In Chapter 1, Cassandra's Approach to High Availability, we discussed Cassandra's tunable consistency characteristics. For any given call, it is possible to achieve either strong consistency or eventual consistency. In the former case, we can know for certain that the copy of the data that Cassandra returns will be the latest. In the case of eventual consistency, the data returned may or may not be the latest, or there may be no data returned at all if the node is unaware of newly inserted data. Under eventual consistency, it is also possible to see deleted data if the node you're reading from has not yet received the delete request.

Depending on the read_repair_chance setting and the consistency level chosen for the read operation (more on this in the anti-entropy section later in this chapter), Cassandra might block the client and resolve the conflict immediately, or this might occur asynchronously. If data in conflict is never requested, the system...