<s>
Dependency	B-Protocol
hell	I-Protocol
is	O
a	O
colloquial	O
term	O
for	O
the	O
frustration	O
of	O
some	O
software	O
users	O
who	O
have	O
installed	O
software	B-Application
packages	I-Application
which	O
have	O
dependencies	B-Application
on	O
specific	O
versions	O
of	O
other	O
software	B-Application
packages	I-Application
.	O
</s>
<s>
The	O
dependency	B-Application
issue	O
arises	O
when	O
several	O
packages	O
have	O
dependencies	B-Application
on	O
the	O
same	O
shared	O
packages	O
or	O
libraries	B-Library
,	O
but	O
they	O
depend	O
on	O
different	O
and	O
incompatible	O
versions	O
of	O
the	O
shared	O
packages	O
.	O
</s>
<s>
This	O
,	O
in	O
turn	O
,	O
may	O
break	O
other	O
dependencies	B-Application
and	O
push	O
the	O
problem	O
to	O
another	O
set	O
of	O
packages	O
.	O
</s>
<s>
Dependency	B-Protocol
hell	I-Protocol
takes	O
several	O
forms	O
:	O
</s>
<s>
An	O
application	O
depends	O
on	O
many	O
libraries	B-Library
,	O
requiring	O
lengthy	O
downloads	O
,	O
large	O
amounts	O
of	O
disk	O
space	O
,	O
and	O
being	O
very	O
portable	O
(	O
all	O
libraries	B-Library
are	O
already	O
ported	O
enabling	O
the	O
application	O
itself	O
to	O
be	O
ported	O
easily	O
)	O
.	O
</s>
<s>
It	O
can	O
also	O
be	O
difficult	O
to	O
locate	O
all	O
the	O
dependencies	B-Application
,	O
which	O
can	O
be	O
fixed	O
by	O
having	O
a	O
repository	B-Architecture
(	O
see	O
below	O
)	O
.	O
</s>
<s>
This	O
is	O
partly	O
inevitable	O
;	O
an	O
application	O
built	O
on	O
a	O
given	O
computing	B-Device
platform	I-Device
(	O
such	O
as	O
Java	B-Device
)	O
requires	O
that	O
platform	O
to	O
be	O
installed	O
,	O
but	O
further	O
applications	O
do	O
not	O
require	O
it	O
.	O
</s>
<s>
This	O
is	O
a	O
particular	O
problem	O
if	O
an	O
application	O
uses	O
a	O
small	O
part	O
of	O
a	O
big	O
library	O
(	O
which	O
can	O
be	O
solved	O
by	O
code	O
refactoring	O
)	O
,	O
or	O
a	O
simple	O
application	O
relies	O
on	O
many	O
libraries	B-Library
.	O
</s>
<s>
If	O
depends	O
on	O
,	O
which	O
depends	O
on	O
,	O
...	O
,	O
which	O
depends	O
on	O
.	O
</s>
<s>
This	O
is	O
distinct	O
from	O
"	O
many	O
dependencies	B-Application
"	O
if	O
the	O
dependencies	B-Application
must	O
be	O
resolved	O
manually	O
,	O
e.g.	O
,	O
on	O
attempting	O
to	O
install	O
,	O
the	O
user	O
is	O
prompted	O
to	O
install	O
first	O
and	O
on	O
attempting	O
to	O
install	O
,	O
the	O
user	O
is	O
then	O
prompted	O
to	O
install	O
,	O
and	O
so	O
on	O
.	O
</s>
<s>
Sometimes	O
,	O
however	O
,	O
during	O
this	O
long	O
chain	O
of	O
dependencies	B-Application
,	O
conflicts	O
arise	O
where	O
two	O
different	O
versions	O
of	O
the	O
same	O
package	O
are	O
required	O
(	O
see	O
conflicting	O
dependencies	B-Application
below	O
)	O
.	O
</s>
<s>
These	O
long	O
chains	O
of	O
dependencies	B-Application
can	O
be	O
solved	O
by	O
having	O
a	O
package	B-Application
manager	I-Application
that	O
resolves	O
all	O
dependencies	B-Application
automatically	O
.	O
</s>
<s>
Other	O
than	O
being	O
a	O
hassle	O
(	O
to	O
resolve	O
all	O
the	O
dependencies	B-Application
manually	O
)	O
,	O
manual	O
resolution	O
can	O
mask	O
dependency	B-Application
cycles	O
or	O
conflicts	O
.	O
</s>
<s>
Solving	O
the	O
dependencies	B-Application
for	O
one	O
software	O
may	O
break	O
the	O
compatibility	O
of	O
another	O
in	O
a	O
similar	O
fashion	O
to	O
whack-a-mole	O
.	O
</s>
<s>
If	O
depends	O
on	O
,	O
and	O
depends	O
on	O
,	O
and	O
different	O
versions	O
of	O
cannot	O
be	O
simultaneously	O
installed	O
,	O
then	O
and	O
cannot	O
simultaneously	O
be	O
used	O
(	O
or	O
installed	O
,	O
if	O
the	O
installer	O
checks	O
dependencies	B-Application
)	O
.	O
</s>
<s>
When	O
possible	O
,	O
this	O
is	O
solved	O
by	O
allowing	O
simultaneous	O
installations	O
of	O
the	O
different	O
dependencies	B-Application
.	O
</s>
<s>
Alternatively	O
,	O
the	O
existing	O
dependency	B-Application
,	O
along	O
with	O
all	O
software	O
that	O
depends	O
on	O
it	O
,	O
must	O
be	O
uninstalled	O
in	O
order	O
to	O
install	O
the	O
new	O
dependency	B-Application
.	O
</s>
<s>
A	O
problem	O
on	O
Linux	B-Application
systems	O
with	O
installing	O
packages	O
from	O
a	O
different	O
distributor	O
(	O
which	O
is	O
not	O
recommended	O
or	O
even	O
supposed	O
to	O
work	O
)	O
is	O
that	O
the	O
resulting	O
long	O
chain	O
of	O
dependencies	B-Application
may	O
lead	O
to	O
a	O
conflicting	O
version	O
of	O
the	B-Language
C	I-Language
standard	I-Language
library	I-Language
(	O
e.g.	O
</s>
<s>
the	O
GNU	B-Language
C	I-Language
Library	I-Language
)	O
,	O
on	O
which	O
thousands	O
of	O
packages	O
depend	O
.	O
</s>
<s>
Its	O
impact	O
can	O
be	O
quite	O
heavy	O
,	O
if	O
it	O
affects	O
core	O
systems	O
or	O
update	O
software	O
itself	O
:	O
a	O
package	B-Application
manager	I-Application
(	O
A	O
)	O
,	O
which	O
requires	O
specific	O
run-time	O
library	O
(	O
B	O
)	O
to	O
function	O
,	O
may	O
brick	O
itself	O
(	O
A	O
)	O
in	O
the	O
middle	O
of	O
the	O
process	O
when	O
upgrading	O
this	O
library	O
(	O
B	O
)	O
to	O
next	O
version	O
.	O
</s>
<s>
Due	O
to	O
incorrect	O
library	O
(	O
B	O
)	O
version	O
,	O
the	O
package	B-Application
manager	I-Application
(	O
A	O
)	O
is	O
now	O
broken	O
-	O
thus	O
no	O
rollback	O
or	O
downgrade	O
of	O
library	O
(	O
B	O
)	O
is	O
possible	O
.	O
</s>
<s>
It	O
is	O
possible	O
for	O
dependency	B-Protocol
hell	I-Protocol
to	O
result	O
from	O
installing	O
a	O
prepared	O
package	O
via	O
a	O
package	B-Application
manager	I-Application
(	O
e.g.	O
</s>
<s>
APT	B-Application
)	O
,	O
but	O
this	O
is	O
unlikely	O
since	O
major	O
package	B-Application
managers	I-Application
have	O
matured	O
and	O
official	O
repositories	O
are	O
well	O
maintained	O
.	O
</s>
<s>
This	O
is	O
the	O
case	O
with	O
current	O
releases	O
of	O
Debian	O
and	O
major	O
derivatives	O
such	O
as	O
Ubuntu	B-Operating_System
.	O
</s>
<s>
Dependency	B-Protocol
hell	I-Protocol
,	O
however	O
,	O
can	O
result	O
from	O
installing	O
a	O
package	O
directly	O
via	O
a	O
package	O
installer	O
(	O
e.g.	O
</s>
<s>
RPM	B-Application
or	O
dpkg	B-Language
)	O
.	O
</s>
<s>
When	O
a	O
library	O
A	O
depends	O
on	O
libraries	B-Library
B	O
and	O
C	O
,	O
both	O
B	O
and	O
C	O
depend	O
on	O
library	O
D	O
,	O
but	O
B	O
requires	O
version	O
D.1	O
and	O
C	O
requires	O
version	O
D.2	O
.	O
</s>
<s>
Package	B-Application
managers	I-Application
like	O
yum	B-Application
,	O
are	O
prone	O
to	O
have	O
conflicts	O
between	O
packages	O
of	O
their	O
repositories	O
,	O
causing	O
dependency	B-Protocol
hell	I-Protocol
in	O
Linux	B-Application
distributions	O
such	O
as	O
CentOS	O
and	O
Red	O
Hat	O
Enterprise	O
Linux	B-Application
.	O
</s>
<s>
Many	O
software	B-Library
libraries	I-Library
are	O
written	O
in	O
a	O
generous	O
way	O
,	O
trying	O
to	O
fulfil	O
most	O
users	O
 '	O
needs	O
,	O
but	O
sometimes	O
only	O
a	O
small	O
portion	O
of	O
functions	O
are	O
required	O
in	O
the	O
host	O
code	O
.	O
</s>
<s>
A	O
very	O
common	O
solution	O
to	O
this	O
problem	O
is	O
to	O
have	O
a	O
standardized	O
numbering	O
system	O
,	O
wherein	O
software	O
uses	O
a	O
specific	O
number	O
for	O
each	O
version	O
(	O
aka	O
major	O
version	O
)	O
,	O
and	O
also	O
a	O
subnumber	O
for	O
each	O
revision	O
(	O
aka	O
minor	B-General_Concept
version	I-General_Concept
)	O
,	O
e.g.	O
</s>
<s>
The	O
minor	B-General_Concept
version	I-General_Concept
might	O
change	O
with	O
even	O
a	O
simple	O
revision	O
that	O
does	O
not	O
prevent	O
other	O
software	O
from	O
working	O
with	O
it	O
.	O
</s>
<s>
In	O
cases	O
like	O
this	O
,	O
software	B-Application
packages	I-Application
can	O
then	O
simply	O
request	O
a	O
component	O
that	O
has	O
a	O
particular	O
major	O
version	O
,	O
and	O
any	O
minor	B-General_Concept
version	I-General_Concept
(	O
greater	O
than	O
or	O
equal	O
to	O
a	O
particular	O
minor	B-General_Concept
version	I-General_Concept
)	O
.	O
</s>
<s>
As	O
such	O
,	O
they	O
will	O
continue	O
to	O
work	O
,	O
and	O
dependencies	B-Application
will	O
be	O
resolved	O
successfully	O
,	O
even	O
if	O
the	O
minor	B-General_Concept
version	I-General_Concept
changes	O
.	O
</s>
<s>
Windows	O
File	O
Protection	O
introduced	O
in	O
Windows	B-Application
2000	I-Application
prevented	O
applications	O
from	O
overwriting	O
system	O
DLLs	O
.	O
</s>
<s>
Developers	O
were	O
instead	O
encouraged	O
to	O
use	O
"	O
Private	O
DLLs	O
"	O
,	O
copies	O
of	O
libraries	B-Library
per	O
application	O
in	O
the	O
directory	B-Application
of	O
the	O
application	O
.	O
</s>
<s>
This	O
uses	O
the	O
Windows	O
search	O
path	O
characteristic	O
that	O
the	O
local	O
path	O
is	O
always	O
prioritized	O
before	O
the	O
system	O
directory	B-Application
with	O
the	O
system	O
wide	O
libraries	B-Library
.	O
</s>
<s>
This	O
allows	O
easy	O
and	O
effective	O
shadowing	O
of	O
library	O
versions	O
by	O
specific	O
application	O
ones	O
,	O
therefore	O
preventing	O
dependency	B-Protocol
hell	I-Protocol
.	O
</s>
<s>
PC-BSD	B-Application
,	O
up	O
to	O
and	O
including	O
version	O
8.2	O
,	O
a	O
predecessor	O
of	O
TrueOS	B-Application
(	O
an	O
operating	O
system	O
based	O
on	O
FreeBSD	B-Operating_System
)	O
places	O
packages	O
and	O
dependencies	B-Application
into	O
self-contained	O
directories	B-Application
in	O
/Programs	O
,	O
which	O
avoids	O
breakage	O
if	O
system	B-Library
libraries	I-Library
are	O
upgraded	O
or	O
changed	O
.	O
</s>
<s>
It	O
uses	O
its	O
own	O
"	O
PBI	O
"	O
(	O
Push	O
Button	O
Installer	O
)	O
for	O
package	B-Application
management	I-Application
.	O
</s>
<s>
A	O
shared	O
module	O
can	O
then	O
be	O
placed	O
in	O
a	O
central	O
repository	B-Architecture
without	O
the	O
risk	O
of	O
breaking	O
applications	O
which	O
are	O
dependent	O
on	O
previous	O
or	O
later	O
versions	O
of	O
the	O
module	O
.	O
</s>
<s>
This	O
solution	O
is	O
used	O
in	O
Microsoft	B-Application
Windows	I-Application
operating	I-Application
systems	I-Application
since	O
Windows	O
Vista	O
,	O
where	O
the	O
Global	O
Assembly	O
Cache	O
is	O
an	O
implementation	O
of	O
such	O
a	O
central	O
registry	O
with	O
associated	O
services	O
and	O
integrated	O
with	O
the	O
installation	O
system/package	O
manager	O
.	O
</s>
<s>
Gentoo	B-Application
Linux	I-Application
solves	O
this	O
problem	O
with	O
a	O
concept	O
called	O
slotting	O
,	O
which	O
allows	O
multiple	O
versions	O
of	O
shared	O
libraries	B-Library
to	O
be	O
installed	O
.	O
</s>
<s>
Some	O
package	B-Application
managers	I-Application
can	O
perform	O
smart	O
upgrades	O
,	O
in	O
which	O
interdependent	O
software	O
components	O
are	O
upgraded	O
at	O
the	O
same	O
time	O
,	O
thereby	O
resolving	O
the	O
major	O
number	O
incompatibility	O
issue	O
too	O
.	O
</s>
<s>
Many	O
current	O
Linux	B-Application
distributions	O
have	O
also	O
implemented	O
repository-based	O
package	B-Application
management	I-Application
systems	I-Application
to	O
try	O
to	O
solve	O
the	O
dependency	B-Application
problem	O
.	O
</s>
<s>
These	O
systems	O
are	O
a	O
layer	O
on	O
top	O
of	O
the	O
RPM	B-Application
,	O
dpkg	B-Language
,	O
or	O
other	O
packaging	O
systems	O
that	O
are	O
designed	O
to	O
automatically	O
resolve	O
dependencies	B-Application
by	O
searching	O
in	O
predefined	O
software	B-Application
repositories	I-Application
.	O
</s>
<s>
Examples	O
of	O
these	O
systems	O
include	O
Apt	B-Application
,	O
Yum	B-Application
,	O
Urpmi	B-Application
,	O
ZYpp	O
,	O
Portage	B-Application
,	O
Pacman	O
and	O
others	O
.	O
</s>
<s>
Typically	O
,	O
the	O
software	B-Application
repositories	I-Application
are	O
FTP	B-Protocol
sites	O
or	O
websites	O
,	O
directories	B-Application
on	O
the	O
local	O
computer	O
or	O
shared	O
across	O
a	O
network	B-Architecture
or	O
,	O
much	O
less	O
commonly	O
,	O
directories	B-Application
on	O
removable	O
media	O
such	O
as	O
CDs	O
or	O
DVDs	O
.	O
</s>
<s>
This	O
eliminates	O
dependency	B-Protocol
hell	I-Protocol
for	O
software	O
packaged	O
in	O
those	O
repositories	O
,	O
which	O
are	O
typically	O
maintained	O
by	O
the	O
Linux	B-Application
distribution	O
provider	O
and	O
mirrored	O
worldwide	O
.	O
</s>
<s>
Although	O
these	O
repositories	O
are	O
often	O
huge	O
,	O
it	O
is	O
not	O
possible	O
to	O
have	O
every	O
piece	O
of	O
software	O
in	O
them	O
,	O
so	O
dependency	B-Protocol
hell	I-Protocol
can	O
still	O
occur	O
.	O
</s>
<s>
In	O
all	O
cases	O
,	O
dependency	B-Protocol
hell	I-Protocol
is	O
still	O
faced	O
by	O
the	O
repository	B-Architecture
maintainers	O
.	O
</s>
<s>
Because	O
different	O
pieces	O
of	O
software	O
have	O
different	O
dependencies	B-Application
,	O
it	O
is	O
possible	O
to	O
get	O
into	O
a	O
vicious	O
circle	O
of	O
dependency	B-Application
requirements	O
,	O
or	O
an	O
ever-expanding	O
tree	B-Data_Structure
of	O
requirements	O
,	O
as	O
each	O
new	O
package	O
demands	O
several	O
more	O
be	O
installed	O
.	O
</s>
<s>
Systems	O
such	O
as	O
Debian	O
's	O
Advanced	B-Application
Packaging	I-Application
Tool	I-Application
can	O
resolve	O
this	O
by	O
presenting	O
the	O
user	O
with	O
a	O
range	O
of	O
solutions	O
,	O
and	O
allowing	O
the	O
user	O
to	O
accept	O
or	O
reject	O
the	O
solutions	O
,	O
as	O
desired	O
.	O
</s>
<s>
If	O
the	O
applications	O
and	O
libraries	B-Library
are	O
developed	O
and	O
maintained	O
with	O
guaranteed	O
downward	O
compatibility	O
in	O
mind	O
,	O
any	O
application	O
or	O
library	O
can	O
be	O
replaced	O
with	O
a	O
newer	O
version	O
at	O
any	O
time	O
without	O
breaking	O
anything	O
.	O
</s>
<s>
While	O
this	O
does	O
not	O
alleviate	O
the	O
multitude	O
of	O
dependency	B-Application
,	O
it	O
does	O
make	O
the	O
jobs	O
of	O
package	B-Application
managers	I-Application
or	O
installers	O
much	O
easier	O
.	O
</s>
<s>
Another	O
approach	O
to	O
avoiding	O
dependency	B-Application
issues	O
is	O
to	O
deploy	O
applications	O
as	O
a	O
software	B-Application
appliance	I-Application
.	O
</s>
<s>
A	O
software	B-Application
appliance	I-Application
encapsulates	O
dependencies	B-Application
in	O
a	O
pre-integrated	O
self-contained	O
unit	O
such	O
that	O
users	O
no	O
longer	O
have	O
to	O
worry	O
about	O
resolving	O
software	B-Library
dependencies	I-Library
.	O
</s>
<s>
Instead	O
the	O
burden	O
is	O
shifted	O
to	O
developers	O
of	O
the	O
software	B-Application
appliance	I-Application
.	O
</s>
<s>
Containers	B-Application
and	O
their	O
images	O
(	O
such	O
as	O
those	O
provided	O
by	O
Docker	B-Language
and	O
Docker	B-Language
Hub	O
)	O
can	O
be	O
seen	O
as	O
an	O
implementation	O
of	O
software	B-Application
appliances	I-Application
.	O
</s>
<s>
It	O
is	O
coded	O
to	O
have	O
all	O
necessary	O
components	O
included	O
,	O
or	O
is	O
designed	O
to	O
keep	O
all	O
necessary	O
files	O
within	O
its	O
own	O
directory	B-Application
,	O
and	O
will	O
not	O
create	O
a	O
dependency	B-Application
problem	O
.	O
</s>
<s>
Applications	O
in	O
RISC	B-Operating_System
OS	I-Operating_System
and	O
the	O
ROX	B-Language
Desktop	I-Language
for	O
Linux	B-Application
use	O
application	B-Application
directories	I-Application
,	O
which	O
work	O
in	O
much	O
the	O
same	O
way	O
:	O
programs	O
and	O
their	O
dependencies	B-Application
are	O
self-contained	O
in	O
their	O
own	O
directories	B-Application
(	O
folders	O
)	O
.	O
</s>
<s>
This	O
method	O
of	O
distribution	O
has	O
also	O
proven	O
useful	O
when	O
porting	O
applications	O
designed	O
for	O
Unix-like	O
platforms	B-Device
to	O
Windows	O
,	O
the	O
most	O
noticeable	O
drawback	O
being	O
multiple	O
installations	O
of	O
the	O
same	O
shared	O
libraries	B-Library
.	O
</s>
<s>
For	O
example	O
,	O
Windows	O
installers	O
for	O
gedit	B-Language
,	O
GIMP	B-Application
,	O
and	O
HexChat	B-Protocol
all	O
include	O
identical	O
copies	O
of	O
the	O
GTK	B-Application
toolkit	I-Application
,	O
which	O
these	O
programs	O
use	O
to	O
render	O
widgets	O
.	O
</s>
<s>
On	O
the	O
other	O
hand	O
,	O
if	O
different	O
versions	O
of	O
GTK	B-Application
are	O
required	O
by	O
each	O
application	O
,	O
then	O
this	O
is	O
the	O
correct	O
behavior	O
and	O
successfully	O
avoids	O
dependency	B-Protocol
hell	I-Protocol
.	O
</s>
<s>
On	O
specific	O
computing	B-Device
platforms	I-Device
,	O
"	O
dependency	B-Protocol
hell	I-Protocol
"	O
often	O
goes	O
by	O
a	O
local	O
specific	O
name	O
,	O
generally	O
the	O
name	O
of	O
components	O
.	O
</s>
<s>
DLL	O
Hell	O
a	O
form	O
of	O
dependency	B-Protocol
hell	I-Protocol
occurring	O
on	O
Microsoft	B-Application
Windows	I-Application
.	O
</s>
<s>
Extension	B-Operating_System
conflict	I-Operating_System
a	O
form	O
of	O
dependency	B-Protocol
hell	I-Protocol
occurring	O
on	O
the	O
classic	B-Application
Mac	I-Application
OS	I-Application
.	O
</s>
<s>
JAR	O
hell	O
a	O
form	O
of	O
dependency	B-Protocol
hell	I-Protocol
occurring	O
in	O
the	O
Java	B-Language
Runtime	I-Language
Environment	I-Language
before	O
build	O
tools	O
like	O
Apache	B-Language
Maven	I-Language
solved	O
this	O
problem	O
in	O
2004	O
.	O
</s>
<s>
RPM	B-Protocol
hell	I-Protocol
a	O
form	O
of	O
dependency	B-Protocol
hell	I-Protocol
occurring	O
in	O
the	O
Red	O
Hat	O
distribution	O
of	O
Linux	B-Application
and	O
other	O
distributions	O
that	O
use	O
RPM	B-Application
as	O
a	O
package	B-Application
manager	I-Application
.	O
</s>
