<s>
Component-based	B-Architecture
software	I-Architecture
engineering	I-Architecture
(	O
CBSE	O
)	O
,	O
also	O
called	O
component-based	B-Architecture
development	I-Architecture
(	O
CBD	O
)	O
,	O
is	O
a	O
branch	O
of	O
software	B-General_Concept
engineering	I-General_Concept
that	O
emphasizes	O
the	O
separation	O
of	O
concerns	O
with	O
respect	O
to	O
the	O
wide-ranging	O
functionality	O
available	O
throughout	O
a	O
given	O
software	O
system	B-General_Concept
.	O
</s>
<s>
Software	B-General_Concept
engineering	I-General_Concept
practitioners	O
regard	O
components	O
as	O
part	O
of	O
the	O
starting	O
platform	O
for	O
service-orientation	O
.	O
</s>
<s>
Components	O
can	O
produce	O
or	O
consume	O
events	O
and	O
can	O
be	O
used	O
for	O
event-driven	B-Application
architectures	I-Application
(	O
EDA	O
)	O
.	O
</s>
<s>
An	O
individual	O
software	O
component	O
is	O
a	O
software	B-Architecture
package	I-Architecture
,	O
a	O
web	O
service	O
,	O
a	O
web	O
resource	O
,	O
or	O
a	O
module	B-Architecture
that	O
encapsulates	O
a	O
set	O
of	O
related	O
functions	O
(	O
or	O
data	O
)	O
.	O
</s>
<s>
All	O
system	B-General_Concept
processes	O
are	O
placed	O
into	O
separate	O
components	O
so	O
that	O
all	O
of	O
the	O
data	O
and	O
functions	O
inside	O
each	O
component	O
are	O
semantically	O
related	O
(	O
just	O
as	O
with	O
the	O
contents	O
of	O
classes	O
)	O
.	O
</s>
<s>
Because	O
of	O
this	O
principle	O
,	O
it	O
is	O
often	O
said	O
that	O
components	O
are	O
modular	B-Architecture
and	O
cohesive	O
.	O
</s>
<s>
With	O
regard	O
to	O
system-wide	O
co-ordination	O
,	O
components	O
communicate	O
with	O
each	O
other	O
via	O
interfaces	B-Application
.	O
</s>
<s>
When	O
a	O
component	O
offers	O
services	O
to	O
the	O
rest	B-Protocol
of	O
the	O
system	B-General_Concept
,	O
it	O
adopts	O
a	O
provided	O
interface	O
that	O
specifies	O
the	O
services	O
that	O
other	O
components	O
can	O
utilize	O
,	O
and	O
how	O
they	O
can	O
do	O
so	O
.	O
</s>
<s>
The	O
UML	B-Language
illustrations	O
within	O
this	O
article	O
represent	O
provided	O
interfaces	B-Application
by	O
a	O
lollipop-symbol	O
attached	O
to	O
the	O
outer	O
edge	O
of	O
the	O
component	O
.	O
</s>
<s>
In	O
the	O
UML	B-Language
illustrations	O
in	O
this	O
article	O
,	O
used	O
interfaces	B-Application
are	O
represented	O
by	O
an	O
open	O
socket	O
symbol	O
attached	O
to	O
the	O
outer	O
edge	O
of	O
the	O
component	O
.	O
</s>
<s>
Another	O
important	O
attribute	O
of	O
components	O
is	O
that	O
they	O
are	O
substitutable	O
,	O
so	O
that	O
a	O
component	O
can	O
replace	O
another	O
(	O
at	O
design	O
time	O
or	O
run-time	O
)	O
,	O
if	O
the	O
successor	O
component	O
meets	O
the	O
requirements	O
of	O
the	O
initial	O
component	O
(	O
expressed	O
via	O
the	O
interfaces	B-Application
)	O
.	O
</s>
<s>
Consequently	O
,	O
components	O
can	O
be	O
replaced	O
with	O
either	O
an	O
updated	O
version	O
or	O
an	O
alternative	O
without	O
breaking	O
the	O
system	B-General_Concept
in	O
which	O
the	O
component	O
operates	O
.	O
</s>
<s>
As	O
a	O
rule	B-Application
of	I-Application
thumb	I-Application
for	O
engineers	O
substituting	O
components	O
,	O
component	O
B	O
can	O
immediately	O
replace	O
component	O
A	O
,	O
if	O
component	O
B	O
provides	O
at	O
least	O
what	O
component	O
A	O
provided	O
and	O
uses	O
no	O
more	O
than	O
what	O
component	O
A	O
used	O
.	O
</s>
<s>
Software	B-Architecture
components	I-Architecture
often	O
take	O
the	O
form	O
of	O
objects	O
(	O
not	O
classes	O
)	O
or	O
collections	O
of	O
objects	O
(	O
from	O
object-oriented	B-Language
programming	I-Language
)	O
,	O
in	O
some	O
binary	O
or	O
textual	O
form	O
,	O
adhering	O
to	O
some	O
interface	B-General_Concept
description	I-General_Concept
language	I-General_Concept
(	O
IDL	B-General_Concept
)	O
so	O
that	O
the	O
component	O
may	O
exist	O
autonomously	O
from	O
other	O
components	O
in	O
a	O
computer	O
.	O
</s>
<s>
When	O
a	O
component	O
is	O
to	O
be	O
accessed	O
or	O
shared	O
across	O
execution	O
contexts	O
or	O
network	O
links	O
,	O
techniques	O
such	O
as	O
serialization	B-Application
or	O
marshalling	B-Application
are	O
often	O
employed	O
to	O
deliver	O
the	O
component	O
to	O
its	O
destination	O
.	O
</s>
<s>
Reusability	B-Application
is	O
an	O
important	O
characteristic	O
of	O
a	O
high-quality	O
software	O
component	O
.	O
</s>
<s>
Programmers	O
should	O
design	O
and	O
implement	O
software	B-Architecture
components	I-Architecture
in	O
such	O
a	O
way	O
that	O
many	O
different	O
programs	O
can	O
reuse	O
them	O
.	O
</s>
<s>
Furthermore	O
,	O
component-based	B-Architecture
usability	O
testing	O
should	O
be	O
considered	O
when	O
software	B-Architecture
components	I-Architecture
directly	O
interact	O
with	O
users	O
.	O
</s>
<s>
Commercial	O
sites	O
routinely	O
created	O
application	O
programs	O
from	O
reusable	O
modules	B-Architecture
written	O
in	O
assembly	B-Language
language	I-Language
,	O
COBOL	B-Application
,	O
PL/1	B-Language
and	O
other	O
second	O
-	O
and	O
third-generation	O
languages	O
using	O
both	O
system	B-General_Concept
and	O
user	O
application	O
libraries	O
.	O
</s>
<s>
Component-based	B-Architecture
software	I-Architecture
engineering	I-Architecture
builds	O
on	O
prior	O
theories	O
of	O
software	O
objects	O
,	O
software	B-Architecture
architectures	I-Architecture
,	O
software	B-Architecture
frameworks	I-Architecture
and	O
software	O
design	O
patterns	O
,	O
and	O
the	O
extensive	O
theory	O
of	O
object-oriented	B-Language
programming	I-Language
and	O
the	O
object-oriented	B-Application
design	I-Application
of	O
all	O
these	O
.	O
</s>
<s>
It	O
claims	O
that	O
software	B-Architecture
components	I-Architecture
,	O
like	O
the	O
idea	O
of	O
hardware	O
components	O
,	O
used	O
for	O
example	O
in	O
telecommunications	O
,	O
can	O
ultimately	O
be	O
made	O
interchangeable	O
and	O
reliable	O
.	O
</s>
<s>
The	O
idea	O
that	O
software	O
should	O
be	O
componentized	O
-	O
built	O
from	O
prefabricated	O
components	O
-	O
first	O
became	O
prominent	O
with	O
Douglas	O
McIlroy	O
's	O
address	O
at	O
the	O
NATO	O
conference	O
on	O
software	B-General_Concept
engineering	I-General_Concept
in	O
Garmisch	O
,	O
Germany	O
,	O
1968	O
,	O
titled	O
Mass	O
Produced	O
Software	B-Architecture
Components	I-Architecture
.	O
</s>
<s>
The	O
conference	O
set	O
out	O
to	O
counter	O
the	O
so-called	O
software	B-Application
crisis	I-Application
.	O
</s>
<s>
McIlroy	O
's	O
subsequent	O
inclusion	O
of	O
pipes	B-Operating_System
and	I-Operating_System
filters	I-Operating_System
into	O
the	O
Unix	B-Application
operating	I-Application
system	I-Application
was	O
the	O
first	O
implementation	O
of	O
an	O
infrastructure	O
for	O
this	O
idea	O
.	O
</s>
<s>
He	O
called	O
them	O
Software	O
ICs	O
and	O
set	O
out	O
to	O
create	O
an	O
infrastructure	O
and	O
market	O
for	O
these	O
components	O
by	O
inventing	O
the	O
Objective-C	B-Language
programming	I-Language
language	I-Language
.	O
</s>
<s>
(	O
He	O
summarizes	O
this	O
view	O
in	O
his	O
book	O
Object-Oriented	B-Language
Programming	I-Language
-	O
An	O
Evolutionary	O
Approach	O
1986	O
.	O
)	O
</s>
<s>
The	O
software	B-Architecture
components	I-Architecture
are	O
used	O
in	O
two	O
different	O
contexts	O
and	O
two	O
kinds	O
:	O
i	O
)	O
using	O
components	O
as	O
parts	O
to	O
build	O
a	O
single	O
executable	O
,	O
or	O
ii	O
)	O
each	O
executable	O
is	O
treated	O
as	O
a	O
component	O
in	O
a	O
distributed	O
environment	O
,	O
where	O
components	O
collaborate	O
with	O
each	O
other	O
using	O
internet	O
or	O
intranet	O
communication	O
protocols	O
for	O
IPC	O
(	O
Inter	O
Process	O
Communications	O
)	O
.	O
</s>
<s>
IBM	O
led	O
the	O
path	O
with	O
their	O
System	B-Application
Object	I-Application
Model	I-Application
(	O
SOM	B-Application
)	O
in	O
the	O
early	O
1990s	O
.	O
</s>
<s>
As	O
a	O
reaction	O
,	O
Microsoft	O
paved	O
the	O
way	O
for	O
actual	O
deployment	O
of	O
component	B-Architecture
software	I-Architecture
with	O
Object	B-Operating_System
linking	I-Operating_System
and	I-Operating_System
embedding	I-Operating_System
(	O
OLE	O
)	O
and	O
Component	B-Application
Object	I-Application
Model	I-Application
(	O
COM	B-Application
)	O
.	O
</s>
<s>
A	O
computer	O
running	O
several	O
software	B-Architecture
components	I-Architecture
is	O
often	O
called	O
an	O
application	B-Application
server	I-Application
.	O
</s>
<s>
This	O
combination	O
of	O
application	B-Application
servers	I-Application
and	O
software	B-Architecture
components	I-Architecture
is	O
usually	O
called	O
distributed	B-Architecture
computing	I-Architecture
.	O
</s>
<s>
Examples	O
of	O
component	O
models	O
are	O
:	O
Enterprise	B-Language
JavaBeans	I-Language
(	O
EJB	B-Language
)	O
model	O
,	O
Component	B-Application
Object	I-Application
Model	I-Application
(	O
COM	B-Application
)	O
model	O
,	O
.NET	B-Application
model	O
,	O
X-MAN	O
component	O
model	O
,	O
and	O
Common	B-Architecture
Object	I-Architecture
Request	I-Architecture
Broker	I-Architecture
Architecture	I-Architecture
(	O
CORBA	B-Architecture
)	O
component	O
model	O
.	O
</s>
<s>
VCL	B-Library
and	O
CLX	B-Language
from	O
Borland	O
and	O
similar	O
free	O
LCL	B-Language
library	O
.	O
</s>
<s>
9P	B-Operating_System
distributed	O
protocol	O
developed	O
for	O
Plan	B-Operating_System
9	I-Operating_System
,	O
and	O
used	O
by	O
Inferno	B-Operating_System
and	O
other	O
systems	O
.	O
</s>
<s>
Part	O
of	O
the	O
Babel	B-Application
Scientific	O
Programming	O
Language	O
Interoperability	O
System	B-General_Concept
(	O
SIDL	O
and	O
Babel	B-Application
are	O
core	O
technologies	O
of	O
the	O
CCA	B-Library
and	O
the	O
SciDAC	O
TASCS	O
Center	O
-	O
see	O
above	O
.	O
)	O
</s>
