Recoverability

    • Recoverable Schedule:If a transaction(e.g. T1) reads data item which is updated by uncommitted transaction(e.g. T2) then commit of T2 should be delayed until commit of T1.
      Example:
      Here T2 executes read operation of A which is written by T1. This is a recoverable schedule as T1 commits before T2.
    • Cascading Rollback: 
      • Failure of one transaction results rollback of set of other transaction.
      • Disadvantage: Wastage of CPU execution time and I/O access time
      • Example
      • When T1 failes; T2,T3 and T4 must also be rollback.
    • Cascadeless Schedules: 
      • Recoverable and no cascading rollback.
      • Not allowed uncommitted reads.
      • If a Transaction(e.g. T1) updates data item(e.g. A) then other transaction(e.g. T2) not allowed to read A until commit of T1.
      • Example:
    • Strict Recoverable Schedule: 
      • Not Allowed uncommitted read and write.
      • If Transaction(e.g. T1) updates a data item(say, A) then other transaction(say T2) not allowed to read or write A until commit of T1.
      • Example: 
      • T2 can do R(A) or W(A) as T1 already commit.
    • Summarize
                                   Hierarchy of Schedules

 

Leave a Reply

avatar
  Subscribe  
Notify of