Sylvain Lebresne created CASSANDRA-8346:
-------------------------------------------
Summary: Paxos operation can use stale data during multiple range
movements
Key: CASSANDRA-8346
URL: https://issues.apache.org/jira/browse/CASSANDRA-8346
Project: Cassandra
Issue Type: Bug
Reporter: Sylvain Lebresne
Assignee: Sylvain Lebresne
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:
{noformat}
"normal quorum" == (RF + 1) / 2 <= (RF + P) - ((RF + P + 1) / 2) ==
"participants considered - blocked for"
{noformat}
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.
--
This message was sent by Atlassian JIRA
(v6.3.4#6332)
