<s>
A	O
Space-based	B-Operating_System
architecture	I-Operating_System
(	O
SBA	O
)	O
is	O
an	O
approach	O
to	O
distributed	B-Architecture
computing	I-Architecture
systems	O
where	O
the	O
various	O
components	O
interact	O
with	O
each	O
other	O
by	O
exchanging	O
tuples	O
or	O
entries	O
via	O
one	O
or	O
more	O
shared	O
spaces	O
.	O
</s>
<s>
This	O
is	O
contrasted	O
with	O
the	O
more	O
common	O
Message	B-General_Concept
queuing	I-General_Concept
service	I-General_Concept
approaches	O
where	O
the	O
various	O
components	O
interact	O
with	O
each	O
other	O
by	O
exchanging	O
messages	O
via	O
a	O
message	O
broker	O
.	O
</s>
<s>
An	O
analogy	O
might	O
be	O
where	O
a	O
message	O
broker	O
is	O
like	O
an	O
Academic	O
conference	O
,	O
where	O
each	O
presenter	O
has	O
the	O
stage	O
,	O
and	O
presents	O
in	O
the	O
order	O
they	O
are	O
scheduled	O
;	O
whereas	O
a	O
tuple	B-Operating_System
space	I-Operating_System
is	O
like	O
an	O
Unconference	O
,	O
where	O
all	O
participants	O
can	O
write	O
on	O
a	O
common	O
whiteboard	O
concurrently	O
,	O
and	O
all	O
can	O
see	O
it	O
at	O
the	O
same	O
time	O
.	O
</s>
<s>
A	O
key	O
goal	O
of	O
both	O
approaches	O
is	O
to	O
create	O
loosely-coupled	B-Application
systems	O
that	O
minimize	O
configuration	O
,	O
especially	O
shared	O
knowledge	O
of	O
who	O
does	O
what	O
,	O
leading	O
to	O
the	O
objectives	O
of	O
better	O
availability	O
,	O
resilience	O
,	O
scalability	B-Architecture
,	O
etc	O
.	O
</s>
<s>
More	O
specifically	O
,	O
an	O
SBA	O
is	O
a	O
distributed-computing	B-Architecture
architecture	O
for	O
achieving	O
linear	O
scalability	B-Architecture
of	O
stateful	O
,	O
high-performance	O
applications	O
using	O
the	O
tuple	B-Operating_System
space	I-Operating_System
paradigm	O
.	O
</s>
<s>
It	O
follows	O
many	O
of	O
the	O
principles	O
of	O
representational	B-Protocol
state	I-Protocol
transfer	I-Protocol
(	O
REST	O
)	O
,	O
service-oriented	O
architecture	O
(	O
SOA	O
)	O
and	O
event-driven	B-Application
architecture	I-Application
(	O
EDA	O
)	O
,	O
as	O
well	O
as	O
elements	O
of	O
grid	B-Architecture
computing	I-Architecture
.	O
</s>
<s>
With	O
a	O
space-based	B-Operating_System
architecture	I-Operating_System
,	O
applications	O
are	O
built	O
out	O
of	O
a	O
set	O
of	O
self-sufficient	O
units	O
,	O
known	O
as	O
processing-units	O
(	O
PU	O
)	O
.	O
</s>
<s>
The	O
SBA	O
model	O
is	O
closely	O
related	O
to	O
other	O
patterns	O
that	O
have	O
been	O
proved	O
successful	O
in	O
addressing	O
the	O
application	O
scalability	B-Architecture
challenge	O
,	O
such	O
as	O
shared	B-Operating_System
nothing	I-Operating_System
architecture	I-Operating_System
(	O
SN	O
)	O
,	O
used	O
by	O
Google	O
,	O
Amazon.com	O
and	O
other	O
well-known	O
companies	O
.	O
</s>
<s>
The	O
model	O
has	O
also	O
been	O
applied	O
by	O
many	O
firms	O
in	O
the	O
securities	O
industry	O
for	O
implementing	O
scalable	B-Architecture
electronic	O
securities	O
trading	O
applications	O
.	O
</s>
<s>
Space-based	B-Operating_System
architecture	I-Operating_System
(	O
SBA	O
)	O
was	O
originally	O
invented	O
and	O
developed	O
at	O
Microsoft	O
in	O
1997	O
–	O
98	O
.	O
</s>
<s>
An	O
application	O
built	O
on	O
the	O
principles	O
of	O
space-based	B-Operating_System
architecture	I-Operating_System
typically	O
has	O
the	O
following	O
components	O
:	O
</s>
<s>
Processing	O
unit	O
The	O
unit	O
of	O
scalability	B-Architecture
and	O
fail-over	O
.	O
</s>
<s>
Normally	O
,	O
a	O
processing	O
unit	O
is	O
built	O
out	O
of	O
a	O
POJO	B-Language
(	O
Plain	B-Language
Old	I-Language
Java	I-Language
Object	I-Language
)	O
container	O
,	O
such	O
as	O
that	O
provided	O
by	O
the	B-Application
Spring	I-Application
Framework	I-Application
.	O
</s>
<s>
Virtual	O
middleware	B-General_Concept
A	O
common	O
runtime	O
and	O
clustering	O
model	O
,	O
used	O
across	O
the	O
entire	O
middleware	B-General_Concept
stack	O
.	O
</s>
<s>
The	O
core	O
middleware	B-General_Concept
components	O
in	O
a	O
typical	O
SBA	O
architecture	O
are	O
:	O
</s>
<s>
POJO-driven	O
services	O
model	O
A	O
lightweight	O
services	O
model	O
that	O
can	O
take	O
any	O
standard	O
Java	O
implementation	O
and	O
turn	O
it	O
into	O
a	O
loosely	B-Application
coupled	I-Application
distributed	O
service	O
.	O
</s>
<s>
SLA	O
definitions	O
include	O
the	O
number	O
of	O
instances	O
that	O
need	O
to	O
run	O
in	O
order	O
to	O
comply	O
with	O
the	O
application	O
scaling	B-Architecture
and	O
fail-over	O
policies	O
,	O
as	O
well	O
as	O
other	O
policies	O
.	O
</s>
