I'm investigating how to create a scalable drupal system where nodes can be shared across different sites. In order to support future growth, it is likely we will need to "plug in" more servers/sites into the network.
I was thinking that this would be best achieved with a central site for storing the master records, users, and permissions, and then the slave sites would hold copies of these where relevant. Not all sites will need all the nodes, only those that are linked.
Alternatively, the central site could store which site has the master record. That might make it easier to grow. Thinking of node id's, the end solution needs to cater for the fact that these numbers could easily hit the millions.
It may be a crazy idea and maybe Drupal is not the right platform for this too.
UPDATE: The concept is similar to that of sharding and map reduce (my basic understanding that it), yet my biggest concern are the node id's. If the system hits the number of users/groups/nodes we are looking at, we could hit the nid int limit and experience performance issues well before that point too. So a distributed model seems to be the best direction.