Session - 18 RECOVERY CONTROL - 2 Matakuliah: M0184 / Pengolahan Data Distribusi Tahun: 2005 Versi:

OBJECTIVE Local recovery protocol Distributed Recovery Protocol Recovery in Multidatabase

Local Recovery Protocol In a DDBMS, we have to consider both global and local atomicity and durability of transaction. Four principal algorithms : 1.Undo/Redo 2.Undo/no-redo 3.No-undo/redo 4.No-und/no-redo

UNDO / REDO The most complex algorithm, since they involve both undoing and redoing transaction following failure This approach has advantage of allowing the buffer manager to decide when to flush the buffers, hence reducing I/O overhead A variation on undo/redo is for the recovery manager to keep an active-list, an abort-list and a commit-list

UNDO/NO-REDO DB buffers are flushed at commit, so there will be never be any need to redo transaction on restart and hence there is no need to store after images on the log. Recovery manager only concern with transaction active at the time of failure which will have to be undone

NO-UNDO/REDO The recovery manager does not write uncommitted transaction to the stable DB. The buffer manager id forced to retain the records in main memory in the DB buffers until commit and this is known as pinning the buffer. Alternatively, update can be written to the log instead of the DB buffers or shadowing can be used

NO-UNDO/NO-REDO In order to avoid having to undo transaction, recovery manager has to ensure that no updates of transactions are written to the stable DB prior to commit To avoid having to redo transaction, recovery manager requires that all updates are written to the stable DB prior to commit This can be resolved by writing to the stable DB in a single atomic action at commit

Distributed Recovery Protocol An important aim of recovery techniques for DDBMS is that the failure of one site should not affect processing at another site Two common commit protocols : 1.Two-phase commit (2PC) 2.Three-phase commit (3PC)

TWO-PHASE COMMIT (2PC) 2PC operates in two phase : voting phase and decision phase All participant are asked by the coordinator whether or not they are prepared to commit the transaction. If one participant votes to abort, then the coordinator instructs all participant to abort the transaction If all vote to commit, the all participant are told to commit the transaction

THREE-PHASE COMMIT (3PC) The basic idea is to remove the uncertainty period for participant who have voted “commit” and are waiting for the “global abort” or “global commit” from the coordinator. The third phase called pre-commit

Recovery in Multidatabase The global transaction must have information about local transaction in order to produce globally serializable schedules. The problem for multidatabase is almost equivalent to fully partitioned networks in which each site runs in an independent partition.