<s>
A	O
database	B-General_Concept
connection	I-General_Concept
is	O
a	O
facility	O
in	O
computer	B-General_Concept
science	I-General_Concept
that	O
allows	O
client	B-Protocol
software	I-Protocol
to	O
talk	O
to	O
database	B-Application
server	I-Application
software	O
,	O
whether	O
on	O
the	O
same	O
machine	O
or	O
not	O
.	O
</s>
<s>
A	O
connection	O
is	O
required	O
to	O
send	O
commands	B-Application
and	O
receive	O
answers	O
,	O
usually	O
in	O
the	O
form	O
of	O
a	O
result	O
set	O
.	O
</s>
<s>
Since	O
some	O
DBMS	O
engines	O
require	O
considerable	O
time	O
to	O
connect	O
,	O
connection	B-General_Concept
pooling	I-General_Concept
was	O
invented	O
to	O
improve	O
performance	O
.	O
</s>
<s>
Connections	O
are	O
built	O
by	O
supplying	O
an	O
underlying	O
driver	B-Application
or	O
provider	O
with	O
a	O
connection	B-Language
string	I-Language
,	O
which	O
is	O
a	O
way	O
of	O
addressing	O
a	O
specific	O
database	O
or	O
server	B-Application
and	O
instance	O
as	O
well	O
as	O
user	O
authentication	O
credentials	O
(	O
for	O
example	O
,	O
Server	B-Application
=	O
sql_box	O
;D	O
atabase	O
=	O
Common	O
;	O
User	O
ID	O
=	O
uid	O
;	O
Pwd	O
=	O
password	O
;	O
)	O
.	O
</s>
<s>
Once	O
a	O
connection	O
has	O
been	O
built	O
it	O
can	O
be	O
opened	O
and	O
closed	O
at	O
will	O
,	O
and	O
properties	O
(	O
such	O
as	O
the	O
command	O
time-out	O
length	O
,	O
or	O
transaction	B-General_Concept
,	O
if	O
one	O
exists	O
)	O
can	O
be	O
set	O
.	O
</s>
<s>
The	O
Connection	B-Language
String	I-Language
is	O
composed	O
of	O
a	O
set	O
of	O
key/value	O
pairs	O
as	O
dictated	O
by	O
the	O
data	O
access	O
interface	O
and	O
data	O
provider	O
being	O
used	O
.	O
</s>
<s>
Many	O
databases	O
(	O
such	O
as	O
PostgreSQL	B-Application
)	O
only	O
allow	O
one	O
operation	O
to	O
be	O
performed	O
at	O
a	O
time	O
on	O
each	O
connection	O
.	O
</s>
<s>
If	O
a	O
request	O
for	O
data	O
(	O
a	O
SQL	B-Language
Select	I-Language
statement	O
)	O
is	O
sent	O
to	O
the	O
database	O
and	O
a	O
result	O
set	O
is	O
returned	O
,	O
the	O
connection	O
is	O
open	O
but	O
not	O
available	O
for	O
other	O
operations	O
until	O
the	O
client	B-Protocol
finishes	O
consuming	O
the	O
result	O
set	O
.	O
</s>
<s>
Other	O
databases	O
,	O
like	O
SQL	B-Application
Server	I-Application
2005	I-Application
(	O
and	O
later	O
)	O
,	O
do	O
not	O
impose	O
this	O
limitation	O
.	O
</s>
<s>
Database	B-General_Concept
connections	I-General_Concept
are	O
finite	O
and	O
expensive	O
and	O
can	O
take	O
a	O
disproportionately	O
long	O
time	O
to	O
create	O
relative	O
to	O
the	O
operations	O
performed	O
on	O
them	O
.	O
</s>
<s>
It	O
is	O
inefficient	O
for	O
an	O
application	O
to	O
create	O
,	O
use	O
,	O
and	O
close	O
a	O
database	B-General_Concept
connection	I-General_Concept
whenever	O
it	O
needs	O
to	O
update	O
a	O
database	O
.	O
</s>
<s>
Connection	B-General_Concept
pooling	I-General_Concept
is	O
a	O
technique	O
designed	O
to	O
alleviate	O
this	O
problem	O
.	O
</s>
<s>
A	O
pool	O
of	O
database	B-General_Concept
connections	I-General_Concept
can	O
be	O
created	O
and	O
then	O
shared	O
among	O
the	O
applications	O
that	O
need	O
to	O
access	O
the	O
database	O
.	O
</s>
<s>
The	O
connection	O
object	O
obtained	O
from	O
the	O
connection	B-General_Concept
pool	I-General_Concept
is	O
often	O
a	O
wrapper	O
around	O
the	O
actual	O
database	B-General_Concept
connection	I-General_Concept
.	O
</s>
<s>
For	O
example	O
,	O
the	O
wrapper	O
object	O
can	O
implement	O
a	O
"	O
close	O
"	O
method	O
that	O
can	O
be	O
called	O
just	O
like	O
the	O
"	O
close	O
"	O
method	O
on	O
the	O
database	B-General_Concept
connection	I-General_Concept
.	O
</s>
<s>
Unlike	O
the	O
method	O
on	O
the	O
database	B-General_Concept
connection	I-General_Concept
,	O
the	O
method	O
on	O
the	O
wrapper	O
may	O
not	O
actually	O
close	O
the	O
database	B-General_Concept
connection	I-General_Concept
,	O
but	O
instead	O
return	O
it	O
to	O
the	O
pool	O
.	O
</s>
<s>
The	O
application	O
need	O
not	O
be	O
aware	O
of	O
the	O
connection	B-General_Concept
pooling	I-General_Concept
when	O
it	O
calls	O
the	O
methods	O
on	O
the	O
wrapper	O
object	O
.	O
</s>
<s>
This	O
is	O
also	O
called	O
multiplexing	B-Architecture
.	O
</s>
<s>
In	O
a	O
client/server	O
architecture	O
,	O
on	O
the	O
other	O
hand	O
,	O
a	O
persistent	O
connection	O
is	O
typically	O
used	O
so	O
that	O
server	B-Application
state	O
can	O
be	O
managed	O
.	O
</s>
<s>
This	O
"	O
state	O
"	O
includes	O
server-side	O
cursors	O
,	O
temporary	O
products	O
,	O
connection-specific	O
functional	O
settings	O
,	O
and	O
so	O
on	O
.	O
</s>
<s>
An	O
application	O
failure	O
occurs	O
when	O
the	O
connection	B-General_Concept
pool	I-General_Concept
overflows	O
.	O
</s>
