Recoverability-

 

Before you go through this article, make sure that you have gone through the previous article on Recoverability in DBMS.

 

We have discussed-

• Non-serial schedules which are not serializable are called as non-serializable schedules.
• Non-serializable schedules may be recoverable or irrecoverable.

 

Recoverable Schedules-

 

If in a schedule,

• A transaction performs a dirty read operation from an uncommitted transaction
• And its commit operation is delayed till the uncommitted transaction either commits or roll backs

then such a schedule is called as a Recoverable Schedule.

 

Types of Recoverable Schedules-

 

A recoverable schedule may be any one of these kinds-

 

 

1. Cascading Schedule
2. Cascadeless Schedule
3. Strict Schedule

 

Cascading Schedule-

 

• If in a schedule, failure of one transaction causes several other dependent transactions to rollback or abort, then such a schedule is called as a Cascading Schedule or Cascading Rollback or Cascading Abort.
• It simply leads to the wastage of CPU time.

 

Example-

 

 

Here,

• Transaction T2 depends on transaction T1.
• Transaction T3 depends on transaction T2.
• Transaction T4 depends on transaction T3.

 

In this schedule,

• The failure of transaction T1 causes the transaction T2 to rollback.
• The rollback of transaction T2 causes the transaction T3 to rollback.
• The rollback of transaction T3 causes the transaction T4 to rollback.

Such a rollback is called as a Cascading Rollback.

 

NOTE-

 

If the transactions T2, T3 and T4 would have committed before the failure of transaction T1, then the schedule would have been irrecoverable.

 

Cascadeless Schedule-

 

If in a schedule, a transaction is not allowed to read a data item until the last transaction that has written it is committed or aborted, then such a schedule is called as a Cascadeless Schedule.

In other words,

• Cascadeless schedule allows only committed read operations.
• Therefore, it avoids cascading roll back and thus saves CPU time.

 

Example-

 

 

NOTE-

 

• Cascadeless schedule allows only committed read operations.
• However, it allows uncommitted write operations.

 

Example-

 

 

Strict Schedule-

 

If in a schedule, a transaction is neither allowed to read nor write a data item until the last transaction that has written it is committed or aborted, then such a schedule is called as a Strict Schedule.

In other words,

• Strict schedule allows only committed read and write operations.
• Clearly, strict schedule implements more restrictions than cascadeless schedule.

 

Example-

 

 

Remember-

 

• Strict schedules are more strict than cascadeless schedules.
• All strict schedules are cascadeless schedules.
• All cascadeless schedules are not strict schedules.

 

 

Next Article- Equivalence of Schedules

 

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