It mentions that 6.x high level rest java client will work with cluster running 7.x, as forward compatibility is guaranteed. However, I want to double confirm that if, for future release, will 7.x high level rest java client work for cluster running 8.x?
We are doing upgrade from 6.x to 7.x, and will need to do a shadow testing on the cluster running 7.x, while keeping 6.x client. We confirmed that 6.x client works for 7.x cluster, but not sure about future releases.
Broadly, yes, a 6.x REST client will work against a 7.x cluster as long as you are not using any deprecated features. The simplest way to tell this is to run your 6.x client against the latest 6.x release, since this will have the most up-to-date deprecation logging to warn you about any deprecated features you're using. If your usage is deprecation-clean according to the latest 6.x release then it should continue to work against a 7.x cluster. The same will be true of a 7.x client running against an 8.x cluster.