Кассандра против Amazon SimpleDB

SimpleDB can only scale by sharding, has 10 GB data size limit per table, and query performance is parallel to record count (eg: poor if you have 1 million records). And google's datastore is slower than simpledb. Cassandra is much more scalable, high traffic sites began to use it, there is nothing better for free if you need high write rates with massive data. cassandra survey

If your read/write ratio is something like %90 for read and %10 for write, then terracotta or infinispan with postgres is a better fit. There some free clustering options for postgresql but none of them matured (mostly prototypes).

Another option is sharding. Hiberntae and NHibernate has sharding support. You can use them with postgres or mysql but you loose joins.

Regards