Here's a number of articles on integrating external and remote data into Drupal, especially via Apache Solr.
- Joe Turgeon: Exposing External Content to Drupal’s Search (15 January 2014)
- Colan Schwartz: Integrating remote data into Drupal 7 and exposing it to Views (16 February 2015)
- Larry Garfield: Remote Data in Drupal: Museums and the Web 2009 (21 April 2009)
- Florian Loretan: Remote entities in Drupal 7 (25 October 2012)