<s>
RESTful	O
(	O
representational	O
state	O
transfer	O
)	O
API	B-General_Concept
(	O
application	O
programming	O
interface	O
)	O
DLs	O
(	O
description	O
languages	O
)	O
are	O
formal	O
languages	O
designed	O
to	O
provide	O
a	O
structured	O
description	O
of	O
a	O
RESTful	O
web	B-General_Concept
API	I-General_Concept
that	O
is	O
useful	O
both	O
to	O
a	O
human	O
and	O
for	O
automated	O
machine	O
processing	O
.	O
</s>
<s>
API	B-General_Concept
description	O
languages	O
are	O
sometimes	O
called	O
interface	B-General_Concept
description	I-General_Concept
languages	I-General_Concept
(	O
IDLs	O
)	O
.	O
</s>
<s>
The	O
structured	O
description	O
might	O
be	O
used	O
to	O
generate	O
documentation	O
for	O
human	O
programmers	B-Application
;	O
such	O
documentation	O
may	O
be	O
easier	O
to	O
read	O
than	O
free-form	O
documentation	O
,	O
since	O
all	O
documentation	O
generated	O
by	O
the	O
same	O
tool	O
follows	O
the	O
same	O
formatting	O
conventions	O
.	O
</s>
<s>
Additionally	O
,	O
the	O
description	O
language	O
is	O
usually	O
precise	O
enough	O
to	O
allow	O
automated	O
generation	O
of	O
various	O
software	O
artifacts	O
,	O
like	O
libraries	O
,	O
to	O
access	O
the	O
API	B-General_Concept
from	O
various	O
programming	O
languages	O
,	O
which	O
takes	O
the	O
burden	O
of	O
manually	O
creating	O
them	O
off	O
the	O
programmers	B-Application
.	O
</s>
<s>
There	O
are	O
two	O
previous	O
major	O
description	O
languages	O
:	O
WSDL2.0	O
(	O
Web	O
Services	O
Description	O
Language	O
)	O
and	O
WADL	B-Language
(	O
Web	B-Language
Application	I-Language
Description	I-Language
Language	I-Language
)	O
.	O
</s>
<s>
Neither	O
is	O
widely	O
adopted	O
in	O
the	O
industry	O
for	O
describing	O
RESTful	O
APIs	B-General_Concept
,	O
citing	O
poor	O
human	O
readability	O
of	O
both	O
and	O
WADL	B-Language
being	O
actually	O
unable	O
to	O
fully	O
describe	O
a	O
RESTful	O
API	B-General_Concept
.	O
</s>
<s>
The	O
principle	O
behind	O
building	O
RESTful	O
APIs	B-General_Concept
is	O
known	O
under	O
the	O
acronym	O
HATEOAS	B-Operating_System
(	O
Hypermedia	B-Operating_System
as	I-Operating_System
the	I-Operating_System
Engine	I-Operating_System
of	I-Operating_System
Application	I-Operating_System
State	I-Operating_System
)	O
.	O
</s>
<s>
Instead	O
,	O
the	O
client	O
is	O
given	O
a	O
set	O
of	O
entry	O
points	O
and	O
the	O
API	B-General_Concept
is	O
discovered	O
dynamically	O
through	O
interaction	O
with	O
these	O
endpoints	O
.	O
</s>
<s>
HATEOAS	B-Operating_System
was	O
introduced	O
in	O
Roy	O
Fielding	O
's	O
doctoral	O
thesis	O
Architectural	O
Styles	O
and	O
the	O
Design	O
of	O
Network-based	O
Software	O
Architectures	O
.	O
</s>
<s>
HATEOAS	B-Operating_System
is	O
one	O
of	O
the	O
key	O
elements	O
distinguishing	O
REST	O
from	O
RPC	B-Operating_System
mechanisms	O
.	O
</s>
