I found another gotcha when embedding MCollective in a larger application: Ruby’s YAML library situation. Historically Ruby has used the Syck library, but since 1.9.2 libyaml is used if available, through a set of bindings called Psych.
While Syck is still available and may be selected at runtime, the default is chosen at compile time, based on the presence or not of libyaml - if it’s there when you build Ruby, you’ll get Psych by default.
To find out which binding your Ruby uses, inspect YAML::ENGINE.yamler:
This wouldn’t be a problem unless there were differences, and although it’s fair to say that Psych is more compliant with the YAML spec, it’s less tolerant of slight deviations - some of which Syck is guilty of.
Coming back to MCollective, this means that an MCollective running on Ruby 1.9.2 with Psych won’t necessarily be able to communicate with other MCollective installations running on Ruby 1.8 with Syck. Mostly this seems to be down to handling of whitespace in multiline YAML values, which MCollective makes heavy use of since it nests one YAML document in another.
The initial fix in my environment was to rebuild Ruby 1.9.2 without libyaml, so Syck is used everywhere. Beyond that, upgrading everything to the same Ruby 1.9.2 build appears to be the best long term answer.