Read-set(t) and timestamps-optimistic concurrency contents, Database Management System

  • 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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