<s>
In	O
computer	B-General_Concept
science	I-General_Concept
,	O
persistence	B-Application
refers	O
to	O
the	O
characteristic	O
of	O
state	B-Application
of	O
a	O
system	O
that	O
outlives	O
(	O
persists	O
more	O
than	O
)	O
the	O
process	B-Operating_System
that	O
created	O
it	O
.	O
</s>
<s>
This	O
is	O
achieved	O
in	O
practice	O
by	O
storing	O
the	O
state	B-Application
as	O
data	O
in	O
computer	B-General_Concept
data	I-General_Concept
storage	I-General_Concept
.	O
</s>
<s>
Programs	O
have	O
to	O
transfer	O
data	O
to	O
and	O
from	O
storage	O
devices	O
and	O
have	O
to	O
provide	O
mappings	O
from	O
the	O
native	O
programming-language	O
data	B-General_Concept
structures	I-General_Concept
to	O
the	O
storage	O
device	O
data	B-General_Concept
structures	I-General_Concept
.	O
</s>
<s>
Picture	O
editing	O
programs	O
or	O
word	B-General_Concept
processors	I-General_Concept
,	O
for	O
example	O
,	O
achieve	O
state	B-Application
persistence	B-Application
by	O
saving	O
their	O
documents	O
to	O
files	B-Operating_System
.	O
</s>
<s>
Persistence	B-Application
is	O
said	O
to	O
be	O
"	O
orthogonal	O
"	O
or	O
"	O
transparent	O
"	O
when	O
it	O
is	O
implemented	O
as	O
an	O
intrinsic	O
property	O
of	O
the	O
execution	O
environment	O
of	O
a	O
program	O
.	O
</s>
<s>
An	O
orthogonal	O
persistence	B-Application
environment	O
does	O
not	O
require	O
any	O
specific	O
actions	O
by	O
programs	O
running	O
in	O
it	O
to	O
retrieve	O
or	O
save	O
their	O
state	B-Application
.	O
</s>
<s>
Non-orthogonal	O
persistence	B-Application
requires	O
data	O
to	O
be	O
written	O
and	O
read	O
to	O
and	O
from	O
storage	O
using	O
specific	O
instructions	O
in	O
a	O
program	O
,	O
resulting	O
in	O
the	O
use	O
of	O
persist	O
as	O
a	O
transitive	O
verb	O
:	O
On	O
completion	O
,	O
the	O
program	O
persists	O
the	O
data	O
.	O
</s>
<s>
The	O
advantage	O
of	O
orthogonal	O
persistence	B-Application
environments	O
is	O
simpler	O
and	O
less	O
error-prone	O
programs	O
.	O
</s>
<s>
The	O
term	O
"	O
persistent	O
"	O
was	O
first	O
introduced	O
by	O
Atkinson	O
and	O
Morrison	O
in	O
the	O
sense	O
of	O
orthogonal	O
persistence	B-Application
:	O
they	O
used	O
an	O
adjective	O
rather	O
than	O
a	O
verb	O
to	O
emphasize	O
persistence	B-Application
as	O
a	O
property	O
of	O
the	O
data	O
,	O
as	O
distinct	O
from	O
an	O
imperative	O
action	O
performed	O
by	O
a	O
program	O
.	O
</s>
<s>
Orthogonal	O
persistence	B-Application
is	O
widely	O
adopted	O
in	O
operating	B-General_Concept
systems	I-General_Concept
for	O
hibernation	O
and	O
in	O
platform	B-General_Concept
virtualization	I-General_Concept
systems	O
such	O
as	O
VMware	O
and	O
VirtualBox	B-Operating_System
for	O
state	B-Application
saving	O
.	O
</s>
<s>
Research	O
prototype	O
languages	O
such	O
as	O
PS-algol	B-Language
,	O
Napier88	B-Language
,	O
Fibonacci	O
and	O
pJama	O
,	O
successfully	O
demonstrated	O
the	O
concepts	O
along	O
with	O
the	O
advantages	O
to	O
programmers	O
.	O
</s>
<s>
Using	O
system	B-Operating_System
images	I-Operating_System
is	O
the	O
simplest	O
persistence	B-Application
strategy	O
.	O
</s>
<s>
Notebook	O
hibernation	O
is	O
an	O
example	O
of	O
orthogonal	O
persistence	B-Application
using	O
a	O
system	B-Operating_System
image	I-Operating_System
because	O
it	O
does	O
not	O
require	O
any	O
actions	O
by	O
the	O
programs	O
running	O
on	O
the	O
machine	O
.	O
</s>
<s>
An	O
example	O
of	O
non-orthogonal	O
persistence	B-Application
using	O
a	O
system	B-Operating_System
image	I-Operating_System
is	O
a	O
simple	O
text	O
editing	O
program	O
executing	O
specific	O
instructions	O
to	O
save	O
an	O
entire	O
document	O
to	O
a	O
file	O
.	O
</s>
<s>
Shortcomings	O
:	O
Requires	O
enough	O
RAM	B-Architecture
to	O
hold	O
the	O
entire	O
system	O
state	B-Application
.	O
</s>
<s>
State	B-Application
changes	O
made	O
to	O
a	O
system	O
after	O
its	O
last	O
image	O
was	O
saved	O
are	O
lost	O
in	O
the	O
case	O
of	O
a	O
system	O
failure	O
or	O
shutdown	O
.	O
</s>
<s>
Saving	O
an	O
image	O
for	O
every	O
single	O
change	O
would	O
be	O
too	O
time-consuming	O
for	O
most	O
systems	O
,	O
so	O
images	O
are	O
not	O
used	O
as	O
the	O
single	O
persistence	B-Application
technique	O
for	O
critical	O
systems	O
.	O
</s>
<s>
Using	O
journals	O
is	O
the	O
second	O
simplest	O
persistence	B-Application
technique	O
.	O
</s>
<s>
Journaling	O
is	O
the	O
process	B-Operating_System
of	O
storing	O
events	O
in	O
a	O
log	O
before	O
each	O
one	O
is	O
applied	O
to	O
a	O
system	O
.	O
</s>
<s>
The	O
entire	O
"	O
Undo/Redo	O
"	O
history	O
of	O
user	O
commands	O
in	O
a	O
picture	O
editing	O
program	O
,	O
for	O
example	O
,	O
when	O
written	O
to	O
a	O
file	O
,	O
constitutes	O
a	O
journal	O
capable	O
of	O
recovering	O
the	O
state	B-Application
of	O
an	O
edited	O
picture	O
at	O
any	O
point	O
in	O
time	O
.	O
</s>
<s>
Journals	O
are	O
used	O
by	O
journaling	B-Application
file	I-Application
systems	I-Application
,	O
prevalent	B-Application
systems	I-Application
and	O
database	B-Application
management	I-Application
systems	I-Application
where	O
they	O
are	O
also	O
called	O
"	O
transaction	O
logs	O
"	O
or	O
"	O
redo	O
logs	O
"	O
.	O
</s>
<s>
As	O
a	O
result	O
,	O
journals	O
are	O
often	O
combined	O
with	O
other	O
persistence	B-Application
techniques	O
.	O
</s>
<s>
This	O
technique	O
is	O
the	O
writing	O
to	O
storage	O
of	O
only	O
those	O
portions	O
of	O
system	O
state	B-Application
that	O
have	O
been	O
modified	O
(	O
are	O
dirty	O
)	O
since	O
their	O
last	O
write	O
.	O
</s>
<s>
Shortcomings	O
:	O
This	O
technique	O
requires	O
state	B-Application
changes	O
to	O
be	O
intercepted	O
within	O
a	O
program	O
.	O
</s>
<s>
This	O
is	O
achieved	O
in	O
a	O
non-transparent	O
way	O
by	O
requiring	O
specific	O
storage-API	O
calls	O
or	O
in	O
a	O
transparent	O
way	O
with	O
automatic	O
program	B-Application
transformation	I-Application
.	O
</s>
<s>
Any	O
software	B-Application
layer	I-Application
that	O
makes	O
it	O
easier	O
for	O
a	O
program	O
to	O
persist	O
its	O
state	B-Application
is	O
generically	O
called	O
a	O
persistence	B-Application
layer	O
.	O
</s>
<s>
Most	O
persistence	B-Application
layers	O
will	O
not	O
achieve	O
persistence	B-Application
directly	O
but	O
will	O
use	O
an	O
underlying	O
database	B-Application
management	I-Application
system	I-Application
.	O
</s>
<s>
System	B-Application
prevalence	I-Application
is	O
a	O
technique	O
that	O
combines	O
system	B-Operating_System
images	I-Operating_System
and	O
transaction	O
journals	O
,	O
mentioned	O
above	O
,	O
to	O
overcome	O
their	O
limitations	O
.	O
</s>
<s>
Shortcomings	O
:	O
A	O
prevalent	O
system	O
must	O
have	O
enough	O
RAM	B-Architecture
to	O
hold	O
the	O
entire	O
system	O
state	B-Application
.	O
</s>
<s>
They	O
provide	O
not	O
only	O
persistence	B-Application
but	O
also	O
other	O
services	O
such	O
as	O
queries	O
,	O
auditing	O
and	O
access	O
control	O
.	O
</s>
<s>
Persistent	O
operating	B-General_Concept
systems	I-General_Concept
are	O
operating	B-General_Concept
systems	I-General_Concept
that	O
remain	O
persistent	O
even	O
after	O
a	O
crash	O
or	O
unexpected	O
shutdown	O
.	O
</s>
