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3/9/2005 Yan Huang - CSCI5330 Database Implementation – Concurrency Control Concurrency Control I
3/9/2005 Yan Huang - CSCI5330 Database Implementation – Concurrency Control
Concurrency Control I
3/9/2005 Yan Huang - CSCI5330 Database Implementation – Concurrency Control
Concurrency Control
T1 T2 … Tn
DB(consistencyconstraints)
3/9/2005 Yan Huang - CSCI5330 Database Implementation – Concurrency Control
Prevent cycles in precedence graph from occurring
T1 T2 ….. Tn
Scheduler
DB
Enforce Conflict Serializable Schedules
3/9/2005 Yan Huang - CSCI5330 Database Implementation – Concurrency Control
A locking protocol
For transaction i Use li to lock an item Use ui to unlock the lock enforced by transaction i
scheduler
T1 T2
locktable
3/9/2005 Yan Huang - CSCI5330 Database Implementation – Concurrency Control
Well behaved transactions
Ti: … li(A) … pi(A) … ui(A) ...
3/9/2005 Yan Huang - CSCI5330 Database Implementation – Concurrency Control
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 ?
3/9/2005 Yan Huang - CSCI5330 Database Implementation – Concurrency Control
Example:
T2: Read(A) T3: Read(A)
A A+100 A A2
Write(A) Write(A)
Read(B) Read(B)
B B+100 B B2
Write(B) Write(B)
Constraint: A=B
3/9/2005 Yan Huang - CSCI5330 Database Implementation – Concurrency Control
Schedule A
T2 T3 25 25
l1(A);Read(A)A A+100;Write(A);u1(A) 125
l2(A);Read(A)A Ax2;Write(A);u2(A) 250l2(B);Read(B)B Bx2;Write(B);u2(B) 50
l1(B);Read(B)B B+100;Write(B);u1(B) 150
250 150
A B
3/9/2005 Yan Huang - CSCI5330 Database Implementation – Concurrency Control
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
3/9/2005 Yan Huang - CSCI5330 Database Implementation – Concurrency Control
Ti = ……. li(A) ………... ui(A) ……...
no unlocks no locks
3/9/2005 Yan Huang - CSCI5330 Database Implementation – Concurrency Control
# locks
held by
Ti
Time
Growing Shrinking
Phase Phase
3/9/2005 Yan Huang - CSCI5330 Database Implementation – Concurrency Control
What happens to a transaction which tries to lock an item but failed?
3/9/2005 Yan Huang - CSCI5330 Database Implementation – Concurrency Control
Schedule B
T2 T3
l1(A);Read(A)
A A+100;Write(A)
l1(B); u1(A)
l2(A);Read(A)
A Ax2;Write(A); l2(B)l2(B)
Read(B);B B+100
Write(B); u1(B)
l2(B); u2(A);Read(B)
B Bx2;Write(B);u2(B);
delayed
3/9/2005 Yan Huang - CSCI5330 Database Implementation – Concurrency Control
2PL conflict-serializable schedules?
To help in proof:Definition Shrink(Ti) = SH(Ti) = first unlock action of Ti
3/9/2005 Yan Huang - CSCI5330 Database Implementation – Concurrency Control
First:
Ti Tj in S SH(Ti) <S SH(Tj)
Proof:Ti Tj means that
S = … pi(A) … ui(A) … lj(A) ... qj(A) …
3/9/2005 Yan Huang - CSCI5330 Database Implementation – Concurrency Control
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
3/9/2005 Yan Huang - CSCI5330 Database Implementation – Concurrency Control
Deadlock
To handle a deadlock one of T4 or T5 must be rolled back and its locks released.
T4 T5
l3(B)
read(B)
write(B)
l4(A)
read(A)
l4(B)
l3(A)
3/9/2005 Yan Huang - CSCI5330 Database Implementation – Concurrency Control
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.
3/9/2005 Yan Huang - CSCI5330 Database Implementation – Concurrency Control
2PL and Deadlock
Are schedules from 2PL transactions deadlock free?
3/9/2005 Yan Huang - CSCI5330 Database Implementation – Concurrency Control
2PL and Possible Schedules
Does 2PL allow all possible conflict serializable schedules?
3/9/2005 Yan Huang - CSCI5330 Database Implementation – Concurrency Control
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
