By default, Atlas runs ElasticSearch in embedded mode as its indexing backend, which works well for proofs of concept or testing in developer environments. For production environments, Hortonworks recommends using Solr in cloud mode for HDP clusters. For further details, see http://s3.thinkaurelius.com/docs/titan/0.5.4/elasticsearch.html and http://s3.thinkaurelius.com/docs/titan/0.5.4/solr.html.