1. Concurrency Control
11/22/2018

2. Concurrency Control
T1 T2 … Tn DB
(consistency
constraints)

3. Enforce Conflict Serializable Schedules
Prevent cycles in precedence graph from occurring
T1 T2 ….. Tn
Scheduler DB

4. A locking protocol T1 T2 lock scheduler table For transaction i
Use li to lock an item
Use ui to unlock the lock enforced by transaction i
T1 T2
lock
table
scheduler

5. Well behaved transactions
Ti: … li(A) … pi(A) … ui(A) ...

6. Example of a transaction performing locking
T1: l1(A);
read (A);
u1(A);
l1(B);
read (B);
u1(B);
display(A+B)
Sufficient to guarantee serializability ?

7. Example: T2: Read(A) T3: Read(A) A  A+100 A  A2 Write(A) Write(A)
Read(B) Read(B)
B  B B B2
Write(B) Write(B)
Constraint: A=B

8. Schedule A A B T2 T3 25 25 l1(A);Read(A) A A+100;Write(A);u1(A) 125
A Ax2;Write(A);u2(A)
l2(B);Read(B)
B Bx2;Write(B);u2(B)
l1(B);Read(B)
B B+100;Write(B);u1(B)

9. Two-Phase Locking Protocol
Phase 1: Growing Phase
transaction may obtain locks
transaction may not release locks
Phase 2: Shrinking Phase
transaction may release locks
transaction may not obtain locks

10. Ti = ……. li(A) ………... ui(A) ……...
no unlocks no locks

11. # locks
held by Ti
Time
Growing Shrinking
Phase Phase

12. What happens to a transaction which tries to lock an item but failed?

13. Schedule B T2 T3 l1(A);Read(A) A A+100;Write(A) l1(B); u1(A)
A Ax2;Write(A); l2(B)
Read(B);B B+100
Write(B); u1(B)
l2(B); u2(A);Read(B)
B Bx2;Write(B);u2(B);
delayed

14. 2PL  conflict-serializable schedules?
To help in proof:
Definition Shrink(Ti) = SH(Ti) = first unlock action of Ti

15. Ti  Tj in S  SH(Ti) <S SH(Tj)
First:
Ti  Tj in S  SH(Ti) <S SH(Tj)
Proof:
Ti  Tj means that
S = … pi(A) … ui(A) … lj(A) ... qj(A) …

16. Then:
(1) Assume P(S) has cycle
T1  T2 …. Tn  T1
(2) By lemma: SH(T1) < SH(T2) < ... < SH(T1)
(3) Impossible, so P(S) acyclic
(4)  S is conflict serializable

17. Deadlock
To handle a deadlock one of T4 or T5 must be rolled back and its locks released.
T T5
l3(B)
read(B)
write(B)
l4(A)
read(A)
l4(B)
l3(A)

18. Starvation A transaction does not get its turn for a long time
Example:
A transaction may be waiting for a lock on an item, while a sequence of other transactions request and are granted an lock on the same item.
The same transaction is repeatedly rolled back due to deadlocks.
Concurrency control manager can be designed to prevent starvation.

19. 2PL and Deadlock
Are schedules from 2PL transactions deadlock free?

20. 2PL and Possible Schedules
Does 2PL allow all possible conflict serializable schedules?

21. Beyond this simple 2PL protocol, it is all a matter of improving performance and allowing more concurrency….
Shared locks
Multiple granularity
Inserts, deletes and phantoms
Other types of C.C. mechanisms