<s>
Z39.50	B-Protocol
is	O
an	O
international	O
standard	O
client	B-Operating_System
–	I-Operating_System
server	I-Operating_System
,	O
application	B-Application
layer	I-Application
communications	O
protocol	O
for	O
searching	O
and	O
retrieving	O
information	O
from	O
a	O
database	O
over	O
a	O
TCP/IP	B-Protocol
computer	B-Architecture
network	I-Architecture
,	O
developed	O
and	O
maintained	O
by	O
the	O
Library	O
of	O
Congress	O
.	O
</s>
<s>
It	O
is	O
covered	O
by	O
ANSI/NISO	O
standard	O
Z39.50	B-Protocol
,	O
and	O
ISO	O
standard	O
23950	O
.	O
</s>
<s>
Z39.50	B-Protocol
is	O
widely	O
used	O
in	O
library	O
environments	O
,	O
for	O
interlibrary	O
catalogue	O
search	O
and	O
loan	O
,	O
often	O
incorporated	O
into	O
integrated	O
library	O
systems	O
and	O
personal	O
bibliographic	B-Application
reference	I-Application
software	I-Application
,	O
and	O
social	O
media	O
such	O
as	O
LibraryThing	B-Library
.	O
</s>
<s>
Work	O
on	O
the	O
Z39.50	B-Protocol
protocol	O
began	O
in	O
the	O
1970s	O
,	O
and	O
led	O
to	O
successive	O
versions	O
in	O
1988	O
,	O
1992	O
,	O
1995	O
and	O
2003	O
.	O
</s>
<s>
The	O
Contextual	B-Language
Query	I-Language
Language	I-Language
(	O
formerly	O
called	O
the	O
Common	B-Language
Query	I-Language
Language	I-Language
)	O
is	O
based	O
on	O
Z39.50	B-Protocol
semantics	O
.	O
</s>
<s>
Search	O
queries	O
contain	O
attributes	O
,	O
typically	O
from	O
the	O
bib-1	B-Protocol
attribute	O
set	O
which	O
defines	O
six	O
attributes	O
to	O
specify	O
information	O
searches	O
on	O
the	O
server	O
computer	O
:	O
use	O
,	O
relation	O
,	O
position	O
,	O
structure	O
,	O
truncation	O
,	O
completeness	O
.	O
</s>
<s>
The	O
syntax	O
of	O
Z39.50	B-Protocol
allows	O
for	O
very	O
complex	O
queries	O
.	O
</s>
<s>
The	O
syntax	O
of	O
Z39.50	B-Protocol
is	O
abstracted	O
from	O
the	O
underlying	O
database	O
structure	O
.	O
</s>
<s>
This	O
allows	O
Z39.50	B-Protocol
queries	O
to	O
be	O
formulated	O
without	O
knowing	O
anything	O
about	O
the	O
target	O
database	O
,	O
but	O
it	O
also	O
means	O
that	O
results	O
for	O
the	O
same	O
query	O
can	O
vary	O
widely	O
among	O
different	O
servers	O
.	O
</s>
<s>
Implementation	O
of	O
the	O
Bath	O
Profile	O
has	O
been	O
slow	O
but	O
is	O
gradually	O
improving	O
the	O
Z39.50	B-Protocol
landscape	O
.	O
</s>
<s>
Z39.50	B-Protocol
is	O
a	O
pre-Web	O
technology	O
,	O
and	O
various	O
working	O
groups	O
are	O
attempting	O
to	O
update	O
it	O
to	O
fit	O
better	O
into	O
the	O
modern	O
environment	O
.	O
</s>
<s>
These	O
attempts	O
fall	O
under	O
the	O
designation	O
ZING	O
(	O
Z39.50	B-Protocol
International	O
:	O
Next	O
Generation	O
)	O
,	O
and	O
pursue	O
various	O
strategies	O
.	O
</s>
<s>
The	O
successors	O
to	O
Z39.50	B-Protocol
are	O
the	O
twin	O
protocols	O
SRU/SRW	O
(	O
Search/Retrieve	O
via	O
URL/Search/Retrieve	O
Web	O
service	O
)	O
,	O
which	O
drop	O
the	O
Z39.50	B-Protocol
communications	O
protocol	O
(	O
replacing	O
it	O
with	O
HTTP	B-Protocol
)	O
while	O
still	O
attempting	O
to	O
preserve	O
the	O
benefits	O
of	O
the	O
query	O
syntax	O
.	O
</s>
<s>
SRU	O
is	O
REST-based	O
,	O
and	O
enables	O
queries	O
to	O
be	O
expressed	O
in	O
URL	O
query	O
strings	O
;	O
SRW	O
uses	O
SOAP	B-Protocol
.	O
</s>
<s>
Both	O
expect	O
search	O
results	O
to	O
be	O
returned	O
as	O
XML	B-Protocol
.	O
</s>
<s>
These	O
projects	O
have	O
a	O
much	O
lower	O
barrier	O
to	O
entry	O
for	O
developers	O
than	O
the	O
original	O
Z39.50	B-Protocol
protocol	O
,	O
allowing	O
the	O
relatively	O
small	O
market	O
for	O
library	O
software	O
to	O
benefit	O
from	O
the	O
web	O
service	O
tools	O
developed	O
for	O
much	O
larger	O
markets	O
.	O
</s>
<s>
The	O
registered	O
internet	O
IP	B-Protocol
port	I-Protocol
for	O
Z39.50	B-Protocol
is	O
210	O
.	O
</s>
<s>
Although	O
the	O
majority	O
of	O
servers	O
use	O
this	O
port	O
,	O
there	O
are	O
dozens	O
of	O
other	O
port	B-Protocol
numbers	I-Protocol
used	O
worldwide	O
by	O
Z39.50	B-Protocol
(	O
e.g.	O
</s>
