Elasticsearch vs Cassandra vs Elasticsearch with Cassandra

Question

One of our applications uses data that is stored into both Cassandra and ElasticSearch. We use Cassandra to access those records whenever we can, and have data duplicated into query tables designed to adhere to specific application-side requests. For a more liberal search than our query tables can allow, ElasticSearch performs that functionality nicely.

We have asked that same question (of ourselves)..."Why don't we just get everything from ElastsicSearch?"

The answer is that ElasticSearch was designed to be a search engine, and not a persistent data store. Sometimes ElasticSearch loses writes. Schema changes are difficult to do in ElasticSearch without blowing everything away and reloading. For that purpose, I have written jobs that are designed to keep ElasticSearch in-sync with our Cassandra cluster. There was also a fairly recent discussion on Quora about this topic, that yielded similar points.

That being said, ElasticSearch works great as a search engine. And Cassandra works great as a scalable, high-performance datastore. But querying data is different from searching for data. There are times that we need one or the other, and a combination of the two works well for our application. It may (or it may not) work well for yours.

As for analytics, I have had some success in using the Cassandra Spark connector, to serve more complex OLAP queries. Hope that helps.

Edit 20200421

I've written a newer answer to a similar question:

ElasticSearch vs. ElasticSearch+Cassandra

Cassandra + Lucene is a great option. There are different initiatives for this issue, for example:

• Stratio’s Cassandra Lucene Index - Derived from Stratio Cassandra, is a plugin for Apache Cassandra that extends its index functionality. (https://github.com/Stratio/cassandra-lucene-index)
• Stratio Cassandra, it's a native integration with Apache Lucene, it is very interesting. (https://github.com/Stratio/stratio-cassandra) - THIS PROJECT HAS BEEN DISCONTINUED IN FAVOUR OF Stratio’s Cassandra Lucene Index
• Tuplejump Calliope, it's like Stratio Cassandra, but it's less active. (https://github.com/tuplejump/stargate-core)
• DSE Search by Datastax. It allows using Cassandra with Apache Solr, but it's a proprietary option.(http://www.datastax.com/what-we-offer/products-services/datastax-enterprise)

After working on this problem myself I have realized that NoSQL databases like casandra are good when you want to make sure you are preserving your data schema with reliable writing operation, and don't want to take advantage of indexing operations that elasticsearch offers. In case you want to preserve some indexes data then elasticsearch is good in case you are trusting your scheme and only going to do far more reads than writes.

My case was data analytics. So I preserved a lot of my Latices in elastic search since later I wanted to traverse through the data a lot to see what should be my next step. I would have used casandra if I wanted to have a lot of changes in the schema of the data in my analytic pilelines.

Also there are many nice representing tools like kibana that you can use to present your data with some good graphics. Maybe I am lazy but they are very good looking and they helped me.

Storing data in a combination of Cassandra and ElasticSearch gives you most functionality. It allows you to lookup key-value tables, and also allows you to search data in indexes.

The combination gives you a lot of flexibility, ideal for your application.

Elassandra is the combined solution of Cassandra + Elastic search , It uses Elastic search to index the data and Cassandra as the data store , i'm not sure about the performance but as per this article , its performance is good.
If your application needs search feature then , Elassandra is the best open source option. DSE search is available but its expensive.