[CASSANDRA-8346] Paxos operation can use stale data during multiple range movements - ASF JIRA

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Details

Type: Bug
Status: Resolved
Priority: Normal
Resolution: Fixed
Fix Version/s: 2.0.12
Component/s: None
Labels:
None

Severity:
Normal

Description

Paxos operations correctly account for pending ranges for all operation pertaining to the Paxos state, but those pending ranges are not taken into account when reading the data to check for the conditions or during a serial read. It's thus possible to break the LWT guarantees by reading a stale value. This require 2 node movements (on the same token range) to be a problem though.

Basically, we have RF replicas + P pending nodes. For the Paxos prepare/propose phases, the number of required participants (the "Paxos QUORUM") is (RF + P + 1) / 2 (SP.getPaxosParticipants), but the read done to check conditions or for serial reads is done at a "normal" QUORUM (or LOCAL_QUORUM), and so a weaker (RF + 1) / 2. We have a problem if it's possible that said read can read only from nodes that were not part of the paxos participants, and so we have a problem if:

"normal quorum" == (RF + 1) / 2 <= (RF + P) - ((RF + P + 1) / 2) == "participants considered - blocked for"

We're good if P = 0 or P = 1 since this inequality gives us respectively RF + 1 <= RF - 1 and RF + 1 <= RF, both of which are impossible. But at P = 2 (2 pending nodes), this inequality is equivalent to RF <= RF and so we might read stale data.