On Sat, Dec 18, 2010 at 11:02 PM, Kevin Irwig <kevinirwig@yahoo.com> wrote:

Say I have two processes on separate machines, and a Cassandra cluster over
several machines. If the first process writes (insert) to a column while the
second process reads (get / get_slice / get_range_slices / others?) from that
column (say the consistency level is QUORUM if that makes a difference), is the
write "atomic", or could the second process get corrupt (half-written) data?
What if Cassandra is not a cluster, but runs on a single machine? I'm currently
using version 0.6.3, but the question is general.

The write is consistent if consistency level of write is QUORUM and read is QUORUM
or according to the popular equation for strong consistency if R + W > N
where N = number of replicas, R = consistency level of read, W = consistency level of write

On single machine with one node, consistency level of 1 for both R and W will make the write consistent as again 1 + 1 > 1 
Cassandra : The definitive guide - is a good resource for most questions besides the wiki and mailing list.