• read-set(T): Each data items that are read by a transaction T
• Timestamps: for all transaction T, the start-time and the end time are reserved for all the three phases.

Let us show this scheme here with the help of the following examples:

Consider the set for transaction T1 and T2.

T1		T2
Phase	Operation	Phase	Operation
-	-	Read	Reads the read set (T2). Let say variables X and Y and performs updating of local values
Read	Reads the read set (T1) lets say variable X and Y and performs updating of local values	-	-
Validate	Validate the values of (T1)	-	-
-	-	Validate	Validate the values of (T2)
Write	Write the updated values in the database and commit	-	-
-	-	Write	Write the updated values in the database and commit

In this given instance both T1 and T2 get committed. Please note that Read set of T1 and Read Set of T2 are both disjoint, also the Write sets are also disjoint and therefore no concurrency related problem can happen.

T1	T2	T3
Operation	Operation	Operation
Read R(A)	--	--
--	Read R(A)	--
--	--	Read (D)
--	--	Update(D)
--	--	Update (A)
--	--	Validate (D,A) finds OK Write (D,A), COMMIT
--	Validate(A):Unsuccessful Value changed by T3	--
Validate(A):Unsuccessful Value changed by T3	--	--
ABORT T1	--	--
--	Abort T2	--

In this scheme both T1 and T2 get aborted as they fail during validate phase whereas only T3 is committed. Optimistic concurrency control performs its checking at the transaction commits point in a validation phase. The serialization order is defined by the time of transaction validation phase.

Posted Date: 3/12/2013 3:14:45 AM | Location : United States

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