Which persistent in-memory database to use?
bsaunder: You threw me off with asking about a "persistent in-memory" database...  kinda like an "invisible multi-color" car...I get what you are saying, seems like you want a write-through cache.  Sure seems that you are in the right ball park for a solution, but may have to write a bit of application code layered to do what you want with those parts.It would probably be helpful if you broke up the read/writes estimates.  Also, are those writes mostly inserts or updates?   There are different types of solutions depending one what you expect your profile to be like.Also are you reads just a key look up or are you doing some sort of aggregation/searching across all records?Seems like you may be prematurely optimizing your solution and underestimating the capabilities of database engines.  Most of them can probably handle the load you are thinking with correct usage and sufficient tuning and hardware.  For example, I believe twitter uses MySQL:http://highscalability.com/scaling-twitter-making-twitter-10...