Classification of Weak Correctness Criteria for Real-Time Database Applications

1. Classification of Weak Correctness Criteria for Real-Time Database Applications 1996. 8. 21 Lee, Kyu-Woong and Park, Seog Sogang Univ., Seoul, Korea

2. Contents • Introduction and Motivation • Forms of Database Consistency • 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. Forms of Database Consistency state consistency write consistency update consistency weak consistency strong consistency strict consistency

5. Forms of Database Consistency U1 • weak consistency[GW82] • multiple query cycles are allowed • update consistency[BC92] • single query cycles are allowed if they can be broken by removing the single read-write edge between update transactions Q2 Q4 U3 single read-write edge U2 U3 Q1

6. U1 Q2 Forms of Database Consistency • write consistency • single query cycles are allowed • update transaction cycles are not allowed • Epsilon-serializability [WYP92] • state consistency • update transaction cycle are restrictively allowed U1 U2 write-write U3

7. 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

8. View Serializability[BH 87] • A schedule H is said to be view-serializable if it is view-equivalent to some serial schedule • Two schedules H1 and H2 are view equivalent 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.

9. 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

10. 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)

11. 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

12. 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

13. 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

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

15. 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