(originally from https://web.archive.org/web/20150422203246/http://ajcsystems.com/blog/page/2/)

At $work, we use Puppet to manage some of our servers, and part of that involves writing custom facts for Facter, Puppet’s tool for gathering node information. (If you’re not familiar, here’s a quick description of Facter.)

These custom facts can use information present on a given host, such as hostname or IP address, to make their determination. However, we’ve also found it useful to be able to ask a custom fact, “what answer would you give if you were run on a given host?”

The answer:

This is a simple Facter fact that takes a hostname and determines which Ganglia grid to place it in. When run as a part of Facter, it will operate as a fact:

$ facter architecture => x86_64 [...] grid => build hardwareisa => x86_64 hardwaremodel => x86_64 [...]

But when called as a standalone script, it will operate as though it were running on that host:

$ ./grid.rb build01 build

Building facts like this will let you keep any custom logic you might have in one place, but reuse it in different contexts. Code reuse FTW.

P.S.- If you’re thinking about posting a comment to the effect that Facter can be run with one or more arguments of facts to output:

$ facter architecture x86_64

you’re missing the point. This technique allows you to run Facter facts on a different host than the target and still get the benefit of the logic embedded in the fact.