<s>
Extensibility	B-Architecture
is	O
a	O
software	B-General_Concept
engineering	I-General_Concept
and	O
systems	O
design	O
principle	O
that	O
provides	O
for	O
future	O
growth	O
.	O
</s>
<s>
Extensibility	B-Architecture
is	O
a	O
measure	O
of	O
the	O
ability	O
to	O
extend	O
a	O
system	O
and	O
the	O
level	O
of	O
effort	O
required	O
to	O
implement	O
the	O
extension	O
.	O
</s>
<s>
An	O
extensible	B-Architecture
system	O
is	O
one	O
whose	O
internal	O
structure	O
and	O
dataflow	B-Application
are	O
minimally	O
or	O
not	O
affected	O
by	O
new	O
or	O
modified	O
functionality	O
,	O
for	O
example	O
recompiling	O
or	O
changing	O
the	O
original	O
source	O
code	O
might	O
be	O
unnecessary	O
when	O
changing	O
a	O
system	O
’s	O
behavior	O
,	O
either	O
by	O
the	O
creator	O
or	O
other	O
programmers	O
.	O
</s>
<s>
Because	O
software	O
systems	O
are	O
long	O
lived	O
and	O
will	O
be	O
modified	O
for	O
new	O
features	O
and	O
added	O
functionalities	O
demanded	O
by	O
users	O
,	O
extensibility	B-Architecture
enables	O
developers	O
to	O
expand	O
or	O
add	O
to	O
the	O
software	O
’s	O
capabilities	O
and	O
facilitates	O
systematic	O
reuse	O
.	O
</s>
<s>
Extensible	B-Architecture
design	O
in	O
software	B-General_Concept
engineering	I-General_Concept
is	O
to	O
accept	O
that	O
not	O
everything	O
can	O
be	O
designed	O
in	O
advance	O
.	O
</s>
<s>
A	O
light	O
software	B-Architecture
framework	I-Architecture
which	O
allows	O
for	O
changes	O
is	O
provided	O
instead	O
.	O
</s>
<s>
Small	O
commands	O
are	O
made	O
to	O
prevent	O
losing	O
the	O
element	O
of	O
extensibility	B-Architecture
,	O
following	O
the	O
principle	O
of	O
separating	O
work	O
elements	O
into	O
comprehensible	O
units	O
,	O
in	O
order	O
to	O
avoid	O
traditional	O
software	O
development	O
issues	O
including	O
low	O
cohesion	O
and	O
high	O
coupling	B-Application
and	O
allow	O
for	O
continued	O
development	O
.	O
</s>
<s>
Embracing	O
change	O
is	O
essential	O
to	O
the	O
extensible	B-Architecture
design	O
,	O
in	O
which	O
additions	O
will	O
be	O
continual	O
.	O
</s>
<s>
Extensible	B-Architecture
design	O
supports	O
frequent	O
re-prioritization	O
and	O
allows	O
functionality	O
to	O
be	O
implemented	O
in	O
small	O
steps	O
upon	O
request	O
,	O
which	O
are	O
the	O
principles	O
advocated	O
by	O
the	O
Agile	O
methodologies	O
and	O
iterative	O
development	O
.	O
</s>
<s>
Extensibility	B-Architecture
imposes	O
fewer	O
and	O
cleaner	O
dependencies	B-Application
during	O
development	O
,	O
as	O
well	O
as	O
reduced	O
coupling	B-Application
and	O
more	O
cohesive	O
abstractions	O
,	O
plus	O
well	O
defined	O
interfaces	B-Application
.	O
</s>
<s>
Many	O
software	O
systems	O
and	O
software	O
product-lines	O
are	O
derived	O
from	O
a	O
base	O
system	O
,	O
which	O
share	O
a	O
common	O
software	B-Architecture
architecture	I-Architecture
or	O
sometimes	O
large	O
parts	O
of	O
the	O
functionality	O
and	O
implementation	O
but	O
are	O
possibly	O
equipped	O
with	O
different	O
components	O
that	O
require	O
an	O
extensible	B-Architecture
base	O
system	O
.	O
</s>
<s>
Building	O
software	O
systems	O
that	O
are	O
independently	O
extensible	B-Architecture
is	O
an	O
important	O
challenge	O
.	O
</s>
<s>
An	O
independently	O
extensible	B-Architecture
system	O
not	O
only	O
allows	O
two	O
people	O
to	O
independently	O
develop	O
extensions	O
to	O
the	O
system	O
,	O
but	O
also	O
allows	O
the	O
two	O
extensions	O
to	O
be	O
combined	O
without	O
a	O
global	O
integrity	O
check	O
.	O
</s>
<s>
There	O
are	O
three	O
different	O
forms	O
of	O
software	O
extensibility	B-Architecture
:	O
white-box	O
extensibility	B-Architecture
,	O
gray-box	O
extensibility	B-Architecture
,	O
and	O
black-box	B-Device
extensibility	B-Architecture
,	O
which	O
are	O
based	O
on	O
what	O
artifacts	O
and	O
the	O
way	O
they	O
are	O
changed	O
.	O
</s>
<s>
Under	O
this	O
form	O
of	O
extensibility	B-Architecture
,	O
a	O
software	O
system	O
can	O
be	O
extended	O
by	O
modifying	O
the	O
source	O
code	O
,	O
and	O
it	O
is	O
the	O
most	O
flexible	O
and	O
the	O
least	O
restrictive	O
form	O
.	O
</s>
<s>
There	O
are	O
two	O
sub-forms	O
of	O
extensibility	B-Architecture
,	O
open-box	O
extensibility	B-Architecture
and	O
glass-box	O
extensibility	B-Architecture
,	O
depending	O
on	O
how	O
changes	O
are	O
applied	O
.	O
</s>
<s>
Changes	O
are	O
performed	O
invasively	O
in	O
open-box	O
extensible	B-Architecture
systems	O
;	O
i.e.	O
</s>
<s>
Open-box	O
extensibility	B-Architecture
is	O
most	O
relevant	O
to	O
bug	O
fixing	O
,	O
internal	O
code	O
refactoring	O
,	O
or	O
production	O
of	O
next	O
version	O
of	O
a	O
software	O
product	O
.	O
</s>
<s>
Glass-box	O
extensibility	B-Architecture
(	O
also	O
called	O
architecture	O
driven	O
frameworks	O
)	O
allows	O
a	O
software	O
system	O
to	O
be	O
extended	O
with	O
available	O
source	O
code	O
,	O
but	O
may	O
not	O
allow	O
the	O
code	O
to	O
be	O
modified	O
.	O
</s>
<s>
One	O
example	O
of	O
this	O
form	O
of	O
extensibility	B-Architecture
is	O
object-oriented	O
application	O
frameworks	O
which	O
achieve	O
extensibility	B-Architecture
typically	O
by	O
using	O
inheritance	O
and	O
dynamic	O
binding	O
.	O
</s>
<s>
In	O
black-box	B-Device
extensibility	B-Architecture
(	O
also	O
called	O
data-driven	O
frameworks	O
)	O
no	O
details	O
about	O
a	O
system	O
’s	O
implementation	O
are	O
used	O
for	O
implementing	O
deployments	O
or	O
extensions	O
;	O
only	O
interface	B-Application
specifications	I-Application
are	O
provided	O
.	O
</s>
<s>
Black-box	B-Device
extensions	O
are	O
typically	O
achieved	O
through	O
system	O
configuration	O
applications	O
or	O
the	O
use	O
of	O
application-specific	O
scripting	B-Language
languages	I-Language
by	O
defining	O
components	O
interfaces	B-Application
.	O
</s>
<s>
Gray-box	O
extensibility	B-Architecture
is	O
a	O
compromise	O
between	O
a	O
pure	O
white-box	O
and	O
a	O
pure	O
black-box	B-Device
approach	O
,	O
which	O
does	O
not	O
rely	O
fully	O
on	O
the	O
exposure	O
of	O
source	O
code	O
.	O
</s>
<s>
Extensibility	B-Architecture
and	O
reusability	O
have	O
many	O
emphasized	O
properties	O
in	O
common	O
,	O
including	O
low	O
coupling	B-Application
,	O
modularity	O
and	O
high	O
risk	O
elements’	O
ability	O
to	O
construct	O
for	O
many	O
different	O
software	O
systems	O
,	O
which	O
is	O
motivated	O
by	O
the	O
observation	O
of	O
software	O
systems	O
often	O
sharing	O
common	O
elements	O
.	O
</s>
<s>
Reusability	O
together	O
with	O
extensibility	B-Architecture
allows	O
a	O
technology	O
to	O
be	O
transferred	O
to	O
another	O
project	O
with	O
less	O
development	O
and	O
maintenance	O
time	O
,	O
as	O
well	O
as	O
enhanced	O
reliability	O
and	O
consistency	O
.	O
</s>
<s>
Modern	O
operating	O
systems	O
support	O
extensibility	B-Architecture
through	O
device	B-Application
drivers	I-Application
and	O
loadable	B-Application
kernel	I-Application
modules	I-Application
.	O
</s>
<s>
Many	O
modern	O
applications	O
support	O
extensibility	B-Architecture
through	O
plug-ins	B-Application
,	O
extension	B-Language
languages	I-Language
,	O
applets	B-Language
,	O
etc	O
.	O
</s>
<s>
The	O
trend	O
of	O
increasing	O
extensibility	B-Architecture
negatively	O
affects	O
software	O
security	O
.	O
</s>
<s>
CGI	O
is	O
one	O
of	O
the	O
primary	O
means	O
by	O
which	O
web	O
servers	O
provide	O
extensibility	B-Architecture
.	O
</s>
