As long as each server hits the same database, I don’t think it would be much of a problem. Load balancing the DB would be the tricky part.
The devil is always in the details [that I left out]. Ideally there would be multiple databases with a Master that the admin and interaction would write to, and the others would be read only replicated versions.
I am guessing that the best way to go is a single db with all the web servers pointing to it.
A master/slave configuration would be exceedingly tricky. It would require code changes. Lots of them. WordPress doesn’t draw any distinction between databases, it has one connection that it uses to read from and write to. So it’s not going to be able to write to the master only while reading from the slaves.
I think MySQL can do circular replication though, where each server can be both master and slave and they replicate both ways. For more than 2 servers, you’d set them up in a circular fashion to achieve this. There may be some locking issues though, but these would be rare. This would work with multiple WordPress/PHP webservers.
A MySQL cluster might be fine. Not certain exactly how the cluster architecture works.
