Would be cool to have ES as Lua plugin for KONG [1]

[1] https://github.com/mashape/kong

Is there any benefit to using Lua as opposed to Java or Ruby as a client for Elasticsearch (assuming one isn't using Lua already in their code)

If you're free to choose how to interact with ES you should use the native Java API [1]. All others are wrappers for the http client where the http client is a wrapper for the native Java client API.

[1] https://www.elastic.co/guide/en/elasticsearch/client/java-ap...

Lua is commonly embedded in other programs, like nginx (via ngx_lua) or haproxy (starting in 1.6). This would let you interacti with elasticsearch from them.

Lua is faster than both

much faster than Ruby