CS4432: Database Systems IILecture #26

Concurrency Control and Recovery

Professor Elke A. Rundensteiner

Concepts

Serial schedule: no interleaving of transactions

S1, S2 are conflict equivalent schedules if S1

transforms into S2 by swaps on non-conflicting actions.

A schedule is conflict serializable if it is conflict equivalent to some serial schedule.

Example: Tj Ti

Wj(A) ri(A) Commit Ti

Abort Tj

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Example: Tj Ti

Wj(A) ri(A) Commit Ti

Abort Tj

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… Non-Persistent Commit (Bad!)

dirty data

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Example: Tj Ti

Wj(A) ri(A) Commit Ti

Abort Tj

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… Problem : Non-Persistent Commit

Can beavoided byrecoverableschedules

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Example: Tj Ti

Wj(A) ri(A) wi(B)

Abort Tj

[Commit Ti]

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…dirty data

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Example: Tj Ti

wj(A) ri(A) wi(B)

Abort Tj

[Commit Ti]

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… Cascading rollback

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Example: Tj Ti

Wj(A) ri(A) wi(B)

Abort Tj

[Commit Ti]

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… Cascading rollback

avoided byavoids-cascading-rollback (ACR)schedules

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Key Observation:

• Schedule is conflict serializable, Yet 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 (final effect will stick)

– 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: “dirty read”

Tj Ti

Wj(A)ri(A)

Ci can we commit

here?

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Definition

Ti reads from Tj in S (denoted as Tj STi) 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)

Definition

Intuition : A schedule S is recoverable,

if transactions only commit after all transactions they read from have already been committed first.

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

then 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

• Another rule: – at most one Ci or Ai per transaction

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How to achieve recoverable schedules?

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

Tj Ti

Wj(A)

Cj

uj(A)ri(A)

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

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

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

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Types of schedules:

Avoids cascading rollback

RC

ACR

ST SERIAL

RC : recoverableACR: avoids cascading rollback ST: strict

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Examples

• Recoverable:– w1(A) w1(B) w2(A) r2(B) c1 c2

• Avoids Cascading Rollback:– w1(A) w1(B) w2(A) c1 r2(B) c2

• Strict:– w1(A) w1(B) c1 w2(A) r2(B) c2

Assumes w2(A) is donewithout reading

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Where are serializable schedules?

Avoids cascading rollback

RC

ACR

ST SERIAL

Recoverable vs Serializable ?

• Every STRICT schedule is serializable.

• Every STRICT schedule is ACR (and thus recoverable).

Recoverable vs Serializable ?

• Recoverable and serializable:– w1(A) w1(B) w2(A) r2(B) c1 c2

• Recoverable and not serializable:– w2(A) w1(B) w1(A) r2(B) c1 c2

• Not recoverable yet serializable:– w1(A) w1(B) w2(A) r2(B) c2 c1

• Not recoverable and not serializable:– w2(A) w1(B) w1(A) r2(B) c2 c1

– 0. Start T1.– 1. L1(A)– 2. R1(A)– 3. W1(A)– 4. O1(A)– 5. Start T2.– 6. L2(C)– 7. L1(B)– 8. U1(A)– 9. L2(A)– 10. W1(B)– 11. Commit T1– 12. R2(A)– 13. W2(C)– 14. U1(B)– 15. Commit T2– 16. U2(A)– 17. U2(C)

Basics of Concurrency Control and Recovery