FWIW, at the end of the day ElasticSeach is just a set of Java classes.

If you’re using the JVM already and want the SQLLite of search, you can instantiate an index in your own JVM, load documents, and get compatible query features.

You can also use Lucene directly, which is simpler, but does have some nuances that differ if you want perfect query compatibility between a deployed ElasticSeach cluster and an in-memory search.