Has anyone tried to query a KDB file using Java standalone program ? Just curiou

What is your use case?

Nothing as of now. Just doing some RND.

Appian offers an API that allows you to interface with your environment through Java. The API can be invoked in different ways: a smart service, a custom function or a servlet custom plug-in.

Those are not the only integration points Appian offers. There's another one called Web API forum.appian.com/.../Web_APIs.html. This would be one of the easiest way of calling Appian from the outside because it allows your external source to retrieve data from Appian via HTTP Request using HTTP basic authentication and the retrieval is expression based which means you could have your expression be a custom function that uses the API to retrieve the data.

In terms of the Web API, your external Java code will be doing a very basic HTTP Request using HTTP basic authentication and the one using the API would be your actual custom function.