<s>
The	O
client	B-Operating_System
–	I-Operating_System
server	I-Operating_System
model	I-Operating_System
is	O
a	O
distributed	B-Architecture
application	I-Architecture
structure	O
that	O
partitions	O
tasks	B-General_Concept
or	O
workloads	O
between	O
the	O
providers	O
of	O
a	O
resource	O
or	O
service	O
,	O
called	O
servers	O
,	O
and	O
service	O
requesters	O
,	O
called	O
clients	O
.	O
</s>
<s>
Often	O
clients	O
and	O
servers	O
communicate	O
over	O
a	O
computer	B-Architecture
network	I-Architecture
on	O
separate	O
hardware	O
,	O
but	O
both	O
client	B-Protocol
and	O
server	B-Application
may	O
reside	O
in	O
the	O
same	O
system	O
.	O
</s>
<s>
A	O
server	B-Application
host	B-Device
runs	O
one	O
or	O
more	O
server	B-Application
programs	B-Application
,	O
which	O
share	O
their	O
resources	O
with	O
clients	O
.	O
</s>
<s>
A	O
client	B-Protocol
usually	O
does	O
not	O
share	O
any	O
of	O
its	O
resources	O
,	O
but	O
it	O
requests	O
content	O
or	O
service	O
from	O
a	O
server	B-Application
.	O
</s>
<s>
Examples	O
of	O
computer	B-Application
applications	I-Application
that	O
use	O
the	O
client	B-Operating_System
–	I-Operating_System
server	I-Operating_System
model	I-Operating_System
are	O
email	O
,	O
network	O
printing	O
,	O
and	O
the	O
World	O
Wide	O
Web	O
.	O
</s>
<s>
The	O
"	O
client	B-Operating_System
–	I-Operating_System
server	I-Operating_System
"	O
characteristic	O
describes	O
the	O
relationship	O
of	O
cooperating	O
programs	B-Application
in	O
an	O
application	O
.	O
</s>
<s>
The	O
server	B-Application
component	O
provides	O
a	O
function	O
or	O
service	O
to	O
one	O
or	O
many	O
clients	O
,	O
which	O
initiate	O
requests	O
for	O
such	O
services	O
.	O
</s>
<s>
For	O
example	O
,	O
a	O
web	B-Application
server	I-Application
serves	O
web	O
pages	O
and	O
a	O
file	B-Application
server	I-Application
serves	O
computer	B-Operating_System
files	I-Operating_System
.	O
</s>
<s>
A	O
shared	B-General_Concept
resource	I-General_Concept
may	O
be	O
any	O
of	O
the	O
server	B-Application
computer	I-Application
's	O
software	O
and	O
electronic	O
components	O
,	O
from	O
programs	B-Application
and	O
data	B-General_Concept
to	O
processors	B-Architecture
and	O
storage	B-General_Concept
devices	I-General_Concept
.	O
</s>
<s>
The	O
sharing	O
of	O
resources	O
of	O
a	O
server	B-Application
constitutes	O
a	O
service	O
.	O
</s>
<s>
Whether	O
a	O
computer	O
is	O
a	O
client	B-Protocol
,	O
a	O
server	B-Application
,	O
or	O
both	O
,	O
is	O
determined	O
by	O
the	O
nature	O
of	O
the	O
application	O
that	O
requires	O
the	O
service	O
functions	O
.	O
</s>
<s>
For	O
example	O
,	O
a	O
single	O
computer	O
can	O
run	O
a	O
web	B-Application
server	I-Application
and	O
file	B-Application
server	I-Application
software	O
at	O
the	O
same	O
time	O
to	O
serve	O
different	O
data	B-General_Concept
to	O
clients	O
making	O
different	O
kinds	O
of	O
requests	O
.	O
</s>
<s>
The	O
client	B-Protocol
software	I-Protocol
can	O
also	O
communicate	O
with	O
server	B-Application
software	I-Application
within	O
the	O
same	O
computer	O
.	O
</s>
<s>
Communication	O
between	O
servers	O
,	O
such	O
as	O
to	O
synchronize	O
data	B-General_Concept
,	O
is	O
sometimes	O
called	O
inter-server	B-Protocol
or	O
server-to-server	B-Protocol
communication	O
.	O
</s>
<s>
Generally	O
,	O
a	O
service	O
is	O
an	O
abstraction	B-Application
of	O
computer	O
resources	O
and	O
a	O
client	B-Protocol
does	O
not	O
have	O
to	O
be	O
concerned	O
with	O
how	O
the	O
server	B-Application
performs	O
while	O
fulfilling	O
the	O
request	O
and	O
delivering	O
the	O
response	O
.	O
</s>
<s>
The	O
client	B-Protocol
only	O
has	O
to	O
understand	O
the	O
response	O
based	O
on	O
the	O
well-known	O
application	B-Application
protocol	I-Application
,	O
i.e.	O
</s>
<s>
the	O
content	O
and	O
the	O
formatting	O
of	O
the	O
data	B-General_Concept
for	O
the	O
requested	O
service	O
.	O
</s>
<s>
Clients	O
and	O
servers	O
exchange	O
messages	O
in	O
a	O
request	B-Protocol
–	I-Protocol
response	I-Protocol
messaging	B-Operating_System
pattern	I-Operating_System
.	O
</s>
<s>
The	O
client	B-Protocol
sends	O
a	O
request	O
,	O
and	O
the	O
server	B-Application
returns	O
a	O
response	O
.	O
</s>
<s>
This	O
exchange	O
of	O
messages	O
is	O
an	O
example	O
of	O
inter-process	B-Operating_System
communication	I-Operating_System
.	O
</s>
<s>
To	O
communicate	O
,	O
the	O
computers	O
must	O
have	O
a	O
common	O
language	O
,	O
and	O
they	O
must	O
follow	O
rules	O
so	O
that	O
both	O
the	O
client	B-Protocol
and	O
the	O
server	B-Application
know	O
what	O
to	O
expect	O
.	O
</s>
<s>
All	O
protocols	O
operate	O
in	O
the	O
application	B-Application
layer	I-Application
.	O
</s>
<s>
The	O
application	B-Application
layer	I-Application
protocol	O
defines	O
the	O
basic	O
patterns	O
of	O
the	O
dialogue	O
.	O
</s>
<s>
To	O
formalize	O
the	O
data	B-General_Concept
exchange	O
even	O
further	O
,	O
the	O
server	B-Application
may	O
implement	O
an	O
application	B-Application
programming	I-Application
interface	I-Application
(	O
API	B-Application
)	O
.	O
</s>
<s>
The	O
API	B-Application
is	O
an	O
abstraction	B-Architecture
layer	I-Architecture
for	O
accessing	O
a	O
service	O
.	O
</s>
<s>
By	O
restricting	O
communication	O
to	O
a	O
specific	O
content	O
format	O
,	O
it	O
facilitates	O
parsing	B-Language
.	O
</s>
<s>
By	O
abstracting	O
access	O
,	O
it	O
facilitates	O
cross-platform	O
data	B-General_Concept
exchange	O
.	O
</s>
<s>
A	O
server	B-Application
may	O
receive	O
requests	O
from	O
many	O
distinct	O
clients	O
in	O
a	O
short	O
period	O
.	O
</s>
<s>
A	O
computer	O
can	O
only	O
perform	O
a	O
limited	O
number	O
of	O
tasks	B-General_Concept
at	O
any	O
moment	O
,	O
and	O
relies	O
on	O
a	O
scheduling	O
system	O
to	O
prioritize	O
incoming	O
requests	O
from	O
clients	O
to	O
accommodate	O
them	O
.	O
</s>
<s>
To	O
prevent	O
abuse	O
and	O
maximize	O
availability	B-Device
,	O
the	O
server	B-Application
software	I-Application
may	O
limit	O
the	O
availability	B-Device
to	O
clients	O
.	O
</s>
<s>
Denial	O
of	O
service	O
attacks	O
are	O
designed	O
to	O
exploit	O
a	O
server	B-Application
's	O
obligation	O
to	O
process	O
requests	O
by	O
overloading	O
it	O
with	O
excessive	O
request	O
rates	O
.	O
</s>
<s>
Encryption	O
should	O
be	O
applied	O
if	O
sensitive	O
information	O
is	O
to	O
be	O
communicated	O
between	O
the	O
client	B-Protocol
and	O
the	O
server	B-Application
.	O
</s>
<s>
When	O
a	O
bank	O
customer	O
accesses	O
online	B-Application
banking	I-Application
services	O
with	O
a	O
web	B-Application
browser	I-Application
(	O
the	O
client	B-Protocol
)	O
,	O
the	O
client	B-Protocol
initiates	O
a	O
request	O
to	O
the	O
bank	O
's	O
web	B-Application
server	I-Application
.	O
</s>
<s>
The	O
customer	O
's	O
login	B-General_Concept
credentials	O
may	O
be	O
stored	O
in	O
a	O
database	O
,	O
and	O
the	O
webserver	B-Application
accesses	O
the	O
database	B-Application
server	I-Application
as	O
a	O
client	B-Protocol
.	O
</s>
<s>
An	O
application	B-Application
server	I-Application
interprets	O
the	O
returned	O
data	B-General_Concept
by	O
applying	O
the	O
bank	O
's	O
business	B-Architecture
logic	I-Architecture
and	O
provides	O
the	O
output	B-General_Concept
to	O
the	O
webserver	B-Application
.	O
</s>
<s>
Finally	O
,	O
the	O
webserver	B-Application
returns	O
the	O
result	O
to	O
the	O
client	B-Protocol
web	B-Application
browser	I-Application
for	O
display	O
.	O
</s>
<s>
In	O
each	O
step	O
of	O
this	O
sequence	O
of	O
client	B-Operating_System
–	I-Operating_System
server	I-Operating_System
message	O
exchanges	O
,	O
a	O
computer	O
processes	O
a	O
request	O
and	O
returns	O
data	B-General_Concept
.	O
</s>
<s>
This	O
is	O
the	O
request-response	B-Protocol
messaging	B-Operating_System
pattern	I-Operating_System
.	O
</s>
<s>
When	O
all	O
the	O
requests	O
are	O
met	O
,	O
the	O
sequence	O
is	O
complete	O
and	O
the	O
web	B-Application
browser	I-Application
presents	O
the	O
data	B-General_Concept
to	O
the	O
customer	O
.	O
</s>
<s>
This	O
example	O
illustrates	O
a	O
design	O
pattern	O
applicable	O
to	O
the	O
client	B-Operating_System
–	I-Operating_System
server	I-Operating_System
model	I-Operating_System
:	O
separation	O
of	O
concerns	O
.	O
</s>
<s>
An	O
early	O
form	O
of	O
client	B-Operating_System
–	I-Operating_System
server	I-Operating_System
architecture	I-Operating_System
is	O
remote	B-Protocol
job	I-Protocol
entry	I-Protocol
,	O
dating	O
at	O
least	O
to	O
OS/360	B-Application
(	O
announced	O
1964	O
)	O
,	O
where	O
the	O
request	O
was	O
to	O
run	O
a	O
job	B-General_Concept
,	O
and	O
the	O
response	O
was	O
the	O
output	B-General_Concept
.	O
</s>
<s>
While	O
formulating	O
the	O
client	B-Operating_System
–	I-Operating_System
server	I-Operating_System
model	I-Operating_System
in	O
the	O
1960s	O
and	O
1970s	O
,	O
computer	O
scientists	O
building	O
ARPANET	O
(	O
at	O
the	O
Stanford	O
Research	O
Institute	O
)	O
used	O
the	O
terms	O
server-host	O
(	O
or	O
serving	O
host	B-Device
)	O
and	O
user-host	O
(	O
or	O
using-host	O
)	O
,	O
and	O
these	O
appear	O
in	O
the	O
early	O
documents	O
RFC	O
5	O
and	O
RFC	O
4	O
.	O
</s>
<s>
One	O
context	O
in	O
which	O
researchers	O
used	O
these	O
terms	O
was	O
in	O
the	O
design	O
of	O
a	O
computer	B-Operating_System
network	I-Operating_System
programming	I-Operating_System
language	O
called	O
Decode-Encode	O
Language	O
(	O
DEL	O
)	O
.	O
</s>
<s>
The	O
purpose	O
of	O
this	O
language	O
was	O
to	O
accept	O
commands	O
from	O
one	O
computer	O
(	O
the	O
user-host	O
)	O
,	O
which	O
would	O
return	O
status	O
reports	O
to	O
the	O
user	O
as	O
it	O
encoded	O
the	O
commands	O
in	O
network	O
packets	O
.	O
</s>
<s>
Another	O
DEL-capable	O
computer	O
,	O
the	O
server-host	O
,	O
received	O
the	O
packets	O
,	O
decoded	O
them	O
,	O
and	O
returned	O
formatted	O
data	B-General_Concept
to	O
the	O
user-host	O
.	O
</s>
<s>
A	O
DEL	O
program	O
on	O
the	O
user-host	O
received	O
the	O
results	O
to	O
present	O
to	O
the	O
user	O
.	O
</s>
<s>
This	O
is	O
a	O
client	B-Operating_System
–	I-Operating_System
server	I-Operating_System
transaction	O
.	O
</s>
<s>
Client-host	O
and	O
server-host	O
have	O
subtly	O
different	O
meanings	O
than	O
client	B-Protocol
and	O
server	B-Application
.	O
</s>
<s>
A	O
host	B-Device
is	O
any	O
computer	O
connected	O
to	O
a	O
network	O
.	O
</s>
<s>
Whereas	O
the	O
words	O
server	B-Application
and	O
client	B-Protocol
may	O
refer	O
either	O
to	O
a	O
computer	O
or	O
to	O
a	O
computer	B-Application
program	I-Application
,	O
server-host	O
and	O
client-host	O
always	O
refer	O
to	O
computers	O
.	O
</s>
<s>
The	O
host	B-Device
is	O
a	O
versatile	O
,	O
multifunction	O
computer	O
;	O
clients	O
and	O
servers	O
are	O
just	O
programs	B-Application
that	O
run	O
on	O
a	O
host	B-Device
.	O
</s>
<s>
In	O
the	O
client	B-Operating_System
–	I-Operating_System
server	I-Operating_System
model	I-Operating_System
,	O
a	O
server	B-Application
is	O
more	O
likely	O
to	O
be	O
devoted	O
to	O
the	O
task	O
of	O
serving	O
.	O
</s>
<s>
An	O
early	O
use	O
of	O
the	O
word	O
client	B-Protocol
occurs	O
in	O
"	O
Separating	O
Data	B-General_Concept
from	O
Function	O
in	O
a	O
Distributed	O
File	O
System	O
"	O
,	O
a	O
1978	O
paper	O
by	O
Xerox	O
PARC	O
computer	O
scientists	O
Howard	O
Sturgis	O
,	O
James	O
Mitchell	O
,	O
and	O
Jay	O
Israel	O
.	O
</s>
<s>
The	O
authors	O
are	O
careful	O
to	O
define	O
the	O
term	O
for	O
readers	O
,	O
and	O
explain	O
that	O
they	O
use	O
it	O
to	O
distinguish	O
between	O
the	O
user	O
and	O
the	O
user	O
's	O
network	B-Protocol
node	I-Protocol
(	O
the	O
client	B-Protocol
)	O
.	O
</s>
<s>
By	O
1992	O
,	O
the	O
word	O
server	B-Application
had	O
entered	O
into	O
general	O
parlance	O
.	O
</s>
<s>
The	O
client	B-Operating_System
–	I-Operating_System
server	I-Operating_System
model	I-Operating_System
does	O
not	O
dictate	O
that	O
server-hosts	O
must	O
have	O
more	O
resources	O
than	O
client-hosts	O
.	O
</s>
<s>
Rather	O
,	O
it	O
enables	O
any	O
general-purpose	O
computer	O
to	O
extend	O
its	O
capabilities	O
by	O
using	O
the	O
shared	B-General_Concept
resources	I-General_Concept
of	O
other	O
hosts	O
.	O
</s>
<s>
Centralized	B-General_Concept
computing	I-General_Concept
,	O
however	O
,	O
specifically	O
allocates	O
a	O
large	O
number	O
of	O
resources	O
to	O
a	O
small	O
number	O
of	O
computers	O
.	O
</s>
<s>
The	O
more	O
computation	O
is	O
offloaded	O
from	O
client-hosts	O
to	O
the	O
central	O
computers	O
,	O
the	O
simpler	O
the	O
client-hosts	O
can	O
be	O
.	O
</s>
<s>
It	O
relies	O
heavily	O
on	O
network	B-General_Concept
resources	I-General_Concept
(	O
servers	O
and	O
infrastructure	O
)	O
for	O
computation	O
and	O
storage	O
.	O
</s>
<s>
A	O
diskless	B-Device
node	I-Device
loads	O
even	O
its	O
operating	B-General_Concept
system	I-General_Concept
from	O
the	O
network	O
,	O
and	O
a	O
computer	B-General_Concept
terminal	I-General_Concept
has	O
no	O
operating	B-General_Concept
system	I-General_Concept
at	O
all	O
;	O
it	O
is	O
only	O
an	O
input/output	B-General_Concept
interface	O
to	O
the	O
server	B-Application
.	O
</s>
<s>
In	O
contrast	O
,	O
a	O
rich	B-Protocol
client	I-Protocol
,	O
such	O
as	O
a	O
personal	B-Device
computer	I-Device
,	O
has	O
many	O
resources	O
and	O
does	O
not	O
rely	O
on	O
a	O
server	B-Application
for	O
essential	O
functions	O
.	O
</s>
<s>
As	O
microcomputers	B-Architecture
decreased	O
in	O
price	O
and	O
increased	O
in	O
power	O
from	O
the	O
1980s	O
to	O
the	O
late	O
1990s	O
,	O
many	O
organizations	O
transitioned	O
computation	O
from	O
centralized	O
servers	O
,	O
such	O
as	O
mainframes	B-Architecture
and	O
minicomputers	B-Architecture
,	O
to	O
rich	B-Protocol
clients	I-Protocol
.	O
</s>
<s>
This	O
afforded	O
greater	O
,	O
more	O
individualized	O
dominion	O
over	O
computer	O
resources	O
,	O
but	O
complicated	O
information	B-Application
technology	I-Application
management	I-Application
.	O
</s>
<s>
During	O
the	O
2000s	O
,	O
web	B-Application
applications	I-Application
matured	O
enough	O
to	O
rival	O
application	B-Application
software	I-Application
developed	O
for	O
a	O
specific	O
microarchitecture	B-General_Concept
.	O
</s>
<s>
This	O
maturation	O
,	O
more	O
affordable	O
mass	B-Device
storage	I-Device
,	O
and	O
the	O
advent	O
of	O
service-oriented	O
architecture	O
were	O
among	O
the	O
factors	O
that	O
gave	O
rise	O
to	O
the	O
cloud	B-Architecture
computing	I-Architecture
trend	O
of	O
the	O
2010s	O
.	O
</s>
<s>
In	O
addition	O
to	O
the	O
client	B-Operating_System
–	I-Operating_System
server	I-Operating_System
model	I-Operating_System
,	O
distributed	B-Architecture
computing	I-Architecture
applications	O
often	O
use	O
the	O
peer-to-peer	O
(	O
P2P	O
)	O
application	O
architecture	O
.	O
</s>
<s>
In	O
the	O
client	B-Operating_System
–	I-Operating_System
server	I-Operating_System
model	I-Operating_System
,	O
the	O
server	B-Application
is	O
often	O
designed	O
to	O
operate	O
as	O
a	O
centralized	O
system	O
that	O
serves	O
many	O
clients	O
.	O
</s>
<s>
The	O
computing	O
power	O
,	O
memory	O
and	O
storage	O
requirements	O
of	O
a	O
server	B-Application
must	O
be	O
scaled	O
appropriately	O
to	O
the	O
expected	O
workload	O
.	O
</s>
<s>
Load-balancing	B-Application
and	O
failover	B-General_Concept
systems	O
are	O
often	O
employed	O
to	O
scale	O
the	O
server	B-Application
beyond	O
a	O
single	O
physical	O
machine	O
.	O
</s>
<s>
Load	B-Application
balancing	I-Application
is	O
defined	O
as	O
the	O
methodical	O
and	O
efficient	O
distribution	O
of	O
network	O
or	O
application	O
traffic	O
across	O
multiple	O
servers	O
in	O
a	O
server	B-Application
farm	O
.	O
</s>
<s>
Each	O
load	B-Application
balancer	I-Application
sits	O
between	O
client	B-Protocol
devices	O
and	O
backend	B-Architecture
servers	O
,	O
receiving	O
and	O
then	O
distributing	O
incoming	O
requests	O
to	O
any	O
available	O
server	B-Application
capable	O
of	O
fulfilling	O
them	O
.	O
</s>
<s>
Peers	O
are	O
coequal	O
,	O
or	O
equipotent	O
nodes	B-Protocol
in	O
a	O
non-hierarchical	O
network	O
.	O
</s>
<s>
Unlike	O
clients	O
in	O
a	O
client	B-Operating_System
–	I-Operating_System
server	I-Operating_System
or	O
client	B-Architecture
–	I-Architecture
queue	I-Architecture
–	I-Architecture
client	I-Architecture
network	O
,	O
peers	O
communicate	O
with	O
each	O
other	O
directly	O
.	O
</s>
<s>
In	O
peer-to-peer	O
networking	O
,	O
an	O
algorithm	O
in	O
the	O
peer-to-peer	O
communications	O
protocol	O
balances	O
load	B-Operating_System
,	O
and	O
even	O
peers	O
with	O
modest	O
resources	O
can	O
help	O
to	O
share	O
the	O
load	B-Operating_System
.	O
</s>
<s>
If	O
a	O
node	O
becomes	O
unavailable	O
,	O
its	O
shared	B-General_Concept
resources	I-General_Concept
remain	O
available	O
as	O
long	O
as	O
other	O
peers	O
offer	O
it	O
.	O
</s>
<s>
Ideally	O
,	O
a	O
peer	O
does	O
not	O
need	O
to	O
achieve	O
high	O
availability	B-Device
because	O
other	O
,	O
redundant	O
peers	O
make	O
up	O
for	O
any	O
resource	O
downtime	O
;	O
as	O
the	O
availability	B-Device
and	O
load	B-Operating_System
capacity	O
of	O
peers	O
change	O
,	O
the	O
protocol	O
reroutes	O
requests	O
.	O
</s>
<s>
Both	O
client	B-Operating_System
–	I-Operating_System
server	I-Operating_System
and	O
master	B-Operating_System
–	I-Operating_System
slave	I-Operating_System
are	O
regarded	O
as	O
sub-categories	O
of	O
distributed	O
peer-to-peer	O
systems	O
.	O
</s>
