Why identity fetch in a transaction is not rolled back ?

Have you ever wondered why the identity values don't rollback even if the underlying transaction is rolled back. With a small example, let's explain

CREATE DATABASE Ident;

-- Create a new table in this database as IdentCol with column c1 as identity

USE Ident

GO

CREATE TABLE IdentCol(c1 intidentity, c2 varchar(10));

-- Lets reset the active transaction log by firing the check point

CHECKPOINT

-- If you want to check what is there in the active transaction log part fire DBCC LOG

DBCCLOG(Ident,4)






-- Begin a transaction insert 1 record into the table and then rollback

BEGIN TRAN T1

INSERT INTO IdentCol VALUES (REPLICATE('a',10));

ROLLBACK TRAN T1

-- Lets see the transaction log again

DBCC LOG(Ident,4)





As (highlighted) can be clearly seen that the generation of the identity column happened outside the transaction (LOP_BEGIN_XACT). This means the identity generation hasn't rolled back even though the insert did. If at the stage we insert 1 row in this table the identity value would be 2



INSERT INTO IdentCol VALUES (REPLICATE('a',10));

SELECT * FROM IdentCol;