Cascadeless schedule in database transaction management
Cascadeless schedule:
A schedule that eliminates cascading
rollback of transactions when a failure occurs is called cascadeless schedule. If
there are no dirty reads in a schedule, then that schedule will be a
cascadeless schedule.
- Every cascadeless schedule is recoverable schedule.
How
do we avoid cascading rollback?
If two transactions T1 and
T2 are in a schedule S, and T2 consumes the data written
by T1 (we say T2 is dependent on T1), then we
must ensure that T1 commits before T2 commits. If this
situation is ensured then we have avoided cascading rollback.
Discussion:
It is an important property that would
save time and other resources in database transaction management.
At first place, we need the schedules
to be recoverable schedules. Recoverability does not ensure cascadeless schedule.
Even if a schedule is recoverable, we may need to rollback several transactions
in case if a transaction Ti fails and other transactions have consumed
the data written by Ti.
Example
1:
Is the following schedule is
cascadeless or not?
|
Instruction
|
T1
|
T2
|
|
1
2
3
4
5
6
|
R(x)
W(y)
Commit
|
R(x)
W(y)
Commit
|
This schedule is cascadeless because
there is no dirty read.
Example
2:
Is the following schedule is
cascadeless or not?
|
Instruction
|
T1
|
T2
|
T3
|
|
1
2
3
4
5
6
|
R(x)
R(y)
W(x)
|
R(x)
W(x)
|
R(x)
|
This schedule is said to be a partial
schedule as it does not consist of either commit or abort statements. In this
schedule, if T1 fails and aborted then T2 which is
dependent on T1 has to be rolled back and T3 which is
dependent on T2 has also to be rolled back. This type of rollback is
called as cascading rollback. Hence, this schedule is not a cascadeless
schedule.
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Go to Transaction management in DBMS page
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what is cascadeless schedule
define cascading rollback
explain with an example the cascadeless schedule
purpose of cascadeless schedule
Example of cascadeless schedule
why all recoverable schedules are not cascadeless schedules?
all cascadeless schedules are recoverable schedules
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