Alfresco by default uses Apache Lucene implementing a complete in-trasaction search engine.
Lucene indexes are stored in the alf_data directory, but if you need, you can configure Alfresco to store indexes in a different folder.
You can configure Alfresco, from version 4, to use Apache Solr as an optional way to manage search indexes, but notice that in this case you don't have an in-transaction management of indexes. This is a good way to scale out and to allow Alfresco to only manage contents and the external search engine manages indexes.
So now, ElasticSearch is not supported.
Hope this helps.
