Facebook has something approaching this, using sleazy DNS and routing tricks to approach optimal cache locality. AFAIK, they have not yet achieved master-master

Facebook has something approaching this, using sleazy DNS and routing tricks to approach optimal cache locality. AFAIK, they have not yet achieved master-master "write anywhere" replication: all writes phone home to the master cluster. In practice for their use-case this is less of a big deal than it may appear.