICT'
RAISERROR ('Celebrity update conflict.',10,0)
END
DELETE #Semaphore WHERE ID = @MyKey
SELECT * FROM   Celebrity
GO
/* CelebrityID     FirstName   LastName     VERSIONSTAMP
1             Celine       Dion         0x000000000000087D
2             Nick         Carter       0x0000000000000877
3             Stevie       Brock        0x0000000000000878
4             Christina   Aguilera     0x0000000000000879
5             Frank        Sinatra      0x000000000000087A
6             Doris        Day          0x000000000000087B
7             Elvis        Presley      0x000000000000087C
*/

-- SQL UPDATE with NO CONFLICT
DECLARE @MyKey int
INSERT INTO #Semaphore (StartVersion, PK)
SELECT  VERSIONSTAMP, 1
FROM Celebrity WHERE CelebrityID=1
SELECT @MyKey = SCOPE_IDENTITY()

-- We are trying to update.
BEGIN TRANSACTION
IF (SELECT StartVersion
FROM   #Semaphore
WHERE  ID = @MyKey) = (SELECT VERSIONSTAMP
FROM   Celebrity
WHERE  CelebrityID = 1)
BEGIN
UPDATE Celebrity
SET     FirstName = 'Lindsay',
LastName = 'Lohan'
WHERE  CelebrityID = 1

COMMIT TRANSACTION
END
ELSE
BEGIN
ROLLBACK TRANSACTION
PRINT 'ROLLBACK - UPDATE CONFLICT'
RAISERROR ('Celebrity update conflict.',10,0)
END
DELETE #Semaphore WHERE ID = @MyKey
SELECT * FROM   Celebrity
GO
/*
CelebrityID FirstName   LastName     VERSIONSTAMP
1             Lindsay      Lohan        0x000000000000087E
2             Nick         Carter       0x0000000000000877
3             Stevie       Brock        0x0000000000000878
4             Christina   Aguilera     0x0000000000000879
5             Frank        Sinatra      0x000000000000087A
6             Doris        Day          0x000000000000087B
7             Elvis        Presley      0x000000000000087C
*/
-- Cleanup
DROP TABLE #Semaphore
DROP TABLE Celebrity

---
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