Comment by wizofaus

Surely having separate DBs all sit on the One Big Server is preferable in many cases. For cases where you really to extract large amounts of data that is derived from multiple DBs, there's no real harm in having some cross-DB joins defined in views somewhere. If there are sensible logical ways to break a monolithic service into component stand-alone services, and good business reasons to do (or it's already been designed that way), then having each talk to their own DB on a shared server should be able to scale pretty well.