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Concurrency III (Timestamps)

Concurrency III (Timestamps). Serializability Via Timestamps (Continued). Every DB element X has two timestamps: 1. RT (X) = highest timestamp of a transaction to read X. 2. WT (X) = highest timestamp of a transaction to write X.

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Concurrency III (Timestamps)

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  1. Concurrency III (Timestamps)

  2. Serializability Via Timestamps (Continued) • Every DB element X has two timestamps: 1. RT (X) = highest timestamp of a transaction to read X. 2. WT (X) = highest timestamp of a transaction to write X. • Every DB element X has a bit C(X) indicating whether the most recent writer of X has committed.

  3. Physically Unrealizable Behaviors • The scheduler assumes the timestamp order of transactions is also the serial order in which they must appear to execute. • Scheduler needs to check that: whenever a read or write occurs, what happens in real time could have happened if each transaction had executed instantaneously at the moment of its timestamp. • If not, we say the behavior is physically unrealizable.

  4. What is Physically Unrealizable? • Read Too Late: Transaction T tries to read X, but TS(T)<WT(X). • Write Too Late: Transaction T tries to write X, but TS(T)<RT(X)

  5. But Wait if Data is Dirty? • T tries to read X, and TS(T)>WT(X), but C(X) = false. • T wants to write X, and TS(T)>WT(X), but C(X) = false.

  6. Abort/Update Decision • Legal = Physically Realizable. • Illegal = Physically Unrealizable. • If illegal, rollback T = abort T and restart it with a new timestamp. • When a transaction finishes with no rollback, commit the transaction by changing all C(X) bits to true.

  7. Rules, in detail… Suppose the scheduler receives a request rT(X), (a) If TS(T)  WT(X), the read is physically realizable. i. If C(X)=true, grant the request. If TS(T) > RT(X), set RT(X) := TS(T); otherwise do not change RT(X). ii. If C(X)=false, delay T until C(X) becomes true, or the transaction that wrote X aborts. (b) If TS(T) < WT(X), the read is physically unrealizable. Rollback T; that is, abort T and restart it with a new, larger timestamp.

  8. Rules, in detail… Suppose the scheduler receives a request wT(X), (a) If TS(T)  RT(X) the write is physically realizable If TS(T)  WT(X), the write must be performed. i. Write the new value for X, ii. Set WT(X) := TS(T), and iii. Set C(X) := false. If TS(T) < WT(X), then there is already a later value in X. If C(X)=true, then the previous writer of X is committed, and we simply ignore the write by T; Otherwise, if C(X)=false, then we must delay T. (b) If TS(T) < RT(X), then the write is physically unrealizable, and T must be rolled back.

  9. Rules, in detail… Suppose the scheduler receives a request to commit T. • It must find all the database elements X written by T, and set c(X) := true. • If any transactions are waiting for X to be committed, these transactions are allowed to proceed. Suppose the scheduler receives a request to abort T or decides to rollback T. • Then any transaction that was waiting on an element X that T wrote must repeat its attempt to read or write, and see whether the action is now legal after T's writes are cancelled.

  10. Exercise I st1; st2; r1(A); r2(B); w2(A); w1(B); T1 T2 A B TS=1 TS=2 r1(A) RT=1 r2(B) RT=2 w2(A) WT=2 w1(B) – illegal Rollback T1

  11. Exercise II st1; st3; st2; r1(A); r2(B); w1(C); r3(B); r3(C); w2(B); w3(A) T1 T2 T3 A B C TS=1 TS=2 TS2=3 r1(A) RT=1 r2(B) RT=3 w1(C) WT=1 r3(B) RT=2 r3(C) RT=2 w2(B) WT=3 w3(A) - Skip if C(X)=1. Wait otherwise.

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