I am looking for an eventually consistent key value data store and i decided to choose between Amazon SimpleDB and Riak ,so can anyone share their valuable experiences comparing both .
Thanks in advance Fedrick
Riak is a key-value store. The data values you store is opaque to the database, so you have no secondary indexes. But you do have the ability to run map-reduce if your data is JSON (or XML, I think). You can run map-reduce over all data, or just a subset ("seed keys"). It also has a "link walking" feature where documents can refer to other documents, which can be auto-fetched. They don't currently have an incremental map-reduce like CouchDB, which means any secondary queries (non-key) are quite expensive. They have plans to fix this.
SimpleDB is actually halfway between a docstore and a keystore: Each key->item supports multiple attributes, but it only goes one level deep. You can query on your key or your attribute values.
In production, Riak should be pretty "hands-off". If it's slow or getting full, just spin up a new server and tell it to join the cluster. (unlike CouchDB or MongoDB where you have to futz with multiple config files).
SimpleDB can take a pounding (tens of thousands of requests per second I've heard), but you are responsible for data scaling (i.e. don't violate their domain size limits or it will slow down).