machines.  In fact, read-only database servers can
  be combined relatively easily too.  Unfortunately, most database
  servers have a read/write mix of requests, and read/write servers
  are much harder to combine.  This is because though read-only
  data needs to be placed on each server only once, a write to any
  server has to be propagated to all servers so that future read
  requests to those servers return consistent results.
 </P
><P
>  This synchronization problem is the fundamental difficulty for
