The language-puppet library is a complete rewrite of the puppetmaster component present in the Puppet configuration management system. It is not yet feature complete, but will let you do things that can’t be done with the original toolset, such as:

• Generating full catalogs locally. This is the most important feature, as it lets you test all parts of your manifests before you git push them and apply them on a host.
• Computing the differences between two versions of the manifests for a single host, or the differences between hosts. This also can be done between Puppet masters, and comes handy when migrating hosts between Puppet masters, upgrading modules or just during development, to check that you didn’t add more (or less) than expected.
• Displaying the output of the template function, so that you can test your complex Erb templates.

All of these points are made possible and convenient thanks to the following features:

• Speed. This is about ten times faster than Puppet on my computer. This means you don’t have to wait for tens of seconds for puppet --test --noop to finish.
• PuppetDB access. Facts and exported resources can now be retrieved from PuppetDB. This means that although you are testing the catalogs on your workstation, they are computed with the facts of the real host, and you can access all that is in PuppetDB (such as exported resources).
• It’s a library ! You can use it to code your own ideas : complex tests, Puppet-master replacement, manifests compilers …

The project, while used daily, is still in an alpha stage. Error messages might be unclear, behaviour might not always be Puppet-perfect, documentation might be incomplete and some important features might be missing. This should still result in a huge productivity increase, especially when developping complex templates or playing with exported resources.

To get an idea of how this works, take a look at this post.