How to enable cache on elasticsearch?

My cluster is using all my cpu and memory is very low always, the query latency are very high and most of the time elasticsearch drop the client connections because it can't handle more connections.

I really don't know more what to do. Anyone can help?

Add more resources!

How many nodes do you have in the cluster? Caching is used and handled by default, so you should probably add more nodes or see if you can optimize the really expensive queries.

If more nodes are not possible, perhaps changes to the application architecture is in place. It's hard to do any general recommendations without more information.