Information Technology Reference
In-Depth Information
4. After completing a partial rollback with action CŒP, all locks acquired after the
corresponding set-savepoint action SŒP are released.
5. All remaining locks are released after committing or completing a total rollback
of the transaction with action C .
Assume that a history H of transactions can be run on database D. We say that
H is
possible under a locking protocol
or
permitted by the protocol
or that the
transactions in H
follow the protocol
on D, if, when executing the history one action
at a time on D in the order the actions appear in H , all the locks requested according
to the protocol can be granted immediately without wait.
Example 6.2
Consider different histories of the transactions
T
1
D
BR
ŒxRŒyW Œ
z
C ,
T
2
D
BR
ŒyW ŒyC
run on a database that contains tuples with keys x, y,and
z
.
The history
H
1
D
T
1
W
BR
Œx
RŒyW Œ
z
C
T
2
W
BR
ŒyW ŒyC
is possible under the read-write locking protocol, because all the locks requested for
the actions can be granted immediately:
1. For RŒx, a commit-duration S lock on x for T
1
.
2. For the first RŒy, a commit-duration S lock on y for T
2
.
3. For WŒy, a commit-duration X lock on y for T
2
.
4. After C , the locks held by T
2
on y are released.
5. For the second RŒy, a commit-duration S lock on y for T
1
.
6. For WŒ
z
, a commit-duration X lock on
z
for T
1
.
7. After C , the locks held by T
1
on x, y and
z
are released.
We note that H
1
contains no isolation anomalies.
On the other hand, the history
H
2
D
T
1
W
BR
Œx
RŒy
W Œ
z
C
T
2
W
BR
Œy
W ŒyC
is not possible under the read-write locking protocol, because of the S lock held by
T
1
on y while T
2
requests an X lock on y:
1. For RŒx, a commit-duration S lock on x for T
1
.
2. For the first RŒy, a commit-duration S lock on y for T
2
.
3. For the second RŒy, a commit-duration S lock on y for T
1
.
4. For WŒy, a commit-duration X lock on y for T
2
not granted.
Because of the lock held by T
1
on y, the exclusive lock requested by T
2
on y cannot
be granted immediately; so T
2
must wait for T
1
to commit and to release its locks,
thus preventing H
2
from occurring. We note that in H
2
, the read action RŒy by T
1
is
