Wednesday, April 19, 2006

row-level locking, dirty-read prevention

explicit row-level locking and its uses

----------

MS SQL: (have tested WITH(ROWLOCK) doesn't seems working)

SET @m_Hit = @m_Hit + (SELECT hit FROM topleveldomainhits WITH(TABLOCKX) WHERE tldsuffix = '.com')

SET @m_Hit = @m_Hit + 1


PostgreSQL:

SELECT hit INTO m_Hit FROM topleveldomainhits WHERE tldsuffix = '.com' FOR UPDATE;

m_Hit = m_Hit + 1;


--------------------------------------------------

Note:

Explicit row-Level Locking is not needed if you are not doing complex computation, for the sake of discussion we just make an example of complex hit computation : )

here is a non-complex hit computation:
-- statement A:
UPDATE topleveldomainhits SET hit = hit + 1 WHERE tldsuffix = '.com'

row-level locking is needed if you are doing complex computation,e.g. passing result to variables(likely scenario is doing multi-line computation) and/or the possibility of parallel processing is very high.

if the statement is is one just one line like statement A, and two users executed it in parallel and even you SET TRANSACTION ISOLATION LEVEL READ UNCOMMITTED, they won't overwrite each other's data. single statements are an atomic operation in itself. so pagehit = 2 :)



-------------------------------
-------------------------------


another example of where explicit row-level locking is necessary, (complex computations)

for the purpose of discussion let's say we have only column field which is Bonus and employeeid



--- bonus computation prodedure run once every month ---



TotalBonus = $80
YearsWork 5
MonthsCount = 3


Final TotalBonus = $1200

MSSQL:

computer A:


CREATE PROCEDURE ComputeYearBonus
@EmployeeID VARCHAR(8)
AS
SET XACT_ABORT ON

BEGIN TRAN
DECLARE @YearsWork INT

SELECT @YearsWork = YearCount FROM YearsWork WITH(TABLOCKX) WHERE Employee =
@EmployeeId

DECLARE @Bonus MONEY
-- process 1. read bonus = 80 * 5 = $400
SET @Bonus = (SELECT TotalBonus FROM Employee WITH(TABLOCKX) WHERE EmployeeId = @EmployeeId) * @YearsWork

-- process 4. write bonus = $400
UPDATE Employee SET TotalBonus = @Bonus WHERE EmployeeId = @EmployeeID

COMMIT TRAN
GO

computer B:


CREATE PROCEDURE ComputeMonthBonus
@EmployeeID VARCHAR(8)

AS
SET XACT_ABORT ON
BEGIN TRAN

DECLARE @MonthsWork INT
SELECT @MonthsWork = MonthCount FROM MonthsWorkThisYear WITH(TABLOCKX) WHERE Employee = @EmployeeId

DECLARE @Bonus MONEY

-- process 2. read bonus = 80 * 3 = $240
SET @Bonus = (SELECT TotalBonus FROM Employee WITH(TABLOCKX) WHERE EmployeeId = @EmployeeId) * @MonthsWork

-- process 4. write bonus = $240
UPDATE Employee SET TotalBonus = @Bonus WHERE EmployeeId = @EmployeeID

COMMIT TRAN

GO



without WITH(TABLOCKX), when computer A and computer B execute at the same time, there is a high probability that they will overwrite each other's bonus computation. see the steps above. last written bonus is $240 instead of $1200.

if we put WITH(TABLOCKX) in our SELECTs, process 2 will wait until computer A commits. after computer A commits, computer B will read the bonus as $400, then $400 multiply by $3 = $1200.

same thing would effect even if computer B executes first, computer B computes $240. then computer A would read the bonus, computer A can't read the bonus until computer B commits. after B commits, computer A will read the bonus of $240. then computer A will multiply this by 5. $240 * 5 = $1200.


you'll notice that in the YearsWork in MonthsWorkThisYear we also put TABLOCKX, same thing, if there are other parallel executing transaction or trigger that updates the two table at the same time with our ComputeBonus. the computebonus will be messed, there will be dirty reads if there is no row-locking, we can't afford to have our bonus miscomputed.


------------------------------------------------



PostgreSQL Version:


postgresql has implicit BEGIN TRAN AND COMMIT TRAN:



CREATE FUNCTION ComputeYearBonus() RETURNS VOID
m_EmployeeID VARCHAR(8)
AS
$$
DECLARE
m_YearsWork INT;
m_Bonus NUMERIC;
BEGIN


SELECT YearCount INTO m_YearsWork FROM YearsWork WHERE Employee = m_EmployeeId FOR UPDATE;

-- process 1. read bonus = 80 * 5 = $400
m_Bonus = (SELECT TotalBonus FROM Employee WHERE EmployeeId = m_EmployeeId FOR UPDATE) * m_YearsWork;


-- process 4. write bonus = $400
UPDATE Employee SET TotalBonus = m_Bonus WHERE EmployeeId = m_EmployeeID;

END;
$$
LANGUAGE 'plpgsql';




CREATE FUNCTION ComputeMonthBonus() RETURNS VOID
m_EmployeeID VARCHAR(8)

AS
$$
DECRLARE
m_MonthsWork INT
m_Bonus NUMERIC
BEGIN


SELECT MonthCount INTO m_MonthsWork FROM MonthsWorkThisYear WHERE Employee = m_EmployeeId FOR UPDATE;


-- process 2. read bonus = 80 * 3 = $240
m_Bonus = (SELECT TotalBonus FROM Employee WHERE EmployeeId = m_EmployeeId FOR UPDATE) * m_MonthsWork;


-- process 4. write bonus = $240
UPDATE Employee SET TotalBonus = m_Bonus WHERE EmployeeId = m_EmployeeID;

END;
$$
LANGUAGE 'plpgsql';



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