<s>
A	O
persistent	B-Language
uniform	I-Language
resource	I-Language
locator	I-Language
(	O
PURL	B-Language
)	O
is	O
a	O
uniform	O
resource	O
locator	O
(	O
URL	O
)	O
(	O
i.e.	O
,	O
location-based	O
uniform	B-Protocol
resource	I-Protocol
identifier	I-Protocol
or	O
URI	B-Protocol
)	O
that	O
is	O
used	O
to	O
redirect	O
to	O
the	O
location	O
of	O
the	O
requested	O
web	O
resource	O
.	O
</s>
<s>
PURLs	B-Language
redirect	O
HTTP	B-Protocol
clients	O
using	O
HTTP	B-Protocol
status	I-Protocol
codes	I-Protocol
.	O
</s>
<s>
Originally	O
,	O
PURLs	B-Language
were	O
recognizable	O
for	O
being	O
hosted	O
at	O
or	O
other	O
hostnames	O
containing	O
purl	B-Language
.	O
</s>
<s>
Early	O
on	O
many	O
of	O
those	O
other	O
hosts	O
used	O
descendants	O
of	O
the	O
original	O
OCLC	O
PURL	B-Language
system	O
software	O
.	O
</s>
<s>
Eventually	O
,	O
however	O
,	O
the	O
PURL	B-Language
concept	O
came	O
to	O
be	O
generic	O
and	O
was	O
used	O
to	O
designate	O
any	O
redirection	O
service	O
(	O
named	O
PURL	B-Language
resolver	O
)	O
that	O
:	O
</s>
<s>
by	O
contract	O
)	O
of	O
the	O
root	O
URL	O
and	O
the	O
PURL	B-Language
resolver	O
services	O
.	O
</s>
<s>
PURLs	B-Language
are	O
used	O
to	O
curate	O
the	O
URL	O
resolution	O
process	O
,	O
thus	O
solving	O
the	O
problem	O
of	O
transitory	O
URIs	B-Protocol
in	O
location-based	O
URI	B-Protocol
schemes	O
like	O
HTTP	B-Protocol
.	I-Protocol
</s>
<s>
Technically	O
the	O
string	O
resolution	O
on	O
PURL	B-Language
is	O
like	O
SEF	B-Protocol
URL	I-Protocol
resolution	O
.	O
</s>
<s>
The	O
remainder	O
of	O
this	O
article	O
is	O
about	O
the	O
OCLC	O
's	O
PURL	B-Language
system	O
,	O
proposed	O
and	O
implemented	O
by	O
OCLC	O
(	O
the	O
Online	O
Computer	O
Library	O
Center	O
)	O
.	O
</s>
<s>
The	O
PURL	B-Language
concept	O
was	O
developed	O
by	O
Stuart	O
Weibel	O
and	O
Erik	O
Jul	O
at	O
OCLC	O
in	O
1995	O
.	O
</s>
<s>
The	O
PURL	B-Language
system	O
was	O
implemented	O
using	O
a	O
forked	O
pre-1.0	O
release	O
of	O
Apache	B-Application
HTTP	I-Application
Server	I-Application
.	O
</s>
<s>
The	O
software	O
was	O
modernized	O
and	O
extended	O
in	O
2007	O
by	O
Zepheira	B-Library
under	O
contract	O
to	O
OCLC	O
and	O
the	O
official	O
website	O
moved	O
to	O
(	O
the	O
'	O
Z	O
 '	O
came	O
from	O
the	O
Zepheira	B-Library
name	O
and	O
was	O
used	O
to	O
differentiate	O
the	O
PURL	B-Language
open-source	B-Application
software	I-Application
site	O
from	O
the	O
PURL	B-Language
resolver	O
operated	O
by	O
OCLC	O
)	O
.	O
</s>
<s>
PURL	B-Language
version	O
numbers	O
may	O
be	O
considered	O
confusing	O
.	O
</s>
<s>
OCLC	O
released	O
versions	O
1	O
and	O
2	O
of	O
the	O
Apache-based	O
source	O
tree	O
,	O
initially	O
in	O
1999	O
under	O
the	O
OCLC	O
Research	O
Public	O
License	O
1.0	O
License	O
and	O
later	O
under	O
the	O
OCLC	O
Research	O
Public	O
License	O
2.0	O
License	O
(	O
)	O
.	O
</s>
<s>
Zepheira	B-Library
released	O
PURLz	O
1.0	O
in	O
2007	O
under	O
the	O
.	O
</s>
<s>
The	O
Callimachus	O
Project	O
implemented	O
PURLs	B-Language
as	O
of	O
its	O
1.0	O
release	O
in	O
2012	O
.	O
</s>
<s>
The	O
oldest	O
PURL	B-Language
HTTP	B-Protocol
resolver	O
was	O
operated	O
by	O
OCLC	O
from	O
1995	O
to	O
September	O
2016	O
and	O
was	O
reached	O
as	O
purl.oclc.org	O
as	O
well	O
as	O
purl.org,	O
purl.net,	O
and	O
purl.com	O
.	O
</s>
<s>
Other	O
notable	O
PURL	B-Language
resolvers	O
include	O
the	O
US	O
Government	O
Printing	O
Office	O
(	O
)	O
,	O
which	O
is	O
operated	O
for	O
the	O
Federal	O
Depository	O
Library	O
Program	O
and	O
has	O
been	O
in	O
operation	O
since	O
1997	O
.	O
</s>
<s>
The	O
PURL	B-Language
concept	O
is	O
used	O
in	O
the	O
,	O
that	O
may	O
replace	O
the	O
old	O
PURL-services	O
and	O
PURL-technologies	O
.	O
</s>
<s>
The	O
transfer	O
re-enabled	O
the	O
ability	O
to	O
manage	O
PURL	B-Language
definitions	O
that	O
had	O
been	O
disabled	O
in	O
the	O
OCLC-hosted	O
service	O
for	O
several	O
months	O
.	O
</s>
<s>
The	O
service	O
hosted	O
on	O
Internet	O
Archive	O
servers	O
supports	O
access	O
via	O
purl.org,	O
purl.net,	O
purl.info,	O
and	O
purl.com	O
.	O
</s>
<s>
OCLC	O
now	O
redirects	O
DNS	O
requests	O
for	O
purl.oclc.org	O
to	O
purl.org	B-Language
.	O
</s>
<s>
The	O
PURL	B-Language
concept	O
allows	O
for	O
generalized	O
URL	O
curation	O
of	O
HTTP	B-Protocol
URIs	B-Protocol
on	O
the	O
World	O
Wide	O
Web	O
.	O
</s>
<s>
PURLs	B-Language
allow	O
third	O
party	O
control	O
over	O
both	O
URL	O
resolution	O
and	O
resource	O
metadata	O
provision	O
.	O
</s>
<s>
A	O
Persistent	B-Language
URL	I-Language
is	O
an	O
address	O
on	O
the	O
World	O
Wide	O
Web	O
that	O
causes	O
a	O
redirection	O
to	O
another	O
Web	O
resource	O
.	O
</s>
<s>
If	O
a	O
Web	O
resource	O
changes	O
location	O
(	O
and	O
hence	O
URL	O
)	O
,	O
a	O
PURL	B-Language
pointing	O
to	O
it	O
can	O
be	O
updated	O
.	O
</s>
<s>
A	O
user	O
of	O
a	O
PURL	B-Language
always	O
uses	O
the	O
same	O
Web	O
address	O
,	O
even	O
though	O
the	O
resource	O
in	O
question	O
may	O
have	O
moved	O
.	O
</s>
<s>
PURLs	B-Language
may	O
be	O
used	O
by	O
publishers	O
to	O
manage	O
their	O
own	O
information	O
space	O
or	O
by	O
Web	O
users	O
to	O
manage	O
theirs	O
;	O
a	O
PURL	B-Language
service	O
is	O
independent	O
of	O
the	O
publisher	O
of	O
information	O
.	O
</s>
<s>
PURL	B-Language
services	O
thus	O
allow	O
the	O
management	O
of	O
hyperlink	O
integrity	O
.	O
</s>
<s>
A	O
simple	O
PURL	B-Language
works	O
by	O
responding	O
to	O
an	O
HTTP	B-Protocol
GET	O
request	O
by	O
returning	O
a	O
response	O
of	O
type	O
302	O
(	O
equivalent	O
to	O
the	O
HTTP	B-Protocol
status	I-Protocol
code	I-Protocol
302	O
,	O
meaning	O
"	O
Found	O
"	O
)	O
.	O
</s>
<s>
The	O
response	O
contains	O
an	O
HTTP	B-Protocol
"	O
Location	O
"	O
header	O
,	O
the	O
value	O
of	O
which	O
is	O
a	O
URL	O
that	O
the	O
client	O
should	O
subsequently	O
retrieve	O
via	O
a	O
new	O
HTTP	B-Protocol
GET	O
request	O
.	O
</s>
<s>
PURLs	B-Language
implement	O
one	O
form	O
of	O
persistent	O
identifier	O
for	O
virtual	O
resources	O
.	O
</s>
<s>
Other	O
persistent	O
identifier	O
schemes	O
include	O
Digital	O
Object	O
Identifiers	O
(	O
DOIs	O
)	O
,	O
Life	O
Sciences	O
Identifiers	O
(	O
LSIDs	O
)	O
and	O
INFO	B-Protocol
URIs	I-Protocol
.	O
</s>
<s>
PURLs	B-Language
have	O
been	O
criticized	O
for	O
their	O
need	O
to	O
resolve	O
a	O
URL	O
,	O
thus	O
tying	O
a	O
PURL	B-Language
to	O
a	O
network	O
location	O
.	O
</s>
<s>
A	O
failure	O
to	O
resolve	O
a	O
PURL	B-Language
could	O
lead	O
to	O
an	O
ambiguous	O
state	O
:	O
It	O
would	O
not	O
be	O
clear	O
whether	O
the	O
PURL	B-Language
failed	O
to	O
resolve	O
because	O
a	O
network	O
failure	O
prevented	O
it	O
or	O
because	O
it	O
did	O
not	O
exist	O
.	O
</s>
<s>
PURLs	B-Language
are	O
themselves	O
valid	O
URLs	O
,	O
so	O
their	O
components	O
must	O
map	O
to	O
the	O
URL	O
specification	O
.	O
</s>
<s>
The	O
scheme	O
used	O
for	O
PURLs	B-Language
is	O
generally	O
HTTP	B-Protocol
.	I-Protocol
</s>
<s>
The	O
host	O
part	O
tells	O
which	O
PURL	B-Language
server	O
to	O
connect	O
to	O
.	O
</s>
<s>
The	O
next	O
part	O
,	O
the	O
PURL	B-Language
domain	O
,	O
is	O
analogous	O
to	O
a	O
resource	O
path	O
in	O
a	O
URL	O
.	O
</s>
<s>
The	O
domain	O
is	O
a	O
hierarchical	O
information	O
space	O
that	O
separates	O
PURLs	B-Language
and	O
allows	O
for	O
PURLs	B-Language
to	O
have	O
different	O
maintainers	O
.	O
</s>
<s>
One	O
or	O
more	O
designated	O
maintainers	O
may	O
administer	O
each	O
PURL	B-Language
domain	O
.	O
</s>
<s>
Finally	O
,	O
the	O
PURL	B-Language
name	O
is	O
the	O
name	O
of	O
the	O
PURL	B-Language
itself	O
.	O
</s>
<s>
The	O
domain	O
and	O
name	O
together	O
constitute	O
the	O
PURL	B-Language
's	O
"	O
id	O
"	O
.	O
</s>
<s>
Both	O
permalink	O
and	O
PURL	B-Language
are	O
used	O
as	O
permanent/persistent	O
URL	O
and	O
redirect	O
to	O
the	O
location	O
of	O
the	O
requested	O
web	O
resource	O
.	O
</s>
<s>
A	O
PURL	B-Language
domain	O
name	O
is	O
independently	O
changeable	O
,	O
and	O
is	O
designed	O
to	O
persist	O
over	O
decades	O
.	O
</s>
<s>
The	O
most	O
common	O
types	O
of	O
PURLs	B-Language
are	O
named	O
to	O
coincide	O
with	O
the	O
HTTP	B-Protocol
response	I-Protocol
code	I-Protocol
that	O
they	O
return	O
.	O
</s>
<s>
Not	O
all	O
HTTP	B-Protocol
response	I-Protocol
codes	I-Protocol
have	O
equivalent	O
PURL	B-Language
types	O
and	O
not	O
all	O
PURL	B-Language
servers	O
implement	O
all	O
PURL	B-Language
types	O
.	O
</s>
<s>
Some	O
HTTP	B-Protocol
response	I-Protocol
codes	I-Protocol
(	O
e.g.	O
</s>
<s>
401	O
,	O
Unauthorized	O
)	O
have	O
clear	O
meanings	O
in	O
the	O
context	O
of	O
an	O
HTTP	B-Protocol
conversation	O
but	O
do	O
not	O
apply	O
to	O
the	O
process	O
of	O
HTTP	B-Protocol
redirection	O
.	O
</s>
<s>
Three	O
additional	O
types	O
of	O
PURLs	B-Language
(	O
"	O
chain	O
"	O
,	O
"	O
partial	O
 '	O
and	O
"	O
clone	O
"	O
)	O
are	O
given	O
mnemonic	O
names	O
related	O
to	O
their	O
functions	O
.	O
</s>
<s>
Most	O
PURLs	B-Language
are	O
so-called	O
"	O
simple	O
PURLs	B-Language
"	O
,	O
which	O
provide	O
a	O
redirection	O
to	O
the	O
desired	O
resource	O
.	O
</s>
<s>
The	O
HTTP	B-Protocol
status	I-Protocol
code	I-Protocol
,	O
and	O
hence	O
of	O
the	O
PURL	B-Language
type	O
,	O
of	O
a	O
simple	O
PURL	B-Language
is	O
302	O
.	O
</s>
<s>
The	O
intent	O
of	O
a	O
302	O
PURL	B-Language
is	O
to	O
inform	O
the	O
Web	O
client	O
and	O
end	O
user	O
that	O
the	O
PURL	B-Language
should	O
always	O
be	O
used	O
to	O
address	O
the	O
requested	O
resource	O
,	O
not	O
the	O
final	O
URI	B-Protocol
resolved	O
.	O
</s>
<s>
This	O
is	O
to	O
allow	O
continued	O
resolution	O
of	O
the	O
resource	O
if	O
the	O
PURL	B-Language
changes	O
.	O
</s>
<s>
Some	O
operators	O
prefer	O
to	O
use	O
PURLs	B-Language
of	O
type	O
301	O
(	O
indicating	O
that	O
the	O
final	O
URI	B-Protocol
should	O
be	O
addressed	O
in	O
future	O
requests	O
)	O
.	O
</s>
<s>
A	O
PURL	B-Language
of	O
type	O
"	O
chain	O
"	O
allows	O
a	O
PURL	B-Language
to	O
redirect	O
to	O
another	O
PURL	B-Language
in	O
a	O
manner	O
identical	O
to	O
a	O
301	O
or	O
302	O
redirection	O
,	O
with	O
the	O
difference	O
that	O
a	O
PURL	B-Language
server	O
will	O
handle	O
the	O
redirection	O
internally	O
for	O
greater	O
efficiency	O
.	O
</s>
<s>
A	O
PURL	B-Language
of	O
type	O
"	O
200	O
"	O
is	O
an	O
"	O
Active	O
PURL	B-Language
"	O
,	O
in	O
which	O
the	O
PURL	B-Language
actively	O
participates	O
in	O
the	O
creation	O
or	O
aggregation	O
of	O
the	O
metadata	O
returned	O
.	O
</s>
<s>
An	O
Active	O
PURL	B-Language
includes	O
some	O
arbitrary	O
computation	O
to	O
produce	O
its	O
output	O
.	O
</s>
<s>
Active	O
PURLs	B-Language
have	O
been	O
implemented	O
in	O
PURLz	O
2.0	O
and	O
The	O
Callimachus	O
Project	O
.	O
</s>
<s>
Active	O
PURLs	B-Language
act	O
similar	O
to	O
a	O
stored	B-General_Concept
procedure	I-General_Concept
in	O
relational	O
databases	O
.	O
</s>
<s>
A	O
PURL	B-Language
of	O
type	O
"	O
303	O
"	O
is	O
used	O
to	O
direct	O
a	O
Web	O
client	O
to	O
a	O
resource	O
that	O
provides	O
additional	O
information	O
regarding	O
the	O
resource	O
they	O
requested	O
,	O
without	O
returning	O
the	O
resource	O
itself	O
.	O
</s>
<s>
This	O
subtlety	O
is	O
useful	O
when	O
the	O
HTTP	B-Protocol
URI	B-Protocol
requested	O
is	O
used	O
as	O
an	O
identifier	O
for	O
a	O
physical	O
or	O
conceptual	O
object	O
that	O
cannot	O
be	O
represented	O
as	O
an	O
information	O
resource	O
.	O
</s>
<s>
PURLs	B-Language
of	O
type	O
303	O
are	O
used	O
most	O
often	O
to	O
redirect	O
to	O
metadata	O
in	O
a	O
serialization	O
format	O
of	O
the	O
Resource	O
Description	O
Framework	O
(	O
RDF	O
)	O
and	O
have	O
relevance	O
for	O
Semantic	O
Web	O
and	O
linked	B-Operating_System
data	I-Operating_System
content	O
.	O
</s>
<s>
This	O
use	O
of	O
the	O
303	O
HTTP	B-Protocol
status	I-Protocol
code	I-Protocol
is	O
conformant	O
with	O
the	O
finding	O
of	O
the	O
of	O
the	O
World	O
Wide	O
Web	O
Consortium	O
.	O
</s>
<s>
A	O
PURL	B-Language
of	O
type	O
"	O
307	O
"	O
informs	O
a	O
user	O
that	O
the	O
resource	O
temporarily	O
resides	O
at	O
a	O
different	O
URL	O
from	O
the	O
norm	O
.	O
</s>
<s>
PURLs	B-Language
of	O
types	O
404	O
and	O
410	O
note	O
that	O
the	O
requested	O
resource	O
could	O
not	O
be	O
found	O
and	O
suggests	O
some	O
information	O
for	O
why	O
that	O
was	O
so	O
.	O
</s>
<s>
Support	O
for	O
the	O
HTTP	B-Protocol
307	O
(	O
Temporary	O
Redirect	O
)	O
,	O
404	O
(	O
Not	O
Found	O
)	O
and	O
410	O
(	O
Gone	O
)	O
response	O
codes	O
are	O
provided	O
for	O
completeness	O
.	O
</s>
<s>
PURLs	B-Language
of	O
types	O
"	O
404	O
"	O
and	O
"	O
410	O
"	O
are	O
provided	O
to	O
assist	O
administrators	O
in	O
marking	O
PURLs	B-Language
that	O
require	O
repair	O
.	O
</s>
<s>
PURLs	B-Language
of	O
these	O
types	O
allow	O
for	O
more	O
efficient	O
indications	O
of	O
resource	O
identification	O
failure	O
when	O
target	O
resources	O
have	O
moved	O
and	O
a	O
suitable	O
replacement	O
has	O
not	O
been	O
identified	O
.	O
</s>
<s>
PURLs	B-Language
of	O
type	O
"	O
clone	O
"	O
are	O
used	O
solely	O
during	O
PURL	B-Language
administration	O
as	O
a	O
convenient	O
method	O
of	O
copying	O
an	O
existing	O
PURL	B-Language
record	O
into	O
a	O
new	O
PURL	B-Language
.	O
</s>
<s>
The	O
PURL	B-Language
service	O
includes	O
a	O
concept	O
known	O
as	O
partial	O
redirection	O
.	O
</s>
<s>
If	O
a	O
request	O
does	O
not	O
match	O
a	O
PURL	B-Language
exactly	O
,	O
the	O
requested	O
URL	O
is	O
checked	O
to	O
determine	O
if	O
some	O
contiguous	O
front	O
portion	O
of	O
the	O
PURL	B-Language
string	O
matches	O
a	O
registered	O
PURL	B-Language
.	O
</s>
<s>
For	O
example	O
,	O
consider	O
a	O
PURL	B-Language
with	O
a	O
URL	O
of	O
http//purl.org/some/path/	O
with	O
a	O
target	O
URL	O
of	O
http://example.com/another/path/	O
.	O
</s>
<s>
An	O
attempt	O
to	O
perform	O
an	O
HTTP	B-Protocol
GET	O
operation	O
on	O
the	O
URL	O
http//purl.org/some/path/and/some/more/data	O
would	O
result	O
in	O
a	O
partial	O
redirection	O
to	O
http://example.com/another/path/and/some/more/data	O
.	O
</s>
<s>
The	O
concept	O
of	O
partial	O
redirection	O
allows	O
hierarchies	O
of	O
Web-based	O
resources	O
to	O
be	O
addressed	O
via	O
PURLs	B-Language
without	O
each	O
resource	O
requiring	O
its	O
own	O
PURL	B-Language
.	O
</s>
<s>
One	O
PURL	B-Language
is	O
sufficient	O
to	O
serve	O
as	O
a	O
top-level	O
node	O
for	O
a	O
hierarchy	O
on	O
a	O
single	O
target	O
server	O
.	O
</s>
<s>
The	O
new	O
PURL	B-Language
service	O
uses	O
the	O
type	O
"	O
partial	O
"	O
to	O
denote	O
a	O
PURL	B-Language
that	O
performs	O
partial	O
redirection	O
.	O
</s>
<s>
Partial	O
redirections	O
at	O
the	O
level	O
of	O
a	O
URL	O
path	O
do	O
not	O
violate	O
common	O
interpretations	O
of	O
the	O
HTTP	B-Protocol
1.1	I-Protocol
specification	O
.	O
</s>
<s>
Fragment	O
identifiers	O
indicate	O
a	O
pointer	O
to	O
more	O
specific	O
information	O
within	O
a	O
resource	O
and	O
are	O
designated	O
as	O
following	O
a	O
#	O
separator	O
in	O
URIs	B-Protocol
.	O
</s>
<s>
If	O
a	O
fragment	O
is	O
attached	O
to	O
a	O
PURL	B-Language
of	O
type	O
"	O
partial	O
"	O
,	O
should	O
a	O
PURL	B-Language
service	O
assume	O
that	O
the	O
fragment	O
has	O
meaning	O
on	O
the	O
target	O
URL	O
or	O
should	O
it	O
discard	O
it	O
in	O
the	O
presumption	O
that	O
a	O
resource	O
with	O
a	O
changed	O
location	O
may	O
have	O
also	O
changed	O
content	O
,	O
thus	O
invalidating	O
fragments	O
defined	O
earlier	O
?	O
</s>
<s>
Bos	O
suggested	O
that	O
fragments	O
should	O
be	O
retained	O
and	O
passed	O
through	O
to	O
target	O
URLs	O
during	O
HTTP	B-Protocol
redirections	O
resulting	O
in	O
300	O
(	O
Multiple	O
Choice	O
)	O
,	O
301	O
(	O
Moved	O
Permanently	O
)	O
,	O
302	O
(	O
Found	O
)	O
or	O
303	O
(	O
See	O
Other	O
)	O
responses	O
unless	O
a	O
designated	O
target	O
URL	O
already	O
includes	O
a	O
fragment	O
identifier	O
.	O
</s>
<s>
Starting	O
with	O
PURLz	O
1.0	O
series	O
,	O
the	O
PURL	B-Language
service	O
implements	O
partial	O
redirections	O
inclusive	O
of	O
fragment	O
identifiers	O
by	O
writing	O
fragments	O
onto	O
target	O
URLs	O
in	O
an	O
attempt	O
to	O
comply	O
with	O
and	O
avoid	O
problematic	O
and	O
inconsistent	O
behavior	O
by	O
browser	O
vendors	O
.	O
</s>
