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class foo(
  $param1 = bigtop_lookup('foo::param1','default-val'),
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jcbollinger says:

A level of indirection would be useful if you want to minimize manifest modifications in the event that you switch data binding mechanisms. Is that a likely eventuality? Hiera can be used with Puppet as old as v2.6, and it doesn't look likely to be pulled from any foreseeable future Puppet. On the cost side, an indirection layer either provides a least-common-denominator feature set (notably, hiera's hiera_array(), hiera_hash(), and hiera_include() functions are not covered by the example), or else ties the indirection layer tightly enough to a particular data provider that switching providers is difficult or even impossible in practice. (extlookup(), for example, has no associated analog of the additional hiera functions I just mentioned, and can supply only string values). Much depends on the nature of the data binding features you want/need to use. As a special case, if you end up relying on Puppet 3's automatic class parameter binding, then you might as well embrace Hiera all the way.

What is the best tool for modeling the data that characterize the configuration to be deployed

It seems like class parameters is an obvious choice here

jcbollinger says:

You are supposing that these will be modeled as class parameters in the first place. Certainly they are data that characterize the configuration to be deployed, and it is possible to model them as class parameters, but that is not the only – and maybe not the best – alternative available to you. Class parametrization is a protocol for declaring and documenting that data on which a class relies, and it enables mechanisms for obtaining that data that non-parametrized classes cannot use, but the same configuration objectives can be accomplished without them.

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