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Merge 'raft: remake the read barrier optimization' from Patryk Jędrzejczak

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The approach taken in 1ae2ae50a6 turned
out to be incorrect. The Raft member requesting a read barrier could
incorrectly advance its commit_idx and break linearizability. We revert that
commit in this PR.

We also remake the read barrier optimization with a completely new approach.
We make the leader replicate to the non-voting requester of a read barrier if
its `commit_idx` is behind.

Fixes https://scylladb.atlassian.net/browse/SCYLLADB-998

No backport: the issue is present only in master.

Closes scylladb/scylladb#29216

* github.com:scylladb/scylladb:
  raft: speed up read barrier requested by non-voters
  Revert "raft: read_barrier: update local commit_idx to read_idx when it's safe"
This commit is contained in:
Dawid Mędrek
2026-03-31 15:11:56 +02:00
parent 216d39883a ba54b2272b
commit f040f1b703
3 changed files with 14 additions and 19 deletions
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23
raft/fsm.cc
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@@ -1098,7 +1098,8 @@ std::optional<std::pair<read_id, index_t>> fsm::start_read_barrier(server_id req
// Make sure that only a leader or a node that is part of the config can request read barrier
// Nodes outside of the config may never get the data, so they will not be able to read it.
if (requester != _my_id && leader_state().tracker.find(requester) == nullptr) {
follower_progress* opt_progress = leader_state().tracker.find(requester);
if (requester != _my_id && opt_progress == nullptr) {
throw std::runtime_error(fmt::format("Read barrier requested by a node outside of the configuration {}", requester));
}
@@ -1109,19 +1110,23 @@ std::optional<std::pair<read_id, index_t>> fsm::start_read_barrier(server_id req
return {};
}
// Optimization for read barriers requested on non-voters. A non-voter doesn't receive the read_quorum message, so
// it might update its commit index only after another leader tick, which would slow down wait_for_apply() at the
// end of the read barrier. Prevent that by replicating to the non-voting requester here.
if (requester != _my_id && opt_progress->commit_idx < _commit_idx && opt_progress->match_idx == _log.last_idx()
&& !opt_progress->can_vote) {
logger.trace("start_read_barrier[{}]: replicate to {} because follower commit_idx={} < commit_idx={}, "
"follower match_idx={} == last_idx={}, and follower can_vote={}",
_my_id, requester, opt_progress->commit_idx, _commit_idx, opt_progress->match_idx,
_log.last_idx(), opt_progress->can_vote);
replicate_to(*opt_progress, true);
}
read_id id = next_read_id();
logger.trace("start_read_barrier[{}] starting read barrier with id {}", _my_id, id);
return std::make_pair(id, _commit_idx);
}
void fsm::maybe_update_commit_idx_for_read(index_t read_idx) {
// read_idx from the leader might not be replicated to the local node yet.
const bool in_local_log = read_idx <= _log.last_idx();
if (in_local_log && log_term_for(read_idx) == get_current_term()) {
advance_commit_idx(read_idx);
}
}
void fsm::stop() {
if (is_leader()) {
// Become follower to stop accepting requests

9
raft/fsm.hh
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@@ -480,15 +480,6 @@ public:
std::optional<std::pair<read_id, index_t>> start_read_barrier(server_id requester);
// Update the commit index to the read index (a read barrier result from the leader) if the local entry with the
// read index belongs to the current term.
//
// Satisfying the condition above guarantees that the local log matches the current leader's log up to the read
// index (the Log Matching Property), so the current leader won't drop the local entry with the read index.
// Moreover, this entry has been committed by the leader, so future leaders also won't drop it (the Leader
// Completeness Property). Hence, updating the commit index is safe.
void maybe_update_commit_idx_for_read(index_t read_idx);
size_t in_memory_log_size() const {
return _log.in_memory_size();
}

1
raft/server.cc
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@@ -1571,7 +1571,6 @@ future<> server_impl::read_barrier(seastar::abort_source* as) {
co_return stop_iteration::no;
}
read_idx = std::get<index_t>(res);
_fsm->maybe_update_commit_idx_for_read(read_idx);
co_return stop_iteration::yes;
});