<s>
In	O
the	O
fields	O
of	O
databases	O
and	O
transaction	B-General_Concept
processing	I-General_Concept
(	O
transaction	O
management	O
)	O
,	O
a	O
schedule	O
(	O
or	O
history	O
)	O
of	O
a	O
system	O
is	O
an	O
abstract	O
model	O
to	O
describe	O
execution	B-General_Concept
of	O
transactions	B-General_Concept
running	O
in	O
the	O
system	O
.	O
</s>
<s>
Often	O
it	O
is	O
a	O
list	O
of	O
operations	O
(	O
actions	O
)	O
ordered	O
by	O
time	O
,	O
performed	O
by	O
a	O
set	O
of	O
transactions	B-General_Concept
that	O
are	O
executed	O
together	O
in	O
the	O
system	O
.	O
</s>
<s>
Examples	O
of	O
such	O
operations	O
are	O
requesting	O
a	O
read	O
operation	O
,	O
reading	O
,	O
writing	O
,	O
aborting	O
,	O
committing	B-Language
,	O
requesting	O
a	O
lock	B-Operating_System
,	O
locking	B-Operating_System
,	O
etc	O
.	O
</s>
<s>
Schedules	O
and	O
schedule	O
properties	O
are	O
fundamental	O
concepts	O
in	O
database	O
concurrency	B-Operating_System
control	I-Operating_System
theory	O
.	O
</s>
<s>
In	O
this	O
example	O
,	O
the	O
horizontal	O
axis	O
represents	O
the	O
different	O
transactions	B-General_Concept
in	O
the	O
schedule	O
D	O
.	O
The	O
vertical	O
axis	O
represents	O
time	O
order	O
of	O
operations	O
.	O
</s>
<s>
Schedule	O
D	O
consists	O
of	O
three	O
transactions	B-General_Concept
T1	O
,	O
T2	O
,	O
T3	O
.	O
</s>
<s>
The	O
schedule	O
describes	O
the	O
actions	O
of	O
the	O
transactions	B-General_Concept
as	O
seen	O
by	O
the	O
DBMS	B-General_Concept
.	O
</s>
<s>
This	O
is	O
an	O
example	O
of	O
a	O
serial	O
schedule	O
,	O
i.e.	O
,	O
sequential	O
with	O
no	O
overlap	O
in	O
time	O
because	O
the	O
actions	O
of	O
all	O
three	O
transactions	B-General_Concept
are	O
sequential	O
,	O
and	O
the	O
transactions	B-General_Concept
are	O
not	O
interleaved	O
in	O
time	O
.	O
</s>
<s>
Usually	O
,	O
for	O
the	O
purpose	O
of	O
reasoning	O
about	O
concurrency	B-Operating_System
control	I-Operating_System
in	O
databases	O
,	O
an	O
operation	O
is	O
modelled	O
as	O
atomic	B-General_Concept
,	O
occurring	O
at	O
a	O
point	O
in	O
time	O
,	O
without	O
duration	O
.	O
</s>
<s>
Real	O
executed	O
operations	O
always	O
have	O
some	O
duration	O
and	O
specified	O
respective	O
times	O
of	O
occurrence	O
of	O
events	O
within	O
them	O
(	O
e.g.	O
,	O
"	O
exact	O
"	O
times	O
of	O
beginning	O
and	O
completion	O
)	O
,	O
but	O
for	O
concurrency	B-Operating_System
control	I-Operating_System
reasoning	O
usually	O
only	O
the	O
precedence	O
in	O
time	O
of	O
the	O
whole	O
operations	O
(	O
without	O
looking	O
into	O
the	O
quite	O
complex	O
details	O
of	O
each	O
operation	O
)	O
matters	O
,	O
i.e.	O
,	O
which	O
operation	O
is	O
before	O
,	O
or	O
after	O
another	O
operation	O
.	O
</s>
<s>
In	O
general	O
,	O
operations	O
of	O
transactions	B-General_Concept
in	O
a	O
schedule	O
can	O
interleave	O
(	O
i.e.	O
,	O
transactions	B-General_Concept
can	O
be	O
executed	O
concurrently	O
)	O
,	O
while	O
time	O
orders	O
between	O
operations	O
in	O
each	O
transaction	O
remain	O
unchanged	O
as	O
implied	O
by	O
the	O
transaction	O
's	O
program	O
.	O
</s>
<s>
Since	O
not	O
always	O
time	O
orders	O
between	O
all	O
operations	O
of	O
all	O
transactions	B-General_Concept
matter	O
and	O
need	O
to	O
be	O
specified	O
,	O
a	O
schedule	O
is	O
,	O
in	O
general	O
,	O
a	O
partial	O
order	O
between	O
operations	O
rather	O
than	O
a	O
total	O
order	O
(	O
where	O
order	O
for	O
each	O
pair	O
is	O
determined	O
,	O
as	O
in	O
a	O
list	O
of	O
operations	O
)	O
.	O
</s>
<s>
Thus	O
,	O
in	O
general	O
,	O
a	O
schedule	O
is	O
a	O
partial	O
order	O
of	O
operations	O
,	O
containing	O
(	O
embedding	O
)	O
the	O
partial	O
orders	O
of	O
all	O
its	O
transactions	B-General_Concept
.	O
</s>
<s>
A	O
schedule	O
that	O
is	O
equivalent	O
(	O
in	O
its	O
outcome	O
)	O
to	O
a	O
serial	O
schedule	O
has	O
the	O
serializability	B-Operating_System
property	O
.	O
</s>
<s>
In	O
schedule	O
E	O
,	O
the	O
order	O
in	O
which	O
the	O
actions	O
of	O
the	O
transactions	B-General_Concept
are	O
executed	O
is	O
not	O
the	O
same	O
as	O
in	O
D	O
,	O
but	O
in	O
the	O
end	O
,	O
E	O
gives	O
the	O
same	O
result	O
as	O
D	O
.	O
</s>
<s>
The	O
actions	O
belong	O
to	O
different	O
transactions	B-General_Concept
.	O
</s>
<s>
Both	O
schedules	O
S1	O
and	O
S2	O
involve	O
the	O
same	O
set	O
of	O
transactions	B-General_Concept
(	O
including	O
the	O
ordering	O
of	O
actions	O
within	O
each	O
transaction	O
)	O
.	O
</s>
<s>
A	O
schedule	O
is	O
said	O
to	O
be	O
conflict-serializable	O
when	O
the	O
schedule	O
is	O
conflict-equivalent	O
to	O
one	O
or	O
more	O
serial	O
schedules	O
.	O
</s>
<s>
Another	O
definition	O
for	O
conflict-serializability	O
is	O
that	O
a	O
schedule	O
is	O
conflict-serializable	O
if	O
and	O
only	O
if	O
its	O
precedence	O
graph/serializability	O
graph	O
,	O
when	O
only	O
committed	O
transactions	B-General_Concept
are	O
considered	O
,	O
is	O
acyclic	O
(	O
if	O
the	O
graph	O
is	O
defined	O
to	O
include	O
also	O
uncommitted	O
transactions	B-General_Concept
,	O
then	O
cycles	O
involving	O
uncommitted	O
transactions	B-General_Concept
may	O
occur	O
without	O
conflict	O
serializability	B-Operating_System
violation	O
)	O
.	O
</s>
<s>
A	O
schedule	O
is	O
said	O
to	O
be	O
commitment-ordered	O
(	O
commit-ordered	O
)	O
,	O
or	O
commitment-order-serializable	O
,	O
if	O
it	O
obeys	O
the	O
Commitment	B-Architecture
ordering	I-Architecture
(	O
CO	O
;	O
also	O
commit-ordering	O
or	O
commit-order-serializability	O
)	O
schedule	O
property	O
.	O
</s>
<s>
This	O
means	O
that	O
the	O
order	O
in	O
time	O
of	O
transactions	B-General_Concept
 '	O
commitment	O
events	O
is	O
compatible	O
with	O
the	O
precedence	O
(	O
partial	O
)	O
order	O
of	O
the	O
respective	O
transactions	B-General_Concept
,	O
as	O
induced	O
by	O
their	O
schedule	O
's	O
acyclic	O
precedence	B-General_Concept
graph	I-General_Concept
(	O
serializability	B-Operating_System
graph	O
,	O
conflict	O
graph	O
)	O
.	O
</s>
<s>
This	O
implies	O
that	O
it	O
is	O
also	O
conflict-serializable	O
.	O
</s>
<s>
The	O
CO	O
property	O
is	O
especially	O
effective	O
for	O
achieving	O
Global	B-Operating_System
serializability	I-Operating_System
in	O
distributed	O
systems	O
.	O
</s>
<s>
Comment	O
:	O
Commitment	B-Architecture
ordering	I-Architecture
,	O
which	O
was	O
discovered	O
in	O
1990	O
,	O
is	O
obviously	O
not	O
mentioned	O
in	O
(	O
Bernstein	O
et	O
al	O
.	O
</s>
<s>
For	O
an	O
extensive	O
coverage	O
of	O
commitment	B-Architecture
ordering	I-Architecture
and	O
its	O
sources	O
see	O
Commitment	B-Architecture
ordering	I-Architecture
and	O
The	O
History	O
of	O
Commitment	B-Architecture
Ordering	I-Architecture
.	O
</s>
<s>
A	O
schedule	O
is	O
said	O
to	O
be	O
view-serializable	O
if	O
it	O
is	O
view-equivalent	O
to	O
some	O
serial	O
schedule	O
.	O
</s>
<s>
Note	O
that	O
by	O
definition	O
,	O
all	O
conflict-serializable	O
schedules	O
are	O
view-serializable	O
.	O
</s>
<s>
Notice	O
that	O
the	O
above	O
example	O
(	O
which	O
is	O
the	O
same	O
as	O
the	O
example	O
in	O
the	O
discussion	O
of	O
conflict-serializable	O
)	O
is	O
both	O
view-serializable	O
and	O
conflict-serializable	O
at	O
the	O
same	O
time	O
.	O
</s>
<s>
There	O
are	O
however	O
view-serializable	O
schedules	O
that	O
are	O
not	O
conflict-serializable	O
:	O
those	O
schedules	O
with	O
a	O
transaction	O
performing	O
a	O
blind	B-General_Concept
write	I-General_Concept
:	O
</s>
<s>
The	O
above	O
example	O
is	O
not	O
conflict-serializable	O
,	O
but	O
it	O
is	O
view-serializable	O
since	O
it	O
has	O
a	O
view-equivalent	O
serial	O
schedule	O
<T1,|T2,|T3>	O
.	O
</s>
<s>
Since	O
determining	O
whether	O
a	O
schedule	O
is	O
view-serializable	O
is	O
NP-complete	O
,	O
view-serializability	O
has	O
little	O
practical	O
interest	O
.	O
</s>
<s>
Transactions	B-General_Concept
commit	O
only	O
after	O
all	O
transactions	B-General_Concept
whose	O
changes	O
they	O
read	O
,	O
commit	O
.	O
</s>
<s>
Note	O
that	O
this	O
Schedule	O
would	O
not	O
be	O
serializable	B-Operating_System
if	O
T1	O
would	O
be	O
committed	O
.	O
</s>
<s>
A	O
schedule	O
is	O
strict	O
-	O
has	O
the	O
strictness	O
property	O
-	O
if	O
for	O
any	O
two	O
transactions	B-General_Concept
T1	O
,	O
T2	O
,	O
if	O
a	O
write	O
operation	O
of	O
T1	O
precedes	O
a	O
conflicting	O
operation	O
of	O
T2	O
(	O
either	O
read	O
or	O
write	O
)	O
,	O
then	O
the	O
commit	O
or	O
abort	O
event	O
of	O
T1	O
also	O
precedes	O
that	O
conflicting	O
operation	O
of	O
T2	O
.	O
</s>
<s>
The	O
following	O
expressions	O
illustrate	O
the	O
hierarchical	O
(	O
containment	O
)	O
relationships	O
between	O
serializability	B-Operating_System
and	O
recoverability	O
classes	O
:	O
</s>
<s>
The	O
Venn	B-Application
diagram	I-Application
(	O
below	O
)	O
illustrates	O
the	O
above	O
clauses	O
graphically	O
.	O
</s>
<s>
In	O
practice	O
,	O
most	O
general	O
purpose	O
database	O
systems	O
employ	O
conflict-serializable	O
and	O
recoverable	O
(	O
primarily	O
strict	O
)	O
schedules	O
.	O
</s>
