<s>
A	O
distributed	B-General_Concept
transaction	I-General_Concept
is	O
a	O
database	B-General_Concept
transaction	I-General_Concept
in	O
which	O
two	O
or	O
more	O
network	O
hosts	O
are	O
involved	O
.	O
</s>
<s>
Distributed	B-General_Concept
transactions	I-General_Concept
,	O
as	O
any	O
other	O
transactions	B-General_Concept
,	O
must	O
have	O
all	O
four	O
ACID	O
(	O
atomicity	O
,	O
consistency	O
,	O
isolation	B-General_Concept
,	O
durability	O
)	O
properties	O
,	O
where	O
atomicity	O
guarantees	O
all-or-nothing	O
outcomes	O
for	O
the	O
unit	B-General_Concept
of	I-General_Concept
work	I-General_Concept
(	O
operations	O
bundle	O
)	O
.	O
</s>
<s>
Open	O
Group	O
,	O
a	O
vendor	O
consortium	O
,	O
proposed	O
the	O
X/Open	O
Distributed	B-General_Concept
Transaction	I-General_Concept
Processing	I-General_Concept
(	O
DTP	B-General_Concept
)	O
Model	O
(	O
X/Open	O
XA	O
)	O
,	O
which	O
became	O
a	O
de	O
facto	O
standard	O
for	O
behavior	O
of	O
transaction	O
model	O
components	O
.	O
</s>
<s>
Databases	O
are	O
common	O
transactional	O
resources	O
and	O
,	O
often	O
,	O
transactions	B-General_Concept
span	O
a	O
couple	O
of	O
such	O
databases	O
.	O
</s>
<s>
In	O
this	O
case	O
,	O
a	O
distributed	B-General_Concept
transaction	I-General_Concept
can	O
be	O
seen	O
as	O
a	O
database	B-General_Concept
transaction	I-General_Concept
that	O
must	O
be	O
synchronized	O
(	O
or	O
provide	O
ACID	O
properties	O
)	O
among	O
multiple	O
participating	O
databases	O
which	O
are	O
distributed	B-Architecture
among	O
different	O
physical	O
locations	O
.	O
</s>
<s>
The	O
isolation	B-General_Concept
property	O
(	O
the	O
I	O
of	O
ACID	O
)	O
poses	O
a	O
special	O
challenge	O
for	O
multi	O
database	B-General_Concept
transactions	I-General_Concept
,	O
since	O
the	O
(	O
global	O
)	O
serializability	B-Operating_System
property	O
could	O
be	O
violated	O
,	O
even	O
if	O
each	O
database	O
provides	O
it	O
(	O
see	O
also	O
global	B-Operating_System
serializability	I-Operating_System
)	O
.	O
</s>
<s>
In	O
practice	O
most	O
commercial	O
database	O
systems	O
use	O
strong	O
strict	O
two	O
phase	O
locking	O
(	O
SS2PL	O
)	O
for	O
concurrency	B-Operating_System
control	I-Operating_System
,	O
which	O
ensures	O
global	B-Operating_System
serializability	I-Operating_System
,	O
if	O
all	O
the	O
participating	O
databases	O
employ	O
it	O
.	O
</s>
<s>
(	O
see	O
also	O
commitment	B-Architecture
ordering	I-Architecture
for	O
multidatabases	O
.	O
)	O
</s>
<s>
A	O
common	O
algorithm	O
for	O
ensuring	O
correct	O
completion	O
of	O
a	O
distributed	B-General_Concept
transaction	I-General_Concept
is	O
the	O
two-phase	B-General_Concept
commit	I-General_Concept
(	O
2PC	B-General_Concept
)	O
.	O
</s>
<s>
This	O
algorithm	O
is	O
usually	O
applied	O
for	O
updates	O
able	O
to	O
commit	B-Language
in	O
a	O
short	O
period	O
of	O
time	O
,	O
ranging	O
from	O
couple	O
of	O
milliseconds	O
to	O
couple	O
of	O
minutes	O
.	O
</s>
<s>
There	O
are	O
also	O
long-lived	O
distributed	B-General_Concept
transactions	I-General_Concept
,	O
for	O
example	O
a	O
transaction	O
to	O
book	O
a	O
trip	O
,	O
which	O
consists	O
of	O
booking	O
a	O
flight	O
,	O
a	O
rental	O
car	O
and	O
a	O
hotel	O
.	O
</s>
<s>
Since	O
booking	O
the	O
flight	O
might	O
take	O
up	O
to	O
a	O
day	O
to	O
get	O
a	O
confirmation	O
,	O
two-phase	B-General_Concept
commit	I-General_Concept
is	O
not	O
applicable	O
here	O
,	O
it	O
will	O
lock	O
the	O
resources	O
for	O
this	O
long	O
.	O
</s>
<s>
In	O
practice	O
,	O
long-lived	O
distributed	B-General_Concept
transactions	I-General_Concept
are	O
implemented	O
in	O
systems	O
based	O
on	O
Web	O
Services	O
.	O
</s>
<s>
Usually	O
these	O
transactions	B-General_Concept
utilize	O
principles	O
of	O
compensating	B-General_Concept
transactions	I-General_Concept
,	O
Optimism	O
and	O
Isolation	B-General_Concept
Without	O
Locking	O
.	O
</s>
<s>
X/Open	O
standard	O
does	O
not	O
cover	O
long-lived	O
DTP	B-General_Concept
.	O
</s>
<s>
Several	O
technologies	O
,	O
including	O
Enterprise	B-Language
Java	I-Language
Beans	I-Language
(	O
EJBs	B-Language
)	O
and	O
Microsoft	B-Operating_System
Transaction	I-Operating_System
Server	I-Operating_System
(	O
MTS	O
)	O
fully	O
support	O
distributed	B-General_Concept
transaction	I-General_Concept
standards	O
.	O
</s>
