Recovarable Schedule, Cascade less Schedule

Shayek Parvez

1. Pundra University Of Science & Technology Name: Shayek Parvez CSE 3 rd Batch, 7 th semester Roll No: C00315306010 Course Title: Database Management System Course Code: 311 Page 1/6

2. Presentation Topic Cascadeless schedules Recoverable & Non recoverable Schedule Precedence Graph Page 2/6

3. Cascadeless schedules Definition : A cascadeless schedule is one where for each pair of transaction Ti and Tj   such that Tj   reads data item, previously written by Ti  the commit operation of Ti appears before the read operation of Tj . Page 3/6

4. Cascadeless schedules 1. Transaction T10 writes a value of A that is read by Transaction T11. 2. Transaction T11 writes a value of A that is read by Transaction T12. Suppose at this point T10 fails. 3. 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. Page 4/6

5. What is recoverable schedule? A recoverable schedule is one where, for each pair of Transaction Ti and Tj such that Tj reads data item previously written by Ti the commit operation of Ti appears before the commit operation Tj . Page 5/6

6. Recoverable & Non recoverable Schedule T1 T2 Read (X) X = X+10 Write (X) Read (X) X =X-10 Write (X) C T1 T2 Read (X) X = X+10 Write (X) C Read (X) X =X-10 Write (X) C * A committed Transaction Should never be rolled back Non recoverable Schedule R ecoverable Schedule Page 6/6

7. Thank You