<s>
A	O
data	B-General_Concept
grid	I-General_Concept
is	O
an	O
architecture	O
or	O
set	O
of	O
services	O
that	O
gives	O
individuals	O
or	O
groups	O
of	O
users	O
the	O
ability	O
to	O
access	O
,	O
modify	O
and	O
transfer	O
extremely	O
large	O
amounts	O
of	O
geographically	O
distributed	O
data	O
for	O
research	O
purposes	O
.	O
</s>
<s>
Data	B-General_Concept
grids	I-General_Concept
make	O
this	O
possible	O
through	O
a	O
host	O
of	O
middleware	B-General_Concept
applications	B-Application
and	O
services	O
that	O
pull	O
together	O
data	O
and	O
resources	B-General_Concept
from	O
multiple	O
administrative	B-Protocol
domains	I-Protocol
and	O
then	O
present	O
it	O
to	O
users	O
upon	O
request	O
.	O
</s>
<s>
The	O
data	O
in	O
a	O
data	B-General_Concept
grid	I-General_Concept
can	O
be	O
located	O
at	O
a	O
single	O
site	O
or	O
multiple	O
sites	O
where	O
each	O
site	O
can	O
be	O
its	O
own	O
administrative	B-Protocol
domain	I-Protocol
governed	O
by	O
a	O
set	O
of	O
security	O
restrictions	O
as	O
to	O
who	O
may	O
access	O
the	O
data	O
.	O
</s>
<s>
Likewise	O
,	O
multiple	O
replicas	B-Operating_System
of	O
the	O
data	O
may	O
be	O
distributed	O
throughout	O
the	O
grid	O
outside	O
their	O
original	O
administrative	B-Protocol
domain	I-Protocol
and	O
the	O
security	O
restrictions	O
placed	O
on	O
the	O
original	O
data	O
for	O
who	O
may	O
access	O
it	O
must	O
be	O
equally	O
applied	O
to	O
the	O
replicas	B-Operating_System
.	O
</s>
<s>
Specifically	O
developed	O
data	B-General_Concept
grid	I-General_Concept
middleware	B-General_Concept
is	O
what	O
handles	O
the	O
integration	O
between	O
users	O
and	O
the	O
data	O
they	O
request	O
by	O
controlling	O
access	O
while	O
making	O
it	O
available	O
as	O
efficiently	O
as	O
possible	O
.	O
</s>
<s>
The	O
adjacent	O
diagram	O
depicts	O
a	O
high	O
level	O
view	O
of	O
a	O
data	B-General_Concept
grid	I-General_Concept
.	O
</s>
<s>
Middleware	B-General_Concept
provides	O
all	O
the	O
services	O
and	O
applications	B-Application
necessary	O
for	O
efficient	O
management	O
of	O
datasets	B-General_Concept
and	O
files	B-Operating_System
within	O
the	O
data	B-General_Concept
grid	I-General_Concept
while	O
providing	O
users	O
quick	O
access	O
to	O
the	O
datasets	B-General_Concept
and	O
files	B-Operating_System
.	O
</s>
<s>
There	O
is	O
a	O
number	O
of	O
concepts	O
and	O
tools	O
that	O
must	O
be	O
available	O
to	O
make	O
a	O
data	B-General_Concept
grid	I-General_Concept
operationally	O
viable	O
.	O
</s>
<s>
However	O
,	O
at	O
the	O
same	O
time	O
not	O
all	O
data	B-General_Concept
grids	I-General_Concept
require	O
the	O
same	O
capabilities	O
and	O
services	O
because	O
of	O
differences	O
in	O
access	O
requirements	O
,	O
security	O
and	O
location	O
of	O
resources	B-General_Concept
in	O
comparison	O
to	O
users	O
.	O
</s>
<s>
In	O
any	O
case	O
,	O
most	O
data	B-General_Concept
grids	I-General_Concept
will	O
have	O
similar	O
middleware	B-General_Concept
services	O
that	O
provide	O
for	O
a	O
universal	O
name	O
space	O
,	O
data	O
transport	O
service	O
,	O
data	O
access	O
service	O
,	O
data	O
replication	O
and	O
resource	O
management	O
service	O
.	O
</s>
<s>
When	O
taken	O
together	O
,	O
they	O
are	O
key	O
to	O
the	O
data	B-General_Concept
grids	I-General_Concept
functional	O
capabilities	O
.	O
</s>
<s>
Since	O
sources	O
of	O
data	O
within	O
the	O
data	B-General_Concept
grid	I-General_Concept
will	O
consist	O
of	O
data	O
from	O
multiple	O
separate	O
systems	O
and	O
networks	B-Architecture
using	O
different	O
file	O
naming	O
conventions	O
,	O
it	O
would	O
be	O
difficult	O
for	O
a	O
user	O
to	O
locate	O
data	O
within	O
the	O
data	B-General_Concept
grid	I-General_Concept
and	O
know	O
they	O
retrieved	O
what	O
they	O
needed	O
based	O
solely	O
on	O
existing	O
physical	O
file	O
names	O
(	O
PFNs	O
)	O
.	O
</s>
<s>
A	O
universal	O
or	O
unified	O
name	O
space	O
makes	O
it	O
possible	O
to	O
create	O
logical	O
file	O
names	O
(	O
LFNs	O
)	O
that	O
can	O
be	O
referenced	O
within	O
the	O
data	B-General_Concept
grid	I-General_Concept
that	O
map	O
to	O
PFNs	O
.	O
</s>
<s>
When	O
an	O
LFN	O
is	O
requested	O
or	O
queried	O
,	O
all	O
matching	O
PFNs	O
are	O
returned	O
to	O
include	O
possible	O
replicas	B-Operating_System
of	O
the	O
requested	O
data	O
.	O
</s>
<s>
The	O
end	O
user	O
can	O
then	O
choose	O
from	O
the	O
returned	O
results	O
the	O
most	O
appropriate	O
replica	B-Operating_System
to	O
use	O
.	O
</s>
<s>
This	O
service	O
is	O
usually	O
provided	O
as	O
part	O
of	O
a	O
management	O
system	O
known	O
as	O
a	O
Storage	B-Operating_System
Resource	I-Operating_System
Broker	I-Operating_System
(	O
SRB	O
)	O
.	O
</s>
<s>
Information	O
about	O
the	O
locations	O
of	O
files	B-Operating_System
and	O
mappings	O
between	O
the	O
LFNs	O
and	O
PFNs	O
may	O
be	O
stored	O
in	O
a	O
metadata	O
or	O
replica	B-Operating_System
catalogue	O
.	O
</s>
<s>
The	O
replica	B-Operating_System
catalogue	O
would	O
contain	O
information	O
about	O
LFNs	O
that	O
map	O
to	O
multiple	O
replica	B-Operating_System
PFNs	O
.	O
</s>
<s>
Another	O
middleware	B-General_Concept
service	O
is	O
that	O
of	O
providing	O
for	O
data	O
transport	O
or	O
data	O
transfer	O
.	O
</s>
<s>
Fault	O
tolerance	O
can	O
be	O
achieved	O
in	O
a	O
data	B-General_Concept
grid	I-General_Concept
by	O
providing	O
mechanisms	O
that	O
ensures	O
data	O
transfer	O
will	O
resume	O
after	O
each	O
interruption	O
until	O
all	O
requested	O
data	O
is	O
received	O
.	O
</s>
<s>
As	O
an	O
example	O
,	O
GridFTP	B-Application
provides	O
for	O
fault	O
tolerance	O
by	O
sending	O
data	O
from	O
the	O
last	O
acknowledged	O
byte	O
without	O
starting	O
the	O
entire	O
transfer	O
from	O
the	O
beginning	O
.	O
</s>
<s>
The	O
data	O
transport	O
service	O
also	O
provides	O
for	O
the	O
low-level	O
access	O
and	O
connections	O
between	O
hosts	B-Device
for	O
file	O
transfer	O
.	O
</s>
<s>
The	O
data	O
transport	O
service	O
may	O
use	O
any	O
number	O
of	O
modes	O
to	O
implement	O
the	O
transfer	O
to	O
include	O
parallel	O
data	O
transfer	O
where	O
two	O
or	O
more	O
data	O
streams	O
are	O
used	O
over	O
the	O
same	O
channel	O
or	O
striped	O
data	O
transfer	O
where	O
two	O
or	O
more	O
steams	O
access	O
different	O
blocks	O
of	O
the	O
file	O
for	O
simultaneous	O
transfer	O
to	O
also	O
using	O
the	O
underlying	O
built-in	O
capabilities	O
of	O
the	O
network	O
hardware	O
or	O
specifically	O
developed	O
protocols	B-Application
to	O
support	O
faster	O
transfer	O
speeds	O
.	O
</s>
<s>
The	O
data	O
transport	O
service	O
might	O
optionally	O
include	O
a	O
network	B-Architecture
overlay	I-Architecture
function	O
to	O
facilitate	O
the	O
routing	O
and	O
transfer	O
of	O
data	O
as	O
well	O
as	O
file	O
I/O	B-General_Concept
functions	O
that	O
allow	O
users	O
to	O
see	O
remote	O
files	B-Operating_System
as	O
if	O
they	O
were	O
local	O
to	O
their	O
system	O
.	O
</s>
<s>
Data	O
access	O
services	O
work	O
hand	O
in	O
hand	O
with	O
the	O
data	O
transfer	O
service	O
to	O
provide	O
security	O
,	O
access	O
controls	O
and	O
management	O
of	O
any	O
data	O
transfers	O
within	O
the	O
data	B-General_Concept
grid	I-General_Concept
.	O
</s>
<s>
Common	O
forms	O
of	O
security	O
for	O
authentication	O
can	O
include	O
the	O
use	O
of	O
passwords	O
or	O
Kerberos	B-Protocol
(	O
protocol	O
)	O
.	O
</s>
<s>
These	O
types	O
of	O
controls	O
can	O
be	O
used	O
to	O
provide	O
granular	O
access	O
to	O
files	B-Operating_System
to	O
include	O
limits	O
on	O
access	O
times	O
,	O
duration	O
of	O
access	O
to	O
granular	O
controls	O
that	O
determine	O
which	O
files	B-Operating_System
can	O
be	O
read	O
or	O
written	O
to	O
.	O
</s>
<s>
The	O
most	O
common	O
form	O
of	O
encryption	O
for	O
this	O
task	O
has	O
been	O
the	O
use	O
of	O
SSL	B-Protocol
while	O
in	O
transport	O
.	O
</s>
<s>
While	O
all	O
of	O
these	O
access	O
services	O
operate	O
within	O
the	O
data	B-General_Concept
grid	I-General_Concept
,	O
access	O
services	O
within	O
the	O
various	O
administrative	B-Protocol
domains	I-Protocol
that	O
host	O
the	O
datasets	B-General_Concept
will	O
still	O
stay	O
in	O
place	O
to	O
enforce	O
access	O
rules	O
.	O
</s>
<s>
The	O
data	B-General_Concept
grid	I-General_Concept
access	O
services	O
must	O
be	O
in	O
step	O
with	O
the	O
administrative	B-Protocol
domains	I-Protocol
access	O
services	O
for	O
this	O
to	O
work	O
.	O
</s>
<s>
To	O
meet	O
the	O
needs	O
for	O
scalability	O
,	O
fast	O
access	O
and	O
user	O
collaboration	O
,	O
most	O
data	B-General_Concept
grids	I-General_Concept
support	O
replication	O
of	O
datasets	B-General_Concept
to	O
points	O
within	O
the	O
distributed	O
storage	O
architecture	O
.	O
</s>
<s>
The	O
use	O
of	O
replicas	B-Operating_System
allows	O
multiple	O
users	O
faster	O
access	O
to	O
datasets	B-General_Concept
and	O
the	O
preservation	O
of	O
bandwidth	O
since	O
replicas	B-Operating_System
can	O
often	O
be	O
placed	O
strategically	O
close	O
to	O
or	O
within	O
sites	O
where	O
users	O
need	O
them	O
.	O
</s>
<s>
However	O
,	O
replication	O
of	O
datasets	B-General_Concept
and	O
creation	O
of	O
replicas	B-Operating_System
is	O
bound	O
by	O
the	O
availability	O
of	O
storage	O
within	O
sites	O
and	O
bandwidth	O
between	O
sites	O
.	O
</s>
<s>
The	O
replication	O
and	O
creation	O
of	O
replica	B-Operating_System
datasets	B-General_Concept
is	O
controlled	O
by	O
a	O
replica	B-Operating_System
management	O
system	O
.	O
</s>
<s>
The	O
replica	B-Operating_System
management	O
system	O
determines	O
user	O
needs	O
for	O
replicas	B-Operating_System
based	O
on	O
input	O
requests	O
and	O
creates	O
them	O
based	O
on	O
availability	O
of	O
storage	O
and	O
bandwidth	O
.	O
</s>
<s>
All	O
replicas	B-Operating_System
are	O
then	O
cataloged	O
or	O
added	O
to	O
a	O
directory	O
based	O
on	O
the	O
data	B-General_Concept
grid	I-General_Concept
as	O
to	O
their	O
location	O
for	O
query	O
by	O
users	O
.	O
</s>
<s>
In	O
order	O
to	O
perform	O
the	O
tasks	O
undertaken	O
by	O
the	O
replica	B-Operating_System
management	O
system	O
,	O
it	O
needs	O
to	O
be	O
able	O
to	O
manage	O
the	O
underlying	O
storage	O
infrastructure	O
.	O
</s>
<s>
The	O
data	O
management	O
system	O
will	O
also	O
ensure	O
the	O
timely	O
updates	O
of	O
changes	O
to	O
replicas	B-Operating_System
are	O
propagated	O
to	O
all	O
nodes	O
.	O
</s>
<s>
There	O
are	O
a	O
number	O
of	O
ways	O
the	O
replication	O
management	O
system	O
can	O
handle	O
the	O
updates	O
of	O
replicas	B-Operating_System
.	O
</s>
<s>
The	O
updates	O
may	O
be	O
designed	O
around	O
a	O
centralized	O
model	O
where	O
a	O
single	O
master	O
replica	B-Operating_System
updates	O
all	O
others	O
,	O
or	O
a	O
decentralized	O
model	O
,	O
where	O
all	O
peers	O
update	O
each	O
other	O
.	O
</s>
<s>
The	O
topology	O
of	O
node	O
placement	O
may	O
also	O
influence	O
the	O
updates	O
of	O
replicas	B-Operating_System
.	O
</s>
<s>
There	O
are	O
a	O
number	O
of	O
ways	O
the	O
replication	O
management	O
system	O
can	O
handle	O
the	O
creation	O
and	O
placement	O
of	O
replicas	B-Operating_System
to	O
best	O
serve	O
the	O
user	O
community	O
.	O
</s>
<s>
If	O
the	O
storage	O
architecture	O
supports	O
replica	B-Operating_System
placement	O
with	O
sufficient	O
site	O
storage	O
,	O
then	O
it	O
becomes	O
a	O
matter	O
of	O
the	O
needs	O
of	O
the	O
users	O
who	O
access	O
the	O
datasets	B-General_Concept
and	O
a	O
strategy	O
for	O
placement	O
of	O
replicas	B-Operating_System
.	O
</s>
<s>
There	O
have	O
been	O
numerous	O
strategies	O
proposed	O
and	O
tested	O
on	O
how	O
to	O
best	O
manage	O
replica	B-Operating_System
placement	O
of	O
datasets	B-General_Concept
within	O
the	O
data	B-General_Concept
grid	I-General_Concept
to	O
meet	O
user	O
requirements	O
.	O
</s>
<s>
It	O
is	O
a	O
matter	O
of	O
the	O
type	O
of	O
data	B-General_Concept
grid	I-General_Concept
and	O
user	O
community	O
requirements	O
for	O
access	O
that	O
will	O
determine	O
the	O
best	O
strategy	O
to	O
use	O
.	O
</s>
<s>
Replicas	B-Operating_System
can	O
even	O
be	O
created	O
where	O
the	O
files	B-Operating_System
are	O
encrypted	O
for	O
confidentiality	O
that	O
would	O
be	O
useful	O
in	O
a	O
research	O
project	O
dealing	O
with	O
medical	O
files	B-Operating_System
.	O
</s>
<s>
The	O
following	O
section	O
contains	O
several	O
strategies	O
for	O
replica	B-Operating_System
placement	O
.	O
</s>
<s>
Dynamic	O
replication	O
is	O
an	O
approach	O
to	O
placement	O
of	O
replicas	B-Operating_System
based	O
on	O
popularity	O
of	O
the	O
data	O
.	O
</s>
<s>
When	O
the	O
number	O
of	O
hits	O
for	O
a	O
specific	O
dataset	B-General_Concept
exceeds	O
the	O
replication	O
threshold	O
it	O
triggers	O
the	O
creation	O
of	O
a	O
replica	B-Operating_System
on	O
the	O
server	O
that	O
directly	O
services	O
the	O
user	O
’s	O
client	O
.	O
</s>
<s>
If	O
the	O
direct	O
servicing	O
server	O
known	O
as	O
a	O
father	O
does	O
not	O
have	O
sufficient	O
space	O
,	O
then	O
the	O
father	O
’s	O
father	O
in	O
the	O
hierarchy	O
is	O
then	O
the	O
target	O
to	O
receive	O
a	O
replica	B-Operating_System
and	O
so	O
on	O
up	O
the	O
chain	O
until	O
it	O
is	O
exhausted	O
.	O
</s>
<s>
The	O
data	O
management	O
system	O
algorithm	O
also	O
allows	O
for	O
the	O
dynamic	O
deletion	O
of	O
replicas	B-Operating_System
that	O
have	O
a	O
null	O
access	O
value	O
or	O
a	O
value	O
lower	O
than	O
the	O
frequency	O
of	O
the	O
data	O
to	O
be	O
stored	O
to	O
free	O
up	O
space	O
.	O
</s>
<s>
This	O
improves	O
system	O
performance	O
in	O
terms	O
of	O
response	O
time	O
,	O
number	O
of	O
replicas	B-Operating_System
and	O
helps	O
load	O
balance	O
across	O
the	O
data	B-General_Concept
grid	I-General_Concept
.	O
</s>
<s>
This	O
method	O
can	O
also	O
use	O
dynamic	O
algorithms	O
that	O
determine	O
whether	O
the	O
cost	O
of	O
creating	O
the	O
replica	B-Operating_System
is	O
truly	O
worth	O
the	O
expected	O
gains	O
given	O
the	O
location	O
.	O
</s>
<s>
This	O
method	O
of	O
replication	O
like	O
the	O
one	O
for	O
dynamic	O
replication	O
has	O
been	O
designed	O
around	O
a	O
hierarchical	O
replication	O
model	O
found	O
in	O
most	O
data	B-General_Concept
grids	I-General_Concept
.	O
</s>
<s>
It	O
works	O
on	O
a	O
similar	O
algorithm	O
to	O
dynamic	O
replication	O
with	O
file	O
access	O
requests	O
being	O
a	O
prime	O
factor	O
in	O
determining	O
which	O
files	B-Operating_System
should	O
be	O
replicated	O
.	O
</s>
<s>
A	O
key	O
difference	O
,	O
however	O
,	O
is	O
the	O
number	O
and	O
frequency	O
of	O
replica	B-Operating_System
creations	O
is	O
keyed	O
to	O
a	O
dynamic	O
threshold	O
that	O
is	O
computed	O
based	O
on	O
request	O
arrival	O
rates	O
from	O
clients	O
over	O
a	O
period	O
of	O
time	O
.	O
</s>
<s>
If	O
the	O
number	O
of	O
requests	O
on	O
average	O
exceeds	O
the	O
previous	O
threshold	O
and	O
shows	O
an	O
upward	O
trend	O
,	O
and	O
storage	O
utilization	O
rates	O
indicate	O
capacity	O
to	O
create	O
more	O
replicas	B-Operating_System
,	O
more	O
replicas	B-Operating_System
may	O
be	O
created	O
.	O
</s>
<s>
As	O
with	O
dynamic	O
replication	O
,	O
the	O
removal	O
of	O
replicas	B-Operating_System
that	O
have	O
a	O
lower	O
threshold	O
that	O
were	O
not	O
created	O
in	O
the	O
current	O
replication	O
interval	O
can	O
be	O
removed	O
to	O
make	O
space	O
for	O
the	O
new	O
replicas	B-Operating_System
.	O
</s>
<s>
Also	O
,	O
like	O
the	O
two	O
before	O
,	O
the	O
popularity	O
of	O
files	B-Operating_System
play	O
a	O
key	O
role	O
in	O
determining	O
which	O
files	B-Operating_System
will	O
be	O
replicated	O
.	O
</s>
<s>
The	O
difference	O
with	O
this	O
method	O
is	O
the	O
placement	O
of	O
the	O
replicas	B-Operating_System
is	O
based	O
on	O
access	O
load	O
and	O
storage	O
load	O
of	O
candidate	O
servers	O
.	O
</s>
<s>
A	O
candidate	O
server	O
may	O
have	O
sufficient	O
storage	O
space	O
but	O
be	O
servicing	O
many	O
clients	O
for	O
access	O
to	O
stored	O
files	B-Operating_System
.	O
</s>
<s>
Therefore	O
,	O
placement	O
of	O
replicas	B-Operating_System
with	O
this	O
method	O
is	O
done	O
by	O
evaluating	O
each	O
candidate	O
node	O
for	O
access	O
load	O
to	O
find	O
a	O
suitable	O
node	O
for	O
the	O
placement	O
of	O
the	O
replica	B-Operating_System
.	O
</s>
<s>
If	O
all	O
candidate	O
nodes	O
are	O
equivalently	O
rated	O
for	O
access	O
load	O
,	O
none	O
or	O
less	O
accessed	O
than	O
the	O
other	O
,	O
then	O
the	O
candidate	O
node	O
with	O
the	O
lowest	O
storage	O
load	O
will	O
be	O
chosen	O
to	O
host	O
the	O
replicas	B-Operating_System
.	O
</s>
<s>
Replicas	B-Operating_System
that	O
are	O
removed	O
might	O
be	O
moved	O
to	O
a	O
parent	O
node	O
for	O
later	O
reuse	O
should	O
they	O
become	O
popular	O
again	O
.	O
</s>
<s>
The	O
above	O
three	O
replica	B-Operating_System
strategies	O
are	O
but	O
three	O
of	O
many	O
possible	O
replication	O
strategies	O
that	O
may	O
be	O
used	O
to	O
place	O
replicas	B-Operating_System
within	O
the	O
data	B-General_Concept
grid	I-General_Concept
where	O
they	O
will	O
improve	O
performance	O
and	O
access	O
.	O
</s>
<s>
Static	O
–	O
uses	O
a	O
fixed	O
replica	B-Operating_System
set	O
of	O
nodes	O
with	O
no	O
dynamic	O
changes	O
to	O
the	O
files	B-Operating_System
being	O
replicated	O
.	O
</s>
<s>
Best	O
Client	O
–	O
Each	O
node	O
records	O
number	O
of	O
requests	O
per	O
file	O
received	O
during	O
a	O
preset	O
time	O
interval	O
;	O
if	O
the	O
request	O
number	O
exceeds	O
the	O
set	O
threshold	O
for	O
a	O
file	O
a	O
replica	B-Operating_System
is	O
created	O
on	O
the	O
best	O
client	O
,	O
one	O
that	O
requested	O
the	O
file	O
the	O
most	O
;	O
stale	O
replicas	B-Operating_System
are	O
removed	O
based	O
on	O
another	O
algorithm	O
.	O
</s>
<s>
If	O
the	O
threshold	O
is	O
exceeded	O
a	O
replica	B-Operating_System
is	O
created	O
at	O
the	O
first	O
tier	O
down	O
from	O
the	O
root	O
,	O
if	O
the	O
threshold	O
is	O
exceeded	O
again	O
a	O
replica	B-Operating_System
is	O
added	O
to	O
the	O
next	O
tier	O
down	O
and	O
so	O
on	O
like	O
a	O
waterfall	O
effect	O
until	O
a	O
replica	B-Operating_System
is	O
placed	O
at	O
the	O
client	O
itself	O
.	O
</s>
<s>
Such	O
characteristics	O
of	O
the	O
data	B-General_Concept
grid	I-General_Concept
systems	O
as	O
large	O
scale	O
and	O
heterogeneity	O
require	O
specific	O
methods	O
of	O
tasks	O
scheduling	O
and	O
resource	O
allocation	O
.	O
</s>
<s>
Another	O
specificity	O
of	O
data	B-General_Concept
grids	I-General_Concept
,	O
dynamics	O
,	O
consists	O
in	O
the	O
continuous	O
process	O
of	O
connecting	O
and	O
disconnecting	O
of	O
nodes	O
and	O
local	O
load	O
imbalance	O
during	O
an	O
execution	O
of	O
tasks	O
.	O
</s>
<s>
As	O
a	O
result	O
,	O
much	O
of	O
the	O
data	B-General_Concept
grids	I-General_Concept
utilize	O
execution-time	O
adaptation	O
techniques	O
that	O
permit	O
the	O
systems	O
to	O
reflect	O
to	O
the	O
dynamic	O
changes	O
:	O
balance	O
the	O
load	O
,	O
replace	O
disconnecting	O
nodes	O
,	O
use	O
the	O
profit	O
of	O
newly	O
connected	O
nodes	O
,	O
recover	O
a	O
task	O
execution	O
after	O
faults	O
.	O
</s>
<s>
The	O
resource	O
management	O
system	O
represents	O
the	O
core	O
functionality	O
of	O
the	O
data	B-General_Concept
grid	I-General_Concept
.	O
</s>
<s>
It	O
is	O
the	O
heart	O
of	O
the	O
system	O
that	O
manages	O
all	O
actions	O
related	O
to	O
storage	O
resources	B-General_Concept
.	O
</s>
<s>
In	O
some	O
data	B-General_Concept
grids	I-General_Concept
it	O
may	O
be	O
necessary	O
to	O
create	O
a	O
federated	O
RMS	O
architecture	O
because	O
of	O
different	O
administrative	O
policies	O
and	O
a	O
diversity	O
of	O
possibilities	O
found	O
within	O
the	O
data	B-General_Concept
grid	I-General_Concept
in	O
place	O
of	O
using	O
a	O
single	O
RMS	O
.	O
</s>
<s>
In	O
such	O
a	O
case	O
the	O
RMSs	O
in	O
the	O
federation	O
will	O
employ	O
an	O
architecture	O
that	O
allows	O
for	O
interoperability	O
based	O
on	O
an	O
agreed	O
upon	O
set	O
of	O
protocols	B-Application
for	O
actions	O
related	O
to	O
storage	O
resources	B-General_Concept
.	O
</s>
<s>
Manage	O
resources	B-General_Concept
,	O
i.e.	O
</s>
<s>
Manage	O
trusts	O
concerning	O
resources	B-General_Concept
in	O
administrative	B-Protocol
domains	I-Protocol
,	O
some	O
domains	O
may	O
place	O
additional	O
restrictions	O
on	O
how	O
they	O
participate	O
requiring	O
adaptation	O
of	O
the	O
RMS	O
or	O
federation	O
.	O
</s>
<s>
Supports	O
adaptability	O
,	O
extensibility	O
,	O
and	O
scalability	O
in	O
relation	O
to	O
the	O
data	B-General_Concept
grid	I-General_Concept
.	O
</s>
<s>
Data	B-General_Concept
grids	I-General_Concept
have	O
been	O
designed	O
with	O
multiple	O
topologies	O
in	O
mind	O
to	O
meet	O
the	O
needs	O
of	O
the	O
scientific	O
community	O
.	O
</s>
<s>
On	O
the	O
right	O
are	O
four	O
diagrams	O
of	O
various	O
topologies	O
that	O
have	O
been	O
used	O
in	O
data	B-General_Concept
grids	I-General_Concept
.	O
</s>
<s>
There	O
are	O
no	O
replicas	B-Operating_System
in	O
this	O
topology	O
as	O
compared	O
to	O
others	O
.	O
</s>
<s>
One	O
project	O
that	O
uses	O
this	O
data	B-General_Concept
grid	I-General_Concept
topology	O
is	O
the	O
Network	O
for	O
Earthquake	O
Engineering	O
Simulation	O
(	O
NEES	O
)	O
in	O
the	O
United	O
States	O
.	O
</s>
<s>
The	O
need	O
for	O
data	B-General_Concept
grids	I-General_Concept
was	O
first	O
recognized	O
by	O
the	O
scientific	O
community	O
concerning	O
climate	B-Application
modeling	I-Application
,	O
where	O
terabyte	O
and	O
petabyte	O
sized	O
data	B-General_Concept
sets	I-General_Concept
were	O
becoming	O
the	O
norm	O
for	O
transport	O
between	O
sites	O
.	O
</s>
<s>
More	O
recent	O
research	O
requirements	O
for	O
data	B-General_Concept
grids	I-General_Concept
have	O
been	O
driven	O
by	O
the	O
Large	O
Hadron	O
Collider	O
(	O
LHC	O
)	O
at	O
CERN	O
,	O
the	O
Laser	O
Interferometer	O
Gravitational	O
Wave	O
Observatory	O
(	O
LIGO	O
)	O
,	O
and	O
the	O
Sloan	O
Digital	O
Sky	O
Survey	O
(	O
SDSS	O
)	O
.	O
</s>
<s>
Other	O
uses	O
for	O
data	B-General_Concept
grids	I-General_Concept
involve	O
governments	O
,	O
hospitals	O
,	O
schools	O
and	O
businesses	O
where	O
efforts	O
are	O
taking	O
place	O
to	O
improve	O
services	O
and	O
reduce	O
costs	O
by	O
providing	O
access	O
to	O
dispersed	O
and	O
separate	O
data	O
systems	O
through	O
the	O
use	O
of	O
data	B-General_Concept
grids	I-General_Concept
.	O
</s>
<s>
From	O
its	O
earliest	O
beginnings	O
,	O
the	O
concept	O
of	O
a	O
Data	B-General_Concept
Grid	I-General_Concept
to	O
support	O
the	O
scientific	O
community	O
was	O
thought	O
of	O
as	O
a	O
specialized	O
extension	O
of	O
the	O
“	O
grid	O
”	O
which	O
itself	O
was	O
first	O
envisioned	O
as	O
a	O
way	O
to	O
link	O
super	O
computers	O
into	O
meta-computers	O
.	O
</s>
<s>
However	O
,	O
that	O
was	O
short	O
lived	O
and	O
the	O
grid	O
evolved	O
into	O
meaning	O
the	O
ability	O
to	O
connect	O
computers	O
anywhere	O
on	O
the	O
web	O
to	O
get	O
access	O
to	O
any	O
desired	O
files	B-Operating_System
and	O
resources	B-General_Concept
,	O
similar	O
to	O
the	O
way	O
electricity	O
is	O
delivered	O
over	O
a	O
grid	O
by	O
simply	O
plugging	O
in	O
a	O
device	O
.	O
</s>
<s>
From	O
this	O
the	O
data	B-General_Concept
grid	I-General_Concept
was	O
proposed	O
as	O
an	O
integrating	O
architecture	O
that	O
would	O
be	O
capable	O
of	O
delivering	O
resources	B-General_Concept
for	O
distributed	O
computations	O
.	O
</s>
<s>
The	O
data	B-General_Concept
grid	I-General_Concept
would	O
include	O
its	O
own	O
management	O
infrastructure	O
capable	O
of	O
managing	O
all	O
aspects	O
of	O
the	O
data	B-General_Concept
grids	I-General_Concept
performance	O
and	O
operation	O
across	O
multiple	O
wide	O
area	O
networks	B-Architecture
while	O
working	O
within	O
the	O
existing	O
framework	O
known	O
as	O
the	O
web	O
.	O
</s>
<s>
The	O
data	B-General_Concept
grid	I-General_Concept
has	O
also	O
been	O
defined	O
more	O
recently	O
in	O
terms	O
of	O
usability	O
;	O
what	O
must	O
a	O
data	B-General_Concept
grid	I-General_Concept
be	O
able	O
to	O
do	O
in	O
order	O
for	O
it	O
to	O
be	O
useful	O
to	O
the	O
scientific	O
community	O
.	O
</s>
<s>
One	O
,	O
users	O
should	O
be	O
able	O
to	O
search	O
and	O
discover	O
applicable	O
resources	B-General_Concept
within	O
the	O
data	B-General_Concept
grid	I-General_Concept
from	O
amongst	O
its	O
many	O
datasets	B-General_Concept
.	O
</s>
<s>
Two	O
,	O
users	O
should	O
be	O
able	O
to	O
locate	O
datasets	B-General_Concept
within	O
the	O
data	B-General_Concept
grid	I-General_Concept
that	O
are	O
most	O
suitable	O
for	O
their	O
requirement	O
from	O
amongst	O
numerous	O
replicas	B-Operating_System
.	O
</s>
<s>
Three	O
,	O
users	O
should	O
be	O
able	O
to	O
transfer	O
and	O
move	O
large	O
datasets	B-General_Concept
between	O
points	O
in	O
a	O
short	O
amount	O
of	O
time	O
.	O
</s>
<s>
Four	O
,	O
the	O
data	B-General_Concept
grid	I-General_Concept
should	O
provide	O
a	O
means	O
to	O
manage	O
multiple	O
copies	O
of	O
datasets	B-General_Concept
within	O
the	O
data	B-General_Concept
grid	I-General_Concept
.	O
</s>
<s>
And	O
finally	O
,	O
the	O
data	B-General_Concept
grid	I-General_Concept
should	O
provide	O
security	O
with	O
user	O
access	O
controls	O
within	O
the	O
data	B-General_Concept
grid	I-General_Concept
,	O
i.e.	O
</s>
<s>
The	O
data	B-General_Concept
grid	I-General_Concept
is	O
an	O
evolving	O
technology	O
that	O
continues	O
to	O
change	O
and	O
grow	O
to	O
meet	O
the	O
needs	O
of	O
an	O
expanding	O
community	O
.	O
</s>
<s>
One	O
of	O
the	O
earliest	O
programs	O
begun	O
to	O
make	O
data	B-General_Concept
grids	I-General_Concept
a	O
reality	O
was	O
funded	O
by	O
the	O
Defense	O
Advanced	O
Research	O
Projects	O
Agency	O
(	O
DARPA	O
)	O
in	O
1997	O
at	O
the	O
University	O
of	O
Chicago	O
.	O
</s>
<s>
This	O
research	O
spawned	O
by	O
DARPA	O
has	O
continued	O
down	O
the	O
path	O
to	O
creating	O
open	O
source	O
tools	O
that	O
make	O
data	B-General_Concept
grids	I-General_Concept
possible	O
.	O
</s>
<s>
As	O
new	O
requirements	O
for	O
data	B-General_Concept
grids	I-General_Concept
emerge	O
projects	O
like	O
the	O
Globus	O
Toolkit	O
will	O
emerge	O
or	O
expand	O
to	O
meet	O
the	O
gap	O
.	O
</s>
<s>
Data	B-General_Concept
grids	I-General_Concept
along	O
with	O
the	O
"	O
Grid	O
"	O
will	O
continue	O
to	O
evolve	O
.	O
</s>
