<s>
A	O
key	B-Data_Structure
–	I-Data_Structure
value	I-Data_Structure
database	I-Data_Structure
,	O
or	O
key	B-Data_Structure
–	I-Data_Structure
value	I-Data_Structure
store	I-Data_Structure
,	O
is	O
a	O
data	O
storage	O
paradigm	O
designed	O
for	O
storing	O
,	O
retrieving	O
,	O
and	O
managing	O
associative	B-Application
arrays	I-Application
,	O
and	O
a	O
data	B-General_Concept
structure	I-General_Concept
more	O
commonly	O
known	O
today	O
as	O
a	O
dictionary	B-Application
or	O
hash	B-Algorithm
table	I-Algorithm
.	O
</s>
<s>
Dictionaries	B-Application
contain	O
a	O
collection	B-Application
of	O
objects	O
,	O
or	O
records	O
,	O
which	O
in	O
turn	O
have	O
many	O
different	O
fields	B-Application
within	O
them	O
,	O
each	O
containing	O
data	O
.	O
</s>
<s>
Key	B-Data_Structure
–	I-Data_Structure
value	I-Data_Structure
databases	I-Data_Structure
work	O
in	O
a	O
very	O
different	O
fashion	O
from	O
the	O
better	O
known	O
relational	B-Application
databases	I-Application
(	O
RDB	O
)	O
.	O
</s>
<s>
RDBs	O
predefine	O
the	O
data	B-General_Concept
structure	I-General_Concept
in	O
the	O
database	O
as	O
a	O
series	O
of	O
tables	O
containing	O
fields	B-Application
with	O
well	O
defined	O
data	O
types	O
.	O
</s>
<s>
In	O
contrast	O
,	O
key	O
–	O
value	O
systems	O
treat	O
the	O
data	O
as	O
a	O
single	O
opaque	O
collection	B-Application
,	O
which	O
may	O
have	O
different	O
fields	B-Application
for	O
every	O
record	O
.	O
</s>
<s>
This	O
offers	O
considerable	O
flexibility	O
and	O
more	O
closely	O
follows	O
modern	O
concepts	O
like	O
object-oriented	B-Language
programming	I-Language
.	O
</s>
<s>
Because	O
optional	O
values	O
are	O
not	O
represented	O
by	O
placeholders	O
or	O
input	O
parameters	O
,	O
as	O
in	O
most	O
RDBs	O
,	O
key	B-Data_Structure
–	I-Data_Structure
value	I-Data_Structure
databases	I-Data_Structure
often	O
use	O
far	O
less	O
memory	B-General_Concept
to	O
store	O
the	O
same	O
data	O
,	O
which	O
can	O
lead	O
to	O
large	O
performance	O
gains	O
in	O
certain	O
workloads	O
.	O
</s>
<s>
Performance	O
,	O
a	O
lack	O
of	O
standardization	O
and	O
other	O
issues	O
limited	O
key	O
–	O
value	O
systems	O
to	O
niche	O
uses	O
for	O
many	O
years	O
,	O
but	O
the	O
rapid	O
move	O
to	O
cloud	B-Architecture
computing	I-Architecture
after	O
2010	O
has	O
led	O
to	O
a	O
renaissance	O
as	O
part	O
of	O
the	O
broader	O
NoSQL	B-General_Concept
movement	O
.	O
</s>
<s>
Some	O
graph	B-Application
databases	I-Application
,	O
such	O
as	O
ArangoDB	B-Operating_System
,	O
are	O
also	O
key	B-Data_Structure
–	I-Data_Structure
value	I-Data_Structure
databases	I-Data_Structure
internally	O
,	O
adding	O
the	O
concept	O
of	O
the	O
relationships	O
(	O
pointers	O
)	O
between	O
records	O
as	O
a	O
first	O
class	O
data	O
type	O
.	O
</s>
<s>
Key	B-Data_Structure
–	I-Data_Structure
value	I-Data_Structure
databases	I-Data_Structure
can	O
use	O
consistency	B-General_Concept
models	I-General_Concept
ranging	O
from	O
eventual	B-General_Concept
consistency	I-General_Concept
to	O
serializability	B-Operating_System
.	O
</s>
<s>
Some	O
maintain	O
data	O
in	O
memory	B-General_Concept
(	O
RAM	O
)	O
,	O
while	O
others	O
employ	O
solid-state	B-Device
drives	I-Device
or	O
rotating	B-Device
disks	I-Device
.	O
</s>
<s>
The	O
Unix	O
system	O
provides	O
dbm	B-General_Concept
(	O
database	O
manager	O
)	O
,	O
which	O
is	O
a	O
1979	O
library	O
originally	O
written	O
by	O
Ken	O
Thompson	O
.	O
</s>
<s>
It	O
is	O
also	O
ported	O
to	O
Microsoft	B-Application
Windows	I-Application
,	O
provided	O
through	O
programming	O
languages	O
such	O
as	O
Perl	O
for	O
Win32	O
.	O
</s>
<s>
The	O
dbm	B-General_Concept
manages	O
associative	B-Application
arrays	I-Application
of	O
arbitrary	O
data	O
by	O
use	O
of	O
a	O
single	O
key	O
(	O
a	O
primary	O
key	O
)	O
.	O
</s>
<s>
Modern	O
implementations	O
include	O
sdbm	B-General_Concept
,	O
GNU	B-General_Concept
dbm	I-General_Concept
,	O
and	O
Berkeley	B-Language
DB	I-Language
.	O
</s>
<s>
Although	O
dbm	B-General_Concept
precedes	O
the	O
concept	O
of	O
a	O
NoSQL	B-General_Concept
and	O
is	O
rarely	O
mentioned	O
in	O
modern	O
discourse	O
,	O
it	O
is	O
used	O
by	O
many	O
pieces	O
of	O
software	O
.	O
</s>
<s>
A	O
more	O
recent	O
example	O
of	O
a	O
key-value	B-Data_Structure
database	I-Data_Structure
is	O
RocksDB	B-Language
which	O
is	O
used	O
as	O
storage	O
engine	O
for	O
other	O
database	O
management	O
systems	O
such	O
as	O
ArangoDB	B-Operating_System
.	O
</s>
