Several sites on multiple servers in 2 data-centers. How would one approach implementing unified user login?

Question

Several sites on multiple servers in 2 data-centers.

How should one approach implementing shared users database, shared way for users to login into those sites so there would be no need to keep separate users database on each server?
Is creating some web-service a way to go? Like some REST endpoint over https? or some oauth authentication is the right way?

Advice wanted:)

Answer 1

Creating Api either rest or SOAP is best way to implement unified user login i would have write this i comments but cant comment till reputation is 50