Realtime Search: Solr vs Elasticsearch by Ryan Sonnek.
Comparison of Solr and Elasticsearch for realtime searching.
Where “realtime” means you are updating the index while performing searches.
I’m not convinced that “realtime” requirements are any more common than those of “BigData.” They do exist and when they do, use the appropriate solution. On the other hand, don’t plan or build for “realtime” or “BigData” unless those are your requirements.