How can I create hierarchical facets for data managed outsider of Drupal?

This depends largely on the backend and the field you are working with.

Definitely need more information,
Chris

Hi Dan.

Thanks for the details.

So the short answer is that if the field had data indexed in a hierarchical manner, i.e. it is a multi-valued field that indexes the item and it's parents, you are half way there. The next thing you have to do is implement hook_factapi_facet_info() to define the facet, paying special attention to the "hierarchy callback" key. This allows you to to define a callback that maps the parents and children.

For some examples, check out the facetapi_facetapi_facet_info() function for defining facets and facetapi_get_taxonomy_hierarchy() for building the hierarchy callback.

Hope this helps,
Chris