meta::hack log

Last week, I attended meta::hack, the MetaCPAN hackathon in Chicago. I'm the maintainer for CPAN Testers, the central database for CPAN users to send in test reports on CPAN distributions and one of MetaCPAN's data sources. I asked to join them so I could improve how MetaCPAN consumes CPAN Testers data, and ensure the stability and reliability of that consumption.

Here's a detailed log of what I was able to accomplish, and information on the new development of CPAN Testers.

Day 1 - Thursday

Day 1 began bright and early, Thursday morning. We all got settled in to one of ServerCentral's conference rooms, passed out extra keyboards and monitors, and got ourselves to work.

My goals for the hackathon were to build an API for MetaCPAN to use to obtain CPAN Testers release summary data: The little bits of CPAN Testers data on the left-hand side of a release page on MetaCPAN. The existing API for this had been causing us some trouble over the last year or so, and it was time to fix it once and for all.

To do this, I started documenting the CPAN Testers database in a DBIx::Class schema. This served a few purposes: I got to go through the database, understand all the data inside, look to see where the data was being generated, and document everything in an easy-to-use ORM. Now that we have this, making new APIs for the data will be extremely easy.

Day 2 - Friday

Starting with day 2, I realized that I had not written a single test for the DBIx::Class schema I had written, so I had no clue whether it worked or not. The early morning was spent fixing this oversight. Once the tests were written and the schema debugged, I released the first CPAN::Testers::Schema to CPAN.

As is the case with free software, I immediately got constructive feedback on the release: Dan Book and Karen Etheridge helped make the dist.ini file (the Dist::Zilla configuration file which manages my releases to CPAN) better for future releases. Thanks to both of them!

With the schema ready, I could finally start building the CPAN Testers release API. This API exposes the release summary data which shows how many test result passes and failures a single CPAN release has. So, if I want to know how many test results were submitted with pass or fail on my newly-released distribution, I can ask the release API. This can be helpful for CPAN author dashboards, and is used by MetaCPAN to display the results right in with the module documentation.

For this API, I picked a couple modern web technologies:

  • Mojolicious is a modern web framework for modern Perl. It's efficient and asynchronous from the ground-up, which is useful for transferring large quantities of data (like CPAN Testers).
  • OpenAPI is a JSON web API specification (similar to WSDL for XML). The API definition file describes and documents the API for easy consumption.
  • Mojolicious::Plugin::OpenAPI reads the OpenAPI specification and produces all the routing necessary for your web app. This plugin made the API a lot easier to build.

I started by writing the API specification. Unfortunately, the OpenAPI spec can be fickle, and the error messages that come out of the JSON validator difficult to discern. Debugging just the spec took me most of the evening. Then I started writing some tests for the API before we all called it a night.

Day 3 - Saturday

By Saturday I was finally able to start implementing the Release API. This involved writing a DBIx::Class::ResultSet class to make querying the release summary table easier. It's almost always better to make your model layer smarter so that the API it defines for consumers can be stable, even if the underlying data schema changes. Since there are likely some schema changes in the future of CPAN Testers, I've been making sure that the CPAN Testers schema API is as forwards-compatible as I can. This resulted in the second release of CPAN::Testers::Schema.

During my testing of the new CPAN::Testers::Schema::ResultSet::Release module, I finally stumbled across DBIx::Class::ResultClass::HashRefInflator. This automatically turns your results into hashrefs instead of objects, which makes them easier to compare with Test::More's is_deeply. After some confusion, I sent in a patch to clarify some documentation.

With the ResultSet class completed, the API itself was trivial: less than 50 lines of code makes up the API. The combination of DBIx::Class and Mojolicious::Plugin::OpenAPI is wonderful.

By midday, I had released the first version of CPAN::Testers::API. Now all that remained was figuring out how to deploy it, which was easier said than done.

I'd been previously working on a CPAN Testers deployment with Rex at the 2016 Perl QA Hackathon in Rugby. Using Rex to install perlbrew, a brand-new modern Perl 5.24, and set up a local::lib was easy. The CPAN Testers Rexfile also sets up a cpantesters user which will run all our apps with reduced privileges.

I decided that the central Rexfile in the cpantesters-deploy repository would set up machines, install the right Perl, install the right OS prereqs, and prepare the machine for working in a CPAN Testers role (with minor adjustments for the particular role). The Rexfile in the individual application (like the Rexfile in CPAN::Testers::API) would deploy the application. This way the project managers can give access to deploy individual apps to anyone, and changes made to an app can be easily deployed when completed.

The key part of any good devops strategy is being able to test the deployment. I was pleased that Vagrant made it amazingly easy to spin up a small VM to test the CPAN Testers deployment. A simple configuration in a Vagrantfile, a special "environment" in the Rexfile, and now I can test deployments over and over until they work the way I want.

Day 4 - Sunday

On Sunday, I woke up to a broken CPAN Testers: The Metabase, the highly-available destination database for incoming test reports, was filled up. Metabase presently uses Amazon SimpleDB, which has a hard limit on the size of a database. You can have up to 250 databases in an account, though, so we just need to manually switch over to a new database. This happens about once every year, and is just one of the annoyances of SimpleDB. After this was fixed, I was able to return to working on the release API.

There was a performance problem with the API: Asking for all the releases took about a minute, which is fine. Asking for a single dist's releases, or a single author's releases took less time, which was great. But asking for all the releases updated since a certain date/time took hours before I simply gave up. I tried adding some new indexes, and I tried playing around with the query, but nothing I did could improve this query's performance to acceptable levels. So, for now, querying for all releases since a certain date/time is disabled. Likely the solution is to denormalize the data just a little to add a date/time field to the release summary, but that will have to wait for another day.

So after removing that small part of the API, and fixing some other bugs, I released the second version of the CPAN::Testers::API. With that done, all I needed was an acceptable deployment strategy: The API daemon needs to start, automatically restart if stopped, log to a predictable location, and all of this needs to be deployed without root privileges.

I've used the BSD daemontools before, and this seems right up its alley: Make the original init system (in this case, systemd) run the svcscan utility as the non-root user to scan a directory that the non-root user has access to. Any services written to this directory get automatically started. The non-root user has complete control over everything without needing root privileges, and I get to avoid messing around with the system's initd (whatever that happens to be).

In the years since I've done this, some new options have appeared, like runit, s6, and nosh. Unfortunately, before I could finish my research, it was time to go. The current front-runner seems like nosh, which means the final step before unleashing this API on the world is just writing a nosh service bundle and some remote monitoring.


Altogether, this was a greatly productive hackathon. It was great to see the MetaCPAN folks, and meet a couple new ones. Thanks to my employer, ServerCentral, for giving me the time, and to all the sponsors for making this happen.

The next steps for me are documenting all the to-do items that I stumbled across or left unfinished in the CPAN Testers API issue tracker and the CPAN Testers Schema issue tracker. These to-do items will be written in the issue tracker so that anyone who wants to help contribute can do so. There is quite a lot of things to do for CPAN Testers, and every little bit can help! If you'd like to contribute, you can talk to us on #cpantesters-discuss on irc.perl.org or e-mail me at doug@preaction.me.

This hackathon wouldn't have been possible without the overwhelming support of our sponsors. Our platinum sponsors were Booking.com and cPanel. Our gold sponsors were Elastic, FastMail, and Perl Careers. Our silver sponsors were ActiveState, Perl Services, and ServerCentral. Our bronze sponsors were Vienna.pm, Easyname, and the Enlightened Perl Organisation (EPO). Please take a moment to thank them for helping our Perl community.

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user-pic I blog about Perl.