1. SECTION 18.8 Timestamps

2. What is Timestamping? Scheduler assign each transaction T a unique number, it’s timestamp TS(T). Timestamps must be issued in ascending order, at the time when a transaction first notifies the scheduler that it is beginning.

3. Timestamp TS(T) Two methods of generating Timestamps. Use the value of system, clock as the timestamp. Use a logical counter that is incremented after a new timestamp has been assigned. Scheduler maintains a table of currently active transactions and their timestamps irrespective of the method used

4. Timestamps for database element X and commit bit RT(X):- The read time of X, which is the highest timestamp of transaction that has read X. WT(X):- The write time of X, which is the highest timestamp of transaction that has write X. C(X):- The commit bit for X, which is true if and only if the most recent transaction to write X has already committed.

5. Physically Unrealizable Behavior Read too late: A transaction U that started after transaction T, but wrote a value for X before T reads X. U writes X T reads X T startU start

6. Physically Unrealizable Behavior Write too late A transaction U that started after T, but read X before T got a chance to write X. U reads X T writes X T startU start Figure: Transaction T tries to write too late

7. Dirty Read It is possible that after T reads the value of X written by U, transaction U will abort. U writes X T reads X U startT startU aborts T could perform a dirty read if it reads X when shown

8. Rules for Timestamps-Based scheduling 1. Scheduler receives a request rT(X) a) If TS(T) ≥ WT(X), the read is physically realizable. 1. If C(X) is true, grant the request, if TS(T) > RT(X), set RT(X) := TS(T); otherwise do not change RT(X). 2. If C(X) is false, delay T until C(X) becomes true or transaction that wrote X aborts. b) If TS(T) < WT(X), the read is physically unrealizable. Rollback T.

9. Rules for Timestamps-Based scheduling (Cont.) 2. Scheduler receives a request WT(X). a) if TS(T) ≥ RT(X) and TS(T) ≥ WT(X), write is physically realizable and must be performed. 1. Write the new value for X, 2. Set WT(X) := TS(T), and 3. Set C(X) := false. b) if TS(T) ≥ RT(X) but TS(T) < WT(X), then the write is physically realizable, but there is already a later values in X. a. If C(X) is true, then the previous writers of X is committed, and ignore the write by T. b. If C(X) is false, we must delay T. c) if TS(T) < RT(X), then the write is physically unrealizable, and T must be rolled back.

10. Rules for Timestamps-Based scheduling (Cont.) 3. Scheduler receives a request to commit T. It must find all the database elements X written by T and set C(X) := true. If any transactions are waiting for X to be committed, these transactions are allowed to proceed. 4. Scheduler receives a request to abort T or decides to rollback T, then any transaction that was waiting on an element X that T wrote must repeat its attempt to read or write.

11. Multiversion Timestamps Multiversion schemes keep old versions of data item to increase concurrency. Each successful write results in the creation of a new version of the data item written. Use timestamps to label versions. When a read(X) operation is issued, select an appropriate version of X based on the timestamp of the transaction, and return the value of the selected version.

12. Timestamps and Locking Generally, timestamping performs better than locking in situations where: Most transactions are read-only. It is rare that concurrent transaction will try to read and write the same element. In high-conflict situation, locking performs better than timestamps