---

Recoverable schedules

---

• Review
  • Serializable schedule:

    A schedule is serializable if: the effect of the execution of the actions in the schedule is equivalent to a serial schedule

    (We also looked at a stronger notion of serializability known as conflict serializability)

  ---

  ---

  ---

  ---

• Recoverable schedule
  • Recoverable schedule:

    A schedule is recoverable if: each transaction commits only after all transactions from which it has read has committed Graphically: The transaction Tk must commit only after the transactions T1, T2 and T3 have committed

    ---

    ---

  • Observation:

    In a recoverable schedule, a transaction Tk can only commit if: transaction Tk has not read (= used) dirty data

    ---

    ---

  • Important property of recoverable schedules:

    If a schedule is recoverable, then: A logging technique (undo, redo or undo/redo) will restore the database to a consistent state !!! (Because each transaction will have used consistent data)

  ---

  ---

  ---

  ---

• New notation (c_i) needed to express recoverable schedules
  • We introduce a new action into the schedule:

    ci = indicates that the transaction Ti commits

    Previously, we have already defined:

    ri(A) = the transaction Ti reads the DB element A wi(A) = the transaction Ti writes the DB element A

  ---

  ---

  ---

  ---

• Example of a recoverable schedule
  • Example of recoverable schedule:

    Consider the following schedule: Transaction T2 has used (read) dirty data written by transaction T1: In order for the schedule to be a recoverable schedule: T1 must commit before T2 !!! Let's check in the schedule: Verified !!! Therefore: The schedule is recoverable.