Relaxed Serializability Based on Consistent States for Real-Time Database Applications

1. Relaxed Serializability Based on Consistent States for Real-Time Database Applications 1996. 4. 15 Lee, Kyu-Woong and Park, Seog

2. Contents • Introduction and Motivation • VSR and Relaxed Serializability • Statewise Serializability(SSR) • Algorithm for SSR • Comparisons to Other Criteria • Conclusion

3. Introductory Example • the example • that the final state of schedule H0 is correct with respect to the situation in the real world schedule H0 : W1(x) R2(x) W2(x) R3(z) W3(y) W1(y) At this time, one of transactions should be aborted to satisfy the conflict serializability. But, the final state is the same as the serial schedule T3T1T2

4. Motivation • possibility • All of non-VSR schedule produce the inconsistent state ? • It is possible that the non-view serializable schedules produce a correct(consistent) state VSR CSR serial correct states possible range of relaxed serializability

5. View Serializability[BH 87] • The schedules H1 and H2 are view serializable 1. They are over the same set of transactions and same set of operations 2. for any data item x, if Ti reads x from Tj in H1, then Ti reads x from Tj in H2. 3. for each data item x, if the write operation Wi(x) is the final write of x in H1, then it is also the final write of x in H2.

6. The Range of Relaxed SR all possible schedules the schedules that produce the correct states (the same final write) schedules that have the same read-from relationships as some serial schedule VSR CSR : final states are consistent : final states are inconsistent

7. a b c T1 W T2 R W an invalid read operation T3 W W T1 R final state of schedule H1 is the same as the serial schedule T1T2T3 Invalid Read Operation • invalid read operation • A read operation Rinvalid in the schedule H is invalid iff • Rinvalid is different from the read-from relationship of a serial schedule that produces the same database state as the result of schedule H. • example of invalid read operation • schedule H1 : W1(a) R2(a) W2(b) W3(b) W3(c) R1(c)

8. Statewise Serializability • [definition] A schedule is statewise serializable if and only if (1) A schedule has the same result as at least one serial schedule and (2) If there exists invalid read operations Rinvalid, they do not reflect any changes on the database state • problem • How can we find the invalid read operations ? • NP problem

9. Algorithm for Relaxed SR • data structures • Wedge : the set of write-write conflict edges • Wseti : the set of data item that is written by Ti • algorithm Tj Wseti ⊇Wsetj W W Ti Tk Ti+1 Tk-1

10. Example of Proposed Algorithm schedule H2 : W1(a) R2(a) W2(b) R3(b) W3(c) W1(c) a b c T1 W T2 R W T3 R W T1 W final state is the same as a serial schedule T3T1T2 T1 the set of write data item Wset1: { a, c} w w ∪ R w T2 T3 the set of write data item Wset3: { c} w R

11. Comparison to Other Criteria SSR(statewise serializability) VSR CSR serial

12. Conclusion • our contribution for real-time database system • provides the higher degree of concurrency • reduces the abort ratio of transaction with urgent deadline • utilizes for other application with semantic transaction • further research • extendibility for applying the relaxed serializability to distributed database system