- Types of operation on
DB elements:
- READ(•)
- WRITE(•)
- INC(•, c)
- For each type of
operation, we use
a specific
type of lock:
- shared lock:
- grants a transaction the privilege to perform a read operation on the DB element
- exclusive lock:
- grants a transaction the privilege to perform a write operation (also read) on the DB element
- increment lock:
- grants a transaction the privilege to perform a increment operation on the DB element
- shared lock:
- Compatibility of the
increment operation:
- Compatibility of the
increment operation with
another increment operation:
- Consider the
outcome of
INC1(A, c1);
INC2(A, c2)
- Consider the
outcome of
INC2(A, c2);
INC1(A, c1)
- The outcome are
identical !!!
- Compatible !!!
Note:
- Although, the
increment operation
reads the
value of A,
it only uses it to
increment the value
- The increment operation does not "pass" to value that it read further !!!
- Consider the
outcome of
INC1(A, c1);
INC2(A, c2)
- Compatibility of the
increment operation with the
read operation:
- Consider the
outcome of
INC1(A, c1);
READ2(A)
- Consider the
outcome of
READ2(A);
INC1(A, c1)
- The outcome are
different !!!
- Not compatible !!!
- Consider the
outcome of
INC1(A, c1);
READ2(A)
- Compatibility of the
increment operation with the
write operation:
- Consider the
outcome of
INC1(A, c1);
WRITE2(A)
- Consider the
outcome of
WRITE2(A);
INC1(A, c1)
- The outcome are
different !!!
- Not compatible !!!
- Consider the
outcome of
INC1(A, c1);
WRITE2(A)
- Compatibility of the
increment operation with
another increment operation:
- Compatibility matrix for
shared/increment/exclusive locking:
- Example transactions:
- Possible
interleaved execution:
- This schedule is
serializable because
we can obtain a
serial schedule by
swapping
non-conflicting operations:
- Original schedule:
- Schedule with
only the
operations that
affect the DB consistency:
Re-written nicer:
- INC2(B, ...) and
INC1(B, ...)
commutes:
- READ2(A, ...) and
INC1(B, ...) are
compatible
because the operate of
different DB elements:
The resulting schedule is serial !!!
- Original schedule: