Information Technology Reference
In-Depth Information
an unrepeatable read. The locking protocol enforces the following execution order
for the actions of transactions T
1
and T
2
:
H
0
2
D
T
1
W
BR
Œx
RŒyW Œ
z
C
T
2
W
BR
Œy
W ŒyC
with locks granted as follows:
4. For WŒ
z
, a commit-duration X lock on
z
for T
1
.
5. After C , the locks held by T
1
on x, y and
z
are released.
6. For WŒy, a commit-duration X lock on y for T
2
.
7. After C , the locks held by T
2
on y are released.
The history H
0
2
contains no isolation anomalies.
t
Rule (4) of the read-write locking protocol states that all locks acquired after
setting a savepoint P are released after a completed partial rollback to P .
Example 6.3
Consider the following transaction, which contains nested partial
rollbacks.
BR
Œx
1
;
u
1
W Œx
1
;
u
1
;
v
1
SŒP
1
RŒx
2
;
u
2
W Œx
2
;
u
2
;
v
2
SŒP
2
W Œx
2
;
v
2
;
w
2
RŒx
3
;
u
3
AŒP
2
W
1
Œx
2
;
v
2
;
w
2
C ŒP
2
W Œx
2
;
v
2
;
w
0
2
AŒP
1
W
1
Œx
2
;
v
2
;
w
0
2
W
1
Œx
2
;
u
2
;
v
2
C ŒP
1
RŒx
2
;
u
2
W Œx
2
;
u
2
;
w
00
2
C .
Locks are acquired and released by the transaction as follows: (cf. Fig.
6.3
):
1. For RŒx
1
;
u
1
,anSlockonkeyx
1
.
2. For WŒx
1
;
u
1
;
v
1
,anXlockonkeyx
1
.
3. For RŒx
2
;
u
2
,anSlockonkeyx
2
.
4. For WŒx
2
;
u
2
;
v
2
,anXlockonkeyx
2
.
5. For WŒx
2
;
v
2
;
w
2
,nonewlock.
6. For RŒx
3
;
u
3
,anSlockonkeyx
3
.
7. For W
1
Œx
2
;
v
2
;
w
2
,nonewlock.
8. After CŒP
2
, the lock on x
3
is released.
9. For WŒx
2
;
v
2
;
w
0
2
,nonewlock.
10. For W
1
Œx
2
;
v
2
;
w
0
2
,nonewlock.
11. For W
1
Œx
2
;
u
2
;
v
2
,nonewlock.
12. After CŒP
1
,thelocksonx
2
are released.
13. For RŒx
2
;
u
2
,anSlockonkeyx
2
.
14. For WŒx
2
;
u
2
;
w
00
2
,anXlockonkeyx
2
.
15. After C ,thelocksonx
2
and x
1
are released.
First we note that each of the tuples with keys x
1
and x
2
written by the transaction is
kept X-locked until it is again in a committed state. Thus dirty writes are prevented.
Second, the transaction holds S locks on keys x
1
, x
2
,andx
3
when reading the tuples
with those keys. Thus, dirty reads are prevented. Third, the S locks are kept until the
transaction commits or completes the partial rollback of an action sequence that
includes the read action for which the S lock was acquired.
