<s>
ReactiveX	B-Application
(	O
also	O
known	O
as	O
Reactive	B-Application
Extensions	I-Application
)	O
is	O
a	O
software	O
library	O
originally	O
created	O
by	O
Microsoft	O
that	O
allows	O
imperative	B-Application
programming	I-Application
languages	I-Application
to	O
operate	O
on	O
sequences	O
of	O
data	O
regardless	O
of	O
whether	O
the	O
data	O
is	O
synchronous	O
or	O
asynchronous	B-Operating_System
.	O
</s>
<s>
It	O
is	O
an	O
implementation	O
of	O
reactive	B-Architecture
programming	I-Architecture
and	O
provides	O
a	O
blueprint	O
for	O
the	O
tools	O
to	O
be	O
implemented	O
in	O
multiple	O
programming	O
languages	O
.	O
</s>
<s>
ReactiveX	B-Application
is	O
an	O
API	B-General_Concept
for	O
asynchronous	B-Operating_System
programming	I-Operating_System
with	O
observable	O
streams	O
.	O
</s>
<s>
Asynchronous	B-Operating_System
programming	I-Operating_System
allows	O
programmers	O
to	O
call	O
functions	O
and	O
then	O
have	O
the	O
functions	O
"	O
callback	O
"	O
when	O
they	O
are	O
done	O
,	O
usually	O
by	O
giving	O
the	O
function	O
the	O
address	O
of	O
another	O
function	O
to	O
execute	O
when	O
it	O
is	O
done	O
.	O
</s>
<s>
streams	O
that	O
can	O
be	O
observed	O
)	O
in	O
the	O
context	O
of	O
Reactive	B-Application
Extensions	I-Application
are	O
like	O
event	O
emitters	O
that	O
emit	O
three	O
events	O
:	O
next	O
,	O
error	O
,	O
and	O
complete	O
.	O
</s>
<s>
For	O
sequences	O
of	O
data	O
,	O
it	O
combines	O
the	O
advantages	O
of	O
iterators	O
with	O
the	O
flexibility	O
of	O
event-based	O
asynchronous	B-Operating_System
programming	I-Operating_System
.	O
</s>
<s>
ReactiveX	B-Application
is	O
a	O
combination	O
of	O
ideas	O
from	O
the	O
observer	B-Language
and	O
the	O
iterator	B-Language
patterns	I-Language
and	O
from	O
functional	B-Language
programming	I-Language
.	O
</s>
<s>
An	O
observer	B-Language
subscribes	O
to	O
an	O
observable	O
sequence	O
.	O
</s>
<s>
The	O
sequence	O
then	O
sends	O
the	O
items	O
to	O
the	O
observer	B-Language
one	O
at	O
a	O
time	O
,	O
usually	O
by	O
calling	O
the	O
provided	O
callback	O
function	O
.	O
</s>
<s>
The	O
observer	B-Language
handles	O
each	O
one	O
before	O
processing	O
the	O
next	O
one	O
.	O
</s>
<s>
In	O
ReactiveX	B-Application
,	O
an	O
observer	B-Language
will	O
never	O
be	O
called	O
with	O
an	O
item	O
out	O
of	O
order	O
or	O
(	O
in	O
a	O
multi-threaded	O
context	O
)	O
called	O
before	O
the	O
callback	O
has	O
returned	O
for	O
the	O
previous	O
item	O
.	O
</s>
<s>
Asynchronous	B-Operating_System
calls	O
remain	O
asynchronous	B-Operating_System
and	O
may	O
be	O
handled	O
by	O
returning	O
an	O
observable	O
.	O
</s>
<s>
It	O
is	O
similar	O
to	O
the	O
iterators	O
pattern	O
in	O
that	O
if	O
a	O
fatal	O
error	O
occurs	O
,	O
it	O
notifies	O
the	O
observer	B-Language
separately	O
(	O
by	O
calling	O
a	O
second	O
function	O
)	O
.	O
</s>
<s>
When	O
all	O
the	O
items	O
have	O
been	O
sent	O
,	O
it	O
completes	O
(	O
and	O
notifies	O
the	O
observer	B-Language
by	O
calling	O
a	O
third	O
function	O
)	O
.	O
</s>
<s>
The	O
Reactive	B-Application
Extensions	I-Application
API	B-General_Concept
also	O
borrows	O
many	O
of	O
its	O
operators	O
from	O
iterator	B-Language
operators	O
in	O
other	O
programming	O
languages	O
.	O
</s>
<s>
Reactive	B-Application
Extensions	I-Application
is	O
different	O
from	O
functional	B-Application
reactive	I-Application
programming	I-Application
as	O
the	O
Introduction	O
to	O
Reactive	B-Application
Extensions	I-Application
explains	O
:	O
</s>
<s>
It	O
is	O
sometimes	O
called	O
"	O
functional	B-Application
reactive	I-Application
programming	I-Application
"	O
but	O
this	O
is	O
a	O
misnomer	O
.	O
</s>
<s>
ReactiveX	B-Application
may	O
be	O
functional	O
,	O
and	O
it	O
may	O
be	O
reactive	O
,	O
but	O
"	O
functional	B-Application
reactive	I-Application
programming	I-Application
"	O
is	O
a	O
different	O
animal	O
.	O
</s>
<s>
One	O
main	O
point	O
of	O
difference	O
is	O
that	O
functional	B-Application
reactive	I-Application
programming	I-Application
operates	O
on	O
values	O
that	O
change	O
continuously	O
over	O
time	O
,	O
while	O
ReactiveX	B-Application
operates	O
on	O
discrete	O
values	O
that	O
are	O
emitted	O
over	O
time	O
.	O
</s>
<s>
(	O
See	O
Conal	O
Elliott	O
's	O
work	O
for	O
more-precise	O
information	O
on	O
functional	B-Application
reactive	I-Application
programming	I-Application
.	O
)	O
</s>
<s>
Some	O
of	O
the	O
operators	O
that	O
can	O
be	O
used	O
in	O
Reactive	B-Application
Extensions	I-Application
may	O
be	O
familiar	O
to	O
programmers	O
who	O
use	O
functional	B-Language
programming	I-Language
language	I-Language
,	O
such	O
as	O
map	O
,	O
reduce	O
,	O
group	O
,	O
and	O
zip	O
.	O
</s>
<s>
There	O
are	O
many	O
other	O
operators	O
available	O
in	O
Reactive	B-Application
Extensions	I-Application
,	O
though	O
the	O
operators	O
available	O
in	O
a	O
particular	O
implementation	O
for	O
a	O
programming	O
language	O
may	O
vary	O
.	O
</s>
<s>
Calling	O
subscribe	O
will	O
register	O
an	O
observer	B-Language
that	O
will	O
observe	O
the	O
values	O
from	O
the	O
observable	O
produced	O
by	O
the	O
chain	O
of	O
operators	O
.	O
</s>
<s>
The	O
above	O
example	O
uses	O
the	O
of	O
Reactive	B-Application
Extensions	I-Application
for	O
the	O
JavaScript	B-Language
programming	I-Language
language	I-Language
.	O
</s>
<s>
Reactive	B-Application
Extensions	I-Application
(	O
Rx	O
)	O
was	O
created	O
by	O
the	O
Cloud	O
Programmability	O
Team	O
at	O
Microsoft	O
around	O
2011	O
,	O
as	O
a	O
byproduct	O
of	O
a	O
larger	O
effort	O
called	O
Volta	B-General_Concept
.	O
</s>
<s>
It	O
was	O
originally	O
intended	O
to	O
provide	O
an	O
abstraction	O
for	O
events	O
across	O
different	O
tiers	O
in	O
an	O
application	O
to	O
support	O
tier	O
splitting	O
in	O
Volta	B-General_Concept
.	O
</s>
<s>
The	O
project	O
's	O
logo	O
represents	O
an	O
electric	O
eel	O
,	O
which	O
is	O
a	O
reference	O
to	O
Volta	B-General_Concept
.	O
</s>
<s>
The	O
extensions	O
suffix	O
in	O
the	O
name	O
is	O
a	O
reference	O
to	O
the	O
Parallel	B-Library
Extensions	I-Library
technology	O
which	O
was	O
invented	O
around	O
the	O
same	O
time	O
;	O
the	O
two	O
are	O
considered	O
complementary	O
.	O
</s>
<s>
The	O
initial	O
implementation	O
of	O
Rx	O
was	O
for	O
.NET	B-Application
Framework	I-Application
and	O
was	O
released	O
on	O
June	O
21	O
,	O
2011	O
.	O
</s>
<s>
Later	O
,	O
the	O
team	O
started	O
the	O
implementation	O
of	O
Rx	O
for	O
other	O
platforms	O
,	O
including	O
JavaScript	B-Language
and	O
C++	B-Language
.	O
</s>
<s>
The	O
technology	O
was	O
released	O
as	O
open	B-License
source	I-License
in	O
late	O
2012	O
,	O
initially	O
on	O
CodePlex	B-Protocol
.	O
</s>
<s>
Later	O
,	O
the	O
code	O
moved	O
to	O
GitHub	B-Application
.	O
</s>
