7.31 Validation Based Protocols In DBMS | Concurrency Control Techniques In DBMS | DBMS Course

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Validation Based Protocols
Validation Based Protocols are used where most of the transactions are readonly and conflicts are low. In these protocols, every transaction Ti have to go through the following phases depending upon its type :
362 Introduction to Database Management System
(i) Read Phase : In read phase Ti reads all the data items and store them in temporary
variables (local variables of Ti
). After reading, all the write operations are made on
the temporary variables instead of actual database. Here changes are local and leaved
in actual database.
(ii) Validation Phase : In validation phase, a validation test is performed to determine
whether changes in actual database can be made.
(iii) Write Phase : If Ti cleared the validation test then actual database is changed according
to temporary variables. Here actual changes are made in database. Validation test
ensures violation free execution of transactions. To determine the time to perform
validation test, use timestamps. Each transaction Ti is associated with three timestamps.
These are:
(a) Start(Ti) : It gives time when Ti starts execution.
(b) Validation(Ti) : It gives time when Ti finished its read phase and starts its validation
phase.
(c) Finish(Ti) : It gives time when Ti finished execution or write phase.
If any transaction Ti failed in validation test then it is aborted and rollback.
To clear the validation test by transaction Tj, for all transactions such that TS(Ti) lessthan Tj,Tj must satisfy one of the following conditions:
(i) If Finish(Tj) lessthan Start(Tj
) then there is no conflict because they are in serial order.
(ii) If Start(Tj) lessthan Finish(Ti
) lessthan Validation(Tj). This condition ensures that actual writes to
database for Ti and Tj does not overlap.
Example. Consider the schedule as shown in Figure 8.16.
T9 T10read(A)
read(B)
B = B * 5;
read(B)
read(A)
A = A + 10;
(Validate)
display(A + B)
(Validate)
Write(B)
Write(A)
FIGURE 8.16. Schedule for transaction T9 and T10 using
validation based protocols.
Suppose, initially A = 10 and B = 5 then T9 display A+B = 15 because T10 first write
new values of A and B to local variables and validate after T9 finished its execution.
Transactions and Concurrency Control 363
Advantages : The advantages of validation based protocols are:
1. It maintains serializability.
2. It is free from cascading rollback.
3. Less overhead then other protocols.
Disadvantages : The disadvantages of validation based protocols are:
1. Starvation of long transactions due to conflicting short transactions.