<s>
Virtuoso	B-Application
Universal	I-Application
Server	I-Application
is	O
a	O
middleware	B-General_Concept
and	O
database	B-General_Concept
engine	I-General_Concept
hybrid	O
that	O
combines	O
the	O
functionality	O
of	O
a	O
traditional	O
relational	B-Application
database	I-Application
management	I-Application
system	I-Application
(	O
RDBMS	B-Application
)	O
,	O
object	B-General_Concept
–	I-General_Concept
relational	I-General_Concept
database	I-General_Concept
(	O
ORDBMS	B-General_Concept
)	O
,	O
virtual	B-General_Concept
database	I-General_Concept
,	O
RDF	O
,	O
XML	B-Protocol
,	O
free-text	B-Application
,	O
web	B-Application
application	I-Application
server	I-Application
and	O
file	B-Application
server	I-Application
functionality	O
in	O
a	O
single	O
system	O
.	O
</s>
<s>
Rather	O
than	O
have	O
dedicated	O
servers	O
for	O
each	O
of	O
the	O
aforementioned	O
functionality	O
realms	O
,	O
Virtuoso	O
is	O
a	O
"	O
universal	O
server	O
"	O
;	O
it	O
enables	O
a	O
single	O
multithreaded	B-Operating_System
server	O
process	B-Operating_System
that	O
implements	O
multiple	O
protocols	O
.	O
</s>
<s>
The	O
free	B-License
and	I-License
open	I-License
source	I-License
edition	O
of	O
Virtuoso	B-Application
Universal	I-Application
Server	I-Application
is	O
also	O
known	O
as	O
OpenLink	B-Application
Virtuoso	I-Application
.	O
</s>
<s>
The	O
software	O
has	O
been	O
developed	O
by	O
OpenLink	O
Software	O
with	O
Kingsley	O
Uyi	O
Idehen	O
and	O
Orri	O
Erling	O
as	O
the	O
chief	B-Architecture
software	I-Architecture
architects	I-Architecture
.	O
</s>
<s>
Virtuoso	O
provides	O
an	O
extended	O
object	B-General_Concept
–	I-General_Concept
relational	I-General_Concept
model	I-General_Concept
,	O
which	O
combines	O
the	O
flexibility	O
of	O
relational	O
access	O
with	O
inheritance	B-Language
,	O
run	O
time	O
data	O
typing	O
,	O
late	O
binding	O
,	O
and	O
identity-based	O
access	O
.	O
</s>
<s>
Virtuoso	B-Application
Universal	I-Application
Server	I-Application
database	O
includes	O
physical	O
file	O
and	O
in	O
memory	O
storage	O
and	O
operating	O
system	O
processes	O
that	O
interact	O
with	O
the	O
storage	O
.	O
</s>
<s>
There	O
is	O
one	O
main	O
process	B-Operating_System
,	O
which	O
has	O
listeners	O
on	O
a	O
specified	O
port	O
for	O
HTTP	B-Protocol
,	O
SOAP	B-Application
,	O
and	O
other	O
protocols	O
.	O
</s>
<s>
It	O
consists	O
of	O
a	O
single	O
process	B-Operating_System
with	O
an	O
adjustable	O
pool	O
of	O
threads	B-Operating_System
shared	O
between	O
clients	O
.	O
</s>
<s>
Multiple	O
threads	B-Operating_System
may	O
work	O
on	O
a	O
single	O
index	O
tree	O
with	O
minimal	O
interference	O
with	O
each	O
other	O
.	O
</s>
<s>
One	O
cache	O
of	O
database	O
pages	O
is	O
shared	O
among	O
all	O
threads	B-Operating_System
and	O
old	O
dirty	O
pages	O
are	O
written	O
back	O
to	O
disk	O
as	O
a	O
background	O
process	B-Operating_System
.	O
</s>
<s>
Virtuoso	O
SQL	B-Language
provides	O
means	O
for	O
exclusive	O
read	O
and	O
for	O
setting	O
transaction	O
isolation	O
.	O
</s>
<s>
Virtuoso	O
ORDBMS	B-General_Concept
database	O
supports	O
entity	O
integrity	O
and	O
referential	B-General_Concept
integrity	I-General_Concept
.	O
</s>
<s>
Virtuoso	O
ensures	O
that	O
relationships	O
between	O
records	O
in	O
related	O
tables	O
are	O
valid	O
by	O
enforcing	O
referential	B-General_Concept
integrity	I-General_Concept
.	O
</s>
<s>
NOT	B-Language
NULL	I-Language
–	O
Within	O
the	O
definition	O
of	O
a	O
table	O
,	O
Virtuoso	O
allows	O
data	O
to	O
contain	O
a	O
NULL	B-Language
value	O
.	O
</s>
<s>
This	O
NULL	B-Language
value	O
is	O
not	O
really	O
a	O
value	O
at	O
all	O
and	O
is	O
considered	O
an	O
absence	O
of	O
value	O
.	O
</s>
<s>
The	O
constraint	O
of	O
NOT	B-Language
NULL	I-Language
forces	O
a	O
value	O
to	O
be	O
given	O
to	O
a	O
column	O
.	O
</s>
<s>
Unique	B-Application
key	I-Application
–	O
Uniqueness	O
for	O
a	O
column	O
or	O
set	O
of	O
columns	O
means	O
that	O
the	O
values	O
in	O
that	O
column	O
or	O
set	O
of	O
columns	O
must	O
be	O
different	O
from	O
all	O
other	O
columns	O
or	O
set	O
of	O
columns	O
in	O
that	O
table	O
.	O
</s>
<s>
A	O
unique	B-Application
key	I-Application
may	O
contain	O
NULL	B-Language
values	O
since	O
they	O
are	O
by	O
definition	O
a	O
unique	O
non-valued	O
value	O
.	O
</s>
<s>
Primary	B-Application
key	I-Application
–	O
Primary	B-Application
key	I-Application
are	O
much	O
like	O
unique	B-Application
keys	I-Application
except	O
that	O
they	O
are	O
designed	O
to	O
uniquely	O
identify	O
a	O
row	O
in	O
a	O
table	O
.	O
</s>
<s>
The	O
primary	B-Application
key	I-Application
cannot	O
contain	O
a	O
NULL	B-Language
value	O
.	O
</s>
<s>
CHECK	B-Language
Constraint	I-Language
–	O
Virtuoso	O
provides	O
on	O
a	O
column	O
an	O
integrity	O
constraint	O
that	O
requires	O
certain	O
conditions	O
to	O
be	O
met	O
before	O
the	O
data	O
is	O
inserted	O
or	O
modified	O
.	O
</s>
<s>
Installations	O
come	O
with	O
two	O
databases	B-Application
:	O
a	O
default	O
and	O
a	O
demo	O
database	O
.	O
</s>
<s>
The	O
Virtuoso	O
project	O
was	O
born	O
in	O
1998	O
from	O
a	O
merger	O
of	O
the	O
OpenLink	O
data	B-General_Concept
access	I-General_Concept
middleware	B-General_Concept
and	O
Kubl	O
RDBMS	B-Application
.	O
</s>
<s>
The	O
Kubl	O
ORDBMS	B-General_Concept
was	O
one	O
of	O
a	O
list	O
of	O
relational	B-Application
database	I-Application
systems	I-Application
with	O
roots	O
in	O
Finland	O
.	O
</s>
<s>
This	O
list	O
also	O
includes	O
MySQL	B-Application
,	O
InnoDB	B-Language
,	O
and	O
Solid	O
RDBMS/Solid	O
Technologies	O
.	O
</s>
<s>
As	O
is	O
the	O
case	O
with	O
most	O
technology	O
products	O
,	O
key	O
personnel	O
behind	O
OpenLink	B-Application
Virtuoso	I-Application
,	O
InnoDB	B-Language
,	O
and	O
Solid	O
share	O
periods	O
of	O
professional	O
overlap	O
that	O
provide	O
noteworthy	O
insight	O
into	O
the	O
history	O
of	O
database	O
technology	O
development	O
in	O
Finland	O
.	O
</s>
<s>
Heikki	O
Tuuri	O
(	O
creator	O
of	O
InnoDB	B-Language
)	O
,	O
Ora	O
Lassila	O
(	O
W3C	O
and	O
Nokia	O
Research	O
,	O
a	O
technology	O
lead	O
and	O
visionary	O
in	O
the	O
areas	O
RDF	O
and	O
Semantic	O
Web	O
in	O
general	O
alongside	O
Tim	O
Berners-Lee	O
)	O
,	O
and	O
Orri	O
Erling	O
(	O
Virtuoso	O
Program	O
Manager	O
at	O
OpenLink	O
Software	O
)	O
all	O
worked	O
together	O
in	O
a	O
startup	O
company	O
called	O
Entity	O
Systems	O
in	O
Finland	O
–	O
where	O
they	O
were	O
developing	O
Common	B-Language
Lisp	I-Language
and	O
Prolog	B-Language
development	O
environments	O
for	O
the	O
early	O
generation	O
of	O
PC	B-Device
's	O
circa	O
.	O
</s>
<s>
Later	O
,	O
Orri	O
Erling	O
worked	O
with	O
VIA	O
International	O
,	O
the	O
developer	O
of	O
VIA/DRE	O
in	O
designing	O
a	O
LISP-based	O
object-oriented	B-Language
data	B-General_Concept
access	I-General_Concept
layer	O
atop	O
the	O
company	O
's	O
DBMS	B-Application
product	O
.	O
</s>
<s>
Heikki	O
Tuuri	O
worked	O
at	O
Solid	O
for	O
a	O
while	O
before	O
starting	O
his	O
own	O
database	B-Application
development	I-Application
project	O
which	O
became	O
InnoDB	B-Language
(	O
acquired	O
by	O
Oracle	B-Application
in	O
2005	O
)	O
.	O
</s>
<s>
Orri	O
Erling	O
started	O
his	O
own	O
DBMS	B-Application
development	O
work	O
in	O
1994	O
,	O
which	O
was	O
to	O
become	O
Kubl	O
.	O
</s>
<s>
Kubl	O
was	O
marketed	O
as	O
a	O
high	O
performance	O
lightweight	O
database	O
for	O
embedded	B-Application
use	O
;	O
the	O
development	O
aim	O
was	O
to	O
achieve	O
top	O
scores	O
in	O
Transactions	B-General_Concept
Per	I-General_Concept
Second	I-General_Concept
tests	O
.	O
</s>
<s>
Pricing	O
of	O
the	O
product	O
was	O
especially	O
favorable	O
to	O
Linux	B-Application
users	O
with	O
a	O
Linux	B-Application
license	O
priced	O
at	O
$199	O
.	O
</s>
<s>
Kubl	O
became	O
the	O
cornerstone	O
of	O
OpenLink	B-Application
Virtuoso	I-Application
,	O
after	O
the	O
technology	O
paths	O
of	O
Kingsley	O
Uyi	O
Idehen	O
and	O
Orri	O
Erling	O
crossed	O
in	O
1998	O
,	O
leading	O
to	O
the	O
acquisition	O
of	O
Kubl	O
by	O
OpenLink	O
Software	O
.	O
</s>
<s>
For	O
the	O
database	O
application	O
developer	O
and	O
systems	O
integrator	O
,	O
Virtuoso	O
implements	O
a	O
variety	O
of	O
industry	O
standard	O
data	B-General_Concept
access	I-General_Concept
APIs	O
(	O
client	O
and	O
server	O
)	O
that	O
includes	O
:	O
ODBC	B-Application
,	O
JDBC	B-Language
,	O
OLE	B-Language
DB	I-Language
,	O
ADO.NET	O
,	O
ADO.NET	B-Application
Entity	I-Application
Framework	I-Application
,	O
XMLA	O
.	O
</s>
<s>
For	O
the	O
Web	B-Application
application	I-Application
developer	I-Application
and	O
content	O
syndicate(s )	O
publishers	O
,	O
and	O
consumers	O
,	O
Virtuoso	O
supports	O
standards	O
such	O
as	O
:	O
Atom	B-Application
,	O
RSS	O
2.0	O
,	O
RSS	O
1.0	O
,	O
OPML	B-Application
,	O
XBEL	B-Language
,	O
FOAF	O
,	O
SIOC	O
.	O
</s>
<s>
OpenLink	O
Data	O
Spaces	O
–	O
a	O
Virtuoso-based	O
platform	O
for	O
cost-effective	O
creation	O
and	O
management	O
of	O
Semantic	O
Web	O
/	O
Linked	B-Operating_System
Data	I-Operating_System
Web	O
presence	O
.	O
</s>
<s>
It	O
provides	O
a	O
data	O
junction	O
box	O
for	O
integrating	O
data	O
across	O
third	O
party	O
Social	O
network	O
service	O
,	O
Blog	O
,	O
File	O
sharing	O
,	O
Shared	O
&	O
Social	B-General_Concept
bookmarking	I-General_Concept
,	O
Wiki	B-Protocol
,	O
E-mail	O
,	O
Photo	O
Sharing	O
,	O
RSS	O
2.0	O
,	O
Atom	B-Application
,	O
and	O
RSS	O
1.1	O
Content	O
Aggregation	O
services	O
.	O
</s>
<s>
In	O
addition	O
,	O
to	O
its	O
third	O
party	O
integration	O
functionality	O
,	O
it	O
also	O
includes	O
its	O
own	O
rich	O
collection	O
of	O
Linked	B-Operating_System
Data	I-Operating_System
compliant	O
distributed	O
collaborative	O
applications	O
,	O
across	O
each	O
of	O
the	O
aforementioned	O
Web	B-Application
application	I-Application
realms	O
.	O
</s>
<s>
Universal	O
Data	B-General_Concept
Access	I-General_Concept
Drivers	O
–	O
High-performance	O
data	B-General_Concept
access	I-General_Concept
drivers	O
for	O
ODBC	B-Application
,	O
JDBC	B-Language
,	O
ADO.NET	O
,	O
and	O
OLE	B-Language
DB	I-Language
that	O
provide	O
transparent	O
access	O
to	O
enterprise	O
databases	B-Application
across	O
multiple	B-Operating_System
platforms	I-Operating_System
and	O
databases	B-Application
.	O
</s>
<s>
Virtuoso	O
is	O
supported	O
on	O
a	O
number	O
of	O
32	O
-	O
and	O
64-bit	O
platforms	B-Device
including	O
cross-platform	B-Operating_System
Windows	B-Application
,	O
UNIX	O
(	O
HP	B-Application
,	O
AIX	B-Application
,	O
Sun	B-Application
,	O
DEC	O
,	O
BSD	O
,	O
SCO	B-Operating_System
)	O
,	O
Linux	B-Application
(	O
Red	O
Hat	O
,	O
SUSE	O
)	O
and	O
macOS	B-Application
.	O
</s>
<s>
In	O
April	O
2006	O
,	O
a	O
free	B-Application
software	I-Application
version	O
of	O
Virtuoso	O
was	O
made	O
available	O
under	O
the	O
GNU	B-License
General	I-License
Public	I-License
License	I-License
version	O
2	O
.	O
</s>
