Why cassandra is considered as partition tolerant by CAP theorem despite we can isolate the coordinator?

Question

Here is the definition of partition tolerance by Gilbert and Lynch

When a network is partitioned, all messages sent from nodes in one component of the partition to nodes in another component are lost.

Let's divide the cluster into two partitions: the first one contains only the coordinator, the second one contains all other nodes. This way coordinator will not be able to contact any replicas and will respond with error. Is it allowed for partition tolerant systems?

Answer 1

More specifically I think the question is which of the other two CAP attributes does Cassandra retain in the face of such a Partition.

The answer is dependent on the configured consistency level. For writes there is the ANY consistency level. At this consistency level, so long as hinted-handoffs are enabled, the coordinator will record the write and maintain Availability. Clients connected to other coordinators will not be able to see the udpated value until the partition is resolved, so reads will not be Consistent. If a stronger consistency level is chosen, then the client is explicitly configuring Consistency over Availability.

So can Cassandra (given that it does not necessarily replicate all data to all nodes) be considered AP when a read coordinator is alone in a partition? If it responds with an error that sounds like Consistency to me, if it responds with an empty result set because the data is not in its partition, then that would be Availability. Since the weakest read consistency level is ONE - requiring at least one replica to respond, Cassandra opts for the former: If the coordinator is not itself one of the replicas owning the requested data then the read will time out and not be Available. As with writes, any stronger read consistency level explicitly configures Cassandra to behave more Consistently at the expense of Availability.

Answer 2

So the "coordinator" node isn't a long-lasting or "leader"-like definition. It changes with practically every query. If there was a non-token-aware operation which needed a coordinator node, and that coordinator was suddenly partitioned-off from the rest, then that one query would fail.

The next query (or a retry) would pick a new node as a coordinator. The only issue, would be that some data rows will be short by one replica (data stored on the partitioned node). But as long as you're querying by ONE and have a RF >= 2, the cluster will continue on like nothing happened.

So "yes," Cassandra is definitely partition-tolerant.

Note: This is why it's important to use a token-aware load balancing policy. That way the driver picks one of the nodes containing the required data as the "coordinator." At consistency ONE, the operation is completed locally, and a network hop is taken out of the equation.