any way to use rest API rather than json/or datasource way

Sabby,

No, REST is not supported. You must load the whole JSON to start reporting. If you want to reduce the size of data to load please look at this article https://www.flexmonster.com/blog/blog-perform-faster/

Thanks,
  Dmytro

Hello Virenda,
 
Thank you for the question.
Currently, you can pass data to Flexmonster from your server in JSON / CSV / OCSV formats.
But the dataset should be sent entirely. If you mean preprocessing/aggregating the data, due to the nature of these formats it can be tricky. But still, it works for some cases.
 
Another option is to use Elasticsearch.
Recently we have released a beta version of Flexmonster that supports it.
It doesn't require loading entire dataset and requests data by composing exact queries.
I think you can try to connect your data source to Elasticsearch and avoid writing complex processing scripts.
Please find more details in the blog post.
 
Hope it helps.
 
Regards,
Ian