DBMS Notes

Types Of Recoverable Schedules

A recoverable schedule may be any one of following

  • Cascading Schedule
  • Cascadeless Schedule
  • Strict Schedule

Cascading Schedule

Casecadless Schedule Strict Schedule
Cascading allows READ or WRITE operation for dependent Transaction (T2) before to T1 committed or abort.

Casecadless Don’t allow READ to dependent Transaction (T2) untill T1 committed or abort.

But it allows WRITE operation for dependent Transaction (T2) before to T1 committed.

Strict don’t allow READ or WRITE operation to dependent Transaction (T2) until T1 committed or abort.

Note: Rollback of one Transaction leads to rollback of all other dependent Transactions 

Note: This case is problematic.

Note: This case is also problematic.

Note: This case if 100% safe.

1. Cascading Schedule (Recoverable)

  • In a schedule, If failure of one transaction (say T1) then several other dependent transactions (i.e. T2,T3,T4) has to rollback or abort.
  • such schedules are called as a Cascading Scheduleor Cascading Rollback or Cascading Abort.
  • It is also called recoverable schedule
  • This case leads toward the wastage of CPU time.

Example:

Here,

  • T2 reads from transaction T1.
  • T3 reads from transaction T2.
  • T4 reads from transaction T3.
  • T5 reads from transaction T
  • And so on..

In this schedule

  • If transaction T1 is fail then the other dependent transactions (T2,T3,T4) has to
  • Note: If any  transactions from T2, T3, T4 or T5 are committed before the failure of transaction T1, then the schedule will not  be

2. Cascadeless Schedule

In a cascadeless schedule, a transaction is never be allowed to perform read operation until the last transaction is committed or aborted.

  • It is also a problematic in some cases which is overcome through third type (strict schedule)

Example

Problem with Cascadeless Schedule

It allows the dependent transaction (T2) to perform Write Operation after Write operation of Transaction (T1). As given below

  • IF T1 committed then T2 has dirty Write value
  • And if T1 abort then rollback of T2 has to perform which cause CPU time wastage

A schedule will be a strict schedule if

“A transaction (Say T2) is neither allowed to read nor write operation until the last transaction (Say T1) that has written it is committed or aborted.

Example

Remember

All serial schedules are strict, cascading and cascadeless and recoverable schedules but not vice versa.

  • As, All cascadeless schedules are may or may not be strict schedules but all cascadeless schedules are always cascading or recoverable.

Descriptive diagram is given below

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