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It's Only a Model!

Fake CPAN - 2011-12-22

Testing the CPAN

Ages ago, Randal Schwarz wrote the minicpan script to produce a minimal, local mirror of the CPAN. With his gracious permission, I converted it to a CPAN distribution so I could install it as easily as anything else. The problem I had was that I really liked writing tests, and writing tests for minicpan was going to be a big pain.

I threw together bunch of pretend CPAN distribution files, all empty tarballs, and wrote an 02packages file by hand, and then realized I'd need to put the thing onto a web server, because minicpan works over HTTP. I'd have to update the tarballs by hand, as well as the index, if I needed to test for new things. The whole thing just started to seem like a huge pain, so testing for the CPAN-Mini distribution devolved to the two lamest (but better than nothing) test methods often used: I ran it by hand before releasing and I shipped a test to make sure that the modules all compiled. Ugh.

Let's take a quick moment to explain how the CPAN works, on a very basic level, and greatly simplified. There are two crucial parts to a CPAN archive (and many more parts that I will happily ignore, here). First, there is a big pile of files, mostly gzipped tar archives, organized into directories by uploader. Almost to a one, these files are "distributions" with a Makefile.PL or Build.PL and a bunch of code and tests. Secondly, there is 02packages.details.txt.gz, which is a very simple list of what Perl packages have been indexed as official parts of the CPAN, and in what archive they can be found. There's an indexer that takes care of updating this file when new distributions are uploaded, and a permissions system, and a whois system, but only the repository of tarballs and the index of packages are relevant for things like the CPAN shell, cpanminus, or minicpan.

Part of the problem with testing against the CPAN is that tests usually want a fixed set of data so they can have a fixed set of expectations. The CPAN is changing all the time. Taking a snapshot is easy, but snapshots are huge! My heavily pruned snapshot is about 1.7 gigs. So, you could prune things down much, much more, but there still might be indexing glitches, affecting what you thought the results should be. Worse, you might care about the contents of distributions, and you might craft those contents yourself, which might lead to things like CPAN::Test::Dummy::Perl5::Build, one of many modules that exist only to serve as test data for live CPAN testing.

These problems, and more, all arose when we decided, at work, to build a CPAN-like system for managing our deployment of Perl code. We needed to test a lot of its tools against simple distributions, but we didn't want to have to upload them, or even make them. We wanted to make a list of distribution names that we wanted, and then have them show up in directories, and then have an 02package file built for us.

I worked on it then, and at several of the annual Perl QA Hackathons, and this spring I delivered the first pass at the final product: Fake CPAN. Before I explain what that is, I'll explain the tools that are under the hood.


The first thing I wanted was a way to quickly build a really boring CPAN distribution from as little input as possible. With that, I could test things like extracting it, running its tests, installing it, and even scanning it to find its author, provided packages, and so on. I sat down and banged out Module::Faker – although it was called ExtUtils::FakeFaker until Schwern begged me not to put it under ExtUtils.

Module::Faker would take a file that looks like a META.yml or META.json file and build the dist that could contain such a file. For example:



name: Email-Infinite
abstract: please make it stop
  cpan_author: RJBS


Given this input YAML, Module::Faker will build a distribution with lib/Email/Infinite.pm, with the right Pod in it, and a package declaration, and a version number. It will add a real META.yml and a Makefile.PL and a bunch of other stuff. The X_Module_Faker key provides some extra information, like who should be listed as the author. There are a bunch of other options that I can specify, too, like extra modules to build, extra files to add, version numbers, and so on.

Module::Faker is, in a way, the Dist::Zilla of bogus code. (There are some who would suggest that Dist::Zilla is the Dist::Zilla of bogus code.)

Later, I went back and added an ever terser way to specify distributions to fake up. These days, you can just create a file with the right sort of name, like: RJBS_Another-Dist-1.24.tar.gz.dist

It will create Another::Dist with version 1.24 and all the required cruft to go along with it, attribute it to me (RJBS), and package it up into a tarball.

Module::Faker was already really useful for testing things like the PAUSE indexer or simple analytical tools. What I wanted, though, was a way to test something that operated on a bunch of distributions in aggregate, with an index file.


CPAN::Faker builds a bunch of distributions, with an index file. (Who saw that coming?) It uses Module::Faker to build a bunch of fake distributions and then builds an index for them. It's pretty darn simple to use, to. First you drop a bunch of Module::Faker source files (like the YAML or .dist files above) into a directory. Then you run:


$ cpanfaker --source ./source-dir --dest ./dest-dir

It will build every dist, put it under the hashed author dir like authors/id/R/RJ/RJBS, and then build an index, listing the highest-versioned, first-indexed source for each package. It also creates a few of the other files things need, like CHECKSUMS and so on. The contents of dest-dir, after cpanfaker runs, will be suitable for use by a CPAN client like cpan or minicpan or cpanm¹.


With CPAN::Faker working, I was finally able to do a whole lot of testing. I could easily produce a tiny but CPAN-like hierarchy. For testing CPAN-Mini, what I needed next was to put the whole thing on a web server.

It seemed like a real waste to just put it on some personal server, though, for use only by me when testing my stuff. I wanted to make it a commodity dataset that anybody could use. Once I started to think about it that way, it seemed like I might want a lot of different kinds of these archives. minicpan, after all, would never really open the tarballs up. Other tools might, though, like cpan or PAUSE. Not only would having some content matter, but having interesting content would matter. After all, CPAN::Test::Dummy::Perl5::Build isn't just there for fun: it has characteristics that make it useful as a test.

Some of these might get pretty big, so having a bunch of fake CPANs seemed like a good way to segment things into different groups. Then, once these became popular, people's tests might be broken if the exact contents of the fake changed. For example, imagine that a fake CPAN was built populated entirely by dists with edge-case installer behaviors. CPAN client authors could use this fake to test that their code handled things correctly. When a new weird behavior was identified, it could be added to the fake for everyone to test.

The problem is that anyone using the data for testing would suddenly start seeing failures. Rather than require that users bundle the whole fake, we can provide every version of every fake. minicpan has a fake called "minicpan." The latest version is 1.002, and you can find its contents at http://fakecpan.org/fake/minicpan/1.002/cpan/

When new dists are added to it, or when its package index file is changed, or whenever it has to change in any way, that can go into http://fakecpan.org/fake/minicpan/1.003/cpan/. Tests sent out into the wild can always run against the 1.002 URL, and tests run by the developer can always target the latest version by using a URL like http://fakecpan.org/fake/minicpan/latest/cpan/

Fake CPAN is currently being used for testing CPAN-Mini, PAUSE, MetaCPAN, CPAN Grep, and other tools.


  1. To use a fake CPAN with minicpan, you'll need to use the --mirror and --mirror-only switches.

See Also