Can I use hiera with a YAML backend to combine arrays?

Question

I'm using hiera with a YAML backend to manage my Puppet configuration and I'd like to append some values to an array.

I have a configuration file which looks a bit like this:

---
some_config:
  - one
  - two
  - three

some_more_config:
  - one
  - two
  - three
  - four

Where some_more_config is always a superset of some_config.

I'd like to improve this file to remove the duplication but I haven't figured out whether it's possible or what the syntax would be:

---
some_config:
  - one
  - two
  - three

some_more_config:
  - "%{::some_config}"
  - four

In words rather than code, some_more_config is the entire contents of some_config plus one additional value.

Answer 1

I don't think you can do that in Hiera because it can only interpolate string-based information (numbers are converted to strings) and not arrays or hashes.

From the Hiera 3: Interpolation Tokens, Variables, and Lookup Functions:

Hiera can interpolate values of any of Puppet’s data types, but the value will be converted to a string.

You can still do this via the Puppet DSL though. Here are a few options:

1. The Hiera 3: Lookup Types documentation covers this in detail, but if these were in multiple levels of the hierarchy you could merge them together using the hiera_array() method. Given how your hierarchy is set up, some hosts could get the smaller list, and some hosts could get the larger list.

2. The puppetlabs-stdlib module can also help. It has concat and union methods for arrays.