Prior to Cassandra 1.1, it was possible to see partial updates in a row when one user was updating the row while another user was reading that same row. For example, if one user was writing a row with two thousand columns, another user could potentially read that same row and see some of the columns, but not all of them if the write was still in progress.
Full row-level isolation is now in place so that writes to a row are isolated to the client performing the write and are not visible to any other user until they are complete.
From a transactional ACID (atomic, consistent, isolated, durable) standpoint, this enhancement now gives Cassandra transactional AID support. A write is isolated at the row-level in the storage engine.