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Serial schedules

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• Review: transaction
  • Recall:

    A transaction will transform a database from a consistent state to another consistent state if:       There are no system failures The transaction is executed in isolation (= no other transaction is active)

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• Correctness of an execution of multiple transactions
  • Correctness of an execution of multiple transactions:

    Multiple transactions will transform a database from a consistent state to another consistent state if:       There are no system failures Each transaction is executed in isolation (= no other transaction is active)

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• Definition: schedule
  • Schedule:

    Schedule = a sequence of (important) operations performed by one or more transactions

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  • In concurrency control, the important operations performed by transactions are:

    read operations (Read(X)) write operations (Write(X))

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  • Example:

    We have 2 DB elements: A = 25 B = 25 (A = B) We have 2 transactions: T1: add 100 to database elements A and B T2: double the value of database elements A and B In code: T1 T2 ================================= READ(A, t) READ(A, s) t = t + 100 s = 2×s WRITE(A, t) WRITE(A, s) READ(B, t) READ(B, s) t = t + 100 s = 2×s WRITE(B, t) WRITE(B, s) Example of a schedule: // Blue: operations of T1 // Red: operations of T2 READ1(A, t) READ2(A, s) t = t + 100 s = 2×s WRITE1(A, t) WRITE2(A, s) READ1(B, t) t = t + 100 WRITE1(B, t) READ2(B, s) s = 2×s WRITE2(B, t)

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• A note of the example transactions to check correctness
  • Correctness consideration when executing the transactions T₁ and T₂:

    When T1 or T2 are executed in isolation: T1: READ(A, t) t = t + 100 WRITE(A, t) READ(B, t) t = t + 100 WRITE(B, t) T2: READ(A, s) s = 2×s WRITE(A, s) READ(B, s) s = 2×s WRITE(B, t) T1 and T2 will preserve the following the property: A = B

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• Serial schedule
  • Serial schedule:

    Serial schedule = a schedule where the all operations of each transactions are executed in sequence I.e.: Operations from different transactions are not interspersed with one another

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  • Example: serial schedule

    Execute T1, then T2: READ1(A, t) // Subscript indicate the transaction t = t + 100 WRITE1(A, t) READ1(B, t) t = t + 100 WRITE1(B, t) READ2(A, s) s = 2×s WRITE2(A, s) READ2(B, s) s = 2×s WRITE2(B, t)

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• Representation of a serial schedule
  • We can represent a serial schedule by:

    A list of the transaction IDs

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  • Example:

    Serial schedule: READ1(A, t) // Subscript indicate the transaction t = t + 100 WRITE1(A, t) READ1(B, t) t = t + 100 WRITE1(B, t) READ2(A, s) s = 2×s WRITE2(A, s) READ2(B, s) s = 2×s WRITE2(B, t) Representation (abriviation): T1 T2

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• Database consistency and serial schedules
  • Fact:

    A serial schedule executes each transaction in isolation

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  • Therefore:

    Every serial schedule will preserve the consistency (correctness) of the database

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  • Conclusion:

    We can use a serial schedule as the basis for the definition of correctness for: Concurrent execution of transactions