<s>
Append-only	B-General_Concept
is	O
a	O
property	O
of	O
computer	B-General_Concept
data	I-General_Concept
storage	I-General_Concept
such	O
that	O
new	O
data	O
can	O
be	O
appended	O
to	O
the	O
storage	O
,	O
but	O
where	O
existing	O
data	O
is	O
immutable	B-Application
.	O
</s>
<s>
Many	O
file	O
systems	O
 '	O
Access	O
Control	O
Lists	O
implement	O
an	O
"	O
append-only	B-General_Concept
"	O
permission	O
:	O
</s>
<s>
chattr	B-Device
in	O
Linux	O
can	O
be	O
used	O
to	O
set	O
the	O
append-only	B-General_Concept
flag	O
to	O
files	O
and	O
directories	O
.	O
</s>
<s>
NTFS	B-Application
ACL	O
has	O
a	O
control	O
for	O
"	O
Create	O
Folders	O
/	O
Append	O
Data	O
"	O
,	O
but	O
it	O
does	O
not	O
seem	O
to	O
keep	O
data	O
immutable	B-Application
.	O
</s>
<s>
Many	O
cloud	O
storage	O
providers	O
provide	O
the	O
ability	O
to	O
limit	O
access	O
as	O
append-only	B-General_Concept
.	O
</s>
<s>
This	O
feature	O
is	O
especially	O
important	O
to	O
mitigate	O
the	O
risk	O
of	O
data	B-General_Concept
loss	I-General_Concept
for	O
backup	B-Protocol
policies	O
in	O
the	O
event	O
that	O
the	O
computer	O
being	O
backed-up	O
becomes	O
infected	O
with	O
ransomware	O
capable	O
of	O
deleting	O
or	O
encrypting	O
the	O
computer	O
's	O
backups	B-Protocol
.	O
</s>
<s>
Many	O
data	B-General_Concept
structures	I-General_Concept
and	O
databases	B-Application
implement	O
immutable	B-Application
objects	I-Application
,	O
effectively	O
making	O
their	O
data	B-General_Concept
structures	I-General_Concept
append-only	B-General_Concept
.	O
</s>
<s>
Implementing	O
an	O
append-only	B-General_Concept
data	B-General_Concept
structure	I-General_Concept
has	O
many	O
benefits	O
,	O
such	O
as	O
ensuring	O
data	O
consistency	B-General_Concept
,	O
improving	O
performance	O
,	O
and	O
permitting	O
rollbacks	O
.	O
</s>
<s>
The	O
prototypical	O
append-only	B-General_Concept
data	B-General_Concept
structure	I-General_Concept
is	O
the	O
log	B-Application
file	I-Application
.	O
</s>
<s>
Log-structured	O
data	B-General_Concept
structures	I-General_Concept
found	O
in	O
Log-structured	B-Application
file	I-Application
systems	I-Application
and	O
databases	B-Application
work	O
in	O
a	O
similar	O
way	O
:	O
every	O
change	O
(	O
transaction	O
)	O
that	O
happens	O
to	O
the	O
data	O
is	O
logged	O
by	O
the	O
program	O
,	O
and	O
on	O
retrieval	O
the	O
program	O
must	O
combine	O
the	O
pieces	O
of	O
data	O
found	O
in	O
this	O
log	B-Application
file	I-Application
.	O
</s>
<s>
Blockchains	B-Application
add	O
cryptography	O
to	O
the	O
logs	O
so	O
that	O
every	O
transaction	O
is	O
verifiable	O
.	O
</s>
<s>
Append-only	B-General_Concept
data	B-General_Concept
structures	I-General_Concept
may	O
also	O
be	O
mandated	O
by	O
the	O
hardware	O
or	O
software	O
environment	O
:	O
</s>
<s>
All	O
objects	O
are	O
immutable	B-Application
in	O
purely	B-Application
functional	I-Application
programming	I-Application
languages	O
,	O
where	O
every	O
function	O
is	O
pure	O
and	O
global	O
states	O
do	O
not	O
exist	O
.	O
</s>
<s>
Flash	B-Device
storage	I-Device
cells	O
can	O
only	O
be	O
written	O
to	O
once	O
before	O
erasing	O
.	O
</s>
<s>
Erasing	O
on	O
a	O
flash	O
drive	O
works	O
on	O
the	O
level	O
of	O
pages	O
with	O
cover	O
many	O
cells	O
at	O
once	O
,	O
so	O
each	O
page	O
is	O
treated	O
as	O
an	O
append-only	B-General_Concept
set	O
of	O
cells	O
until	O
it	O
fills	O
up	O
.	O
</s>
<s>
Hard	O
drives	O
that	O
use	O
shingled	B-Device
magnetic	I-Device
recording	I-Device
cannot	O
be	O
written	O
to	O
randomly	O
because	O
writing	O
on	O
a	O
track	O
would	O
clobber	O
a	O
neighboring	O
,	O
usually	O
later	O
,	O
track	O
.	O
</s>
<s>
As	O
a	O
result	O
,	O
each	O
"	O
zone	O
"	O
on	O
the	O
drive	O
is	O
append-only	B-General_Concept
.	O
</s>
<s>
Append-only	B-General_Concept
data	B-General_Concept
structures	I-General_Concept
grow	O
over	O
time	O
,	O
with	O
more	O
and	O
more	O
space	O
dedicated	O
to	O
"	O
stale	O
"	O
data	O
found	O
only	O
in	O
the	O
history	O
and	O
more	O
time	O
wasted	O
on	O
parsing	O
these	O
data	O
.	O
</s>
<s>
A	O
number	O
of	O
append-only	B-General_Concept
systems	O
implement	O
rewriting	O
(	O
copying	O
garbage	B-General_Concept
collection	I-General_Concept
)	O
,	O
so	O
that	O
a	O
new	O
structure	O
is	O
created	O
only	O
containing	O
the	O
current	O
version	O
and	O
optionally	O
a	O
few	O
older	O
ones	O
.	O
</s>
