1. Distributed Database Management Systems
Lecture 28

2. In the previous lecture
Types of Transaction
Transaction in DDBS
Serial Transactions
Conflicting Ops/Tr.

3. In this Lecture
Serializability Theory
Serializability Theory in DDBS.

4. Equivalent Schedules
Two schedules S1, S2 defined over same T are equivalent if they have same effect on the database, that is, leave database in same final state.

5. Formally, if for each pair of conflicting operations Oij and Okl (i ≠ k) if Oij <1 Okl then Oij <2 Okl
The phenomena is also called conflict equivalence

6. Serializable Schedule

7. If it is conflict equivalent to a serial schedule, i. e
If it is conflict equivalent to a serial schedule, i.e., the final state in which it leaves the database is equivalent to a serial schedule-

8. Ss ={W2(x), W2(y), R2(z), C2, R1(x), W1(x), C1, R3(x), R3(y), R3(z), C3}
S1 = {W2(x), R1(x), R3 (x),W1(x),C1, W2(y), R3(y), R2(z),C2 ,R3(z), C3} No

9. Ss ={W2(x), W2(y), R2(z), C2, R1(x), W1(x), C1, R3(x), R3(y), R3(z), C3}
S2 ={W2(x), R1(x), W1(x), C1, R3(x), W2(y), R3(y), R2(z), C2, R3(z), C3} yes

10. The function of the concurrency controller is to generate serializable schedule
Local and Global Schedules.

11. Fragmented Databases

12. The serializability is straight forward.
Local transaction are independent of each other; each concerns local data.

13. In case of global transactions
Local subtransactions will be treated as different transactions-

14. Replicated Databases

15. T1: T2: Read(x) x = x + 5 Write(x) Commit x = x*10
LS1={R1(x), W1(x),C1, R2(x), W2(x), C2}
LS2={R2(x), W2(x), C2, R1(x), W1(x), C1}

16. All values of replicated data should be same
Local Schedule same
Conflicting Ops in same relative order on all sites.

17. Logical and physical data items
User issues Ops on logical data items
Replica control maps to physical ones-

18. ROWA Protocol
Reduces availability in case of failure
Different alogos, different replications.

19. Concurrency Control Algorithms

20. Different categorizations possible
Like, mode of distribution, network topology-

21. Synchronization primitive is the most common
Locking and Ordering
Pessimistic & Optimistic.

22. Pessimistic approach synchronizes transactions early
Optimistic do this late in execution life cycle of transactions

23. Pessimistic Locking-based Centralized Locking Primary Copy Locking
Distributed Locking-

24. Pessimistic Timestamp Ordering (TO) Hybrid Basic TO Multiversion TO
Conservative TO
Hybrid

25. Optimistic
Locking-based
Timestamp ordering-based.

26. Locking based Concurrency Control

27. Basic idea is that data items accessed by conflicting operations are accessed by one operation at a time
Data Items locked by Lock Manager

28. Two major types of locks, read lock and write lock
Transaction need to apply lock first.

29. For improved accessibility, compatibility of locks to be established
rli(x)
wli(x)
rlj(x)
Yes No
wlj(x)

30. Locking is job of DDBMS, not the user
Scheduler is the Lock Manager
TM and LM interact.