This is a YAML+ERB backend for hiera. It works like the regular YAML backend but allows you to also template the YAML file using ERB syntax on the entire fact scope.

This can be used to prevent a sprawl of hiera hierarchy levels that are only used once or twice. Can also be used to reduce data duplication where several different fact values should evaluate to the same value.

However you might not want to put too much code back into your data once you have actually separated your data and code, but up to you :)

Examples:

Match on two different fact values instead of repeating that value in two different YAML files:

<% if ["Linux", "Darwin"].include? @kernel -%>
puppet::version: '3.7.3'
<% else -%>
puppet::version: '3.7.2'
<% end %>

Use a hiera value for a range of fact values

<% if @processorcount.between?(4,8) -%>
apache::max_keepalive_requests: 200
<% end -%>

Calculate a value for a hiera key:

puppetdb::max_treads: <%= [@processorcount - 1,1].max %>