Recoverable and Cascadeless Schedules 15

Recoverability
A recoverable schedule is one where, for each pair of Transaction Ti and Tsuch that T reads data item previously written by Ti   the commit operation of Ti  appears before the commit operation T.
 
 
Suppose that the system allows T9 to commit immediately after execution of read(A) instruction.Thus T9 commit before T8 does.
Now suppose that T8 fails before it commits. Since T9 has read the value of data item A written by T8 we must abort T9 to ensure transaction Atomicity.
However, T9 has already committed and cannot be aborted.Thus we have a situation where it is impossible to recover correctly from the failure of T8.
 
Cascadeless schedules
 
 
Transaction T10 writes a value of A that is read by Transaction T11. Transaction T11 writes a value of A that is read by Transaction T12. Suppose at this point T10 fails. T10 must be rolled back, since T11 is dependent on T10, T11 must be rolled back, T12 is dependent on T11, T12 must be rolled back.
This phenomenon, in which a single transaction failure leads to a series of transaction rollbacks is called Cascading rollback.
 
  • Cascading rollback is undesirable, since it leads to the undoing of a significant amount of work.
  • It is desirable to restrict the schedules to those where cascading rollbacks cannot occur, Such schedules are called Cascadeless Schedules.
  • Formally, a cascadeless schedule is one where for each pair of transaction Ti  and T such that T reads data item, previously written by Ti   the commit operation of Ti  appears before the read operation of T.
Every Cascadeless schedule is also recoverable schedule.
 
Cascadeless Schedule
 
Recoverable and Cascadeless Schedules 4.33/5 (86.67%) 33 votes

15 thoughts on “Recoverable and Cascadeless Schedules

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  13. Reply TUSHAR Nov 28,2014 11:57 am

    HOW THESE R RECOVERABLE XPLAIN

    • Reply Anupam Jain Dec 2,2014 11:21 am

      Hello Tushar,
      The scenario you’ve given is a classic example of simultaneous read ‘n write of same record.
      Consider this setup: There is a bank account. At some instant it has a balance of let’s say Rs. 500/-.
      Now, T1 begins and reads this value of 500. Simultaneously, T2 comes and reads the same value of 500/-.
      Now both transactions perform whatever operation they have on the same value of 500. At the time of writing, T2 writes the value let’s say 600 by adding 100 to the value read.
      After this, T1 comes and write the value 400 by subtracting 100 from the originally read value of 500.
      Here both the transactions are independent of each other ’cause one’s written value is not being used by the other transaction.
      So T1 would simply overwrite any effect produced by T2.
      Hence if any commit fails, that particular transaction can be rolled back without affecting the other one. Although, word of caution is, we won’t be able to predict the final state of the record in case of failures.
      Thus the transactions are independent and recoverable.
      Hope the explanation is easy enough to understand.
      :)

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