Hello! first of all congratulate you for erpnext. I’d like to use ERPNext as frontend, but doing some processes in the backend with another program developed with another languaje, but accesing ERPNext database.
Of cource custom app in ERPNext has to be opensource, but my backend app should? or only if I want?
THX!!!
Good job!!!
IANAL.
I don’t think what you connect with ERPNext needs to be open source.
If that becomes true then what happens to proprietary integrations like dropbox, google, shopify etc?
yes you are true, then Dropbox should be open source 
THX!!!