Validation Based Protocol

It imposes less overhead

Also based on Timestamp Protocol. It has three phases:

1. Read Phase: During this phase, the system executes transaction T_i. . It reads the values of the various data items and stores them in variable local to T_i. It performs all the write operations on temporary local variables without update of the actual database.
2. Validation Phase: Transaction T_i performs a validation test to determine whether it can copy to database the temporary local variables that hold the result of write operations without causing a violation of serializability.
3. Write Phase: If Transaction T_i succeeds in validation, then the system applies the actual updates to the database, otherwise the system rolls back T_i.

To perform the validation test, we need to know when the various phases of transaction T_itook place. We shall therefore associate three different timestamps with transaction T_i.

1.       Start (T_i): the time when T_i, started its execution.

2.       Validation (T_i): the time when T_i finished its read phase and started its validation phase.

3.       Finish (T_i): the time when T_i finished its write phase.

The Validation Test for T_j  requires that, for all transaction T_i  with TS(T_i  ) < TS(T_j  ) one of the following condition must hold

1.  Finish (T_i) < Start (T_j): Since T_i  completes its execution before T_j  started, the serializability order is indeed maintained.
2.   Start(T_j )<Finish(T_i  ) <validation(T_j  ): The validation phase of T_j should occur after T_ifinishes.

Few Points

1. Resolves the cascade rollbacks
2.  Suffers from the starvation.
3. Optimistic concurrency control.