Sign Up/Login system for multi-domain/multi-server

Question

I'm thinking about a good solution for implementing a sign up/login system that works across different domains and servers. A working example is Olx (you can register in one domain, and your login will work on the rest of domains).

The scenario is that every domain (one per country) has its own database. And there will be 2 servers (for example), each one will have the 50% of the domains (and so the 50% of databases).

What would you suggest to start with?

Answer 1

I would make one central login service (e.g. http://login.domain.com) with it's own database. This way all login data will be on one place.

The other sites would query this service with login credentials to authorize the user. (Similart to OpenID here on SO)