[Rails] question about host web based application using rails

[Carl Youngblood]

Joe Van Dyk wrote:

>Why do you think you need multiple databases?
>
I know this is somewhat late, but I have some experience with a massive 
multiple database system where each user's information had the potential 
of residing on a separate database.  Our load was so intense that it was 
ridiculously expensive to "scale up" to a larger big-iron database 
server.  Instead we devised a "scale-out" load-balancing scheme that 
used a user's username to determine which database their data was on and 
connected to that for each request.  user-db mappings were stored in 
memory in a dynamic trie structure that could be accessed with custom 
PHP functions.  Tables holding infrequently-updated common data were 
duplicated on each host and synchronized when necessary.  Transferring 
user data to a different server when load got too high had too be 
accomplished programmatically, since we had inter-server transactions in 
MySQL.  In short, it was complicated, but it did work and it saved us a 
lot of money.  This was for a web analytics product where our servers 
got a hit every time someone accessed our clients' web sites, one of 
which was eBay, for example.  That may give you some notion of the 
magnitude of the problem.  So for some specialized cases, a multiple 
database strategy can be very useful.

Carl