share users or nodes across multiple sites without sharing database I have one master drupal site that offers basic functionality for users.(it's a shop); and we have some other sites, for example one news site, one marketplace, one job finding service etc. I want every "spin off" site be a separate site, but users sign up once, then login on each subsite without needing to sign up again. and I want to show nodes of subsites at master site in the user profile. which is the best way of sharing users and contents, since domain access module is not a good option, because we are going to produce lots of data and keeping all data in one database is hard. we want to be able to separate any high traffic spin off in the future, but with domain access, all should be at one database.
what options do I have for sharing users and contents without sharing database?


what I mean is someting like stackexchange that there is no need to sign up anytime. is it possible to do that with OpenID or OAUTH etc?
I tried almost every possible scenarios, 

The best thing for me was use of Oauth2 server module for master site to act as authentication server and OpenID Connect module for other sites which use out-of-box authentication.

This solution can be done without needing to merge databases, or host sites on same server, or periodic synchronization. the only challenge I have is that authentication server must use https, and I'm going to buy certificate for my host.