Can I use 7.x elasticsearch client library "org.elasticsearch.client:elasticsearch-rest-client" with 6.x elasticsearch server

Hi,

Can I use 7.x elasticsearch client library "org.elasticsearch.client:elasticsearch-rest-client" with 6.x elasticsearch server. Is there backward compatibility in client library ?

I believe the recommendation is the other way around.
Using newer server than the REST library.
Upgrade your database before upgrading the REST library.

There are also several breaking changes you need to be aware of.
The biggest one is the removal of "_type". So REST 7.x calls don't allow you to specify _type; which will not work for ES 6.x.