[CASSANDRA-19904] Deprecate use of gossip state for paxos electorate verification

• Type: Improvement
• Status: Resolved
• Priority: Normal
• Resolution: Fixed
• Fix Version/s: 5.x
• Component/s: Consistency/Coordination
• Change Category: Semantic
• Complexity: Normal
• Platform: All

Source Control Link

https://github.com/apache/cassandra/commit/5539f9b9996ec40fe933ee487968b1595d7a9fee

Test and Documentation Plan

Updates to existing tests

• Assignee: Sam Tunnicliffe
• Reporter: Sam Tunnicliffe

Authors

Sam Tunnicliffe

Reviewers

Benedict Elliott Smith, Marcus Eriksson

• Created: 09/Sep/24 08:51
• Updated: 20/Sep/24 16:24
• Resolved: 18/Sep/24 11:00

Description

In C* 5.0 and earlier, paxos uses gossip state to verify the paxos electorate remains stable during a round.
In 5.1, we mostly use the current epoch for this purpose, but some usage of gossip state has been left in place and should be removed.

An Electorate is now constructed from ClusterMetadata and no longer relies on anything in gossip, so updating gossip in PaxosPrepare::onResponse is no longer necessary. Instead, in the case where the respondent has signalled a mismatch the requester should perform a metadata catchup to the appropriate epoch.

Currently, this mismatch is communicated by the presence of a non-empty map of endpoint states, but this is no longer necessary as Paxos::verifyElectorate can provide the epoch of the placements used to construct its local electorate if it differs from the remote one. That epoch can then be included in the Permitted response instead of the gossip state. When handling the Permitted response, the paxos coordinator can use this epoch to determine whether it needs to call permittedOrTerminateIfElectorateMismatch.

For now, we will need to include both the electorate epoch and gossip state in the Permitted response to ensure continuity during upgrades but eventually the gossip state can be removed from these messages.

Issue Links

• fixes: CASSANDRA-19934 NPE in AffectedRangesImpl#intersects when other has different keyspaces than local
• links to: Trunk PR