CS542

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CS542

• Concurrency Control
• Aborts and Deadlocks

Professor Elke A. Rundensteiner
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Two More Topics

• Strict 2P Locking
• Aborts can cause Cascading Aborts
• Recoverable Schedules
• Handling Deadlocks
• Optimistic
• Pessimistic

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2 Phase Locking Protocol

• locks
• held by
• Ti
• Time
• Growing Shrinking
• Phase Phase

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Concurrency control recovery

• Example Ti To
• Wi(A)
• rj(A)
• Commit To
• Abort Ti

• reads A even though Ti written A has not
  committed.
• Rollback ? Cascading rollback? (But committed!)

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• Note
• Schedule is conflict serializable
• But Schedule is 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 ( takes
  effect )
• 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 Tj Ti Wj(A) ri(A
) Ci ? can we commit here?

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...
...
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Back to example Tj Ti Wj(A) ri(A
) Ci ? No, cannot commit Dont
commit if you use (read) a variable written by
any other Tj that did not yet commit.
...
...
...
...
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Definition

• Ti reads from Tj in S (Tj ?S Ti) if
• wj(A) ltS ri(A)
• aj ltS ri(A) (lt does not precede)
• (3) If wj(A) ltS wk(A) ltS ri(A) then
• ak ltS ri(A)

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Definition

• Schedule S is recoverable if
• whenever Tj ?S Ti and j ? i and Ci ? S
• then Cj ltS Ci
• Idea Transactions only commit after all
  transactions they read from have been committed
  first.
• (allows dirty reads for maximal concurrency)

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Back to example Tj Ti Wj(A) ri(A
) Cj Ci ? Yes.
...
...
...
...
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• Note in transactions, reads and writes
  precede commit or abort
• ? If Ci ? Ti, then ri(A) lt Ci
• wi(A) lt Ci
• ? If Ai ? Ti, then ri(A) lt Ai
• wi(A) lt Ai

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How to achieve recoverable schedules?
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Strict 2PL With 2PL, hold locks until commit

• Ti Tj
• Wi(A)
• Ci
• ui(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.
• S is strict if each transaction may read and
  write only items previously written by committed
  transactions.

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Recoverable
Avoids cascading rollback
Strict
SERIAL
Avoid Cascading Aborts
Conflict Serializable
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Now what to do about deadlocks? (which
can arise for our 2PL and even
strict-2P protocol).
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Schedule with Deadlock

• T1 T2
• l1(A) Read(A) l2(B)Read(B)
• A A100Write(A) B Bx2Write(B)
• l1(B) l2(A)

delayed
delayed
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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

T5
T2
T1
T7
T4
T6
T3
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Deadlock Prevention Resource Ordering

• Order all elements A1, A2, , An
• A transaction T can lock Ai after Aj only if i gt
  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 are given a timestamp when they
  arrive . ts(Ti)
• Ti can only wait for Tj if ts(Ti)lt ts(Tj)
  ...else die

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Example

• T1
• (ts 10)
• T2
• (ts 20)
• T3
• (ts 25)

wait
wait
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Wound-wait

• Transactions given a timestamp when they arrive
  ts(Ti)
• Ti wounds Tj if ts(Ti)lt ts(Tj)
• else Ti waits
• Wound Tj rolls back and gives lock to Ti

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Example

• T1
• (ts 10)
• T2
• (ts 20)
• T3
• (ts 15)

Not wait (instead wound T2)
wait
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Summary

• Cascading rollbacks and
• Recoverable schedules
• Deadlocks
• Prevention
• Detection