<s>
In	O
computer	B-General_Concept
science	I-General_Concept
,	O
request	B-Protocol
–	I-Protocol
response	I-Protocol
or	O
request	O
–	O
reply	O
is	O
one	O
of	O
the	O
basic	O
methods	O
computers	O
use	O
to	O
communicate	O
with	O
each	O
other	O
in	O
a	O
network	B-Architecture
,	O
in	O
which	O
the	O
first	O
computer	O
sends	O
a	O
request	O
for	O
some	O
data	B-General_Concept
and	O
the	O
second	O
responds	O
to	O
the	O
request	O
.	O
</s>
<s>
More	O
specifically	O
,	O
it	O
is	O
a	O
message	B-Operating_System
exchange	I-Operating_System
pattern	I-Operating_System
in	O
which	O
a	O
requestor	O
sends	O
a	O
request	O
message	O
to	O
a	O
replier	O
system	O
,	O
which	O
receives	O
and	O
processes	O
the	O
request	O
,	O
ultimately	O
returning	O
a	O
message	O
in	O
response	O
.	O
</s>
<s>
This	O
is	O
a	O
simple	O
but	O
powerful	O
messaging	B-Operating_System
pattern	I-Operating_System
which	O
allows	O
two	O
applications	O
to	O
have	O
a	O
two-way	O
conversation	O
with	O
one	O
another	O
over	O
a	O
channel	O
;	O
it	O
is	O
especially	O
common	O
in	O
client	B-Operating_System
–	I-Operating_System
server	I-Operating_System
architectures	I-Operating_System
.	O
</s>
<s>
For	O
simplicity	O
,	O
this	O
pattern	O
is	O
typically	O
implemented	O
in	O
a	O
purely	O
synchronous	B-Protocol
fashion	O
,	O
as	O
in	O
web	O
service	O
calls	O
over	O
HTTP	B-Protocol
,	O
which	O
holds	O
a	O
connection	O
open	O
and	O
waits	O
until	O
the	O
response	O
is	O
delivered	O
or	O
the	O
timeout	O
period	O
expires	O
.	O
</s>
<s>
However	O
,	O
request	B-Protocol
–	I-Protocol
response	I-Protocol
may	O
also	O
be	O
implemented	O
asynchronously	O
,	O
with	O
a	O
response	O
being	O
returned	O
at	O
some	O
unknown	O
later	O
time	O
.	O
</s>
<s>
When	O
a	O
synchronous	B-Protocol
system	O
communicates	O
with	O
an	O
asynchronous	O
system	O
,	O
it	O
is	O
referred	O
to	O
as	O
"	O
sync	O
over	O
async	O
"	O
or	O
"	O
sync/async	O
"	O
.	O
</s>
<s>
This	O
is	O
common	O
in	O
enterprise	B-Architecture
application	I-Architecture
integration	I-Architecture
(	O
EAI	O
)	O
implementations	O
where	O
slow	O
aggregations	O
,	O
time-intensive	O
functions	O
,	O
or	O
human	O
workflow	B-Operating_System
must	O
be	O
performed	O
before	O
a	O
response	O
can	O
be	O
constructed	O
and	O
delivered	O
.	O
</s>
<s>
In	O
contrast	O
,	O
one-way	O
computer	B-Architecture
communication	I-Architecture
,	O
which	O
is	O
like	O
the	O
push-to-talk	O
or	O
"	O
barge	O
in	O
"	O
feature	O
found	O
on	O
some	O
phones	O
and	O
two-way	O
radios	O
,	O
sends	O
a	O
message	O
without	O
waiting	O
for	O
a	O
response	O
.	O
</s>
<s>
Sending	O
an	O
email	O
is	O
an	O
example	O
of	O
one-way	O
communication	O
,	O
and	O
another	O
example	O
are	O
fieldbus	O
sensors	O
,	O
such	O
as	O
most	O
CAN	B-Protocol
bus	I-Protocol
sensors	O
,	O
which	O
periodically	O
and	O
autonomously	O
send	O
out	O
their	O
data	B-General_Concept
,	O
whether	O
or	O
not	O
any	O
other	O
devices	O
on	O
the	O
bus	O
are	O
listening	O
for	O
it	O
.	O
</s>
<s>
(	O
Most	O
of	O
these	O
systems	O
use	O
a	O
"	O
listen	B-General_Concept
before	I-General_Concept
talk	I-General_Concept
"	O
or	O
other	O
contention-based	B-General_Concept
protocol	I-General_Concept
so	O
multiple	O
sensors	O
can	O
transmit	O
periodic	O
updates	O
without	O
any	O
pre-coordination	O
.	O
)	O
</s>
