<s>
Google	B-Architecture
data	I-Architecture
centers	I-Architecture
are	O
the	O
large	O
data	B-Operating_System
center	I-Operating_System
facilities	O
Google	B-Application
uses	O
to	O
provide	O
their	B-Application
services	I-Application
,	O
which	O
combine	O
large	O
drives	O
,	O
computer	O
nodes	O
organized	O
in	O
aisles	O
of	O
racks	O
,	O
internal	O
and	O
external	O
networking	O
,	O
environmental	O
controls	O
(	O
mainly	O
cooling	O
and	O
humidification	O
control	O
)	O
,	O
and	O
operations	O
software	O
(	O
especially	O
as	O
concerns	O
load	B-Application
balancing	I-Application
and	O
fault	B-General_Concept
tolerance	I-General_Concept
)	O
.	O
</s>
<s>
There	O
is	O
no	O
official	O
data	O
on	O
how	O
many	O
servers	B-Application
are	O
in	O
Google	B-Architecture
data	I-Architecture
centers	I-Architecture
,	O
but	O
Gartner	O
estimated	O
in	O
a	O
July	O
2016	O
report	O
that	O
Google	B-Application
at	O
the	O
time	O
had	O
2.5	O
million	O
servers	B-Application
.	O
</s>
<s>
This	O
number	O
is	O
changing	O
as	O
the	O
company	O
expands	O
capacity	O
and	O
refreshes	O
its	O
hardware	B-Architecture
.	O
</s>
<s>
The	O
locations	O
of	O
Google	B-Application
's	I-Application
various	O
data	B-Operating_System
centers	I-Operating_System
by	O
continent	O
are	O
as	O
follows	O
:	O
</s>
<s>
The	O
original	O
hardware	B-Architecture
(	O
circa	O
1998	O
)	O
that	O
was	O
used	O
by	O
Google	B-Application
when	O
it	O
was	O
located	O
at	O
Stanford	O
University	O
included	O
:	O
</s>
<s>
Sun	B-Device
Microsystems	I-Device
Ultra	I-Device
II	I-Device
with	O
dual	O
200MHz	O
processors	O
,	O
and	O
256MB	O
of	O
RAM	B-Architecture
.	O
</s>
<s>
2	O
×	O
300MHz	O
dual	O
Pentium	B-General_Concept
II	I-General_Concept
servers	B-Application
donated	O
by	O
Intel	O
,	O
they	O
included	O
512MB	O
of	O
RAM	B-Architecture
and	O
10	O
×	O
9GB	O
hard	B-Device
drives	I-Device
between	O
the	O
two	O
.	O
</s>
<s>
F50	O
IBM	B-Device
RS/6000	I-Device
donated	O
by	O
IBM	O
,	O
included	O
4	O
processors	O
,	O
512MB	O
of	O
memory	O
and	O
8	O
×	O
9GB	O
hard	B-Device
disk	I-Device
drives	I-Device
.	O
</s>
<s>
Two	O
additional	O
boxes	O
included	O
3	O
×	O
9GB	O
hard	B-Device
drives	I-Device
and	O
6	O
x	O
4GB	O
hard	B-Device
disk	I-Device
drives	I-Device
respectively	O
(	O
the	O
original	O
storage	O
for	O
Backrub	O
)	O
.	O
</s>
<s>
These	O
were	O
attached	O
to	O
the	O
Sun	B-Device
Ultra	I-Device
II	O
.	O
</s>
<s>
SDD	O
disk	O
expansion	O
box	O
with	O
another	O
8	O
×	O
9GB	O
hard	B-Device
disk	I-Device
drives	I-Device
donated	O
by	O
IBM	O
.	O
</s>
<s>
Homemade	O
disk	O
box	O
which	O
contained	O
10	O
×	O
9GB	O
SCSI	B-Architecture
hard	B-Device
disk	I-Device
drives	I-Device
.	O
</s>
<s>
As	O
of	O
2014	O
,	O
Google	B-Application
has	O
used	O
a	O
heavily	O
customized	O
version	O
of	O
Debian	O
Linux	B-Application
.	O
</s>
<s>
Servers	B-Application
as	O
of	O
2009	O
–	O
2010	O
consisted	O
of	O
custom-made	O
open-top	O
systems	O
containing	O
two	O
processors	O
(	O
each	O
with	O
several	O
cores	O
)	O
,	O
a	O
considerable	O
amount	O
of	O
RAM	B-Architecture
spread	O
over	O
8	O
DIMM	O
slots	O
housing	O
double-height	O
DIMMs	O
,	O
and	O
at	O
least	O
two	O
SATA	O
hard	B-Device
disk	I-Device
drives	I-Device
connected	O
through	O
a	O
non-standard	O
ATX-sized	O
power	O
supply	O
unit	O
.	O
</s>
<s>
The	O
servers	B-Application
were	O
open	O
top	O
so	O
more	O
servers	B-Application
could	O
fit	O
into	O
a	O
rack	O
.	O
</s>
<s>
According	O
to	O
Google	B-Application
,	O
their	O
global	O
data	B-Operating_System
center	I-Operating_System
operation	O
electrical	O
power	O
ranges	O
between	O
500	O
and	O
681	O
megawatts	O
.	O
</s>
<s>
The	O
combined	O
processing	O
power	O
of	O
these	O
servers	B-Application
might	O
have	O
reached	O
from	O
20	O
to	O
100	O
petaflops	O
in	O
2008	O
.	O
</s>
<s>
Details	O
of	O
the	B-Application
Google	I-Application
worldwide	O
private	O
networks	O
are	O
not	O
publicly	O
available	O
,	O
but	O
Google	B-Application
publications	O
make	O
references	O
to	O
the	O
"	O
Atlas	O
Top	O
10	O
"	O
report	O
that	O
ranks	O
Google	B-Application
as	O
the	O
third	O
largest	O
ISP	O
behind	O
Level	O
3	O
.	O
</s>
<s>
In	O
order	O
to	O
run	O
such	O
a	O
large	O
network	O
,	O
with	O
direct	O
connections	O
to	O
as	O
many	O
ISPs	O
as	O
possible	O
at	O
the	O
lowest	O
possible	O
cost	O
,	O
Google	B-Application
has	O
a	O
very	O
open	O
peering	B-Protocol
policy	O
.	O
</s>
<s>
From	O
this	O
site	O
,	O
we	O
can	O
see	O
that	O
the	B-Application
Google	I-Application
network	O
can	O
be	O
accessed	O
from	O
67	O
public	O
exchange	O
points	O
and	O
69	O
different	O
locations	O
across	O
the	O
world	O
.	O
</s>
<s>
As	O
of	O
May	O
2012	O
,	O
Google	B-Application
had	O
882	O
Gbit/s	O
of	O
public	O
connectivity	O
(	O
not	O
counting	O
private	O
peering	B-Protocol
agreements	O
that	O
Google	B-Application
has	O
with	O
the	O
largest	O
ISPs	O
)	O
.	O
</s>
<s>
This	O
public	O
network	O
is	O
used	O
to	O
distribute	O
content	O
to	O
Google	B-Application
users	O
as	O
well	O
as	O
to	O
crawl	O
the	O
internet	O
to	O
build	O
its	O
search	O
indexes	O
.	O
</s>
<s>
The	O
private	O
side	O
of	O
the	O
network	O
is	O
a	O
secret	O
,	O
but	O
a	O
recent	O
disclosure	O
from	O
Google	B-Application
indicate	O
that	O
they	O
use	O
custom	O
built	O
high-radix	O
switch-routers	B-Application
(	O
with	O
a	O
capacity	O
of	O
128	O
×	O
10	O
Gigabit	O
Ethernet	O
port	O
)	O
for	O
the	O
wide	O
area	O
network	O
.	O
</s>
<s>
Running	O
no	O
less	O
than	O
two	O
routers	O
per	O
datacenter	B-Operating_System
(	O
for	O
redundancy	O
)	O
we	O
can	O
conclude	O
that	O
the	B-Application
Google	I-Application
network	O
scales	O
in	O
the	O
terabit	O
per	O
second	O
range	O
(	O
with	O
two	O
fully	O
loaded	O
routers	O
the	O
bi-sectional	O
bandwidth	O
amount	O
to	O
1,280	O
Gbit/s	O
)	O
.	O
</s>
<s>
These	O
custom	O
switch-routers	B-Application
are	O
connected	O
to	O
DWDM	B-Architecture
devices	O
to	O
interconnect	O
data	B-Operating_System
centers	I-Operating_System
and	O
point	O
of	O
presences	O
(	O
PoP	O
)	O
via	O
dark	B-Protocol
fiber	I-Protocol
.	O
</s>
<s>
From	O
a	O
datacenter	B-Operating_System
view	O
,	O
the	O
network	O
starts	O
at	O
the	O
rack	O
level	O
,	O
where	O
19-inch	B-Application
racks	I-Application
are	O
custom-made	O
and	O
contain	O
40	O
to	O
80	O
servers	B-Application
(	O
20	O
to	O
40	O
1U	O
servers	B-Application
on	O
either	O
side	O
,	O
while	O
new	O
servers	B-Application
are	O
2U	O
rackmount	B-Application
systems	O
.	O
</s>
<s>
Each	O
rack	O
has	O
an	O
Ethernet	B-Protocol
switch	I-Protocol
)	O
.	O
</s>
<s>
Servers	B-Application
are	O
connected	O
via	O
a	O
1Gbit/s	O
Ethernet	O
link	O
to	O
the	O
top	O
of	O
rack	O
switch	O
(	O
TOR	O
)	O
.	O
</s>
<s>
The	O
cluster	O
switches	O
themselves	O
are	O
interconnected	O
and	O
form	O
the	O
datacenter	B-Operating_System
interconnect	O
fabric	O
(	O
most	O
likely	O
using	O
a	O
dragonfly	O
design	O
rather	O
than	O
a	O
classic	O
butterfly	O
or	O
flattened	O
butterfly	O
layout	O
)	O
.	O
</s>
<s>
From	O
an	O
operation	O
standpoint	O
,	O
when	O
a	O
client	O
computer	O
attempts	O
to	O
connect	O
to	O
Google	B-Application
,	O
several	O
DNS	B-Protocol
servers	I-Protocol
resolve	O
www.google.com	O
into	O
multiple	O
IP	O
addresses	O
via	O
Round	B-Protocol
Robin	I-Protocol
policy	O
.	O
</s>
<s>
Furthermore	O
,	O
this	O
acts	O
as	O
the	O
first	O
level	O
of	O
load	B-Application
balancing	I-Application
and	O
directs	O
the	O
client	O
to	O
different	O
Google	B-Application
clusters	O
.	O
</s>
<s>
A	O
Google	B-Application
cluster	O
has	O
thousands	O
of	O
servers	B-Application
,	O
and	O
once	O
the	O
client	O
has	O
connected	O
to	O
the	O
server	O
additional	O
load	B-Application
balancing	I-Application
is	O
done	O
to	O
send	O
the	O
queries	B-Library
to	O
the	O
least	O
loaded	O
web	O
server	O
.	O
</s>
<s>
This	O
makes	O
Google	B-Application
one	O
of	O
the	O
largest	O
and	O
most	O
complex	O
content	B-Operating_System
delivery	I-Operating_System
networks	I-Operating_System
.	O
</s>
<s>
Google	B-Application
has	O
numerous	O
data	B-Operating_System
centers	I-Operating_System
scattered	O
around	O
the	O
world	O
.	O
</s>
<s>
At	O
least	O
12	O
significant	O
Google	B-Architecture
data	I-Architecture
center	I-Architecture
installations	O
are	O
located	O
in	O
the	O
United	O
States	O
.	O
</s>
<s>
Google	B-Application
's	I-Application
Oceania	O
Data	B-Operating_System
Center	I-Operating_System
is	O
located	O
in	O
Sydney	O
,	O
Australia	O
.	O
</s>
<s>
To	O
support	O
fault	B-General_Concept
tolerance	I-General_Concept
,	O
increase	O
the	O
scale	O
of	O
data	B-Operating_System
centers	I-Operating_System
and	O
accommodate	O
low-radix	O
switches	O
,	O
Google	B-Application
has	O
adopted	O
various	O
modified	O
Clos	B-Device
topologies	O
in	O
the	O
past	O
.	O
</s>
<s>
One	O
of	O
the	O
largest	O
Google	B-Architecture
data	I-Architecture
centers	I-Architecture
is	O
located	O
in	O
the	O
town	O
of	O
The	O
Dalles	O
,	O
Oregon	O
,	O
on	O
the	O
Columbia	O
River	O
,	O
approximately	O
80	O
miles	O
(	O
129km	O
)	O
from	O
Portland	O
.	O
</s>
<s>
The	O
site	O
was	O
chosen	O
to	O
take	O
advantage	O
of	O
inexpensive	O
hydroelectric	O
power	O
,	O
and	O
to	O
tap	O
into	O
the	O
region	O
's	O
large	O
surplus	B-Protocol
of	O
fiber	B-Architecture
optic	I-Architecture
cable	O
,	O
a	O
remnant	O
of	O
the	O
dot-com	O
boom	O
.	O
</s>
<s>
In	O
February	O
2009	O
,	O
Stora	O
Enso	O
announced	O
that	O
they	O
had	O
sold	O
the	O
Summa	O
paper	O
mill	O
in	O
Hamina	O
,	O
Finland	O
to	O
Google	B-Application
for	O
40	O
million	O
Euros	O
.	O
</s>
<s>
Google	B-Application
invested	O
200	O
million	O
euros	O
on	O
the	O
site	O
to	O
build	O
a	O
data	B-Operating_System
center	I-Operating_System
and	O
announced	O
additional	O
150	O
million	O
euro	O
investment	O
in	O
2012	O
.	O
</s>
<s>
Google	B-Application
chose	O
this	O
location	O
due	O
to	O
the	O
availability	O
and	O
proximity	O
of	O
renewable	O
energy	O
sources	O
.	O
</s>
<s>
In	O
2005	O
,	O
Google	B-Application
was	O
researching	O
a	O
containerized	O
modular	B-Application
data	I-Application
center	I-Application
.	O
</s>
<s>
Google	B-Application
filed	O
a	O
patent	O
application	O
for	O
this	O
technology	O
in	O
2003	O
.	O
</s>
<s>
In	O
2013	O
,	O
the	O
press	O
revealed	O
the	O
existence	O
of	O
Google	B-Application
's	I-Application
floating	O
data	B-Operating_System
centers	I-Operating_System
along	O
the	O
coasts	O
of	O
the	O
states	O
of	O
California	O
(	O
Treasure	O
Island	O
's	O
Building	O
3	O
)	O
and	O
Maine	O
.	O
</s>
<s>
The	O
data	B-Operating_System
centers	I-Operating_System
are	O
250	O
feet	O
long	O
,	O
72	O
feet	O
wide	O
,	O
16	O
feet	O
deep	O
.	O
</s>
<s>
The	O
patent	O
for	O
an	O
in-ocean	O
data	B-Operating_System
center	I-Operating_System
cooling	O
technology	O
was	O
bought	O
by	O
Google	B-Application
in	O
2009	O
(	O
along	O
with	O
a	O
wave-powered	O
ship-based	O
data	B-Operating_System
center	I-Operating_System
patent	O
in	O
2008	O
)	O
.	O
</s>
<s>
Shortly	O
thereafter	O
,	O
Google	B-Application
declared	O
that	O
the	O
two	O
massive	O
and	O
secretly-built	O
infrastructures	O
were	O
merely	O
"	O
interactive	O
learning	O
centers	O
,	O
 [ ... ] 	O
a	O
space	O
where	O
people	O
can	O
learn	O
about	O
new	O
technology.	O
"	O
</s>
<s>
Google	B-Application
halted	O
work	O
on	O
the	O
barges	O
in	O
late	O
2013	O
and	O
began	O
selling	O
off	O
the	O
barges	O
in	O
2014	O
.	O
</s>
<s>
Most	O
of	O
the	O
software	B-Application
stack	I-Application
that	O
Google	B-Application
uses	O
on	O
their	O
servers	B-Application
was	O
developed	O
in-house	O
.	O
</s>
<s>
According	O
to	O
a	O
well-known	O
Google	B-Application
employee	O
,	O
C++	B-Language
,	O
Java	B-Language
,	O
Python	B-Language
and	O
(	O
more	O
recently	O
)	O
Go	B-Application
are	O
favored	O
over	O
other	O
programming	O
languages	O
.	O
</s>
<s>
For	O
example	O
,	O
the	O
back	O
end	O
of	O
Gmail	O
is	O
written	O
in	O
Java	B-Language
and	O
the	O
back	O
end	O
of	O
Google	B-Application
Search	O
is	O
written	O
in	O
C++	B-Language
.	O
</s>
<s>
Google	B-Application
has	O
acknowledged	O
that	O
Python	B-Language
has	O
played	O
an	O
important	O
role	O
from	O
the	O
beginning	O
,	O
and	O
that	O
it	O
continues	O
to	O
do	O
so	O
as	O
the	O
system	O
grows	O
and	O
evolves	O
.	O
</s>
<s>
The	O
software	O
that	O
runs	O
the	B-Application
Google	I-Application
infrastructure	O
includes	O
:	O
</s>
<s>
Google	B-Protocol
Web	I-Protocol
Server	I-Protocol
(	O
GWS	O
)	O
custom	O
Linux-based	O
Web	O
server	O
that	O
Google	B-Application
uses	O
for	O
its	O
online	O
services	O
.	O
</s>
<s>
Google	B-Operating_System
F1	I-Operating_System
a	O
distributed	O
,	O
quasi-SQL	O
DBMS	B-General_Concept
based	O
on	O
Spanner	B-Operating_System
,	O
substituting	O
a	O
custom	O
version	O
of	O
MySQL	O
.	O
</s>
<s>
Google	B-Application
has	O
developed	O
several	O
abstractions	O
which	O
it	O
uses	O
for	O
storing	O
most	O
of	O
its	O
data	O
:	O
</s>
<s>
Protocol	B-Protocol
Buffers	I-Protocol
"	O
Google	B-Application
's	I-Application
lingua	O
franca	O
for	O
data	O
"	O
,	O
a	O
binary	O
serialization	O
format	O
which	O
is	O
widely	O
used	O
within	O
the	O
company	O
.	O
</s>
<s>
It	O
is	O
also	O
used	O
as	O
one	O
of	O
the	O
building	O
blocks	O
of	O
Bigtable	B-Application
.	O
</s>
<s>
When	O
an	O
update	O
is	O
required	O
,	O
queries	B-Library
are	O
redirected	O
to	O
other	O
servers	B-Application
,	O
so	O
as	O
to	O
simplify	O
consistency	O
issues	O
.	O
</s>
<s>
Queries	B-Library
are	O
divided	O
into	O
sub-queries	O
,	O
where	O
those	O
sub-queries	O
may	O
be	O
sent	O
to	O
different	O
ducts	O
in	O
parallel	B-Operating_System
,	O
thus	O
reducing	O
the	O
latency	O
time	O
.	O
</s>
<s>
To	O
lessen	O
the	O
effects	O
of	O
unavoidable	O
hardware	B-Architecture
failure	O
,	O
software	O
is	O
designed	O
to	O
be	O
fault	B-General_Concept
tolerant	I-General_Concept
.	O
</s>
<s>
Thus	O
,	O
when	O
a	O
system	O
goes	O
down	O
,	O
data	O
is	O
still	O
available	O
on	O
other	O
servers	B-Application
,	O
which	O
increases	O
reliability	O
.	O
</s>
<s>
Like	O
most	O
search	O
engines	O
,	O
Google	B-Application
indexes	O
documents	O
by	O
building	O
a	O
data	O
structure	O
known	O
as	O
inverted	B-Algorithm
index	I-Algorithm
.	O
</s>
<s>
Such	O
an	O
index	O
obtains	O
a	O
list	O
of	O
documents	O
by	O
a	O
query	B-Library
word	O
.	O
</s>
<s>
The	O
index	O
is	O
very	O
large	O
due	O
to	O
the	O
number	O
of	O
documents	O
stored	O
in	O
the	O
servers	B-Application
.	O
</s>
<s>
The	O
index	O
is	O
partitioned	O
by	O
document	O
IDs	O
into	O
many	O
pieces	O
called	O
shards	B-General_Concept
.	O
</s>
<s>
Each	O
shard	B-General_Concept
is	O
replicated	B-Architecture
onto	O
multiple	O
servers	B-Application
.	O
</s>
<s>
Initially	O
,	O
the	O
index	O
was	O
being	O
served	O
from	O
hard	B-Device
disk	I-Device
drives	I-Device
,	O
as	O
is	O
done	O
in	O
traditional	O
information	B-Library
retrieval	I-Library
(	O
IR	O
)	O
systems	O
.	O
</s>
<s>
Google	B-Application
dealt	O
with	O
the	O
increasing	O
query	B-Library
volume	O
by	O
increasing	O
number	O
of	O
replicas	B-Architecture
of	O
each	O
shard	B-General_Concept
and	O
thus	O
increasing	O
number	O
of	O
servers	B-Application
.	O
</s>
<s>
Soon	O
they	O
found	O
that	O
they	O
had	O
enough	O
servers	B-Application
to	O
keep	O
a	O
copy	O
of	O
the	O
whole	O
index	O
in	O
main	O
memory	O
(	O
although	O
with	O
low	O
replication	B-Architecture
or	O
no	O
replication	B-Architecture
at	O
all	O
)	O
,	O
and	O
in	O
early	O
2001	O
Google	B-Application
switched	O
to	O
an	O
in-memory	O
index	O
system	O
.	O
</s>
<s>
This	O
switch	O
"	O
radically	O
changed	O
many	O
design	O
parameters	O
"	O
of	O
their	O
search	O
system	O
,	O
and	O
allowed	O
for	O
a	O
significant	O
increase	O
in	O
throughput	O
and	O
a	O
large	O
decrease	O
in	O
latency	O
of	O
queries	B-Library
.	O
</s>
<s>
In	O
June	O
2010	O
,	O
Google	B-Application
rolled	O
out	O
a	O
next-generation	O
indexing	O
and	O
serving	O
system	O
called	O
"	O
Caffeine	O
"	O
which	O
can	O
continuously	O
crawl	O
and	O
update	O
the	O
search	O
index	O
.	O
</s>
<s>
Previously	O
,	O
Google	B-Application
updated	O
its	O
search	O
index	O
in	O
batches	O
using	O
a	O
series	O
of	O
MapReduce	B-Operating_System
jobs	O
.	O
</s>
<s>
Later	O
Google	B-Application
revealed	O
a	O
distributed	O
data	O
processing	O
system	O
called	O
"	O
Percolator	O
"	O
which	O
is	O
said	O
to	O
be	O
the	O
basis	O
of	O
Caffeine	O
indexing	O
system	O
.	O
</s>
<s>
Google	B-Application
's	I-Application
server	O
infrastructure	O
is	O
divided	O
into	O
several	O
types	O
,	O
each	O
assigned	O
to	O
a	O
different	O
purpose	O
:	O
</s>
<s>
Web	O
servers	B-Application
coordinate	O
the	O
execution	O
of	O
queries	B-Library
sent	O
by	O
users	O
,	O
then	O
format	O
the	O
result	O
into	O
an	O
HTML	B-Language
page	O
.	O
</s>
<s>
The	O
execution	O
consists	O
of	O
sending	O
queries	B-Library
to	O
index	O
servers	B-Application
,	O
merging	O
the	O
results	O
,	O
computing	O
their	O
rank	O
,	O
retrieving	O
a	O
summary	O
for	O
each	O
hit	O
(	O
using	O
the	O
document	O
server	O
)	O
,	O
asking	O
for	O
suggestions	O
from	O
the	O
spelling	O
servers	B-Application
,	O
and	O
finally	O
getting	O
a	O
list	O
of	O
advertisements	O
from	O
the	O
ad	O
server	O
.	O
</s>
<s>
Data-gathering	O
servers	B-Application
are	O
permanently	O
dedicated	O
to	O
spidering	B-Application
the	O
Web	O
.	O
</s>
<s>
Google	B-Application
's	I-Application
web	B-Application
crawler	I-Application
is	O
known	O
as	O
GoogleBot	O
.	O
</s>
<s>
They	O
update	O
the	O
index	O
and	O
document	O
databases	O
and	O
apply	O
Google	B-Application
's	I-Application
algorithms	O
to	O
assign	O
ranks	O
to	O
pages	O
.	O
</s>
<s>
Each	O
index	O
server	O
contains	O
a	O
set	O
of	O
index	O
shards	B-General_Concept
.	O
</s>
<s>
They	O
return	O
a	O
list	O
of	O
document	O
IDs	O
(	O
"	O
docid	O
"	O
)	O
,	O
such	O
that	O
documents	O
corresponding	O
to	O
a	O
certain	O
docid	O
contain	O
the	O
query	B-Library
word	O
.	O
</s>
<s>
These	O
servers	B-Application
need	O
less	O
disk	O
space	O
,	O
but	O
suffer	O
the	O
greatest	O
CPU	O
workload	O
.	O
</s>
<s>
Document	O
servers	B-Application
store	O
documents	O
.	O
</s>
<s>
Each	O
document	O
is	O
stored	O
on	O
dozens	O
of	O
document	O
servers	B-Application
.	O
</s>
<s>
When	O
performing	O
a	O
search	O
,	O
a	O
document	O
server	O
returns	O
a	O
summary	O
for	O
the	O
document	O
based	O
on	O
query	B-Library
words	O
.	O
</s>
<s>
These	O
servers	B-Application
need	O
more	O
disk	O
space	O
.	O
</s>
<s>
Ad	O
servers	B-Application
manage	O
advertisements	O
offered	O
by	O
services	O
like	O
AdWords	B-Application
and	O
AdSense	O
.	O
</s>
<s>
Spelling	O
servers	B-Application
make	O
suggestions	O
about	O
the	O
spelling	O
of	O
queries	B-Library
.	O
</s>
<s>
In	O
October	O
2013	O
,	O
The	O
Washington	O
Post	O
reported	O
that	O
the	O
U.S.	O
National	O
Security	O
Agency	O
intercepted	O
communications	O
between	O
Google	B-Application
's	I-Application
data	B-Operating_System
centers	I-Operating_System
,	O
as	O
part	O
of	O
a	O
program	O
named	O
MUSCULAR	B-Application
.	O
</s>
<s>
This	O
wiretapping	O
was	O
made	O
possible	O
because	O
,	O
at	O
the	O
time	O
,	O
Google	B-Application
did	O
not	O
encrypt	O
data	O
passed	O
inside	O
its	O
own	O
network	O
.	O
</s>
<s>
This	O
was	O
rectified	O
when	O
Google	B-Application
began	O
encrypting	O
data	O
sent	O
between	O
data	B-Operating_System
centers	I-Operating_System
in	O
2013	O
.	O
</s>
<s>
Google	B-Application
's	I-Application
most	O
efficient	O
data	B-Operating_System
center	I-Operating_System
runs	O
at	O
using	O
only	O
fresh	O
air	O
cooling	O
,	O
requiring	O
no	O
electrically	O
powered	O
air	O
conditioning	O
.	O
</s>
<s>
In	O
December	O
2016	O
,	O
Google	B-Application
announced	O
that	O
—	O
starting	O
in	O
2017	O
—	O
it	O
would	O
purchase	O
enough	O
renewable	O
energy	O
to	O
match	O
100%	O
of	O
the	O
energy	O
usage	O
of	O
its	O
data	B-Operating_System
centers	I-Operating_System
and	O
offices	O
.	O
</s>
<s>
The	O
commitment	O
will	O
make	O
Google	B-Application
"	O
the	O
world	O
's	O
largest	O
corporate	O
buyer	O
of	O
renewable	O
power	O
,	O
with	O
commitments	O
reaching	O
2.6	O
gigawatts	O
(	O
2,600	O
megawatts	O
)	O
of	O
wind	O
and	O
solar	O
energy	O
"	O
.	O
</s>
