<s>
In	O
software	B-General_Concept
engineering	I-General_Concept
,	O
multitier	B-Operating_System
architecture	I-Operating_System
(	O
often	O
referred	O
to	O
as	O
n-tier	B-Operating_System
architecture	I-Operating_System
)	O
is	O
a	O
client	B-Operating_System
–	I-Operating_System
server	I-Operating_System
architecture	I-Operating_System
in	O
which	O
presentation	O
,	O
application	O
processing	O
and	O
data	B-Application
management	O
functions	O
are	O
physically	O
separated	O
.	O
</s>
<s>
The	O
most	O
widespread	O
use	O
of	O
multitier	B-Operating_System
architecture	I-Operating_System
is	O
the	O
three-tier	B-Operating_System
architecture	I-Operating_System
.	O
</s>
<s>
N-tier	B-Operating_System
application	O
architecture	O
provides	O
a	O
model	O
by	O
which	O
developers	O
can	O
create	O
flexible	O
and	O
reusable	O
applications	O
.	O
</s>
<s>
By	O
segregating	O
an	O
application	O
into	O
tiers	B-Operating_System
,	O
developers	O
acquire	O
the	O
option	O
of	O
modifying	O
or	O
adding	O
a	O
specific	O
tier	B-Operating_System
,	O
instead	O
of	O
reworking	O
the	O
entire	O
application	O
.	O
</s>
<s>
A	O
three-tier	B-Operating_System
architecture	I-Operating_System
is	O
typically	O
composed	O
of	O
a	O
presentation	O
tier	B-Operating_System
,	O
a	O
logic	B-Architecture
tier	B-Operating_System
,	O
and	O
a	O
data	B-Application
tier	B-Operating_System
.	O
</s>
<s>
While	O
the	O
concepts	O
of	O
layer	O
and	O
tier	B-Operating_System
are	O
often	O
used	O
interchangeably	O
,	O
one	O
fairly	O
common	O
point	O
of	O
view	O
is	O
that	O
there	O
is	O
indeed	O
a	O
difference	O
.	O
</s>
<s>
This	O
view	O
holds	O
that	O
a	O
layer	O
is	O
a	O
logical	O
structuring	O
mechanism	O
for	O
the	O
conceptual	O
elements	O
that	O
make	O
up	O
the	O
software	O
solution	O
,	O
while	O
a	O
tier	B-Operating_System
is	O
a	O
physical	O
structuring	O
mechanism	O
for	O
the	O
hardware	O
elements	O
that	O
make	O
up	O
the	O
system	O
infrastructure	O
.	O
</s>
<s>
For	O
example	O
,	O
a	O
three-layer	B-Operating_System
solution	O
could	O
easily	O
be	O
deployed	O
on	O
a	O
single	O
tier	B-Operating_System
,	O
such	O
in	O
the	O
case	O
of	O
an	O
extreme	O
database-centric	B-Operating_System
architecture	I-Operating_System
called	O
RDBMS-only	O
architecture	O
or	O
in	O
a	O
personal	B-Device
workstation	I-Device
.	O
</s>
<s>
In	O
a	O
logical	O
multilayer	O
architecture	O
for	O
an	O
information	O
system	O
with	O
an	O
object-oriented	B-Application
design	I-Application
,	O
the	O
following	O
four	O
are	O
the	O
most	O
common	O
:	O
</s>
<s>
Business	B-Architecture
layer	I-Architecture
(	O
a.k.a.	O
</s>
<s>
Data	B-Application
access	I-Application
layer	I-Application
(	O
a.k.a.	O
</s>
<s>
If	O
the	O
application	O
architecture	O
has	O
no	O
explicit	O
distinction	O
between	O
the	O
business	B-Architecture
layer	I-Architecture
and	O
the	O
presentation	O
layer	O
(	O
i.e.	O
,	O
the	O
presentation	O
layer	O
is	O
considered	O
part	O
of	O
the	O
business	B-Architecture
layer	I-Architecture
)	O
,	O
then	O
a	O
traditional	O
client-server	B-Operating_System
(	O
two-tier	O
)	O
model	O
has	O
been	O
implemented	O
.	O
</s>
<s>
The	O
more	O
usual	O
convention	O
is	O
that	O
the	O
application	O
layer	O
(	O
or	O
service	B-Protocol
layer	I-Protocol
)	O
is	O
considered	O
a	O
sublayer	O
of	O
the	O
business	B-Architecture
layer	I-Architecture
,	O
typically	O
encapsulating	O
the	O
API	B-Application
definition	O
surfacing	O
the	O
supported	O
business	O
functionality	O
.	O
</s>
<s>
For	O
example	O
,	O
if	O
the	O
model	O
–	O
view	O
–	O
presenter	O
pattern	O
is	O
used	O
,	O
the	O
presenter	O
sublayer	O
might	O
be	O
used	O
as	O
an	O
additional	O
layer	O
between	O
the	O
user	B-Application
interface	I-Application
layer	O
and	O
the	O
business/application	O
layer	O
(	O
as	O
represented	O
by	O
the	O
model	O
sublayer	O
)	O
.	O
</s>
<s>
It	O
's	O
also	O
sometimes	O
called	O
the	O
"	O
low-level	O
business	B-Architecture
layer	I-Architecture
"	O
or	O
the	O
"	O
business	O
services	O
layer	O
"	O
.	O
</s>
<s>
This	O
layer	O
is	O
very	O
general	O
and	O
can	O
be	O
used	O
in	O
several	O
application	O
tiers	B-Operating_System
(	O
e.g.	O
</s>
<s>
Developers	O
often	O
focus	O
on	O
the	O
persistence	O
(	O
data	B-General_Concept
access	I-General_Concept
)	O
capabilities	O
of	O
the	O
infrastructure	O
layer	O
and	O
therefore	O
only	O
talk	O
about	O
the	O
persistence	O
layer	O
or	O
the	O
data	B-Application
access	I-Application
layer	I-Application
(	O
instead	O
of	O
an	O
infrastructure	O
layer	O
or	O
technical	O
services	O
layer	O
)	O
.	O
</s>
<s>
Three-tier	B-Operating_System
architecture	I-Operating_System
is	O
a	O
client-server	B-Operating_System
software	O
architecture	O
pattern	O
in	O
which	O
the	O
user	B-Application
interface	I-Application
(	O
presentation	O
)	O
,	O
functional	B-Architecture
process	I-Architecture
logic	I-Architecture
(	O
"	O
business	O
rules	O
"	O
)	O
,	O
computer	B-General_Concept
data	I-General_Concept
storage	I-General_Concept
and	O
data	B-General_Concept
access	I-General_Concept
are	O
developed	O
and	O
maintained	O
as	O
independent	O
modules	B-Architecture
,	O
most	O
often	O
on	O
separate	O
platforms	B-Device
.	O
</s>
<s>
Apart	O
from	O
the	O
usual	O
advantages	O
of	O
modular	B-Architecture
software	I-Architecture
with	O
well-defined	O
interfaces	O
,	O
the	O
three-tier	B-Operating_System
architecture	I-Operating_System
is	O
intended	O
to	O
allow	O
any	O
of	O
the	O
three	B-Operating_System
tiers	I-Operating_System
to	O
be	O
upgraded	O
or	O
replaced	O
independently	O
in	O
response	O
to	O
changes	O
in	O
requirements	O
or	O
technology	O
.	O
</s>
<s>
For	O
example	O
,	O
a	O
change	O
of	O
operating	B-General_Concept
system	I-General_Concept
in	O
the	O
presentation	O
tier	B-Operating_System
would	O
only	O
affect	O
the	O
user	B-Application
interface	I-Application
code	O
.	O
</s>
<s>
Typically	O
,	O
the	O
user	B-Application
interface	I-Application
runs	O
on	O
a	O
desktop	O
PC	B-Device
or	O
workstation	B-Device
and	O
uses	O
a	O
standard	O
graphical	B-Application
user	I-Application
interface	I-Application
,	O
functional	B-Architecture
process	I-Architecture
logic	I-Architecture
that	O
may	O
consist	O
of	O
one	O
or	O
more	O
separate	O
modules	B-Architecture
running	O
on	O
a	O
workstation	B-Device
or	O
application	B-Application
server	I-Application
,	O
and	O
an	O
RDBMS	O
on	O
a	O
database	B-Application
server	I-Application
or	O
mainframe	B-Architecture
that	O
contains	O
the	O
computer	B-General_Concept
data	I-General_Concept
storage	I-General_Concept
logic	B-Architecture
.	O
</s>
<s>
The	O
middle	B-Operating_System
tier	I-Operating_System
may	O
be	O
multitiered	O
itself	O
(	O
in	O
which	O
case	O
the	O
overall	O
architecture	O
is	O
called	O
an	O
"	O
n-tier	B-Operating_System
architecture	I-Operating_System
"	O
)	O
.	O
</s>
<s>
The	O
presentation	O
tier	B-Operating_System
displays	O
information	O
related	O
to	O
such	O
services	O
as	O
browsing	O
merchandise	O
,	O
purchasing	O
and	O
shopping	O
cart	O
contents	O
.	O
</s>
<s>
It	O
communicates	O
with	O
other	O
tiers	B-Operating_System
by	O
which	O
it	O
puts	O
out	O
the	O
results	O
to	O
the	O
browser/client	O
tier	B-Operating_System
and	O
all	O
other	O
tiers	B-Operating_System
in	O
the	O
network	O
.	O
</s>
<s>
In	O
simple	O
terms	O
,	O
it	O
is	O
a	O
layer	O
that	O
users	O
can	O
access	O
directly	O
(	O
such	O
as	O
a	O
web	O
page	O
,	O
or	O
an	O
operating	B-General_Concept
system	I-General_Concept
's	O
GUI	B-Application
)	O
.	O
</s>
<s>
The	O
logical	O
tier	B-Operating_System
is	O
pulled	O
out	O
from	O
the	O
presentation	O
tier	B-Operating_System
and	O
,	O
as	O
its	O
layer	O
,	O
it	O
controls	O
an	O
application	O
’s	O
functionality	O
by	O
performing	O
detailed	O
processing	O
.	O
</s>
<s>
The	O
data	B-Application
tier	B-Operating_System
includes	O
the	O
data	B-Application
persistence	O
mechanisms	O
(	O
database	B-Application
servers	I-Application
,	O
file	O
shares	O
,	O
etc	O
.	O
)	O
</s>
<s>
and	O
the	O
data	B-Application
access	I-Application
layer	I-Application
that	O
encapsulates	O
the	O
persistence	O
mechanisms	O
and	O
exposes	O
the	O
data	B-Application
.	O
</s>
<s>
The	O
data	B-Application
access	I-Application
layer	I-Application
should	O
provide	O
an	O
API	B-Application
to	O
the	O
application	O
tier	B-Operating_System
that	O
exposes	O
methods	O
of	O
managing	O
the	O
stored	O
data	B-Application
without	O
exposing	O
or	O
creating	O
dependencies	O
on	O
the	O
data	B-Application
storage	O
mechanisms	O
.	O
</s>
<s>
Avoiding	O
dependencies	O
on	O
the	O
storage	O
mechanisms	O
allows	O
for	O
updates	O
or	O
changes	O
without	O
the	O
application	O
tier	B-Operating_System
clients	O
being	O
affected	O
by	O
or	O
even	O
aware	O
of	O
the	O
change	O
.	O
</s>
<s>
As	O
with	O
the	O
separation	O
of	O
any	O
tier	B-Operating_System
,	O
there	O
are	O
costs	O
for	O
implementation	O
and	O
often	O
costs	O
to	O
performance	O
in	O
exchange	O
for	O
improved	O
scalability	O
and	O
maintainability	O
.	O
</s>
<s>
In	O
the	O
web	O
development	O
field	O
,	O
three-tier	B-Operating_System
is	O
often	O
used	O
to	O
refer	O
to	O
websites	O
,	O
commonly	O
electronic	O
commerce	O
websites	O
,	O
which	O
are	O
built	O
using	O
three	B-Operating_System
tiers	I-Operating_System
:	O
</s>
<s>
A	O
front-end	B-Architecture
web	B-Application
server	I-Application
serving	O
static	O
content	O
,	O
and	O
potentially	O
some	O
cached	B-Protocol
dynamic	O
content	O
.	O
</s>
<s>
In	O
web-based	B-Application
application	I-Application
,	O
front	B-Architecture
end	I-Architecture
is	O
the	O
content	O
rendered	O
by	O
the	O
browser	O
.	O
</s>
<s>
A	O
middle	O
dynamic	O
content	O
processing	O
and	O
generation	O
level	O
application	B-Application
server	I-Application
(	O
e.g.	O
,	O
Symfony	B-Language
,	O
Spring	B-Application
,	O
ASP.NET	B-Application
,	O
Django	B-Language
,	O
Rails	B-Application
,	O
Node.js	B-Language
)	O
.	O
</s>
<s>
A	O
back-end	B-Architecture
database	O
or	O
data	B-General_Concept
store	I-General_Concept
,	O
comprising	O
both	O
data	B-Application
sets	O
and	O
the	O
database	B-Application
management	I-Application
system	I-Application
software	O
that	O
manages	O
and	O
provides	O
access	O
to	O
the	O
data	B-Application
.	O
</s>
<s>
Data	B-Application
transfer	O
between	O
tiers	B-Operating_System
is	O
part	O
of	O
the	O
architecture	O
.	O
</s>
<s>
Protocols	O
involved	O
may	O
include	O
one	O
or	O
more	O
of	O
SNMP	B-Protocol
,	O
CORBA	B-Architecture
,	O
Java	B-Language
RMI	I-Language
,	O
.NET	B-General_Concept
Remoting	I-General_Concept
,	O
Windows	B-Application
Communication	I-Application
Foundation	I-Application
,	O
sockets	B-Protocol
,	O
UDP	B-Protocol
,	O
web	O
services	O
or	O
other	O
standard	O
or	O
proprietary	O
protocols	O
.	O
</s>
<s>
Often	O
middleware	B-General_Concept
is	O
used	O
to	O
connect	O
the	O
separate	O
tiers	B-Operating_System
.	O
</s>
<s>
Separate	O
tiers	B-Operating_System
often	O
(	O
but	O
not	O
necessarily	O
)	O
run	O
on	O
separate	O
physical	O
servers	O
,	O
and	O
each	O
tier	B-Operating_System
may	O
itself	O
run	O
on	O
a	O
cluster	B-Architecture
.	O
</s>
<s>
The	O
end-to-end	O
traceability	O
of	O
data	B-Application
flows	O
through	O
n-tier	B-Operating_System
systems	O
is	O
a	O
challenging	O
task	O
which	O
becomes	O
more	O
important	O
when	O
systems	O
increase	O
in	O
complexity	O
.	O
</s>
<s>
The	O
Application	O
Response	O
Measurement	O
defines	O
concepts	O
and	O
APIs	B-Application
for	O
measuring	O
performance	O
and	O
correlating	O
transactions	O
between	O
tiers	B-Operating_System
.	O
</s>
<s>
Generally	O
,	O
the	O
term	O
"	O
tiers	B-Operating_System
"	O
is	O
used	O
to	O
describe	O
physical	O
distribution	O
of	O
components	O
of	O
a	O
system	O
on	O
separate	O
servers	O
,	O
computers	O
,	O
or	O
networks	O
(	O
processing	O
nodes	O
)	O
.	O
</s>
<s>
A	O
three-tier	B-Operating_System
architecture	I-Operating_System
then	O
will	O
have	O
three	O
processing	O
nodes	O
.	O
</s>
