Page 194 - DCAP402_DCAO204_DATABASE MANAGEMENT SYSTEM_MANAGING DATABASE

Unit 11: Recovery System

If log on stable storage at the time of crash as per (a) (b) and (c) then in: Notes
(a) No redo action needs to be performed.
(b) redo(T ) must be performed since <T commit> is present
1 1
(c) redo(T ) must be performed followed by redo(T ) since
2 2
<T commit> and <T commit> are present.
1 2

Notes This sequence of redo operation as suggested in (c) any number of times, it will still
bring the value of X, Y, Z to consistent redo values. This property of the redo operation is
called idempotent.

Immediate Database Modification

The immediate database modification scheme allows database updates on the stored database
even of an uncommitted transaction. These updates are made as the writes are issued (since
undoing may be needed, update logs must have both the old value as well as the new value).
Updated log records must be written before database item is written (assume that the log record
is output directly to a stable storage and can be extended to postpone log record output, as long
as prior to execution of an output (Y) operation for a data block Y all log records corresponding
to items Y must be flushed to stable storage).
Output of updated blocks can take place at any time before or after transaction commit. Order in
which blocks are output can be different from the order in which they are written.

Example:
Log Write operation Output
<T start>
1
<T , X, 10000, 9000>
1
T , Y, 8000, 9000
1
X = 9000 Output Block of X
Y = 9000 Output Block of Y
<T commit>
1
<T start>
2
<T , Z, 20,000, 19,000>
2
Z= 19000 Output Block of Z
<T commit>
2
The recovery procedure in such has two operations instead of one:
1. undo(T ) restores the value of all data items updated by T to their old values, moving
i i
backwards from the last log record for T ,
i
2. redo(T ) sets the value of all data items updated by T to the new values, moving forward
i i
from the first log record for T .
i
Both operations are idempotent, that is, even if the operation is executed multiple times the
effect is the same as it is executed once. (This is necessary because operations may have to be re-
executed during recovery).

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