<s>
A	O
federated	B-General_Concept
database	I-General_Concept
system	I-General_Concept
(	O
FDBS	O
)	O
is	O
a	O
type	O
of	O
meta-database	O
management	O
system	O
(	O
DBMS	B-Application
)	O
,	O
which	O
transparently	O
maps	O
multiple	O
autonomous	O
database	B-Application
systems	I-Application
into	O
a	O
single	O
federated	B-General_Concept
database	I-General_Concept
.	O
</s>
<s>
The	O
constituent	O
databases	B-Application
are	O
interconnected	O
via	O
a	O
computer	B-Architecture
network	I-Architecture
and	O
may	O
be	O
geographically	O
decentralized	O
.	O
</s>
<s>
Since	O
the	O
constituent	O
database	B-Application
systems	I-Application
remain	O
autonomous	O
,	O
a	O
federated	B-General_Concept
database	I-General_Concept
system	I-General_Concept
is	O
a	O
contrastable	O
alternative	O
to	O
the	O
(	O
sometimes	O
daunting	O
)	O
task	O
of	O
merging	O
several	O
disparate	O
databases	B-Application
.	O
</s>
<s>
A	O
federated	B-General_Concept
database	I-General_Concept
,	O
or	O
virtual	B-General_Concept
database	I-General_Concept
,	O
is	O
a	O
composite	O
of	O
all	O
constituent	O
databases	B-Application
in	O
a	O
federated	B-General_Concept
database	I-General_Concept
system	I-General_Concept
.	O
</s>
<s>
There	O
is	O
no	O
actual	O
data	B-General_Concept
integration	I-General_Concept
in	O
the	O
constituent	O
disparate	O
databases	B-Application
as	O
a	O
result	O
of	O
data	O
federation	O
.	O
</s>
<s>
Through	O
data	B-Application
abstraction	I-Application
,	O
federated	B-General_Concept
database	I-General_Concept
systems	I-General_Concept
can	O
provide	O
a	O
uniform	O
user	B-Application
interface	I-Application
,	O
enabling	O
users	O
and	O
clients	B-Protocol
to	O
store	O
and	O
retrieve	O
data	O
from	O
multiple	O
noncontiguous	O
databases	B-Application
with	O
a	O
single	O
query	B-Library
—	O
even	O
if	O
the	O
constituent	O
databases	B-Application
are	O
heterogeneous	B-General_Concept
.	O
</s>
<s>
To	O
this	O
end	O
,	O
a	O
federated	B-General_Concept
database	I-General_Concept
system	I-General_Concept
must	O
be	O
able	O
to	O
decompose	O
the	O
query	B-Library
into	O
subqueries	O
for	O
submission	O
to	O
the	O
relevant	O
constituent	O
DBMSs	B-Application
,	O
after	O
which	O
the	O
system	O
must	O
composite	O
the	O
result	B-Application
sets	I-Application
of	O
the	O
subqueries	O
.	O
</s>
<s>
Because	O
various	O
database	B-Application
management	I-Application
systems	I-Application
employ	O
different	O
query	B-Language
languages	I-Language
,	O
federated	B-General_Concept
database	I-General_Concept
systems	I-General_Concept
can	O
apply	O
wrappers	O
to	O
the	O
subqueries	O
to	O
translate	O
them	O
into	O
the	O
appropriate	O
query	B-Language
languages	I-Language
.	O
</s>
<s>
McLeod	O
and	O
Heimbigner	O
were	O
among	O
the	O
first	O
to	O
define	O
a	O
federated	B-General_Concept
database	I-General_Concept
system	I-General_Concept
in	O
the	O
mid	O
1980s	O
.	O
</s>
<s>
A	O
FDBS	O
is	O
one	O
which	O
"define[s]	O
the	O
architecture	O
and	O
interconnect[s]	O
databases	B-Application
that	O
minimize	O
central	O
authority	O
yet	O
support	O
partial	O
sharing	O
and	O
coordination	O
among	O
database	B-Application
systems	I-Application
"	O
.	O
</s>
<s>
This	O
description	O
might	O
not	O
accurately	O
reflect	O
the	O
McLeod/Heimbigner	O
definition	O
of	O
a	O
federated	B-General_Concept
database	I-General_Concept
.	O
</s>
<s>
McLeod/Heimbigner	O
'	O
s	O
federated	B-General_Concept
database	I-General_Concept
is	O
a	O
collection	O
of	O
autonomous	O
components	O
that	O
make	O
their	O
data	O
available	O
to	O
other	O
members	O
of	O
the	O
federation	O
through	O
the	O
publication	O
of	O
an	O
export	O
schema	B-Application
and	O
access	O
operations	O
;	O
there	O
is	O
no	O
unified	O
,	O
central	O
schema	B-Application
that	O
encompasses	O
the	O
information	O
available	O
from	O
the	O
members	O
of	O
the	O
federation	O
.	O
</s>
<s>
Among	O
other	O
surveys	O
,	O
practitioners	O
define	O
a	O
Federated	B-General_Concept
Database	I-General_Concept
as	O
a	O
collection	O
of	O
cooperating	O
component	O
systems	O
which	O
are	O
autonomous	O
and	O
are	O
possibly	O
heterogeneous	B-General_Concept
.	O
</s>
<s>
The	O
three	O
important	O
components	O
of	O
an	O
FDBS	O
are	O
autonomy	O
,	O
heterogeneity	B-General_Concept
and	O
distribution	O
.	O
</s>
<s>
Another	O
dimension	O
which	O
has	O
also	O
been	O
considered	O
is	O
the	O
Networking	O
Environment	O
Computer	B-Architecture
Network	I-Architecture
,	O
e.g.	O
,	O
many	O
DBSs	O
over	O
a	O
LAN	B-General_Concept
or	O
many	O
DBSs	O
over	O
a	O
WAN	O
update	O
related	O
functions	O
of	O
participating	O
DBSs	O
(	O
e.g.	O
,	O
no	O
updates	O
,	O
nonatomic	O
transitions	O
,	O
atomic	B-General_Concept
updates	I-General_Concept
)	O
.	O
</s>
<s>
A	O
DBMS	B-Application
can	O
be	O
classified	O
as	O
either	O
centralized	O
or	O
distributed	O
.	O
</s>
<s>
A	O
centralized	O
system	O
manages	O
a	O
single	O
database	O
while	O
distributed	O
manages	O
multiple	O
databases	B-Application
.	O
</s>
<s>
A	O
component	O
DBS	O
in	O
a	O
DBMS	B-Application
may	O
be	O
centralized	O
or	O
distributed	O
.	O
</s>
<s>
A	O
nonfederated	O
database	B-Application
system	I-Application
is	O
an	O
integration	O
of	O
component	O
DBMS	B-Application
that	O
are	O
not	O
autonomous	O
.	O
</s>
<s>
A	O
federated	B-General_Concept
database	I-General_Concept
system	I-General_Concept
consists	O
of	O
component	O
DBS	O
that	O
are	O
autonomous	O
yet	O
participate	O
in	O
a	O
federation	O
to	O
allow	O
partial	O
and	O
controlled	O
sharing	O
of	O
their	O
data	O
.	O
</s>
<s>
Federated	O
architectures	O
differ	O
based	O
on	O
levels	O
of	O
integration	O
with	O
the	O
component	O
database	B-Application
systems	I-Application
and	O
the	O
extent	O
of	O
services	O
offered	O
by	O
the	O
federation	O
.	O
</s>
<s>
Loosely	O
Coupled	O
require	O
component	O
databases	B-Application
to	O
construct	O
their	O
own	O
federated	O
schema	B-Application
.	O
</s>
<s>
A	O
user	O
will	O
typically	O
access	O
other	O
component	O
database	B-Application
systems	I-Application
by	O
using	O
a	O
multidatabase	O
language	O
but	O
this	O
removes	O
any	O
levels	O
of	O
location	O
transparency	O
,	O
forcing	O
the	O
user	O
to	O
have	O
direct	O
knowledge	O
of	O
the	O
federated	O
schema	B-Application
.	O
</s>
<s>
A	O
user	O
imports	O
the	O
data	O
they	O
require	O
from	O
other	O
component	O
databases	B-Application
and	O
integrates	O
it	O
with	O
their	O
own	O
to	O
form	O
a	O
federated	O
schema	B-Application
.	O
</s>
<s>
Tightly	O
coupled	O
system	O
consists	O
of	O
component	O
systems	O
that	O
use	O
independent	O
processes	O
to	O
construct	O
and	O
publicize	O
an	O
integrated	O
federated	O
schema	B-Application
.	O
</s>
<s>
Multiple	O
DBS	O
of	O
which	O
FDBS	O
are	O
a	O
specific	O
type	O
can	O
be	O
characterized	O
along	O
three	O
dimensions	O
:	O
Distribution	O
,	O
Heterogeneity	B-General_Concept
and	O
Autonomy	O
.	O
</s>
<s>
Another	O
characterization	O
could	O
be	O
based	O
on	O
the	O
dimension	O
of	O
networking	O
,	O
for	O
example	O
single	O
databases	B-Application
or	O
multiple	O
databases	B-Application
in	O
a	O
LAN	B-General_Concept
or	O
WAN	O
.	O
</s>
<s>
Data	O
can	O
be	O
distributed	O
among	O
multiple	O
databases	B-Application
which	O
could	O
be	O
stored	O
in	O
a	O
single	O
computer	O
or	O
multiple	O
computers	O
.	O
</s>
<s>
Heterogeneities	B-General_Concept
in	O
databases	B-Application
arise	O
due	O
to	O
factors	O
such	O
as	O
differences	O
in	O
structures	O
,	O
semantics	O
of	O
data	O
,	O
the	O
constraints	O
supported	O
or	O
query	B-Language
language	I-Language
.	O
</s>
<s>
Differences	O
in	O
structure	O
occur	O
when	O
two	O
data	B-Application
models	I-Application
provide	O
different	O
primitives	O
such	O
as	O
object	O
oriented	O
(	O
OO	O
)	O
models	O
that	O
support	O
specialization	O
and	O
inheritance	O
and	O
relational	B-Architecture
models	I-Architecture
that	O
do	O
not	O
.	O
</s>
<s>
For	O
example	O
,	O
the	O
set	O
type	O
in	O
CODASYL	B-Language
schema	B-Application
may	O
be	O
partially	O
modeled	O
as	O
a	O
referential	O
integrity	O
constraint	O
in	O
a	O
relationship	O
schema	B-Application
.	O
</s>
<s>
CODASYL	B-Language
supports	O
insertion	O
and	O
retention	O
that	O
are	O
not	O
captured	O
by	O
referential	O
integrity	O
alone	O
.	O
</s>
<s>
The	O
query	B-Language
language	I-Language
supported	O
by	O
one	O
DBMS	B-Application
can	O
also	O
contribute	O
to	O
heterogeneity	B-General_Concept
between	O
other	O
component	O
DBMSs	B-Application
.	O
</s>
<s>
For	O
example	O
,	O
differences	O
in	O
query	B-Language
languages	I-Language
with	O
the	O
same	O
data	B-Application
models	I-Application
or	O
different	O
versions	O
of	O
query	B-Language
languages	I-Language
could	O
contribute	O
to	O
heterogeneity	B-General_Concept
.	O
</s>
<s>
Semantic	O
heterogeneities	B-General_Concept
arise	O
when	O
there	O
is	O
a	O
disagreement	O
about	O
meaning	O
,	O
interpretation	O
or	O
intended	O
use	O
of	O
data	O
.	O
</s>
<s>
At	O
the	O
schema	B-Application
and	O
data	O
level	O
,	O
classification	O
of	O
possible	O
heterogeneities	B-General_Concept
include	O
:	O
</s>
<s>
databases	B-Application
using	O
different	O
names	O
to	O
represent	O
the	O
same	O
concept	O
.	O
</s>
<s>
databases	B-Application
using	O
different	O
values	O
to	O
represent	O
same	O
concept	O
.	O
</s>
<s>
databases	B-Application
using	O
same	O
data	O
values	O
from	O
domains	O
of	O
different	O
cardinalities	B-Application
for	O
same	O
data	O
.	O
</s>
<s>
same	O
concepts	O
are	O
represented	O
at	O
schema	B-Application
level	O
and	O
instance	O
level	O
.	O
</s>
<s>
Schema	B-Application
conflicts	O
e.g.	O
</s>
<s>
In	O
creating	O
a	O
federated	O
schema	B-Application
,	O
one	O
has	O
to	O
resolve	O
such	O
heterogeneities	B-General_Concept
before	O
integrating	O
the	O
component	O
DB	O
schemas	B-Application
.	O
</s>
<s>
Dealing	O
with	O
incompatible	O
data	O
types	O
or	O
query	B-Library
syntax	O
is	O
not	O
the	O
only	O
obstacle	O
to	O
a	O
concrete	O
implementation	O
of	O
an	O
FDBS	O
.	O
</s>
<s>
In	O
systems	O
that	O
are	O
not	O
planned	O
top-down	O
,	O
a	O
generic	O
problem	O
lies	O
in	O
matching	O
semantically	O
equivalent	O
,	O
but	O
differently	O
named	O
parts	O
from	O
different	O
schemas	B-Application
(=	O
data	B-Application
models	I-Application
)	O
(	O
tables	O
,	O
attributes	B-General_Concept
)	O
.	O
</s>
<s>
A	O
pairwise	O
mapping	O
between	O
n	O
attributes	B-General_Concept
would	O
result	O
in	O
mapping	O
rules	O
(	O
given	O
equivalence	O
mappings	O
)	O
-	O
a	O
number	O
that	O
quickly	O
gets	O
too	O
large	O
for	O
practical	O
purposes	O
.	O
</s>
<s>
A	O
common	O
way	O
out	O
is	O
to	O
provide	O
a	O
global	O
schema	B-Application
that	O
comprises	O
the	O
relevant	O
parts	O
of	O
all	O
member	O
schemas	B-Application
and	O
provide	O
mappings	O
in	O
the	O
form	O
of	O
database	B-Language
views	I-Language
.	O
</s>
<s>
Both	O
are	O
examples	O
of	O
data	B-General_Concept
integration	I-General_Concept
,	O
called	O
the	O
schema	B-General_Concept
matching	I-General_Concept
problem	O
.	O
</s>
<s>
It	O
is	O
important	O
to	O
understand	O
the	O
aspects	O
of	O
autonomy	O
for	O
component	O
databases	B-Application
and	O
how	O
they	O
can	O
be	O
addressed	O
when	O
a	O
component	O
DBS	O
participates	O
in	O
an	O
FDBS	O
.	O
</s>
<s>
Design	O
Autonomy	O
which	O
refers	O
to	O
ability	O
to	O
choose	O
its	O
design	O
irrespective	O
of	O
data	O
,	O
query	B-Language
language	I-Language
or	O
conceptualization	O
,	O
functionality	O
of	O
the	O
system	O
implementation	O
.	O
</s>
<s>
Heterogeneities	B-General_Concept
in	O
an	O
FDBS	O
are	O
primarily	O
due	O
to	O
design	O
autonomy	O
.	O
</s>
<s>
Communication	O
autonomy	O
refers	O
to	O
the	O
general	O
operation	O
of	O
the	O
DBMS	B-Application
to	O
communicate	O
with	O
other	O
DBMS	B-Application
or	O
not	O
.	O
</s>
<s>
Execution	O
autonomy	O
allows	O
a	O
component	O
DBMS	B-Application
to	O
control	O
the	O
operations	O
requested	O
by	O
local	O
and	O
external	O
operations	O
.	O
</s>
<s>
The	O
ANSI/X3/SPARC	O
Study	O
Group	O
outlined	O
a	O
three	O
level	O
data	O
description	O
architecture	O
,	O
the	O
components	O
of	O
which	O
are	O
the	O
conceptual	O
schema	B-Application
,	O
internal	O
schema	B-Application
and	O
external	O
schema	B-Application
of	O
databases	B-Application
.	O
</s>
<s>
It	O
was	O
therefore	O
extended	O
to	O
support	O
the	O
three	O
dimensions	O
of	O
the	O
FDBS	O
namely	O
Distribution	O
,	O
Autonomy	O
and	O
Heterogeneity	B-General_Concept
.	O
</s>
<s>
The	O
five	O
level	O
schema	B-Application
architecture	O
is	O
explained	O
below	O
.	O
</s>
<s>
The	O
Heterogeneity	B-General_Concept
and	O
Autonomy	O
requirements	O
pose	O
special	O
challenges	O
concerning	O
concurrency	B-Operating_System
control	I-Operating_System
in	O
an	O
FDBS	O
,	O
which	O
is	O
crucial	O
for	O
the	O
correct	O
execution	O
of	O
its	O
concurrent	O
transactions	B-General_Concept
(	O
see	O
also	O
Global	B-Operating_System
concurrency	I-Operating_System
control	I-Operating_System
)	O
.	O
</s>
<s>
Achieving	O
global	B-Operating_System
serializability	I-Operating_System
,	O
the	O
major	O
correctness	O
criterion	O
,	O
under	O
these	O
requirements	O
has	O
been	O
characterized	O
as	O
very	O
difficult	O
and	O
unsolved	O
.	O
</s>
<s>
Commitment	B-Architecture
ordering	I-Architecture
,	O
introduced	O
in	O
1991	O
,	O
has	O
provided	O
a	O
general	O
solution	O
for	O
this	O
issue	O
(	O
See	O
Global	B-Operating_System
serializability	I-Operating_System
;	O
See	O
Commitment	B-Architecture
ordering	I-Architecture
also	O
for	O
the	O
architectural	O
aspects	O
of	O
the	O
solution	O
)	O
.	O
</s>
<s>
The	O
five	O
level	O
schema	B-Application
architecture	O
includes	O
the	O
following	O
:	O
</s>
<s>
Local	O
Schema	B-Application
is	O
basically	O
the	O
conceptual	O
model	O
of	O
a	O
component	O
database	O
expressed	O
in	O
a	O
native	O
data	B-Application
model	I-Application
.	O
</s>
<s>
Component	O
schema	B-Application
is	O
the	O
subset	O
of	O
the	O
local	O
schema	B-Application
that	O
the	O
owner	O
organisation	O
is	O
willing	O
to	O
share	O
with	O
other	O
users	O
of	O
the	O
FDBS	O
and	O
it	O
is	O
translated	O
into	O
a	O
common	B-Application
data	I-Application
model	I-Application
.	O
</s>
<s>
Export	O
Schema	B-Application
represents	O
a	O
subset	O
of	O
a	O
component	O
schema	B-Application
that	O
is	O
available	O
to	O
a	O
particular	O
federation	O
.	O
</s>
<s>
The	O
export	O
schema	B-Application
helps	O
in	O
managing	O
flow	O
of	O
control	O
of	O
data	O
.	O
</s>
<s>
Federated	O
Schema	B-Application
is	O
an	O
integration	O
of	O
multiple	O
export	O
schemas	B-Application
.	O
</s>
<s>
It	O
includes	O
information	O
on	O
data	O
distribution	O
that	O
is	O
generated	O
when	O
integrating	O
export	O
schemas	B-Application
.	O
</s>
<s>
External	O
schema	B-Application
is	O
extracted	O
from	O
a	O
federated	O
schema	B-Application
,	O
and	O
is	O
defined	O
for	O
the	O
users/applications	O
of	O
a	O
particular	O
federation	O
.	O
</s>
<s>
While	O
accurately	O
representing	O
the	O
state	O
of	O
the	O
art	O
in	O
data	B-General_Concept
integration	I-General_Concept
,	O
the	O
Five	O
Level	O
Schema	B-Application
Architecture	O
above	O
does	O
suffer	O
from	O
a	O
major	O
drawback	O
,	O
namely	O
IT	O
imposed	O
look	O
and	O
feel	O
.	O
</s>
<s>
Modern	O
data	O
users	O
demand	O
control	O
over	O
how	O
data	O
is	O
presented	O
;	O
their	O
needs	O
are	O
somewhat	O
in	O
conflict	O
with	O
such	O
bottom-up	O
approaches	O
to	O
data	B-General_Concept
integration	I-General_Concept
.	O
</s>
