1. Temple University – CIS Dept. CIS661 – Principles of Data Management
V. Megalooikonomou
Concurrency control- Deadlocks
(based on slides by C. Faloutsos at CMU and on notes by Silberchatz,Korth, and Sudarshan)

2. General Overview Relational model - SQL
Functional Dependencies & Normalization
Physical Design &Indexing
Query optimization
Transaction processing
concurrency control
recovery

3. Transactions - dfn = unit of work, eg. Atomicity (all or none)
move $10 from savings to checking
Atomicity (all or none)
Consistency
Isolation (as if alone)
Durability
recovery
concurrency control

4. Isolation - concurrency control
serializability <-> correctness
precedence graph
automatically correct interleavings:
locks + protocol (2PL, 2PLC)
but: deadlocks!

5. Deadlocks
detection
handling
(prevention)

6. Deadlock detection T1 T2 lock manager L(A) Yes L(B) Yes ... L(B) No
<waits>
L(A) No
time
DEADLOCK

7. Algo for deadlock detection?
wait-for graph:
nodes -> transactions
arcs -> Tsource waits for Tsink
if cycle, then deadlock!
Must invoke a deadlock-detection algorithm periodically to look for cycles.

8. Eg: time for ‘B’ T1 T2 ‘A’ ‘B’ for ‘A’ T1 T2 lock manager L(A) Yes
L(B) Yes
...
L(B) No
<waits>
L(A) No
time
for ‘B’ T1 T2
‘A’
‘B’
for ‘A’

9. Another example is there a deadlock? if yes, which xacts are involved?

10. Another example now, is there a deadlock?
if yes, which xacts are involved? T1 T2 T3 T4

11. Deadlock detection how often should we run the algo?
how many transactions are typically involved?

12. Deadlock handling
Q: what to do? T1 T2 T3 T4

13. Deadlock handling Q0: what to do? A: select a ‘victim’ & ‘rollback’
Q1: which/how to choose? T1 T2 T3 T4

14. Deadlock handling Q1: which/how to choose? A1.1: by age
A1.2: by progress
A1.3: by # items locked already...
A1.4: by # xacts to rollback
Q2: How far to rollback? T1 T2 T3 T4

15. Deadlock handling Q2: How far to rollback? A2.1: completely
A2.2: minimally
Q3: Starvation?? T1 T2 T3 T4

16. Deadlock handling Q3: Starvation??
A3.1: include #rollbacks in victim selection criterion. T1 T2 T3 T4

17. SQL statement usually, conc. control is transparent to the user, but
LOCK <table-name> [EXCLUSIVE|SHARED]

18. Concurrency control - conclusions
serializability <-> correctness
automatically correct interleavings:
locks + protocol (2PL, 2PLC, ...)
deadlock detection + handling

19. serializable schedules serializable schedules
Conclusions
2PL schedules
serializable schedules
serializable schedules
2PLC
serial sch’s