What type of database is best for high number of users and high concurrency?

We are building a web-based application that needs to support large number of users in a very high concurrency environment. Users will be attempting to change the same record at the same time. In terms of data volume in the database, we expect it to be very low (we're not trying to build the next Facebook), instead we need to provide each user very quick turnaround time for each request, so from the database perspective we need a solution that scales very easily as we add more users and records.

We are currently looking at relational and object-based databases, and also distributed database systems such as Cassandra and Hypertable. We prefer the open source solutions over commercial.

We'开发者_如何学Cre just looking for some direction, we don't need details on how to build the solution. Any suggestions would be greatly appreciated.

Amazon's SimpleDB supports conditional puts and consistent reads, but at that point, you're defeating the purpose and might as well just use mysql/percona and scale out vertically.

do you really need ACID? something's gotta give. and eventual consistency isn't all that bad, right? :)