Question

Question 6: Briefly explain what is recoverability and what is cascade freedom. Mention a protocol that ensures both properties, and explain why it ensures both the above properties.

Answer 1

Recoverability: It means a property by using which we recover the lost data or the process of getting back our lost data or erased data 6 It is done by following steps: 1. by creating database backup 2. Use of hardware standards 3. from maintaining hardware record 4. from maintaining software record 5. Planning hardware contingencies 6. Providing training and documentation Explanation 2 Let two transaction pairs T and T Tj reads data items written by Ti Commit operation of Ti present before T T8 Read(A) Write(A) T9 T9 is dependent on T8 Non recoverable schedule if T9 commits before T8 0 0 Read(a) Read(B)

System used T9 to commit immediately after execution of read (A) instruction Thus T9 commit before T8 completed Cascade freedom T10 Read(A) Read(B) Write(A) T11 T12 Read(A) Write(A) Read(A) Transaction T10 writes value of A and T11 read it T11 writes and T1 read If at this point T10 fails then T10 rolled back so T11 dependent on T10, T11 must be rolled back, T12 dependent on T11 in this case in which a single transaction failure leads to series of transaction rollback is called cascading rollback.

Protocol that ensure property of recoverability and cascade freedom Timestamp-ordering protocol Let Ti aborts, but Тј has read a data item written by Ti Then Tj must terminate, if Tj had been allowed to commit earlier, the Schedule is not recoverable. Further, any transaction that has read a data item written by Tj must terminate This can lead to cascading rollback -that is, a chain of rollbacks Solution of Timestamp-ordering protocol A transaction is designed such that its writes are all performed at the end of its processing All writes of a transaction form an atomic action, any transaction cannot execute while a transaction is being written A transaction that terminates is restarted with a new timestamp