I meant that you probably have an application which is writing data to Cassandra.
In this application, while you send data to Cassandra, send also data to elasticsearch.
I explained something similar but for RDBMS at https://david.pilato.fr/blog/2015-05-09-advanced-search-for-your-legacy-application/.
To me you can apply the same concept to any other datastore and Elasticsearch.
I'm not sure if there are projects to synchronize from Cassandra to elasticsearch.
You can look at using Logstash JDBC input with a Cassandra JDBC driver as someone explained at:
But it's more a batch which run every X minutes so not really real time. Might fit your needs though.