<s>
A	O
database	B-Application
abstraction	I-Application
layer	I-Application
(	O
DBAL	B-Application
or	O
DAL	O
)	O
is	O
an	O
application	B-Application
programming	I-Application
interface	I-Application
which	O
unifies	O
the	O
communication	O
between	O
a	O
computer	O
application	O
and	O
databases	O
such	O
as	O
SQL	B-Application
Server	I-Application
,	O
IBM	B-Application
Db2	I-Application
,	O
MySQL	B-Application
,	O
PostgreSQL	B-Application
,	O
Oracle	B-General_Concept
or	O
SQLite	B-Language
.	O
</s>
<s>
Database	B-Application
abstraction	I-Application
layers	I-Application
reduce	O
the	O
amount	O
of	O
work	O
by	O
providing	O
a	O
consistent	O
API	B-Application
to	O
the	O
developer	O
and	O
hide	O
the	O
database	O
specifics	O
behind	O
this	O
interface	O
as	O
much	O
as	O
possible	O
.	O
</s>
<s>
Implementation	O
of	O
the	O
physical	O
layer	O
may	O
use	O
database-specific	O
APIs	B-Application
or	O
use	O
the	O
underlying	O
language	O
standard	O
database	O
access	O
technology	O
and	O
the	O
database	O
's	O
version	O
SQL	B-Language
.	O
</s>
<s>
Additionally	O
it	O
needs	O
to	O
span	O
all	O
the	O
supported	O
databases	O
and	O
their	O
quirks	O
,	O
APIs	B-Application
,	O
and	O
problems	O
.	O
</s>
<s>
Database	O
operations	O
are	O
represented	O
only	O
loosely	O
as	O
SQL	B-Language
or	O
even	O
database	O
access	O
at	O
this	O
level	O
.	O
</s>
<s>
Libraries	O
unify	O
access	O
to	O
databases	O
by	O
providing	O
a	O
single	O
low-level	O
programming	B-Application
interface	I-Application
to	O
the	O
application	O
developer	O
.	O
</s>
<s>
Their	O
advantages	O
are	O
most	O
often	O
speed	O
and	O
flexibility	O
because	O
they	O
are	O
not	O
tied	O
to	O
a	O
specific	O
query	B-Language
language	I-Language
(	O
subset	O
)	O
and	O
only	O
have	O
to	O
implement	O
a	O
thin	O
layer	O
to	O
reach	O
their	O
goal	O
.	O
</s>
<s>
As	O
all	O
SQL	B-Language
dialects	O
are	O
similar	O
to	O
one	O
another	O
,	O
application	O
developers	O
can	O
use	O
all	O
the	O
language	O
features	O
,	O
possibly	O
providing	O
configurable	O
elements	O
for	O
database-specific	O
cases	O
,	O
such	O
as	O
typically	O
user-IDs	O
and	O
credentials	O
.	O
</s>
<s>
Popular	O
use	O
for	O
database	B-Application
abstraction	I-Application
layers	I-Application
are	O
among	O
object-oriented	B-Language
programming	I-Language
languages	I-Language
,	O
which	O
are	O
similar	O
to	O
API-level	O
abstraction	O
layers	O
.	O
</s>
<s>
In	O
an	O
object-oriented	B-Language
language	I-Language
like	O
C++	O
or	O
Java	O
,	O
a	O
database	O
can	O
be	O
represented	O
through	O
an	O
object	O
,	O
whose	O
methods	O
and	O
members	O
(	O
or	O
the	O
equivalent	O
thereof	O
in	O
other	O
programming	O
languages	O
)	O
represent	O
various	O
functionalities	O
of	O
the	O
database	O
.	O
</s>
<s>
They	O
also	O
share	O
advantages	O
and	O
disadvantages	O
with	O
API-level	O
interfaces	O
.	O
</s>
<s>
An	O
example	O
of	O
a	O
database	B-Application
abstraction	I-Application
layer	I-Application
on	O
the	O
language	O
level	O
would	O
be	O
ODBC	B-Application
that	O
is	O
a	O
platform-independent	O
implementation	O
of	O
a	O
database	B-Application
abstraction	I-Application
layer	I-Application
.	O
</s>
<s>
The	O
user	O
installs	O
specific	O
driver	B-Application
software	I-Application
,	O
through	O
which	O
ODBC	B-Application
can	O
communicate	O
with	O
a	O
database	O
or	O
set	O
of	O
databases	O
.	O
</s>
<s>
The	O
user	O
then	O
has	O
the	O
ability	O
to	O
have	O
programs	O
communicate	O
with	O
ODBC	B-Application
,	O
which	O
then	O
relays	O
the	O
results	O
back	O
and	O
forth	O
between	O
the	O
user	O
programs	O
and	O
the	O
database	O
.	O
</s>
<s>
Finally	O
,	O
large	O
projects	O
may	O
develop	O
their	O
own	O
libraries	O
,	O
such	O
as	O
,	O
for	O
example	O
,	O
libgda	O
for	O
GNOME	B-Application
.	O
</s>
<s>
Development	O
period	O
:	O
software	O
developers	O
only	O
have	O
to	O
know	O
the	O
database	B-Application
abstraction	I-Application
layer	I-Application
's	O
API	B-Application
instead	O
of	O
all	O
APIs	B-Application
of	O
the	O
databases	O
their	O
application	O
should	O
support	O
.	O
</s>
<s>
Wider	O
potential	O
install-base	O
:	O
using	O
a	O
database	B-Application
abstraction	I-Application
layer	I-Application
means	O
that	O
there	O
is	O
no	O
requirement	O
for	O
new	O
installations	O
to	O
utilise	O
a	O
specific	O
database	O
,	O
i.e.	O
</s>
<s>
This	O
is	O
especially	O
true	O
for	O
database	B-Application
abstraction	I-Application
layers	I-Application
that	O
try	O
to	O
unify	O
the	O
query	B-Language
language	I-Language
as	O
well	O
like	O
ODBC	B-Application
.	O
</s>
<s>
Dependency	O
:	O
a	O
database	B-Application
abstraction	I-Application
layer	I-Application
provides	O
yet	O
another	O
functional	O
dependency	O
for	O
a	O
software	O
system	O
,	O
i.e.	O
</s>
<s>
a	O
given	O
database	B-Application
abstraction	I-Application
layer	I-Application
,	O
like	O
anything	O
else	O
,	O
may	O
eventually	O
become	O
obsolete	O
,	O
outmoded	O
or	O
unsupported	O
.	O
</s>
<s>
Masked	O
operations	O
:	O
database	B-Application
abstraction	I-Application
layers	I-Application
may	O
limit	O
the	O
number	O
of	O
available	O
database	O
operations	O
to	O
a	O
subset	O
of	O
those	O
supported	O
by	O
the	O
supported	O
database	O
backends	O
.	O
</s>
<s>
In	O
particular	O
,	O
database	B-Application
abstraction	I-Application
layers	I-Application
may	O
not	O
fully	O
support	O
database	O
backend-specific	O
optimizations	O
or	O
debugging	O
features	O
.	O
</s>
