On Sun, 22 Nov 2009, Dennis Kibbe wrote:
> "The upstream maintainer of yum, Seth Vidal, had the following to say
> about 'yum priorities' in September 2009:
>
> Gosh, I hope people do not set up yum priorities. There are so many things
> about priorities that make me cringe all over. It could just be that it
> reminds me of apt 'pinning' and that makes me want to hurl."
>
> This note was placed on the wiki (PackageManagement/Yum?Priorities)
> without any explanation why yum-priorities isn't a good idea.
Hi, Dennis
That page is outlinked from the general discussion on
Respositories, which runs through a discussion of
'exclude' and 'includepkg' as earlier options to consider
before these two non-stock install addons to yum that you
mentioned.
The problem with priorities, and pinning generally, is that it
cannot anticipate the growth of package dependencies, and
tries to solve with a static rule, a shifting problem. It may
work to get what is initially wanted, but it is a durable
solution, nor the right solution, because eventually, some
combination of enhanced weighting will cause an unintended
consequence, blocking some more important upgrade [a
point version bump, or worse a security async update].
We see it a lot in the IRC channel with people who don't or
won't read, and with the intermitent availability of some
non-CentOS archives, and yet want the system to solve
integrating encumbered sound driver codecs and extensions.
They do, sometimes withthis approach, or forcing or much worse
--nodeps, and later have the 'wheels come off' when some
library dependency on a main archive is blocked by an upgrade
path not anticipated or tested by the adjunct archive
maintainer.
It is usually safe to drill in a binary package out at the
leaf nodes from an external archive -- but these encumbered
packages have a witches brew of libraries they need as well,
and when upgrades on the main line are issued, one can end up
with an unsolvable set of dependencies for the old, and
requirements by the new.
'priorities' falls over and dies at that point from
self-induced dependency hell, and CentOS is blamed for it in
the back splatter. I was the wiki article editor who
initially added that caveat section, after seeing priorities
being pushed as the 'best' alternative.
It is not. It is more like Russian roulette without peeking
at the state of the chamber, for your installation. The
mentioned 'exclude' and 'includepkg' approach is more correct,
but also requires reading the yum and rpm man pages, and
gaining some understanding of dependencies.
-- Russ herrold
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