Grokbase Groups Perl qa May 2011
FAQ

On Tue, May 24, 2011 at 10:19:49PM -0700, Jeffrey Thalhammer wrote:

It's easy for me to get carried away with all the intricacies of dependency management. But for my task at hand, I think it might be the wrong problem. So for the sake of this discussion, let's put aside the issue of how a private CPAN (really, the 02packages) could be constructed to have a magical mixture of modules. Instead, I want to think about how a private CPAN mirror fits into the development cycle. This might be easiest by walking through a hypothetical scenario...

Let's suppose I wan to write a new application in Perl. By "application", I really just mean some set of modules and scripts. And let's assume that my development, testing, and deployment environments all have a perfectly clean version of perl -- no modules have been added or updated from the core.

I want my app to leverage the CPAN tool chain, so I setup a CPAN-style distribution for my source code. It has lib/, bin/, and t/ directories, and all the usual trappings. I'm also going to use Module::Build. And of course, all this is in some source control system (assume svn for now).

After a couple of Mountain-Dews, I have some code and tests and everything passes. Commit. Now I want to add some feature that requires a module from CPAN. So I go shopping on search.cpan.org and find one I like called Frobulator::Simple...

So instead, let's say I add Frobulator::Simple to the "requires" list (or "build_requires", or whatever) in my Build.PL. Now I run the "installdeps" target, which fetches the latest Frobulator::Simple and all its dependencies from a public CPAN and installs them.

Now this is where it gets interesting. Rather than install in site_perl, it installs into a directory right inside my project (let's call it "dlib" for "dependent libraries"). At the same time, it stashes the tar balls in a directory structure suitable for a CPAN mirror, ...
FWIW, my solution (which I haven't tried) would be:

* make an up-to-date mirror of the most recent versions of everything on
the CPAN, using CPAN::Mini;
* use CPAN::Mini::Inject to add your own modules to the mirror.

To keep your mirror up to date with new releases to the CPAN, use the
funky fast update mechanism to get change notifications, and, for each
new distribution:
* download it;
* inject it into a "staging" CPAN;
* try to test your application, using a clean perl and the staging
CPAN to resolve dependencies;
* if it passes, inject the new distribution into your "real" CPAN;
* if it fails, remove it from the staging CPAN

Optionally, if adding a distribution to the CPAN makes your app fail its
tests, put it into a third CPAN mirror and notify your devs that it's
there and that they should fix the app and then re-inject the
distribution into your CPAN mirror. However, I can see problems
here. First, they won't do it because they have more important* things
to do such as adding features or fixing their own bugs; second, you
should probably address those failures in order, meaning that that work
stream will eventually get blocked entirely when something like Catalyst
has an incompatible change that completely fucks your application over;
third, what do you do about dists which fail their own tests but don't
make your app fail its tests?

* to management

--
David Cantrell | Hero of the Information Age

"IMO, the primary historical significance of Unix is that it marks the
time in computer history where CPUs became so cheap that it was possible
to build an operating system without adult supervision."
-- Russ Holsclaw in a.f.c

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