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Concurrency control by validation

Concurrency control by validation. Concurrency control by validation. Architecture of Validation based scheduler The validation rules Comparison of three concurrency-control mechanism Exercises. How can concurrency control be performed with validation?.

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Concurrency control by validation

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  1. Concurrency control by validation

  2. Concurrency control by validation • Architecture of Validation based scheduler • The validation rules • Comparison of three concurrency-control mechanism • Exercises

  3. How can concurrency control be performed with validation? • In this method, transactions are allowed to access data without locks. • At the appropriate time a check is performed that the transaction has behaved in a serializable manner. • Just before a transaction starts to write values of database elements, it goes through a “validation phase”. • In this phase the risk of a physically unrealizable behavior is checked, if a risk exists the transaction is rolled back. • The above processes are taken care of, by a validation scheduler.

  4. Validation-Based scheduler • The validation-based scheduler must know the following: * The set of database elements the transaction T reads. * The set of database elements T writes. • T executes in three phases: * Read – elements are read into read set RS(T) and local address space for write is calculated. * Validate – Validation of clashing elements with other transactions is done. * Write – transaction writes elements to write set WS(T)

  5. Validation based Scheduler sets • Validation based scheduler assumed serial order of transactions. • It maintains three sets of transactions: • START( ): set of T’s started but not completed validation. • VAL( ): set of T’s validated but not finished the writing phase. • FIN( ): set of T’s that have finished.

  6. Potential problems with validation scheduler • Case 1: • U is in VAL or FIN; that is, U has validated, • FIN(U)>START(T); that is, U did not finish before T started • RS(T) ∩WS(T) ≠φ; let it contain database element X. Since in this case we don’t know if T got to read the value that U wrote, we must roll back T to avoid the risk that the actions of T and U will not be consistent with the assumed serial order.

  7. Potential problems with validation scheduler • Case 2: - U is in VAL; U has successfully validated. - FIN(U)>VAL(T); U did not finish before T entered its validation phase. - WS(T) ∩ WS(U) ≠φ; let x be in both write sets. T and U must both write values of X and if we let T validate, it is possible that it will write X before U does. Since we cannot be sure, we rollback T to make sure it does not violate the assumed serial order in which it follows U.

  8. Sample Problem

  9. Solution • Validation of U: Nothing to check • Validation of T: WS(U) ∩ RS(T)= {D} ∩{A,B}=φ WS(U) ∩ WS(T)= {D}∩ {A,C}=φ • Validation of V: RS(V) ∩ WS(T)= {B}∩{A,C}=φ WS(V) ∩ WS(T)={D,E}∩ {A,C}=φ RS(V) ∩ WS(U)={B} ∩{D}=φ • Validation of W: RS(W) ∩ WS(T)= {A,D}∩{A,C}={A} RS(W) ∩ WS(V)= {A,D}∩{D,E}={D} WS(W) ∩ WS(V)= {A,C}∩{D,E}=φ (W is not validated)

  10. Thank You

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