How to sync cassandra and elasticsearch
One way would be to develop a capture process that runs on a schedule or in a loop and reads "new" data from Cassandra and sends it to Elasticsearch. The definition of "new" data depends on your Cassandra schema and your applications' insert/update/delete patterns.
This can be either done in real time or as the separate batch process .
As and when you get the data to the Cassandra based on the retrieval pattern /query pattern index the data to the Elastic search.
Have the real time processing layer(such as Strom)/Distributed Message broke like Kafka to sync up the data .
Or periodically query the data from Cassandra and ingest to the ES as the separate batch job.