Explain Concurrency Control with Time Stamping and Optimistic Methods.?

Time Stamping : The order in which the transaction enter in to the data base.

The time stamp approach to scheduling concurrent transaction & assigns a unique time – stamp to each transaction.

The time stamp value produces on explicit order in which transaction are submitted to the DBMS.

Time  stamp have 2–properties. Uniqueness                           Monotonicity

Uniqueness ensures that no equal time stamp values can exist.

Monotonicity ensures that time stamp values always increase.

The dis advantages of time stamping approach is that each value stored in this data base requires two additional time stamp fields.

one for the last time the field was read & one for the last update .

Wait /die :  If the transaction requesting the lock is the order of the two transactions , it will wait until the other transaction is completed & the locks are released.

If the transaction requesting the lock is the younger of the two transactions , it will die & rescheduled.

If short  the order transaction waits for the younger to complete & release the locks.

Wound / wait  :   If the transaction requesting the lock is the order of 2- transactions . It will preempt (wound /stop) the younger transaction.

If the transaction requesting lock is younger of two it will wait until other transaction is completed & locks released.

In short in wound/wait schema. The order transaction rolls back the younger transactions & reschedules it.

Concurrency Control with Optimistic Methods

An optimistic approach each transaction moves through two or three phases. Retrieved as read, validation & write.

During the read phase : In this the transaction reads the data base, executes the needed computations & makes the updates to a private copy of the data base values. All update operations of the transaction are recorded in a temporary update file, which is not accessed by the remaining transactions.

During the validation phase : The transaction is validated to ensure that the changes made will not affect the integrity & consistency of the data base. If the validation test is ‘’+ve’ the transaction goes to the write phase. If the validation test is ‘–ve’ the transaction restarted & changes are discarded.

During the write phase : The changes are permanently applied to the data base.