Modules that will allow search for external data using either Apache Solr or Elastic Search
You need Search API with either of the plugin modules. Search API is a fantastic way to abstract out the engine from the logic. Although I've only use the Solr implementation I've spoken with several developers who have switched to Elastic.


https://www.drupal.org/project/search_api
https://www.drupal.org/project/search_api_solr
https://www.drupal.org/project/search_api_elasticsearch


Search API also works with Facet API (which supports the ApacheSolr module too -- the 'competitor' to Search API).
https://www.drupal.org/project/facetapi

In Drupal 8 those two modules (Search API and ApacheSolr) have become one.