Cassandra as a primary data store for Elasticsearch
Cassandra  commitlogs, backup and restore ensure durability for Elasticseach.
Cross datacenter replication for Elasticsearch
Cassandra  asynchronous replication allow to synchronize many Elasticsearch clusters sharing the same mapping.
Realtime Search and Analytics with Cassandra
Elasticsearch  API, plugins and tools like Kibana  provide additional features to cassandra.
 Elasticsearch and Kibana are registered trademarks of Elasticsearch BV.
 Apache Cassandra, Apache, and Cassandra are trademarks of the Apache Software Foundation.