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CS4432: Database Systems IILecture #23

Transaction Management

Professor Elke A. Rundensteiner

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# locksheld byTi

Time Growing Shrinking Phase Phase

2 Phase Locking Protocol

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Now :

Show that rules #1,2,3 conflict- serializable schedules

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Conflict rules for li(A), ui(A):

• li(A), lj(A) conflict • li(A), uj(A) conflict

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Theorem Rules #1,2,3 conflict (2PL) serializable

schedule

To help in proof:Definition Shrink(Ti) = SH(Ti) =

first unlock action of Ti

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LemmaTi Tj in S SH(Ti) <S SH(Tj)

Proof of lemma:Ti Tj means that

S = … pi(A) … qj(A) …; p,q conflictBy rules 1,2:

S = … pi(A) … ui(A) … lj(A) ... qj(A) … SH(Ti) SH(Tj)

By rule 3:

So, SH(Ti) <S SH(Tj)

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Proof:(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

Theorem Rules #1,2,3 conflict (2PL) serializable

schedule

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• Beyond this simple 2PL protocol, it is all a matter of improving

performance and allowing more concurrency….– Shared locks– Multiple granularity– Other types of C.C. mechanisms

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Chapter 10 More on transaction processing

Topics:• Cascading rollback • Recoverable schedule• Deadlocks

– Prevention– Detection

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Schedule H (T2 reversed)

T1 T2l1(A); Read(A) l2(B);Read(B)A A+100;Write(A) B Bx2;Write(B)

ll11(B)(B) l l22(A)(A)delayeddelayed

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• Assume deadlocked transactions are rolled back– They have no effect– They do not appear in schedule

E.g., Schedule H =This space intentionally

left blank!

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Example: Ti To

Wi(A) rj(A) Commit To

Abort Ti

Concurrency control & recovery

……

… ……

… Cascading rollback (Bad!)

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• Schedule is conflict serializable• Ti To

• But not recoverable

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• Need to make “final’ decision for each transaction:– commit decision - system guarantees

transaction will or has completed, no matter what

– abort decision - system guarantees transaction will or has been rolled back

(has no effect)

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To model this, two new actions:

• Ci - transaction Ti commits• Ai - transaction Ti aborts

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...

...

...

...

Back to example:

Ti Tj

Wi(A)rj(A)

Cj can we commit here?

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Definition

Ti read from Tj in S (Tj S Tj) if

(1) wj(A) <S ri(A)

(2) aj <S ri(A) (< : does not precede)

(3) If wj(A) <S wk(A) <S ri(A) then

ak <S ri(A)

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Definition

Schedule S is recoverable if whenever Tj S Ti and j i and Ci

Sthen Cj <S Ci

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Note: in transactions, reads and writes precede commit or abort

If Ci Ti, then ri(A) < Ci

wi(A) < Ci

If Ai Ti, then ri(A) < Ai

wi(A) < Ai

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How to achieve recoverable schedules?

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With 2PL, hold write locks to commit (strict 2PL)

Ti Tj

Wi(A)

Ciui(A)

rj(A)

...

...

...

...

...

...

...

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• S is recoverable if each transaction commits only after all transactions from which it read have committed.

• S avoids cascading rollback if each transaction may read only those values written by committed transactions.

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• S is strict if each transaction may read and write only items previously written by committed transactions.

Avoids cascading rollback

RC

ACA

ST SERIAL

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Schedule with Deadlock

T1 T2l1(A); Read(A) l2(B);Read(B)A A+100;Write(A) B Bx2;Write(B)

ll11(B)(B) l l22(A)(A)delayeddelayed

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Deadlocks

• Detection– Wait-for graph

• Prevention– Resource ordering– Timeout– Wait-die– Wound-wait

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Deadlock Detection

• Build Wait-For graph• Use lock table structures• Build incrementally or periodically• When cycle found, rollback victim

T1

T3

T2

T6

T5

T4T7

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Resource Ordering

• Order all elements A1, A2, …, An

• A transaction T can lock Ai after Aj only if i > j

Problem : Ordered lock requests not realistic in most cases

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Timeout

• If transaction waits more than L sec., roll it back!

• Simple scheme• Hard to select L

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Wait-die

• Transactions given a timestamp when they arrive …. ts(Ti)

• Ti can only wait for Tj if ts(Ti)< ts(Tj) ...else die

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T1

(ts =10)

T2

(ts =20)

T3

(ts =25)

wait

wait

Example:

wait?

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Wound-wait

• Transactions given a timestamp when they arrive … ts(Ti)

• Ti wounds Tj if ts(Ti)< ts(Tj) else Ti waits

“Wound”: Tj rolls back and gives lock to Ti

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Summary

• Cascading rollbackRecoverable schedule

• Deadlock– Prevention– Detection