As mentioned in yesterday’s standup blog, my pair and I encountered some problems with YAML parsing over the last few days, and now that I think I understand it I wanted to document it for posterity.

Psych is a new YAML parser which presumably is better than what came before it, but can’t merge hash keys correctly and doesn’t work with delayed job. The merging of hash keys is serious, as our standard databse.yml defines a common section, and back references it merging in individual database name and settings. When psych is loaded, we get a blank database name, which makes active record pretty much useless.

Ruby 1.9.2 optionally compiles psych into ruby if you have libyaml installed on the computer. Some gems will require psych if it’s available, thus poisoning any future YAML parsing which does not expect psych’s pedantic (and currently broken) behavior.

You can always look at the value of the YAML constant – it can varry between YAML, Syck and Psych, depending on what’s loaded. You can switch the yamler by adding YAML::ENGINE.yamler = ‘syck’, but you need to make sure this happens at every code entry point.

For now, I’m no longer install libyaml and libyaml-devel on servers, which got there because I had been following RVM’s information ruby prerequisites. I’ll also write a chef recipe to assert that libyaml is not installed.