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 .
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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.
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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 .
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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
Recoverable Schedule
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