<s>
A	O
virtual	B-Application
appliance	I-Application
is	O
a	O
pre-configured	O
virtual	O
machine	O
image	O
,	O
ready	O
to	O
run	O
on	O
a	O
hypervisor	B-Operating_System
;	O
virtual	B-Application
appliances	I-Application
are	O
a	O
subset	O
of	O
the	O
broader	O
class	O
of	O
software	B-Application
appliances	I-Application
.	O
</s>
<s>
Installation	O
of	O
a	O
software	B-Application
appliance	I-Application
on	O
a	O
virtual	O
machine	O
and	O
packaging	O
that	O
into	O
an	O
image	O
creates	O
a	O
virtual	B-Application
appliance	I-Application
.	O
</s>
<s>
Like	O
software	B-Application
appliances	I-Application
,	O
virtual	B-Application
appliances	I-Application
are	O
intended	O
to	O
eliminate	O
the	O
installation	O
,	O
configuration	O
and	O
maintenance	O
costs	O
associated	O
with	O
running	O
complex	O
stacks	O
of	O
software	O
.	O
</s>
<s>
A	O
virtual	B-Application
appliance	I-Application
is	O
not	O
a	O
complete	O
virtual	O
machine	O
platform	O
,	O
but	O
rather	O
a	O
software	B-Operating_System
image	I-Operating_System
containing	O
a	O
software	O
stack	O
designed	O
to	O
run	O
on	O
a	O
virtual	O
machine	O
platform	O
which	O
may	O
be	O
a	O
Type	O
1	O
or	O
Type	O
2	O
hypervisor	B-Operating_System
.	O
</s>
<s>
Like	O
a	O
physical	O
computer	O
,	O
a	O
hypervisor	B-Operating_System
is	O
merely	O
a	O
platform	O
for	O
running	O
an	O
operating	O
system	O
environment	O
and	O
does	O
not	O
provide	O
application	B-Application
software	I-Application
itself	O
.	O
</s>
<s>
Many	O
virtual	B-Application
appliances	I-Application
provide	O
a	O
Web	O
page	O
user	B-Application
interface	I-Application
to	O
permit	O
their	O
configuration	O
.	O
</s>
<s>
A	O
virtual	B-Application
appliance	I-Application
is	O
usually	O
built	O
to	O
host	O
a	O
single	O
application	O
;	O
it	O
therefore	O
represents	O
a	O
new	O
way	O
to	O
deploy	B-Application
applications	O
on	O
a	O
network	O
.	O
</s>
<s>
Virtual	B-Application
appliances	I-Application
are	O
provided	O
to	O
the	O
user	O
or	O
customer	O
as	O
files	O
,	O
via	O
either	O
electronic	O
downloads	O
or	O
physical	O
distribution	O
.	O
</s>
<s>
It	O
may	O
also	O
be	O
distributed	O
as	O
Open	O
Virtual	B-Application
Appliance	I-Application
(	O
OVA	O
)	O
,	O
the	O
.ova	O
file	O
format	O
is	O
interchangeable	O
with	O
.ovf	O
.	O
</s>
<s>
Most	O
virtualization	O
platforms	O
,	O
including	O
those	O
from	O
VMware	O
,	O
Microsoft	O
,	O
Oracle	B-General_Concept
,	O
and	O
Citrix	O
,	O
can	O
install	O
virtual	B-Application
appliances	I-Application
from	O
an	O
OVF	O
file	O
.	O
</s>
<s>
Virtualization	O
solves	O
a	O
key	O
problem	O
in	O
the	O
grid	B-Architecture
computing	I-Architecture
arena	O
–	O
namely	O
,	O
the	O
reality	O
that	O
any	O
sufficiently	O
large	O
grid	O
will	O
inevitably	O
consist	O
of	O
a	O
wide	O
variety	O
of	O
heterogeneous	O
hardware	O
and	O
operating	O
system	O
configurations	O
.	O
</s>
<s>
Adding	O
virtual	B-Application
appliances	I-Application
into	O
the	O
picture	O
allows	O
for	O
extremely	O
rapid	O
provisioning	O
of	O
grid	O
nodes	O
and	O
importantly	O
,	O
cleanly	O
decouples	O
the	O
grid	O
operator	O
from	O
the	O
grid	O
consumer	O
by	O
encapsulating	O
all	O
knowledge	O
of	O
the	O
application	O
within	O
the	O
virtual	B-Application
appliance	I-Application
.	O
</s>
<s>
Virtual	B-Application
appliances	I-Application
are	O
critical	O
resources	O
in	O
infrastructure	B-General_Concept
as	I-General_Concept
a	I-General_Concept
service	I-General_Concept
cloud	B-Architecture
computing	I-Architecture
.	O
</s>
<s>
The	O
file	O
format	O
of	O
the	O
virtual	B-Application
appliance	I-Application
is	O
the	O
concern	O
of	O
the	O
cloud	O
provider	O
and	O
usually	O
not	O
relevant	O
to	O
the	O
cloud	O
user	O
even	O
though	O
the	O
cloud	O
user	O
may	O
be	O
the	O
owner	O
of	O
the	O
virtual	B-Application
appliance	I-Application
.	O
</s>
<s>
However	O
,	O
challenges	O
may	O
arise	O
with	O
the	O
transfer	O
of	O
virtual	B-Application
appliance	I-Application
ownership	O
or	O
transfer	O
of	O
virtual	B-Application
appliances	I-Application
between	O
cloud	O
data	O
centers	O
.	O
</s>
<s>
In	O
this	O
case	O
,	O
virtual	B-Application
appliance	I-Application
copy	O
or	O
export/import	O
features	O
can	O
be	O
used	O
to	O
overcome	O
this	O
problem	O
.	O
</s>
<s>
With	O
the	O
rise	O
of	O
virtualization	O
as	O
a	O
platform	O
for	O
hosted	O
services	O
provision	O
,	O
virtual	B-Application
appliances	I-Application
have	O
come	O
to	O
provide	O
a	O
direct	O
route	O
for	O
traditional	O
on-premises	O
applications	O
to	O
be	O
rapidly	O
redeployed	O
in	O
a	O
software	B-Architecture
as	I-Architecture
a	I-Architecture
service	I-Architecture
(	O
SaaS	B-Architecture
)	O
mode	O
–	O
without	O
requiring	O
major	O
application	O
re-architecture	O
for	O
multi-tenancy	B-Architecture
.	O
</s>
<s>
By	O
decoupling	O
the	O
hardware	O
and	O
operating	O
system	O
infrastructure	O
provider	O
from	O
the	O
application	O
stack	O
provider	O
,	O
virtual	B-Application
appliances	I-Application
allow	O
economies	O
of	O
scale	O
on	O
the	O
one	O
side	O
to	O
be	O
leveraged	O
by	O
the	O
economy	O
of	O
simplicity	O
on	O
the	O
other	O
.	O
</s>
<s>
Traditional	O
approaches	O
to	O
SaaS	B-Architecture
,	O
such	O
as	O
that	O
touted	O
by	O
Salesforce.com,	O
leverage	O
shared	O
infrastructure	O
by	O
forcing	O
massive	O
change	O
and	O
increased	O
complexity	O
on	O
the	O
software	O
stack	O
.	O
</s>
<s>
A	O
concrete	O
example	O
of	O
the	O
virtual	B-Application
appliances	I-Application
approach	O
to	O
delivering	O
SaaS	B-Architecture
is	O
the	O
Amazon	B-General_Concept
Elastic	I-General_Concept
Compute	I-General_Concept
Cloud	I-General_Concept
(	O
EC2	O
)	O
—	O
a	O
grid	O
of	O
Xen	O
hypervisor	B-Operating_System
nodes	O
coupled	O
with	O
the	O
availability	O
of	O
pre-packaged	O
virtual	B-Application
appliances	I-Application
in	O
the	O
Amazon	B-Application
Machine	I-Application
Image	I-Application
format	O
.	O
</s>
<s>
Amazon	B-General_Concept
EC2	I-General_Concept
reduces	O
the	O
cost-barrier	O
to	O
the	O
point	O
where	O
it	O
becomes	O
feasible	O
to	O
have	O
each	O
customer	O
of	O
the	O
hosted	O
service	O
provisioned	O
with	O
their	O
own	O
virtual	B-Application
appliance	I-Application
instance(s )	O
rather	O
than	O
forcing	O
them	O
to	O
share	O
common	O
instances	O
.	O
</s>
<s>
Furthermore	O
,	O
in	O
contrast	O
to	O
the	O
multi-tenancy	B-Architecture
approaches	O
to	O
SaaS	B-Architecture
,	O
a	O
virtual	B-Application
appliance	I-Application
can	O
also	O
be	O
deployed	O
on-premises	O
for	O
customers	O
that	O
need	O
local	O
network	O
access	O
to	O
the	O
running	O
application	O
,	O
or	O
have	O
security	O
requirements	O
that	O
a	O
third-party	O
hosting	O
model	O
does	O
not	O
meet	O
.	O
</s>
<s>
The	O
underlying	O
virtualization	O
technology	O
also	O
allows	O
for	O
rapid	O
movement	O
of	O
virtual	B-Application
appliances	I-Application
instances	O
between	O
physical	O
execution	O
environments	O
.	O
</s>
<s>
Traditional	O
approaches	O
to	O
SaaS	B-Architecture
fix	O
the	O
application	O
in	O
place	O
on	O
the	O
hosted	O
infrastructure	O
.	O
</s>
