<s>
A	O
model	B-Language
transformation	I-Language
,	O
in	O
model-driven	B-Language
engineering	I-Language
,	O
is	O
an	O
automated	O
way	O
of	O
modifying	O
and	O
creating	O
models	O
.	O
</s>
<s>
An	O
example	O
use	O
of	O
model	B-Language
transformation	I-Language
is	O
ensuring	O
that	O
a	O
family	O
of	O
models	O
is	O
consistent	O
,	O
in	O
a	O
precise	O
sense	O
which	O
the	O
software	O
engineer	O
can	O
define	O
.	O
</s>
<s>
The	O
aim	O
of	O
using	O
a	O
model	B-Language
transformation	I-Language
is	O
to	O
save	O
effort	O
and	O
reduce	O
errors	O
by	O
automating	O
the	O
building	O
and	O
modification	O
of	O
models	O
where	O
possible	O
.	O
</s>
<s>
Model	B-Language
transformations	I-Language
can	O
be	O
thought	O
of	O
as	O
programs	O
that	O
take	O
models	O
as	O
input	O
.	O
</s>
<s>
There	O
is	O
a	O
wide	O
variety	O
of	O
kinds	O
of	O
model	B-Language
transformation	I-Language
and	O
uses	O
of	O
them	O
,	O
which	O
differ	O
in	O
their	O
inputs	O
and	O
outputs	O
and	O
also	O
in	O
the	O
way	O
they	O
are	O
expressed	O
.	O
</s>
<s>
A	O
model	B-Language
transformation	I-Language
usually	O
specifies	O
which	O
models	O
are	O
acceptable	O
as	O
input	O
,	O
and	O
if	O
appropriate	O
what	O
models	O
it	O
may	O
produce	O
as	O
output	O
,	O
by	O
specifying	O
the	O
metamodel	O
to	O
which	O
a	O
model	B-Application
must	O
conform	O
.	O
</s>
<s>
Model	B-Language
transformations	I-Language
and	O
languages	O
for	O
them	O
have	O
been	O
classified	O
in	O
many	O
ways	O
.	O
</s>
<s>
In	O
principle	O
a	O
model	B-Language
transformation	I-Language
may	O
have	O
many	O
inputs	O
and	O
outputs	O
of	O
various	O
types	O
;	O
the	O
only	O
absolute	O
limitation	O
is	O
that	O
a	O
model	B-Language
transformation	I-Language
will	O
take	O
at	O
least	O
one	O
model	B-Application
as	O
input	O
.	O
</s>
<s>
However	O
,	O
a	O
model	B-Language
transformation	I-Language
that	O
did	O
not	O
produce	O
any	O
model	B-Application
as	O
output	O
would	O
more	O
commonly	O
be	O
called	O
a	O
model	B-Application
analysis	O
or	O
model	B-Application
query	O
.	O
</s>
<s>
For	O
example	O
,	O
in	O
a	O
process	O
conforming	O
to	O
the	O
OMG	B-Architecture
Model	B-Architecture
Driven	I-Architecture
Architecture	I-Architecture
,	O
a	O
platform-independent	B-Architecture
model	I-Architecture
might	O
be	O
transformed	O
into	O
a	O
platform-specific	B-Architecture
model	I-Architecture
by	O
an	O
exogenous	O
model	B-Language
transformation	I-Language
.	O
</s>
<s>
A	O
unidirectional	O
model	B-Language
transformation	I-Language
has	O
only	O
one	O
mode	O
of	O
execution	O
:	O
that	O
is	O
,	O
it	O
always	O
takes	O
the	O
same	O
type	O
of	O
input	O
and	O
produces	O
the	O
same	O
type	O
of	O
output	O
.	O
</s>
<s>
Unidirectional	O
model	B-Language
transformations	I-Language
are	O
useful	O
in	O
compilation-like	O
situations	O
,	O
where	O
any	O
output	O
model	B-Application
is	O
read-only	O
.	O
</s>
<s>
The	O
relevant	O
notion	O
of	O
consistency	O
is	O
then	O
very	O
simple	O
:	O
the	O
input	O
model	B-Application
is	O
consistent	O
with	O
the	O
model	B-Application
that	O
the	O
transformation	O
would	O
produce	O
as	O
output	O
,	O
only	O
.	O
</s>
<s>
For	O
a	O
bidirectional	O
model	B-Language
transformation	I-Language
,	O
the	O
same	O
type	O
of	O
model	B-Application
can	O
sometimes	O
be	O
input	O
and	O
other	O
times	O
be	O
output	O
.	O
</s>
<s>
Bidirectional	B-Algorithm
transformations	I-Algorithm
are	O
necessary	O
in	O
situations	O
where	O
people	O
are	O
working	O
on	O
more	O
than	O
one	O
model	B-Application
and	O
the	O
models	O
must	O
be	O
kept	O
consistent	O
.	O
</s>
<s>
Then	O
a	O
change	O
to	O
either	O
model	B-Application
might	O
necessitate	O
a	O
change	O
to	O
the	O
other	O
,	O
in	O
order	O
to	O
maintain	O
consistency	O
between	O
the	O
models	O
.	O
</s>
<s>
Because	O
each	O
model	B-Application
can	O
incorporate	O
information	O
which	O
is	O
not	O
reflected	O
in	O
the	O
other	O
,	O
there	O
may	O
be	O
many	O
models	O
which	O
are	O
consistent	O
with	O
a	O
given	O
model	B-Application
.	O
</s>
<s>
bijective	O
transformations	O
,	O
in	O
which	O
there	O
is	O
exactly	O
one	O
model	B-Application
which	O
is	O
consistent	O
with	O
any	O
given	O
model	B-Application
;	O
that	O
is	O
,	O
the	O
consistency	O
relation	O
is	O
bijective	O
.	O
</s>
<s>
view	O
transformations	O
,	O
in	O
which	O
a	O
concrete	O
model	B-Application
determines	O
a	O
single	O
view	O
model	B-Application
,	O
but	O
the	O
same	O
view	O
model	B-Application
might	O
be	O
produced	O
from	O
many	O
different	O
concrete	O
models	O
.	O
</s>
<s>
The	O
view	O
model	B-Application
is	O
an	O
abstraction	O
of	O
the	O
concrete	O
model	B-Application
.	O
</s>
<s>
If	O
the	O
view	O
may	O
be	O
updated	O
,	O
a	O
bidirectional	B-Algorithm
transformation	I-Algorithm
is	O
needed	O
.	O
</s>
<s>
This	O
situation	O
is	O
known	O
in	O
the	O
database	O
field	O
as	O
view-update	B-Language
.	O
</s>
<s>
Any	O
concrete	O
model	B-Application
is	O
consistent	O
with	O
its	O
view	O
.	O
</s>
<s>
It	O
is	O
particularly	O
important	O
that	O
a	O
bidirectional	O
model	B-Language
transformation	I-Language
has	O
appropriate	O
properties	O
to	O
make	O
it	O
behave	O
sensibly	O
:	O
for	O
example	O
,	O
not	O
making	O
changes	O
unnecessarily	O
,	O
or	O
discarding	O
deliberately	O
made	O
changes	O
.	O
</s>
<s>
A	O
model	B-Language
transformation	I-Language
may	O
be	O
written	O
in	O
a	O
general	O
purpose	O
programming	O
language	O
,	O
but	O
specialised	O
model	B-Language
transformation	I-Language
languages	I-Language
are	O
also	O
available	O
.	O
</s>
<s>
Bidirectional	B-Algorithm
transformations	I-Algorithm
,	O
in	O
particular	O
,	O
are	O
best	O
written	O
in	O
a	O
language	O
that	O
ensures	O
the	O
directions	O
are	O
appropriately	O
related	O
.	O
</s>
<s>
The	O
OMG-standardised	O
model	B-Language
transformation	I-Language
languages	I-Language
are	O
collectively	O
known	O
as	O
QVT	B-Language
.	O
</s>
<s>
In	O
some	O
model	B-Language
transformation	I-Language
languages	I-Language
,	O
for	O
example	O
the	O
QVT	B-Language
languages	O
,	O
a	O
model	B-Language
transformation	I-Language
is	O
itself	O
a	O
model	B-Application
,	O
that	O
is	O
,	O
it	O
conforms	O
to	O
a	O
metamodel	O
which	O
is	O
part	O
of	O
the	O
model	B-Language
transformation	I-Language
language	I-Language
's	O
definition	O
.	O
</s>
