<s>
In	O
configuration	B-General_Concept
management	I-General_Concept
(	O
CM	O
)	O
,	O
one	O
has	O
to	O
control	O
(	O
among	O
other	O
things	O
)	O
changes	O
made	O
to	O
software	O
and	O
documentation	O
.	O
</s>
<s>
This	O
is	O
called	O
revision	B-Architecture
control	I-Architecture
,	O
which	O
manages	O
multiple	O
versions	O
of	O
the	O
same	O
unit	O
of	O
information	O
.	O
</s>
<s>
Although	O
revision	B-Architecture
control	I-Architecture
is	O
important	O
to	O
CM	O
,	O
it	O
is	O
not	O
equal	O
to	O
it	O
.	O
</s>
<s>
Synchronization	B-General_Concept
Models	I-General_Concept
,	O
also	O
known	O
as	O
Configuration	B-General_Concept
Management	I-General_Concept
Models	O
(	O
Feiler	O
,	O
1991	O
)	O
,	O
describe	O
methods	O
to	O
enable	O
revision	B-Architecture
control	I-Architecture
through	O
allowing	O
simultaneous	O
,	O
concurrent	O
changes	O
to	O
individual	O
files	O
.	O
</s>
<s>
Feiler	O
(	O
1991	O
)	O
reports	O
on	O
four	O
different	O
synchronization	B-General_Concept
models	I-General_Concept
,	O
shortly	O
described	O
below	O
.	O
</s>
<s>
In	O
the	O
check-out/check	O
-in	O
model	O
,	O
files	O
are	O
stored	O
individually	O
in	O
a	O
repository	B-Protocol
from	O
which	O
they	O
are	O
checked	O
out	O
whenever	O
the	O
files	O
are	O
accessed	O
,	O
and	O
checked	O
in	O
when	O
they	O
have	O
changed	O
.	O
</s>
<s>
This	O
repository	B-Protocol
can	O
store	O
multiple	O
versions	O
of	O
the	O
files	O
.	O
</s>
<s>
The	O
basic	O
mechanism	O
used	O
to	O
prevent	O
conflicts	O
by	O
simultaneous	O
modifications	O
is	O
that	O
of	O
locking	B-Operating_System
.	O
</s>
<s>
This	O
model	O
allows	O
developers	O
to	O
think	O
in	O
configurations	B-General_Concept
instead	O
of	O
individual	O
files	O
.	O
</s>
<s>
Although	O
the	O
complete	O
check-out/check	O
-in	O
model	O
is	O
represented	O
in	O
the	O
composition	O
model	O
,	O
it	O
enables	O
the	O
use	O
of	O
different	O
strategies	O
for	O
updating	O
through	O
the	O
use	O
of	O
improved	O
support	O
for	O
the	O
management	O
of	O
configurations	B-General_Concept
.	O
</s>
<s>
Basically	O
,	O
it	O
uses	O
versions	O
of	O
configurations	B-General_Concept
and	O
versions	O
of	O
files	O
.	O
</s>
<s>
When	O
the	O
change	O
is	O
completed	O
,	O
the	O
complete	O
configuration	O
is	O
stored	O
back	O
into	O
the	O
repository	B-Protocol
and	O
integrated	O
with	O
other	O
changes	O
.	O
</s>
<s>
New	O
configurations	B-General_Concept
of	O
the	O
product	O
are	O
then	O
created	O
by	O
applying	O
sets	O
of	O
the	O
independently	O
stored	O
changes	O
on	O
the	O
baseline	O
version	O
.	O
</s>
<s>
This	O
entry	O
covers	O
the	O
check-out/check	O
-in	O
synchronization	B-General_Concept
model	I-General_Concept
,	O
including	O
a	O
meta-model	O
(	O
a	O
process-data	B-Application
diagram	I-Application
)	O
.	O
</s>
<s>
Issues	O
that	O
are	O
not	O
discussed	O
in	O
detail	O
are	O
the	O
three	O
remaining	O
synchronization	B-General_Concept
models	I-General_Concept
and	O
the	O
actual	O
editing	O
of	O
CIs	O
together	O
with	O
the	O
methods	O
related	O
to	O
this	O
.	O
</s>
<s>
Concept	O
DefinitionVersionA	O
version	O
is	O
a	O
state	O
of	O
an	O
object	O
or	O
concept	O
that	O
varies	O
from	O
its	O
previous	O
state	O
or	O
condition.Configuration	O
itemAn	O
element	O
of	O
software	O
or	O
a	O
document	O
placed	O
under	O
version	B-Architecture
control	I-Architecture
.	O
</s>
<s>
A	O
computer	O
program	O
's	O
source	O
code	O
is	O
the	O
collection	O
of	O
files	O
that	O
can	O
be	O
converted	O
from	O
human-readable	O
form	O
to	O
an	O
equivalent	O
computer-executable	O
form.RepositoryA	O
repository	B-Protocol
is	O
also	O
called	O
a	O
vault	O
.	O
</s>
<s>
A	O
repository	B-Protocol
contains	O
only	O
one	O
complete	O
version	O
of	O
a	O
configuration	O
item	O
.	O
</s>
<s>
This	O
section	O
contains	O
an	O
elaboration	O
on	O
the	O
check-out/check	O
-in	O
synchronization	B-General_Concept
model	I-General_Concept
.	O
</s>
<s>
The	O
process-data	B-Application
diagram	I-Application
above	O
describes	O
the	O
different	O
concepts	O
that	O
are	O
applicable	O
in	O
the	O
check-out/check	O
-in	O
synchronization	B-General_Concept
model	I-General_Concept
and	O
their	O
relation	O
to	O
the	O
activities	O
that	O
take	O
place	O
.	O
</s>
<s>
The	O
repository	B-Protocol
can	O
contain	O
multiple	O
branches	O
and	O
revisions	O
of	O
files	O
.	O
</s>
<s>
Activity	O
Sub-activity	O
DefinitionCheck-Out	O
Files	O
are	O
not	O
read	O
or	O
changed	O
directly	O
from	O
the	O
repository	B-Protocol
.	O
</s>
<s>
Checking	O
out	O
describes	O
these	O
activities.Copy	O
CI	O
A	O
particular	O
version	O
of	O
a	O
CI	O
is	O
copied	O
from	O
the	O
repository.Lock	O
CIIf	O
write	O
access	O
is	O
required	O
and	O
if	O
the	O
CI	O
is	O
not	O
already	O
locked	O
by	O
someone	O
else	O
,	O
the	O
CI	O
is	O
locked	O
.	O
</s>
<s>
Otherwise	O
the	O
CI	O
cannot	O
be	O
written	O
back	O
to	O
the	O
repository	B-Protocol
(	O
read-only	O
access	O
)	O
.Modify	O
CIThe	O
person	O
editing	O
the	O
CI	O
makes	O
changes	O
to	O
it	O
.	O
</s>
<s>
This	O
is	O
out	O
of	O
the	O
scope	O
of	O
the	O
current	O
meta-model	O
regarding	O
version	O
management.Check-InThe	O
CI	O
needs	O
to	O
be	O
placed	O
back	O
into	O
the	O
repository	B-Protocol
.	O
</s>
<s>
Checking	O
in	O
describes	O
these	O
activities.Select	O
versioning	O
strategyThe	O
new	O
CI	O
has	O
to	O
be	O
placed	O
back	O
in	O
the	O
repository	B-Protocol
using	O
a	O
versioning	O
strategy	O
.	O
</s>
<s>
This	O
describes	O
the	O
selection	O
of	O
the	O
strategy	O
used	O
to	O
place	O
the	O
CI	O
back	O
into	O
the	O
repository.Create	O
branchThe	O
CI	O
is	O
selected	O
to	O
be	O
the	O
start	O
of	O
a	O
new	O
branch.Create	O
revisionThe	O
CI	O
is	O
selected	O
to	O
be	O
a	O
revision	B-Architecture
of	O
another	O
CI.Select	O
development	O
stateThe	O
CI	O
is	O
determined	O
to	O
be	O
of	O
a	O
certain	O
development	O
state.Write	O
CIThe	O
CI	O
is	O
stored	O
in	O
the	O
repository.Unlock	O
CIThe	O
CI	O
is	O
unlocked	O
.	O
</s>
<s>
Feiler	O
(	O
1991	O
)	O
evaluated	O
the	O
check-out/check	O
-in	O
synchronization	B-General_Concept
model	I-General_Concept
.	O
</s>
<s>
However	O
,	O
this	O
simplicity	O
results	O
in	O
a	O
lack	O
of	O
management	O
of	O
configurations	B-General_Concept
,	O
such	O
as	O
product	O
version	O
tracking	O
and	O
checking	O
version	O
history	O
across	O
multiple	O
logically	O
connected	O
files	O
.	O
</s>
<s>
The	O
turn-taking	O
mechanism	O
of	O
locking	B-Operating_System
is	O
a	O
real	O
problem	O
as	O
well	O
when	O
working	O
with	O
many	O
developers	O
,	O
as	O
these	O
files	O
cannot	O
be	O
edited	O
by	O
others	O
once	O
it	O
has	O
been	O
locked	O
.	O
</s>
<s>
To	O
illustrate	O
the	O
check-out/check	O
-in	O
synchronization	B-General_Concept
model	I-General_Concept
,	O
this	O
section	O
contains	O
an	O
example	O
of	O
how	O
this	O
process	O
works	O
.	O
</s>
<s>
The	O
figure	O
below	O
contains	O
a	O
state	B-Application
transition	I-Application
diagram	I-Application
of	O
a	O
CI	O
.	O
</s>
<s>
When	O
a	O
CI	O
is	O
first	O
created	O
,	O
it	O
is	O
modified	O
and	O
stored	O
in	O
the	O
repository	B-Protocol
.	O
</s>
<s>
When	O
someone	O
requests	O
to	O
open	O
the	O
CI	O
,	O
it	O
is	O
first	O
copied	O
to	O
the	O
local	O
machine	O
of	O
the	O
developer	O
(	O
note	O
:	O
there	O
are	O
systems	O
where	O
editing	O
occurs	O
directly	O
in	O
the	O
repository	B-Protocol
.	O
</s>
<s>
After	O
modifications	O
have	O
been	O
done	O
,	O
it	O
is	O
stored	O
back	O
in	O
the	O
repository	B-Protocol
and	O
unlocked	O
.	O
</s>
<s>
Now	O
,	O
assume	O
that	O
the	O
developer	O
that	O
was	O
just	O
discussed	O
is	O
in	O
the	O
process	O
of	O
editing	O
a	O
CI	O
that	O
is	O
already	O
in	O
the	O
repository	B-Protocol
.	O
</s>
<s>
You	O
want	O
to	O
open	O
a	O
CI	O
from	O
the	O
repository	B-Protocol
and	O
so	O
it	O
is	O
copied	O
to	O
your	O
local	O
drive	O
.	O
</s>
