Array overwrite or append pattern

[Andreas Zuber]

Hello

I recently came across a module where I had the following problem:

I have some arrays with OS specific defaults which get written into a
config file via template. The user of the module may want to do one of
the following things:

1. Use the default settings provided by the module
2. Completely overwrite the array
3. Append some values to the array from a hiera_array lookup
4. Overwrite the defaults and Append some values to the array from a
hiera_array lookup

I wondered if there is an easy solution to this or an already
established pattern. The code I came up with which implements all the
requirements above looks something like this:

manifests/init.pp
class example (
$myvar = $example::params::myvar,
) inherits example::params {
notify{$myvar: }
}

manifests/params.pp
class example::params {

$myvar_defaults = [
'some',
'actual',
'defaults',
]

$myvar_real_defaults = hiera('example::myvar_defaults', $myvar_defaults)
$myvar_append = hiera_array('example::myvar_append', [])
$myvar = $myvar_real_defaults + $myvar_append
}

Here is how this works for the requirements:

1. Simply use the class
2. Completely overwrite by setting the class parameter example::myvar as
usual
3. Values set in hiera for variable example::myvar_append will be
appended to example::myvar
4. Setting example::myvar_defaults (priority lookup) will overwrite the
defaults but still append example::myvar_append

Would be great if I can get some feedback on this. Is there a simpler
solution I missed? Is this too confusing to be useful at all? Is there
an already established pattern as a solution to this problem I should
use instead?

Thanks in advance
Andreas

[jcbollinger]

If I understand you correctly, you are dividing the wanted data into two pieces, one of which has a static default and must be subject to priority lookup, and the other of which must be subject to an array-merge lookup.  This naturally maps to two different Hiera keys.  Moreover, I suppose you are targeting a master older than v4.3.0, since v4.3.0 has the resolution for PUP-5395, and that would make it pretty straightforward to implement a solution based simply on two class parameters.

With that said, I don't think your solution is too bad, but as a stylistic consideration I do think it's poor form to put much logic into params classes.  Personally, I would instead put most of it into the main class:

class example::params {
  $myvar_defaults = [
    'some',
    'actual',
    'defaults',
  ]
}

class example(
    $myvar = $example::params::myvar
  ) inherits example::params {

  $myvar_append = hiera_array('example::myvar_append', [])

  $myvar_full = $myvar + $myvar_append
}

Of course, your template would then interpolate values from $example::myvar_full instead of from $example::myvar.  That's slightly simpler code-wise, uses one fewer Hiera key, and I account it better style.  Moreover, it doesn't muddle with the natural idea of "defaults", as your code does.  All four of your use cases are supported.


John