<s>
In	O
version	B-Architecture
control	I-Architecture
systems	I-Architecture
,	O
a	O
repository	B-General_Concept
is	O
a	O
data	O
structure	O
that	O
stores	O
metadata	O
for	O
a	O
set	O
of	O
files	O
or	O
directory	B-Architecture
structure	I-Architecture
.	O
</s>
<s>
Depending	O
on	O
whether	O
the	O
version	B-Architecture
control	I-Architecture
system	I-Architecture
in	O
use	O
is	O
distributed	O
,	O
like	O
Git	B-Application
or	O
Mercurial	B-Protocol
,	O
or	O
centralized	O
,	O
like	O
Subversion	B-Application
,	O
CVS	B-Application
,	O
or	O
Perforce	B-General_Concept
,	O
the	O
whole	O
set	O
of	O
information	O
in	O
the	O
repository	B-General_Concept
may	O
be	O
duplicated	O
on	O
every	O
user	O
's	O
system	O
or	O
may	O
be	O
maintained	O
on	O
a	O
single	O
server	B-Application
.	O
</s>
<s>
Some	O
of	O
the	O
metadata	O
that	O
a	O
repository	B-General_Concept
contains	O
includes	O
,	O
among	O
other	O
things	O
,	O
a	O
historical	O
record	O
of	O
changes	O
in	O
the	O
repository	B-General_Concept
,	O
a	O
set	O
of	O
commit	O
objects	O
,	O
and	O
a	O
set	O
of	O
references	O
to	O
commit	O
objects	O
,	O
called	O
heads	O
.	O
</s>
<s>
The	O
main	O
purpose	O
of	O
a	O
repository	B-General_Concept
is	O
to	O
store	O
a	O
set	O
of	O
files	O
,	O
as	O
well	O
as	O
the	O
history	O
of	O
changes	O
made	O
to	O
those	O
files	O
.	O
</s>
<s>
Exactly	O
how	O
each	O
version	B-Architecture
control	I-Architecture
system	I-Architecture
handles	O
storing	O
those	O
changes	O
,	O
however	O
,	O
differs	O
greatly	O
.	O
</s>
<s>
For	O
instance	O
,	O
Subversion	B-Application
in	O
the	O
past	O
relied	O
on	O
a	O
database	O
instance	O
but	O
has	O
since	O
moved	O
to	O
storing	O
its	O
changes	O
directly	O
on	O
the	O
filesystem	O
.	O
</s>
<s>
These	O
differences	O
in	O
storage	O
techniques	O
have	O
generally	O
led	O
to	O
diverse	O
uses	O
of	O
version	B-Architecture
control	I-Architecture
by	O
different	O
groups	O
,	O
depending	O
on	O
their	O
needs	O
.	O
</s>
<s>
In	O
software	O
engineering	O
,	O
a	O
version	B-Architecture
control	I-Architecture
system	I-Architecture
is	O
used	O
to	O
keep	O
track	O
of	O
versions	O
of	O
a	O
set	O
of	O
files	O
,	O
usually	O
to	O
allow	O
multiple	O
developers	O
to	O
collaborate	O
on	O
a	O
project	O
.	O
</s>
<s>
The	O
repository	B-General_Concept
keeps	O
track	O
of	O
the	O
files	O
in	O
the	O
project	O
,	O
which	O
is	O
represented	O
as	O
a	O
graph	O
.	O
</s>
<s>
A	O
Distributed	B-Protocol
version	I-Protocol
control	I-Protocol
system	I-Protocol
is	O
made	O
up	O
of	O
central	O
and	O
branch	O
repositories	B-General_Concept
.	O
</s>
<s>
A	O
central	O
repository	B-General_Concept
exists	O
on	O
the	O
server	B-Application
.	O
</s>
<s>
To	O
make	O
changes	O
to	O
it	O
,	O
a	O
developer	O
first	O
works	O
on	O
a	O
branch	O
repository	B-General_Concept
,	O
and	O
proceeds	O
to	O
commit	O
the	O
change	O
to	O
the	O
former	O
.	O
</s>
<s>
A	O
code	B-Protocol
forge	I-Protocol
is	O
a	O
web	O
interface	O
to	O
a	O
version	B-Architecture
control	I-Architecture
system	I-Architecture
.	O
</s>
<s>
A	O
user	O
can	O
commonly	O
browse	O
repositories	B-General_Concept
and	O
their	O
constituent	O
files	O
on	O
the	O
page	O
itself	O
.	O
</s>
<s>
While	O
forges	B-Protocol
are	O
mainly	O
used	O
to	O
perform	O
version	B-Architecture
control	I-Architecture
operations	O
,	O
some	O
forges	B-Protocol
allow	O
users	O
to	O
host	O
static	O
web	O
pages	O
by	O
uploading	O
its	O
source	O
code	O
(	O
such	O
as	O
HTML	B-Language
and	O
JavaScript	B-Language
,	O
but	O
not	O
PHP	B-Application
)	O
to	O
a	O
repository	B-General_Concept
.	O
</s>
<s>
The	O
use	O
of	O
repositories	B-General_Concept
as	O
a	O
place	O
to	O
upload	O
web	O
documents	O
allows	O
version	B-Architecture
control	I-Architecture
to	O
be	O
integrated	O
,	O
and	O
additionally	O
allows	O
quick	O
iteration	O
because	O
changes	O
are	O
pushed	O
through	O
the	O
Version	B-Architecture
Control	I-Architecture
System	I-Architecture
instead	O
of	O
having	O
to	O
upload	O
the	O
file	O
through	O
a	O
protocol	O
like	O
FTP	B-Protocol
.	O
</s>
<s>
Examples	O
of	O
this	O
kind	O
of	O
service	O
include	O
GitHub	O
Pages	O
and	O
GitLab	B-Application
Pages	O
.	O
</s>
