<s>
Database	B-General_Concept
scalability	I-General_Concept
is	O
the	O
ability	O
of	O
a	O
database	O
to	O
handle	O
changing	O
demands	O
by	O
adding/removing	O
resources	O
.	O
</s>
<s>
The	O
initial	O
history	O
of	O
database	B-General_Concept
scalability	I-General_Concept
was	O
to	O
provide	O
service	O
on	O
ever	O
smaller	O
computers	O
.	O
</s>
<s>
The	O
first	O
database	O
management	O
systems	O
such	O
as	O
IMS	B-Application
ran	O
on	O
mainframe	B-Architecture
computers	I-Architecture
.	O
</s>
<s>
The	O
second	O
generation	O
,	O
including	O
Ingres	B-Application
,	O
Informix	B-Application
,	O
Sybase	B-Application
,	O
RDB	B-Application
and	O
Oracle	B-General_Concept
emerged	O
on	O
minicomputers	B-Architecture
.	O
</s>
<s>
The	O
third	O
generation	O
,	O
including	O
dBase	B-Application
and	O
Oracle	B-General_Concept
(	O
again	O
)	O
,	O
ran	O
on	O
personal	O
computers	O
.	O
</s>
<s>
This	O
eliminated	O
a	O
critical	O
scalability	B-Architecture
bottleneck	O
,	O
as	O
coarser	O
locks	O
could	O
delay	O
access	O
to	O
rows	O
even	O
though	O
they	O
were	O
not	O
directly	O
involved	O
in	O
a	O
transaction	O
.	O
</s>
<s>
The	O
first	O
was	O
to	O
support	O
multiprocessor	B-Operating_System
computers	I-Operating_System
.	O
</s>
<s>
This	O
evolved	O
into	O
support	O
for	O
multi-core	B-Architecture
processors	I-Architecture
.	O
</s>
<s>
A	O
much	O
more	O
significant	O
change	O
involved	O
allowing	O
distributed	B-General_Concept
transactions	I-General_Concept
to	O
affect	O
data	O
stored	O
on	O
separate	O
computers	O
,	O
using	O
the	O
two-phase	B-General_Concept
commit	I-General_Concept
protocol	I-General_Concept
,	O
establishing	O
the	O
shared-nothing	B-Operating_System
architecture	I-Operating_System
.	O
</s>
<s>
Still	O
later	O
,	O
Oracle	B-General_Concept
introduced	O
the	O
shared-everything	O
architecture	O
,	O
which	O
provided	O
full	O
functionality	O
on	O
multi-server	O
clusters	O
.	O
</s>
<s>
Another	O
innovation	O
was	O
storing	O
copies	O
of	O
tables	O
on	O
multiple	O
computers	O
(	O
database	B-Architecture
replication	I-Architecture
)	O
,	O
which	O
both	O
improved	O
availability	O
(	O
processing	O
could	O
continue	O
on	O
a	O
copy	O
even	O
if	O
the	O
main	O
system	O
was	O
unavailable	O
)	O
and	O
scalability	B-Architecture
particularly	O
for	O
query/analysis	O
,	O
in	O
that	O
requests	O
could	O
be	O
routed	O
to	O
the	O
copy	O
if	O
the	O
primary	O
reached	O
its	O
capacity	O
.	O
</s>
<s>
In	O
the	O
early	O
twenty-first	O
century	O
,	O
NoSQL	B-General_Concept
systems	O
gained	O
favor	O
over	O
relational	B-Application
databases	I-Application
for	O
some	O
workloads	O
.	O
</s>
<s>
Motivations	O
included	O
still	O
greater	O
scalability	B-Architecture
and	O
support	O
for	O
documents	O
and	O
other	O
"	O
non-relational	O
"	O
data	O
types	O
.	O
</s>
<s>
Often	O
sacrificed	O
was	O
the	O
strict	O
ACID	O
consistency	O
protocols	O
that	O
guaranteed	O
perfect	O
consistency	O
at	O
all	O
times	O
in	O
favor	O
of	O
eventual	B-General_Concept
consistency	I-General_Concept
that	O
ensured	O
that	O
all	O
nodes	O
would	O
eventually	O
return	O
the	O
latest	O
data	O
.	O
</s>
<s>
The	O
most	O
prominent	O
early	O
system	O
was	O
Google	O
's	O
BigTable/MapReduce	O
,	O
developed	O
in	O
2004	O
.	O
</s>
<s>
It	O
achieved	O
near-linear	O
scalability	B-Architecture
across	O
multiple	O
server	B-Operating_System
farms	I-Operating_System
,	O
at	O
the	O
cost	O
of	O
features	O
such	O
as	O
multi-row	O
transactions	O
and	O
joins	O
.	O
</s>
<s>
In	O
2007	O
,	O
the	O
first	O
NewSQL	B-General_Concept
system	O
,	O
H-Store	B-Application
,	O
was	O
developed	O
.	O
</s>
<s>
NewSQL	B-General_Concept
systems	O
attempt	O
to	O
combine	O
NoSQL	B-General_Concept
scalability	B-Architecture
with	O
ACID	O
transactions	O
and	O
SQL	O
interfaces	O
.	O
</s>
<s>
Database	B-General_Concept
scalability	I-General_Concept
has	O
three	O
basic	O
dimensions	O
:	O
amount	O
of	O
data	O
,	O
volume	O
of	O
requests	O
and	O
size	O
of	O
requests	O
.	O
</s>
<s>
Vertical	O
database	O
scaling	B-Architecture
implies	O
that	O
the	O
database	O
system	O
can	O
fully	O
exploit	O
maximally	O
configured	O
systems	O
,	O
including	O
typically	O
multiprocessors	B-Operating_System
with	O
large	O
memories	O
and	O
vast	O
storage	O
capacity	O
.	O
</s>
<s>
If	O
workloads	O
expand	O
beyond	O
that	O
limit	O
,	O
the	O
choices	O
are	O
either	O
to	O
migrate	O
to	O
a	O
different	O
,	O
still	O
larger	O
system	O
,	O
or	O
to	O
rearchitect	O
the	O
system	O
to	O
achieve	O
horizontal	O
scalability	B-Architecture
.	O
</s>
<s>
Horizontal	O
database	O
scaling	B-Architecture
involves	O
adding	O
more	O
servers	O
to	O
work	O
on	O
a	O
single	O
workload	O
.	O
</s>
<s>
Most	O
horizontally	O
scalable	B-Architecture
systems	I-Architecture
come	O
with	O
functionality	O
compromises	O
.	O
</s>
<s>
Databases	O
run	O
on	O
individual	O
hardware	O
ranging	O
in	O
capacity	O
from	O
smartwatches	O
to	O
supercomputers	O
to	O
multiple	O
transparently	O
reconfigurable	O
server	B-Operating_System
farms	I-Operating_System
.	O
</s>
<s>
Databases	O
also	O
scaled	O
vertically	O
to	O
run	O
on	O
64-bit	O
microprocessors	B-Architecture
,	O
multi-core	B-Architecture
CPUs	I-Architecture
,	O
and	O
large	O
SMP	B-Operating_System
multiprocessors	I-Operating_System
,	O
using	O
multi-threaded	B-Operating_System
implementations	O
.	O
</s>
<s>
Alternatively	O
,	O
some	O
databases	O
use	O
multi-version	B-Operating_System
read	I-Operating_System
consistency	I-Operating_System
to	O
avoid	O
(	O
blocking	O
)	O
read	O
locks	O
while	O
still	O
providing	O
consistent	O
query	O
results	O
.	O
</s>
<s>
In	O
a	O
shared	B-Operating_System
nothing	I-Operating_System
system	O
,	O
at	O
any	O
given	O
moment	O
,	O
all	O
such	O
inserts	O
are	O
processed	O
by	O
the	O
single	O
server	O
that	O
manages	O
that	O
partition	B-General_Concept
(	O
shard	O
)	O
of	O
the	O
table	O
,	O
possibly	O
overwhelming	O
it	O
,	O
while	O
the	O
rest	O
of	O
the	O
system	O
has	O
little	O
to	O
do	O
.	O
</s>
<s>
One	O
solution	O
for	O
this	O
is	O
to	O
reverse	B-Data_Structure
the	I-Data_Structure
digits	I-Data_Structure
of	I-Data_Structure
the	I-Data_Structure
primary	I-Data_Structure
key	I-Data_Structure
.	O
</s>
<s>
A	O
basic	O
technique	O
is	O
to	O
split	B-General_Concept
large	O
tables	O
into	O
multiple	O
partitions	B-General_Concept
based	O
on	O
ranges	O
of	O
values	O
in	O
a	O
key	O
field	O
.	O
</s>
<s>
This	O
scaling	B-Architecture
technique	O
is	O
particularly	O
convenient	O
for	O
seldom	O
or	O
never-updated	O
data	O
,	O
such	O
as	O
transaction	O
history	O
or	O
tax	O
tables	O
.	O
</s>
<s>
HP	O
Enterprise	O
's	O
NonStop	B-Application
SQL	I-Application
uses	O
the	O
shared	B-Operating_System
nothing	I-Operating_System
architecture	I-Operating_System
in	O
which	O
neither	O
data	O
nor	O
memory	O
are	O
shared	O
across	O
server	O
boundaries	O
.	O
</s>
<s>
This	O
architecture	O
provides	O
near-linear	O
scalability	B-Architecture
.	O
</s>
<s>
The	O
widely	O
supported	O
X/Open	O
XA	O
standard	O
employs	O
a	O
global	O
transaction	O
monitor	O
to	O
coordinate	O
distributed	B-General_Concept
transactions	I-General_Concept
among	O
semi-autonomous	O
XA-compliant	O
transaction	O
resources	O
.	O
</s>
<s>
Oracle	B-Operating_System
RAC	I-Operating_System
uses	O
a	O
different	O
model	O
to	O
achieve	O
scalability	B-Architecture
,	O
based	O
on	O
a	O
"	O
shared-everything	O
"	O
architecture	O
.	O
</s>
<s>
This	O
approach	O
incorporates	O
the	O
shared	B-Operating_System
disk	I-Operating_System
approach	O
that	O
allows	O
multiple	O
computers	O
to	O
access	O
any	O
disk	O
in	O
the	O
cluster	O
.	O
</s>
<s>
Network-attached	B-Application
storage	I-Application
(	O
NAS	B-Application
)	O
and	O
Storage	O
area	O
networks	O
(	O
SANs	O
)	O
coupled	O
with	O
local	O
area	O
networks	O
and	O
Fibre	B-Architecture
Channel	I-Architecture
technology	O
enable	O
such	O
configurations	O
.	O
</s>
<s>
Some	O
researchers	O
question	O
the	O
inherent	O
limitations	O
of	O
relational	B-Application
database	I-Application
management	I-Application
systems	I-Application
.	O
</s>
<s>
GigaSpaces	O
,	O
for	O
example	O
,	O
contends	O
that	O
space-based	B-Operating_System
architecture	I-Operating_System
is	O
required	O
to	O
achieve	O
performance	O
and	O
scalability	B-Architecture
.	O
</s>
<s>
Base	O
One	O
makes	O
the	O
case	O
for	O
extreme	O
scalability	B-Architecture
within	O
mainstream	O
relational	B-Application
database	I-Application
technology	O
.	O
</s>
