<s>
An	O
(	O
)	O
is	O
a	O
way	O
for	O
two	O
or	O
more	O
computer	B-Application
programs	I-Application
to	O
communicate	O
with	O
each	O
other	O
.	O
</s>
<s>
It	O
is	O
a	O
type	O
of	O
software	O
interface	B-Application
,	O
offering	O
a	O
service	O
to	O
other	O
pieces	O
of	O
software	O
.	O
</s>
<s>
A	O
document	O
or	O
standard	O
that	O
describes	O
how	O
to	O
build	O
or	O
use	O
such	O
a	O
connection	O
or	O
interface	B-Application
is	O
called	O
an	O
.	O
</s>
<s>
A	O
computer	O
system	O
that	O
meets	O
this	O
standard	O
is	O
said	O
to	O
or	O
an	O
API	B-Application
.	O
</s>
<s>
The	O
term	O
API	B-Application
may	O
refer	O
either	O
to	O
the	O
specification	O
or	O
to	O
the	O
implementation	O
.	O
</s>
<s>
In	O
contrast	O
to	O
a	O
user	B-Application
interface	I-Application
,	O
which	O
connects	O
a	O
computer	O
to	O
a	O
person	O
,	O
an	O
application	B-Application
programming	I-Application
interface	I-Application
connects	O
computers	O
or	O
pieces	O
of	O
software	O
to	O
each	O
other	O
.	O
</s>
<s>
It	O
is	O
not	O
intended	O
to	O
be	O
used	O
directly	O
by	O
a	O
person	O
(	O
the	O
end	O
user	O
)	O
other	O
than	O
a	O
computer	B-Application
programmer	I-Application
who	O
is	O
incorporating	O
it	O
into	O
the	O
software	O
.	O
</s>
<s>
An	O
API	B-Application
is	O
often	O
made	O
up	O
of	O
different	O
parts	O
which	O
act	O
as	O
tools	O
or	O
services	O
that	O
are	O
available	O
to	O
the	O
programmer	B-Application
.	O
</s>
<s>
A	O
program	O
or	O
a	O
programmer	B-Application
that	O
uses	O
one	O
of	O
these	O
parts	O
is	O
said	O
to	O
that	O
portion	O
of	O
the	O
API	B-Application
.	O
</s>
<s>
The	O
calls	O
that	O
make	O
up	O
the	O
API	B-Application
are	O
also	O
known	O
as	O
subroutines	O
,	O
methods	O
,	O
requests	O
,	O
or	O
endpoints	O
.	O
</s>
<s>
An	O
API	B-Application
specification	O
these	O
calls	O
,	O
meaning	O
that	O
it	O
explains	O
how	O
to	O
use	O
or	O
implement	O
them	O
.	O
</s>
<s>
One	O
purpose	O
of	O
APIs	B-Application
is	O
to	O
hide	B-Application
the	I-Application
internal	I-Application
details	I-Application
of	O
how	O
a	O
system	O
works	O
,	O
exposing	O
only	O
those	O
parts	O
a	O
programmer	B-Application
will	O
find	O
useful	O
and	O
keeping	O
them	O
consistent	O
even	O
if	O
the	O
internal	O
details	O
later	O
change	O
.	O
</s>
<s>
An	O
API	B-Application
may	O
be	O
custom-built	O
for	O
a	O
particular	O
pair	O
of	O
systems	O
,	O
or	O
it	O
may	O
be	O
a	O
shared	O
standard	O
allowing	O
interoperability	O
among	O
many	O
systems	O
.	O
</s>
<s>
There	O
are	O
APIs	B-Application
for	O
programming	O
languages	O
,	O
software	B-Library
libraries	I-Library
,	O
computer	B-General_Concept
operating	I-General_Concept
systems	I-General_Concept
,	O
and	O
computer	B-Architecture
hardware	I-Architecture
.	O
</s>
<s>
APIs	B-Application
originated	O
in	O
the	O
1940s	O
,	O
though	O
the	O
term	O
did	O
not	O
emerge	O
until	O
the	O
1960s	O
and	O
1970s	O
.	O
</s>
<s>
Contemporary	O
usage	O
of	O
the	O
term	O
API	B-Application
often	O
refers	O
to	O
web	B-General_Concept
APIs	I-General_Concept
,	O
which	O
allow	O
communication	O
between	O
computers	O
that	O
are	O
joined	O
by	O
the	O
internet	O
.	O
</s>
<s>
Recent	O
developments	O
in	O
APIs	B-Application
have	O
led	O
to	O
the	O
rise	O
in	O
popularity	O
of	O
microservices	O
,	O
which	O
are	O
loosely	O
coupled	O
services	O
accessed	O
through	O
public	O
APIs	B-Application
.	O
</s>
<s>
In	O
building	O
applications	O
,	O
an	O
API	B-Application
simplifies	O
programming	O
by	O
abstracting	B-Application
the	O
underlying	O
implementation	O
and	O
only	O
exposing	O
objects	O
or	O
actions	O
the	O
developer	B-Application
needs	O
.	O
</s>
<s>
While	O
a	O
graphical	O
interface	B-Application
for	O
an	O
email	B-Protocol
client	I-Protocol
might	O
provide	O
a	O
user	O
with	O
a	O
button	O
that	O
performs	O
all	O
the	O
steps	O
for	O
fetching	O
and	O
highlighting	O
new	O
emails	O
,	O
an	O
API	B-Application
for	O
file	O
input/output	B-General_Concept
might	O
give	O
the	O
developer	B-Application
a	O
function	O
that	O
copies	O
a	O
file	O
from	O
one	O
location	O
to	O
another	O
without	O
requiring	O
that	O
the	O
developer	B-Application
understand	O
the	O
file	B-Application
system	I-Application
operations	O
occurring	O
behind	O
the	O
scenes	O
.	O
</s>
<s>
The	O
term	O
API	B-Application
initially	O
described	O
an	O
interface	B-Application
only	O
for	O
end-user-facing	O
programs	O
,	O
known	O
as	O
application	B-Application
programs	I-Application
.	O
</s>
<s>
Today	O
,	O
the	O
term	O
is	O
broader	O
,	O
including	O
also	O
utility	B-Application
software	I-Application
and	O
even	O
hardware	B-Application
interfaces	I-Application
.	O
</s>
<s>
The	O
idea	O
of	O
the	O
API	B-Application
is	O
much	O
older	O
than	O
the	O
term	O
itself	O
.	O
</s>
<s>
British	O
computer	O
scientists	O
Maurice	O
Wilkes	O
and	O
David	O
Wheeler	O
worked	O
on	O
a	O
modular	B-Architecture
software	I-Architecture
library	O
in	O
the	O
1940s	O
for	O
EDSAC	B-Device
,	O
an	O
early	O
computer	O
.	O
</s>
<s>
Today	O
,	O
such	O
a	O
catalog	O
would	O
be	O
called	O
an	O
API	B-Application
(	O
or	O
an	O
API	B-Application
specification	O
or	O
API	B-Application
documentation	I-Application
)	O
because	O
it	O
instructs	O
a	O
programmer	B-Application
on	O
how	O
to	O
use	O
(	O
or	O
"	O
call	O
"	O
)	O
each	O
subroutine	O
that	O
the	O
programmer	B-Application
needs	O
.	O
</s>
<s>
Wilkes	O
and	O
Wheeler	O
's	O
1951	O
book	O
The	O
Preparation	O
of	O
Programs	O
for	O
an	O
Electronic	O
Digital	O
Computer	O
contains	O
the	O
first	O
published	O
API	B-Application
specification	O
.	O
</s>
<s>
Joshua	O
Bloch	O
considers	O
that	O
Wilkes	O
and	O
Wheeler	O
"	O
latently	O
invented	O
"	O
the	O
API	B-Application
because	O
it	O
is	O
more	O
of	O
a	O
concept	O
that	O
is	O
discovered	O
than	O
invented	O
.	O
</s>
<s>
The	O
term	O
"	O
application	B-Application
program	I-Application
interface	I-Application
"	O
(	O
without	O
an	O
-ing	O
suffix	O
)	O
is	O
first	O
recorded	O
in	O
a	O
paper	O
called	O
Data	O
structures	O
and	O
techniques	O
for	O
remote	O
computer	O
graphics	O
presented	O
at	O
an	O
AFIPS	O
conference	O
in	O
1968	O
.	O
</s>
<s>
A	O
consistent	O
application	O
interface	B-Application
(	O
consisting	O
of	O
Fortran	B-Application
subroutine	O
calls	O
)	O
was	O
intended	O
to	O
free	O
the	O
programmer	B-Application
from	O
dealing	O
with	O
idiosyncrasies	O
of	O
the	O
graphics	O
display	O
device	O
,	O
and	O
to	O
provide	O
hardware	B-Architecture
independence	O
if	O
the	O
computer	O
or	O
the	O
display	O
were	O
replaced	O
.	O
</s>
<s>
The	O
term	O
was	O
introduced	O
to	O
the	O
field	O
of	O
databases	B-Application
by	O
C	O
.	O
J	O
.	O
</s>
<s>
Date	O
in	O
a	O
1974	O
paper	O
called	O
The	O
Relational	B-Application
and	O
Network	B-General_Concept
Approaches	O
:	O
Comparison	O
of	O
the	O
Application	B-Application
Programming	I-Application
Interface	I-Application
.	O
</s>
<s>
An	O
API	B-Application
became	O
a	O
part	O
of	O
the	O
ANSI/SPARC	B-General_Concept
framework	I-General_Concept
for	O
database	B-Application
management	I-Application
systems	I-Application
.	O
</s>
<s>
This	O
framework	O
treated	O
the	O
application	B-Application
programming	I-Application
interface	I-Application
separately	O
from	O
other	O
interfaces	B-Application
,	O
such	O
as	O
the	O
query	O
interface	B-Application
.	O
</s>
<s>
Database	O
professionals	O
in	O
the	O
1970s	O
observed	O
these	O
different	O
interfaces	B-Application
could	O
be	O
combined	O
;	O
a	O
sufficiently	O
rich	O
application	O
interface	B-Application
could	O
support	O
the	O
other	O
interfaces	B-Application
as	O
well	O
.	O
</s>
<s>
This	O
observation	O
led	O
to	O
APIs	B-Application
that	O
supported	O
all	O
types	O
of	O
programming	O
,	O
not	O
just	O
application	O
programming	O
.	O
</s>
<s>
By	O
1990	O
,	O
the	O
API	B-Application
was	O
defined	O
simply	O
as	O
"	O
a	O
set	O
of	O
services	O
available	O
to	O
a	O
programmer	B-Application
for	O
performing	O
certain	O
tasks	O
"	O
by	O
technologist	O
Carl	O
Malamud	O
.	O
</s>
<s>
The	O
idea	O
of	O
the	O
API	B-Application
was	O
expanded	O
again	O
with	O
the	O
dawn	O
of	O
remote	B-Operating_System
procedure	I-Operating_System
calls	I-Operating_System
and	O
web	B-General_Concept
APIs	I-General_Concept
.	O
</s>
<s>
As	O
computer	B-Architecture
networks	I-Architecture
became	O
common	O
in	O
the	O
1970s	O
and	O
1980s	O
,	O
programmers	B-Application
wanted	O
to	O
call	O
libraries	O
located	O
not	O
only	O
on	O
their	O
local	O
computers	O
but	O
on	O
computers	O
located	O
elsewhere	O
.	O
</s>
<s>
These	O
remote	B-Operating_System
procedure	I-Operating_System
calls	I-Operating_System
were	O
well	O
supported	O
by	O
the	O
Java	B-Language
language	I-Language
in	O
particular	O
.	O
</s>
<s>
In	O
the	O
1990s	O
,	O
with	O
the	O
spread	O
of	O
the	O
internet	O
,	O
standards	O
like	O
CORBA	B-Architecture
,	O
COM	B-Application
,	O
and	O
DCOM	B-Operating_System
competed	O
to	O
become	O
the	O
most	O
common	O
way	O
to	O
expose	O
API	B-Application
services	O
.	O
</s>
<s>
Roy	O
Fielding	O
's	O
dissertation	O
Architectural	O
Styles	O
and	O
the	O
Design	O
of	O
Network-based	O
Software	B-Architecture
Architectures	I-Architecture
at	O
UC	O
Irvine	O
in	O
2000	O
outlined	O
Representational	B-Protocol
state	I-Protocol
transfer	I-Protocol
(	O
REST	O
)	O
and	O
described	O
the	O
idea	O
of	O
a	O
"	O
network-based	O
Application	B-Application
Programming	I-Application
Interface	I-Application
"	O
that	O
Fielding	O
contrasted	O
with	O
traditional	O
"	O
library-based	O
"	O
APIs	B-Application
.	O
</s>
<s>
XML	B-Protocol
and	O
JSON	B-General_Concept
web	B-General_Concept
APIs	I-General_Concept
saw	O
widespread	O
commercial	O
adoption	O
beginning	O
in	O
2000	O
and	O
continuing	O
as	O
of	O
2022	O
.	O
</s>
<s>
The	O
web	B-General_Concept
API	I-General_Concept
is	O
now	O
the	O
most	O
common	O
meaning	O
of	O
the	O
term	O
API	B-Application
.	O
</s>
<s>
The	O
Semantic	O
Web	O
proposed	O
by	O
Tim	O
Berners-Lee	O
in	O
2001	O
included	O
"	O
semantic	O
APIs	B-Application
"	O
that	O
recasts	O
the	O
API	B-Application
as	O
an	O
open	O
,	O
distributed	O
data	O
interface	B-Application
rather	O
than	O
a	O
software	O
behavior	O
interface	B-Application
.	O
</s>
<s>
Proprietary	B-Application
interfaces	B-Application
and	O
agents	O
became	O
more	O
widespread	O
than	O
open	O
ones	O
,	O
but	O
the	O
idea	O
of	O
the	O
API	B-Application
as	O
a	O
data	O
interface	B-Application
took	O
hold	O
.	O
</s>
<s>
Because	O
web	B-General_Concept
APIs	I-General_Concept
are	O
widely	O
used	O
to	O
exchange	O
data	O
of	O
all	O
kinds	O
online	O
,	O
API	B-Application
has	O
become	O
a	O
broad	O
term	O
describing	O
much	O
of	O
the	O
communication	O
on	O
the	O
internet	O
.	O
</s>
<s>
When	O
used	O
in	O
this	O
way	O
,	O
the	O
term	O
API	B-Application
has	O
overlap	O
in	O
meaning	O
with	O
the	O
term	O
communication	O
protocol	O
.	O
</s>
<s>
The	O
interface	B-Application
to	O
a	O
software	B-Library
library	I-Library
is	O
one	O
type	O
of	O
API	B-Application
.	O
</s>
<s>
The	O
API	B-Application
describes	O
and	O
prescribes	O
the	O
"	O
expected	O
behavior	O
"	O
(	O
a	O
specification	O
)	O
while	O
the	O
library	O
is	O
an	O
"	O
actual	O
implementation	O
"	O
of	O
this	O
set	O
of	O
rules	O
.	O
</s>
<s>
A	O
single	O
API	B-Application
can	O
have	O
multiple	O
implementations	O
(	O
or	O
none	O
,	O
being	O
abstract	O
)	O
in	O
the	O
form	O
of	O
different	O
libraries	O
that	O
share	O
the	O
same	O
programming	B-Application
interface	I-Application
.	O
</s>
<s>
The	O
separation	O
of	O
the	O
API	B-Application
from	O
its	O
implementation	O
can	O
allow	O
programs	O
written	O
in	O
one	O
language	O
to	O
use	O
a	O
library	O
written	O
in	O
another	O
.	O
</s>
<s>
For	O
example	O
,	O
because	O
Scala	B-Application
and	O
Java	B-Language
compile	B-Language
to	O
compatible	O
bytecode	O
,	O
Scala	B-Application
developers	B-Application
can	O
take	O
advantage	O
of	O
any	O
Java	B-Language
API	I-Language
.	O
</s>
<s>
API	B-Application
use	O
can	O
vary	O
depending	O
on	O
the	O
type	O
of	O
programming	O
language	O
involved	O
.	O
</s>
<s>
An	O
API	B-Application
for	O
a	O
procedural	B-Application
language	I-Application
such	O
as	O
Lua	B-Language
could	O
consist	O
primarily	O
of	O
basic	O
routines	O
to	O
execute	O
code	O
,	O
manipulate	O
data	O
or	O
handle	O
errors	O
while	O
an	O
API	B-Application
for	O
an	O
object-oriented	B-Language
language	I-Language
,	O
such	O
as	O
Java	B-Language
,	O
would	O
provide	O
a	O
specification	O
of	O
classes	O
and	O
its	O
class	B-Application
methods	I-Application
.	O
</s>
<s>
Hyrum	O
's	O
law	O
states	O
that	O
"	O
With	O
a	O
sufficient	O
number	O
of	O
users	O
of	O
an	O
API	B-Application
,	O
it	O
does	O
not	O
matter	O
what	O
you	O
promise	O
in	O
the	O
contract	O
:	O
all	O
observable	O
behaviors	O
of	O
your	O
system	O
will	O
be	O
depended	O
on	O
by	O
somebody.	O
"	O
</s>
<s>
Meanwhile	O
,	O
several	O
studies	O
show	O
that	O
most	O
applications	O
that	O
use	O
an	O
API	B-Application
tend	O
to	O
use	O
a	O
small	O
part	O
of	O
the	O
API	B-Application
.	O
</s>
<s>
Language	B-Application
bindings	I-Application
are	O
also	O
APIs	B-Application
.	O
</s>
<s>
By	O
mapping	O
the	O
features	O
and	O
capabilities	O
of	O
one	O
language	O
to	O
an	O
interface	B-Application
implemented	O
in	O
another	O
language	O
,	O
a	O
language	B-Application
binding	I-Application
allows	O
a	O
library	O
or	O
service	O
written	O
in	O
one	O
language	O
to	O
be	O
used	O
when	O
developing	O
in	O
another	O
language	O
.	O
</s>
<s>
Tools	O
such	O
as	O
SWIG	B-Application
and	O
F2PY	O
,	O
a	O
Fortran-to-Python	O
interface	B-Application
generator	O
,	O
facilitate	O
the	O
creation	O
of	O
such	O
interfaces	B-Application
.	O
</s>
<s>
An	O
API	B-Application
can	O
also	O
be	O
related	O
to	O
a	O
software	B-Architecture
framework	I-Architecture
:	O
a	O
framework	O
can	O
be	O
based	O
on	O
several	O
libraries	O
implementing	O
several	O
APIs	B-Application
,	O
but	O
unlike	O
the	O
normal	O
use	O
of	O
an	O
API	B-Application
,	O
the	O
access	O
to	O
the	O
behavior	O
built	O
into	O
the	O
framework	O
is	O
mediated	O
by	O
extending	O
its	O
content	O
with	O
new	O
classes	O
plugged	O
into	O
the	O
framework	O
itself	O
.	O
</s>
<s>
Moreover	O
,	O
the	O
overall	O
program	O
flow	O
of	O
control	O
can	O
be	O
out	O
of	O
the	O
control	O
of	O
the	O
caller	O
and	O
in	O
the	O
framework	O
's	O
hands	O
by	O
inversion	B-Language
of	I-Language
control	I-Language
or	O
a	O
similar	O
mechanism	O
.	O
</s>
<s>
An	O
API	B-Application
can	O
specify	O
the	O
interface	B-Application
between	O
an	O
application	O
and	O
the	O
operating	B-General_Concept
system	I-General_Concept
.	O
</s>
<s>
POSIX	O
,	O
for	O
example	O
,	O
provides	O
a	O
set	O
of	O
common	O
API	B-Application
specifications	O
that	O
aim	O
to	O
enable	O
an	O
application	O
written	O
for	O
a	O
POSIX	O
conformant	O
operating	B-General_Concept
system	I-General_Concept
to	O
be	O
compiled	B-Language
for	O
another	O
POSIX	O
conformant	O
operating	B-General_Concept
system	I-General_Concept
.	O
</s>
<s>
Linux	B-Application
and	O
Berkeley	B-Operating_System
Software	I-Operating_System
Distribution	I-Operating_System
are	O
examples	O
of	O
operating	B-General_Concept
systems	I-General_Concept
that	O
implement	O
the	O
POSIX	O
APIs	B-Application
.	O
</s>
<s>
Microsoft	O
has	O
shown	O
a	O
strong	O
commitment	O
to	O
a	O
backward-compatible	O
API	B-Application
,	O
particularly	O
within	O
its	O
Windows	B-Library
API	I-Library
(	O
Win32	B-Library
)	O
library	O
,	O
so	O
older	O
applications	O
may	O
run	O
on	O
newer	O
versions	O
of	O
Windows	O
using	O
an	O
executable-specific	O
setting	O
called	O
"	O
Compatibility	O
Mode	O
"	O
.	O
</s>
<s>
An	O
API	B-Application
differs	O
from	O
an	O
application	B-Operating_System
binary	I-Operating_System
interface	I-Operating_System
(	O
ABI	O
)	O
in	O
that	O
an	O
API	B-Application
is	O
source	O
code	O
based	O
while	O
an	O
ABI	O
is	O
binary	O
based	O
.	O
</s>
<s>
For	O
instance	O
,	O
POSIX	O
provides	O
APIs	B-Application
while	O
the	O
Linux	B-Application
Standard	I-Application
Base	I-Application
provides	O
an	O
ABI	O
.	O
</s>
<s>
Remote	O
APIs	B-Application
allow	O
developers	B-Application
to	O
manipulate	O
remote	O
resources	O
through	O
protocols	O
,	O
specific	O
standards	O
for	O
communication	O
that	O
allow	O
different	O
technologies	O
to	O
work	O
together	O
,	O
regardless	O
of	O
language	O
or	O
platform	O
.	O
</s>
<s>
For	O
example	O
,	O
the	O
Java	B-Language
Database	O
Connectivity	O
API	B-Application
allows	O
developers	B-Application
to	O
query	O
many	O
different	O
types	O
of	O
databases	B-Application
with	O
the	O
same	O
set	O
of	O
functions	O
,	O
while	O
the	O
Java	B-Language
remote	I-Language
method	I-Language
invocation	I-Language
API	B-Application
uses	O
the	O
Java	B-Language
Remote	O
Method	O
Protocol	O
to	O
allow	O
invocation	B-Operating_System
of	O
functions	O
that	O
operate	O
remotely	O
but	O
appear	O
local	O
to	O
the	O
developer	B-Application
.	O
</s>
<s>
Therefore	O
,	O
remote	O
APIs	B-Application
are	O
useful	O
in	O
maintaining	O
the	O
object	O
abstraction	B-Application
in	O
object-oriented	B-Language
programming	I-Language
;	O
a	O
method	B-Language
call	I-Language
,	O
executed	O
locally	O
on	O
a	O
proxy	O
object	O
,	O
invokes	O
the	O
corresponding	O
method	O
on	O
the	O
remote	O
object	O
,	O
using	O
the	O
remoting	B-Operating_System
protocol	O
,	O
and	O
acquires	O
the	O
result	O
to	O
be	O
used	O
locally	O
as	O
a	O
return	O
value	O
.	O
</s>
<s>
Web	B-General_Concept
APIs	I-General_Concept
are	O
a	O
service	O
accessed	O
from	O
client	O
devices	O
(	O
Mobile	O
Phones	O
,	O
Laptop	O
,	O
etc	O
.	O
)	O
</s>
<s>
to	O
a	O
web	O
server	O
using	O
the	O
Hypertext	B-Protocol
Transfer	I-Protocol
Protocol	I-Protocol
(	O
HTTP	B-Protocol
)	O
.	O
</s>
<s>
Client	O
devices	O
send	O
a	O
request	O
in	O
the	O
form	O
of	O
an	O
HTTP	B-Protocol
request	O
,	O
and	O
are	O
met	O
with	O
a	O
response	O
message	O
usually	O
in	O
JavaScript	B-General_Concept
Object	I-General_Concept
Notation	I-General_Concept
(	O
JSON	B-General_Concept
)	O
or	O
Extensible	B-Protocol
Markup	I-Protocol
Language	I-Protocol
(	O
XML	B-Protocol
)	O
format	O
.	O
</s>
<s>
Developers	B-Application
typically	O
use	O
Web	B-General_Concept
APIs	I-General_Concept
to	O
query	O
a	O
server	O
for	O
a	O
specific	O
set	O
of	O
data	O
from	O
that	O
server	O
.	O
</s>
<s>
An	O
example	O
might	O
be	O
a	O
shipping	O
company	O
API	B-Application
that	O
can	O
be	O
added	O
to	O
an	O
eCommerce-focused	O
website	O
to	O
facilitate	O
ordering	O
shipping	O
services	O
and	O
automatically	O
include	O
current	O
shipping	O
rates	O
,	O
without	O
the	O
site	O
developer	B-Application
having	O
to	O
enter	O
the	O
shipper	O
's	O
rate	O
table	O
into	O
a	O
web	O
database	O
.	O
</s>
<s>
While	O
"	O
web	B-General_Concept
API	I-General_Concept
"	O
historically	O
has	O
been	O
virtually	O
synonymous	O
with	O
web	O
service	O
,	O
the	O
recent	O
trend	O
(	O
so-called	O
Web	O
2.0	O
)	O
has	O
been	O
moving	O
away	O
from	O
Simple	O
Object	O
Access	O
Protocol	O
(	O
SOAP	B-Application
)	O
based	O
web	O
services	O
and	O
service-oriented	O
architecture	O
(	O
SOA	O
)	O
towards	O
more	O
direct	O
representational	B-Protocol
state	I-Protocol
transfer	I-Protocol
(	O
REST	O
)	O
style	O
web	O
resources	O
and	O
resource-oriented	B-Architecture
architecture	I-Architecture
(	O
ROA	O
)	O
.	O
</s>
<s>
Web	B-General_Concept
APIs	I-General_Concept
allow	O
the	O
combination	O
of	O
multiple	O
APIs	B-Application
into	O
new	O
applications	O
known	O
as	O
mashups	B-Application
.	O
</s>
<s>
In	O
the	O
social	O
media	O
space	O
,	O
web	B-General_Concept
APIs	I-General_Concept
have	O
allowed	O
web	O
communities	O
to	O
facilitate	O
sharing	O
content	O
and	O
data	O
between	O
communities	O
and	O
applications	O
.	O
</s>
<s>
For	O
example	O
,	O
Twitter	O
's	O
REST	B-Protocol
API	I-Protocol
allows	O
developers	B-Application
to	O
access	O
core	O
Twitter	O
data	O
and	O
the	O
Search	O
API	B-Application
provides	O
methods	O
for	O
developers	B-Application
to	O
interact	O
with	O
Twitter	O
Search	O
and	O
trends	O
data	O
.	O
</s>
<s>
The	O
design	O
of	O
an	O
API	B-Application
has	O
a	O
significant	O
impact	O
on	O
its	O
usage	O
.	O
</s>
<s>
First	O
of	O
all	O
,	O
the	O
design	O
of	O
programming	B-Application
interfaces	I-Application
represents	O
an	O
important	O
part	O
of	O
software	B-Architecture
architecture	I-Architecture
,	O
the	O
organization	O
of	O
a	O
complex	O
piece	O
of	O
software	O
.	O
</s>
<s>
The	O
principle	O
of	O
information	B-Application
hiding	I-Application
describes	O
the	O
role	O
of	O
programming	B-Application
interfaces	I-Application
as	O
enabling	O
modular	B-Architecture
programming	I-Architecture
by	O
hiding	O
the	O
implementation	O
details	O
of	O
the	O
modules	B-Architecture
so	O
that	O
users	O
of	O
modules	B-Architecture
need	O
not	O
understand	O
the	O
complexities	O
inside	O
the	O
modules	B-Architecture
.	O
</s>
<s>
Aside	O
from	O
the	O
previous	O
underlying	O
principle	O
,	O
other	O
metrics	O
for	O
measuring	O
the	O
usability	O
of	O
an	O
API	B-Application
may	O
include	O
properties	O
such	O
as	O
functional	O
efficiency	O
,	O
overall	O
correctness	O
,	O
and	O
learnability	O
for	O
novices	O
.	O
</s>
<s>
One	O
straightforward	O
and	O
commonly	O
adopted	O
way	O
of	O
designing	O
APIs	B-Application
is	O
to	O
follow	O
Nielsen	O
's	O
heuristic	O
evaluation	O
guidelines	O
.	O
</s>
<s>
The	O
Factory	B-Language
method	I-Language
pattern	I-Language
is	O
also	O
typical	O
in	O
designing	O
APIs	B-Application
due	O
to	O
their	O
reusable	O
nature	O
.	O
</s>
<s>
Thus	O
,	O
the	O
design	O
of	O
an	O
API	B-Application
attempts	O
to	O
provide	O
only	O
the	O
tools	O
a	O
user	O
would	O
expect	O
.	O
</s>
<s>
An	O
application	B-Application
programming	I-Application
interface	I-Application
can	O
be	O
synchronous	B-Operating_System
or	I-Operating_System
asynchronous	I-Operating_System
.	O
</s>
<s>
A	O
synchronous	O
API	B-Application
call	O
is	O
a	O
design	O
pattern	O
where	O
the	O
call	O
site	O
is	O
blocked	O
while	O
waiting	O
for	O
the	O
called	O
code	O
to	O
finish	O
.	O
</s>
<s>
With	O
a	O
asynchronous	O
API	B-Application
call	O
,	O
however	O
,	O
the	O
call	O
site	O
is	O
not	O
blocked	O
while	O
waiting	O
for	O
the	O
called	O
code	O
to	O
finish	O
,	O
and	O
instead	O
the	O
calling	O
thread	O
is	O
notified	O
when	O
the	O
reply	O
arrives	O
.	O
</s>
<s>
API	B-Application
security	O
is	O
very	O
critical	O
when	O
developing	O
a	O
public	O
facing	O
API	B-Application
.	O
</s>
<s>
Common	O
threats	O
include	O
SQL	B-Language
injection	I-Language
,	O
Denial-of-service	O
attack	O
(	O
DoS	O
)	O
,	O
broken	O
authentication	O
,	O
and	O
exposing	O
sensitive	O
data	O
.	O
</s>
<s>
Some	O
common	O
security	O
practices	O
include	O
proper	O
connection	O
security	O
using	O
HTTPS	B-Protocol
,	O
content	O
security	O
to	O
mitigate	O
data	O
injection	O
attacks	O
,	O
and	O
requiring	O
an	O
API	B-Application
key	O
to	O
use	O
your	O
service	O
.	O
</s>
<s>
Many	O
public	O
facing	O
API	B-Application
services	O
require	O
you	O
to	O
use	O
an	O
assigned	O
API	B-Application
key	O
,	O
and	O
will	O
refuse	O
to	O
serve	O
data	O
without	O
sending	O
the	O
key	O
with	O
your	O
request	O
.	O
</s>
<s>
APIs	B-Application
are	O
one	O
of	O
the	O
more	O
common	O
ways	O
technology	O
companies	O
integrate	O
.	O
</s>
<s>
Those	O
that	O
provide	O
and	O
use	O
APIs	B-Application
are	O
considered	O
as	O
being	O
members	O
of	O
a	O
business	O
ecosystem	O
.	O
</s>
<s>
The	O
main	O
policies	O
for	O
releasing	O
an	O
API	B-Application
are	O
:	O
</s>
<s>
Private	O
:	O
The	O
API	B-Application
is	O
for	O
internal	O
company	O
use	O
only	O
.	O
</s>
<s>
Partner	O
:	O
Only	O
specific	O
business	O
partners	O
can	O
use	O
the	O
API	B-Application
.	O
</s>
<s>
For	O
example	O
,	O
vehicle	O
for	O
hire	O
companies	O
such	O
as	O
Uber	B-Application
and	O
Lyft	B-Application
allow	O
approved	O
third-party	O
developers	B-Application
to	O
directly	O
order	O
rides	O
from	O
within	O
their	O
apps	O
.	O
</s>
<s>
This	O
allows	O
the	O
companies	O
to	O
exercise	O
quality	O
control	O
by	O
curating	O
which	O
apps	O
have	O
access	O
to	O
the	O
API	B-Application
and	O
provides	O
them	O
with	O
an	O
additional	O
revenue	O
stream	O
.	O
</s>
<s>
Public	O
:	O
The	O
API	B-Application
is	O
available	O
for	O
use	O
by	O
the	O
public	O
.	O
</s>
<s>
For	O
example	O
,	O
Microsoft	O
makes	O
the	O
Windows	B-Library
API	I-Library
public	O
,	O
and	O
Apple	O
releases	O
its	O
API	B-Application
Cocoa	B-Operating_System
so	O
that	O
software	O
can	O
be	O
written	O
for	O
their	O
platforms	B-Device
.	O
</s>
<s>
Not	O
all	O
public	O
APIs	B-Application
are	O
generally	O
accessible	O
by	O
everybody	O
.	O
</s>
<s>
For	O
example	O
,	O
Internet	O
service	O
providers	O
like	O
Cloudflare	O
or	O
Voxility	O
,	O
use	O
RESTful	B-Protocol
APIs	I-Protocol
to	O
allow	O
customers	O
and	O
resellers	O
access	O
to	O
their	O
infrastructure	O
information	O
,	O
DDoS	O
stats	O
,	O
network	B-General_Concept
performance	O
,	O
or	O
dashboard	O
controls	O
.	O
</s>
<s>
Access	O
to	O
such	O
APIs	B-Application
is	O
granted	O
either	O
by	O
"	O
API	B-Application
tokens	O
"	O
,	O
or	O
customer	O
status	O
validations	O
.	O
</s>
<s>
An	O
important	O
factor	O
when	O
an	O
API	B-Application
becomes	O
public	O
is	O
its	O
"	O
interface	B-Application
stability	O
"	O
.	O
</s>
<s>
Changes	O
to	O
the	O
API	B-Application
—	O
for	O
example	O
adding	O
new	O
parameters	O
to	O
a	O
function	O
call	O
—	O
could	O
break	O
compatibility	O
with	O
the	O
clients	O
that	O
depend	O
on	O
that	O
API	B-Application
.	O
</s>
<s>
When	O
parts	O
of	O
a	O
publicly	O
presented	O
API	B-Application
are	O
subject	O
to	O
change	O
and	O
thus	O
not	O
stable	O
,	O
such	O
parts	O
of	O
a	O
particular	O
API	B-Application
should	O
be	O
documented	O
explicitly	O
as	O
"	O
unstable	O
"	O
.	O
</s>
<s>
For	O
example	O
,	O
in	O
the	O
Google	B-Language
Guava	I-Language
library	O
,	O
the	O
parts	O
that	O
are	O
considered	O
unstable	O
,	O
and	O
that	O
might	O
change	O
soon	O
,	O
are	O
marked	O
with	O
the	O
Java	B-Language
annotation	I-Language
@Beta	O
.	O
</s>
<s>
A	O
public	O
API	B-Application
can	O
sometimes	O
declare	O
parts	O
of	O
itself	O
as	O
deprecated	O
or	O
rescinded	O
.	O
</s>
<s>
This	O
usually	O
means	O
that	O
part	O
of	O
the	O
API	B-Application
should	O
be	O
considered	O
a	O
candidate	O
for	O
being	O
removed	O
,	O
or	O
modified	O
in	O
a	O
backward	O
incompatible	O
way	O
.	O
</s>
<s>
Therefore	O
,	O
these	O
changes	O
allow	O
developers	B-Application
to	O
transition	O
away	O
from	O
parts	O
of	O
the	O
API	B-Application
that	O
will	O
be	O
removed	O
or	O
not	O
supported	O
in	O
the	O
future	O
.	O
</s>
<s>
On	O
February	O
19	O
,	O
2020	O
,	O
Akamai	B-Application
published	O
their	O
annual	O
"	O
State	O
of	O
the	O
Internet	O
"	O
report	O
,	O
showcasing	O
the	O
growing	O
trend	O
of	O
cybercriminals	O
targeting	O
public	O
API	B-Application
platforms	B-Device
at	O
financial	O
services	O
worldwide	O
.	O
</s>
<s>
From	O
December	O
2017	O
through	O
November	O
2019	O
,	O
Akamai	B-Application
witnessed	O
85.42	O
billion	O
credential	O
violation	O
attacks	O
.	O
</s>
<s>
About	O
20%	O
,	O
or	O
16.55	O
billion	O
,	O
were	O
against	O
hostnames	O
defined	O
as	O
API	B-Application
endpoints	O
.	O
</s>
<s>
API	B-Application
documentation	I-Application
describes	O
the	O
services	O
an	O
API	B-Application
offers	O
and	O
how	O
to	O
use	O
those	O
services	O
,	O
aiming	O
to	O
cover	O
everything	O
a	O
client	O
would	O
need	O
to	O
know	O
for	O
practical	O
purposes	O
.	O
</s>
<s>
Documentation	O
is	O
crucial	O
for	O
the	O
development	O
and	O
maintenance	O
of	O
applications	O
using	O
the	O
API	B-Application
.	O
</s>
<s>
API	B-Application
documentation	I-Application
is	O
traditionally	O
found	O
in	O
documentation	O
files	O
but	O
can	O
also	O
be	O
found	O
in	O
social	O
media	O
such	O
as	O
blogs	O
,	O
forums	O
,	O
and	O
Q&A	O
websites	O
.	O
</s>
<s>
However	O
,	O
the	O
types	O
of	O
content	O
included	O
in	O
the	O
documentation	O
differ	O
from	O
API	B-Application
to	O
API	B-Application
.	O
</s>
<s>
In	O
the	O
interest	O
of	O
clarity	O
,	O
API	B-Application
documentation	I-Application
may	O
include	O
a	O
description	O
of	O
classes	O
and	O
methods	O
in	O
the	O
API	B-Application
as	O
well	O
as	O
"	O
typical	O
usage	O
scenarios	O
,	O
code	O
snippets	O
,	O
design	O
rationales	O
,	O
performance	O
discussions	O
,	O
and	O
contracts	O
"	O
,	O
but	O
implementation	O
details	O
of	O
the	O
API	B-Application
services	O
themselves	O
are	O
usually	O
omitted	O
.	O
</s>
<s>
Reference	O
documentation	O
for	O
a	O
REST	B-Protocol
API	I-Protocol
can	O
be	O
generated	O
automatically	O
from	O
an	O
OpenAPI	O
document	O
,	O
which	O
is	O
a	O
machine-readable	O
text	O
file	O
that	O
uses	O
a	O
prescribed	O
format	O
and	O
syntax	O
defined	O
in	O
the	O
OpenAPI	B-Architecture
Specification	I-Architecture
.	O
</s>
<s>
The	O
OpenAPI	O
document	O
defines	O
basic	O
information	O
such	O
as	O
the	O
API	B-Application
's	O
name	O
and	O
description	O
,	O
as	O
well	O
as	O
describing	O
operations	O
the	O
API	B-Application
provides	O
access	O
to	O
.	O
</s>
<s>
API	B-Application
documentation	I-Application
can	O
be	O
enriched	O
with	O
metadata	O
information	O
like	O
Java	B-Language
annotations	I-Language
.	O
</s>
<s>
This	O
metadata	O
can	O
be	O
used	O
by	O
the	O
compiler	B-Language
,	O
tools	O
,	O
and	O
by	O
the	O
run-time	O
environment	O
to	O
implement	O
custom	O
behaviors	O
or	O
custom	O
handling	O
.	O
</s>
<s>
In	O
2010	O
,	O
Oracle	O
Corporation	O
sued	O
Google	O
for	O
having	O
distributed	O
a	O
new	O
implementation	O
of	O
Java	B-Language
embedded	O
in	O
the	O
Android	O
operating	B-General_Concept
system	I-General_Concept
.	O
</s>
<s>
Google	O
had	O
not	O
acquired	O
any	O
permission	O
to	O
reproduce	O
the	O
Java	B-Language
API	I-Language
,	O
although	O
permission	O
had	O
been	O
given	O
to	O
the	O
similar	O
OpenJDK	O
project	O
.	O
</s>
<s>
Google	O
had	O
approached	O
Oracle	O
to	O
negotiate	O
a	O
license	O
for	O
their	O
API	B-Application
,	O
but	O
were	O
turned	O
down	O
due	O
to	O
trust	O
issues	O
.	O
</s>
<s>
Judge	O
William	O
Alsup	O
ruled	O
in	O
the	O
Oracle	B-Language
v	I-Language
.	I-Language
Google	I-Language
case	O
that	O
APIs	B-Application
cannot	O
be	O
copyrighted	O
in	O
the	O
U.S	O
and	O
that	O
a	O
victory	O
for	O
Oracle	O
would	O
have	O
widely	O
expanded	O
copyright	O
protection	O
to	O
a	O
"	O
functional	O
set	O
of	O
symbols	O
"	O
and	O
allowed	O
the	O
copyrighting	O
of	O
simple	O
software	O
commands	O
:	O
</s>
<s>
Alsup	O
's	O
ruling	O
was	O
overturned	O
in	O
2014	O
on	O
appeal	O
to	O
the	O
Court	O
of	O
Appeals	O
for	O
the	O
Federal	O
Circuit	O
,	O
though	O
the	O
question	O
of	O
whether	O
such	O
use	O
of	O
APIs	B-Application
constitutes	O
fair	O
use	O
was	O
left	O
unresolved	O
.	O
</s>
<s>
In	O
2016	O
,	O
following	O
a	O
two-week	O
trial	O
,	O
a	O
jury	O
determined	O
that	O
Google	O
's	O
reimplementation	O
of	O
the	O
Java	B-Language
API	I-Language
constituted	O
fair	O
use	O
,	O
but	O
Oracle	O
vowed	O
to	O
appeal	O
the	O
decision	O
.	O
</s>
<s>
Oracle	O
won	O
on	O
its	O
appeal	O
,	O
with	O
the	O
Court	O
of	O
Appeals	O
for	O
the	O
Federal	O
Circuit	O
ruling	O
that	O
Google	O
's	O
use	O
of	O
the	O
APIs	B-Application
did	O
not	O
qualify	O
for	O
fair	O
use	O
.	O
</s>
<s>
Justice	O
Stephen	O
Breyer	O
delivered	O
the	O
opinion	O
of	O
the	O
court	O
and	O
at	O
one	O
point	O
mentioned	O
that	O
"	O
The	O
declaring	O
code	O
is	O
,	O
if	O
copyrightable	O
at	O
all	O
,	O
further	O
than	O
are	O
most	O
computer	B-Application
programs	I-Application
from	O
the	O
core	O
of	O
copyright.	O
"	O
</s>
<s>
This	O
means	O
the	O
code	O
used	O
in	O
APIs	B-Application
are	O
more	O
similar	O
to	O
dictionaries	O
than	O
novels	O
in	O
terms	O
of	O
copyright	O
protection	O
.	O
</s>
<s>
OpenMP	B-Application
API	B-Application
that	O
supports	O
multi-platform	O
shared	O
memory	O
multiprocessing	O
programming	O
in	O
C	O
,	O
C++	O
,	O
and	O
Fortran	B-Application
on	O
many	O
architectures	O
,	O
including	O
Unix	O
and	O
Microsoft	B-Application
Windows	I-Application
platforms	B-Device
.	O
</s>
