<s>
System	B-Application
prevalence	I-Application
is	O
a	O
simple	O
software	O
architectural	O
pattern	O
that	O
combines	O
system	B-Operating_System
images	I-Operating_System
(	O
snapshots	O
)	O
and	O
transaction	B-General_Concept
journaling	B-General_Concept
to	O
provide	O
speed	O
,	O
performance	O
scalability	O
,	O
transparent	O
persistence	O
and	O
transparent	O
live	B-General_Concept
mirroring	I-General_Concept
of	O
computer	O
system	O
state	B-Application
.	O
</s>
<s>
In	O
a	O
prevalent	O
system	O
,	O
state	B-Application
is	O
kept	O
in	O
memory	B-General_Concept
in	O
native	O
format	O
,	O
all	O
transactions	O
are	O
journaled	B-General_Concept
and	O
System	B-Operating_System
images	I-Operating_System
are	O
regularly	O
saved	O
to	O
disk	O
.	O
</s>
<s>
System	B-Operating_System
images	I-Operating_System
and	O
transaction	B-General_Concept
journals	I-General_Concept
can	O
be	O
stored	O
in	O
language-specific	O
serialization	B-Application
format	O
for	O
speed	O
or	O
in	O
XML	B-Protocol
format	O
for	O
cross-language	O
portability	O
.	O
</s>
<s>
The	O
first	O
usage	O
of	O
the	O
term	O
and	O
generic	O
,	O
publicly	O
available	O
implementation	O
of	O
a	O
system	B-Application
prevalence	I-Application
layer	B-Application
was	O
Prevayler	B-Application
,	O
written	O
for	O
Java	O
by	O
Klaus	O
Wuestefeld	O
in	O
2001	O
.	O
</s>
<s>
Simply	O
keeping	O
system	O
state	B-Application
in	O
RAM	O
in	O
its	O
normal	O
,	O
natural	O
,	O
language-specific	O
format	O
is	O
orders	O
of	O
magnitude	O
faster	O
and	O
more	O
programmer-friendly	O
than	O
the	O
multiple	O
conversions	O
that	O
are	O
needed	O
when	O
it	O
is	O
stored	O
and	O
retrieved	O
from	O
a	O
DBMS	B-General_Concept
.	O
</s>
<s>
As	O
an	O
example	O
,	O
Martin	O
Fowler	O
describes	O
"	O
The	O
LMAX	O
Architecture	O
"	O
with	O
a	O
transaction-journal	O
and	O
system-image	O
(	O
snapshot	O
)	O
based	O
business	O
system	O
at	O
its	O
core	O
,	O
which	O
can	O
process	O
6	O
million	O
transactions	O
per	O
second	O
on	O
a	O
single	O
thread	O
.	O
</s>
<s>
A	O
prevalent	O
system	O
needs	O
enough	O
memory	B-General_Concept
to	O
hold	O
its	O
entire	O
state	B-Application
in	O
RAM	O
(	O
the	O
"	O
prevalent	O
hypothesis	O
"	O
)	O
.	O
</s>
<s>
Prevalence	O
advocates	O
claim	O
this	O
is	O
continuously	O
alleviated	O
by	O
decreasing	O
RAM	O
prices	O
,	O
and	O
the	O
fact	O
that	O
many	O
business	O
databases	O
are	O
small	O
enough	O
already	O
to	O
fit	O
in	O
memory	B-General_Concept
.	O
</s>
<s>
Programmers	O
need	O
skill	O
in	O
working	O
with	O
business	O
state	B-Application
natively	O
in	O
RAM	O
,	O
rather	O
than	O
using	O
explicit	O
API	B-General_Concept
calls	O
for	O
storage	O
and	O
queries	O
for	O
retrieval	O
.	O
</s>
<s>
The	O
system	O
's	O
events	O
must	O
be	O
capturable	O
for	O
journaling	B-General_Concept
.	O
</s>
