r/consensus: prefer availability over voter priority

Redpanda Raft implementation exposes an API allowing to override a voter priority. This is used by the drain manager when a node is in maintenance mode. In current implementation when the only voter is in maintenance mode the Raft group is not able to elect a leader as the reported priority it to low (the priority override in maintenance is set to 0). Fixed Raft implementation to make sure that it prioritize an availability over the user priority preference. If a node is the only voter the priority override is ignored. Fixes: redpanda-data/cloudv2#6174 Signed-off-by: Michal Maslanka <[email protected]>
mmaslankaprv · Jun 16, 2023 · 1b21f83 · 1b21f83
1 parent e13a04d
commit 1b21f83
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Showing 3 changed files with 26 additions and 11 deletions.
diff --git a/src/v/raft/consensus.cc b/src/v/raft/consensus.cc
@@ -883,25 +883,35 @@ void consensus::dispatch_vote(bool leadership_transfer) {
     // lower our required priority for next time.
     _target_priority = next_target_priority();
 
+    const auto& latest_config = _configuration_manager.get_latest();
     // skip sending vote request if current node is not a voter in current
     // configuration
-    if (!_configuration_manager.get_latest().is_allowed_to_request_votes(
-          _self)) {
+    if (!latest_config.is_allowed_to_request_votes(_self)) {
         arm_vote_timeout();
         return;
     }
 
     // if priority is to low, skip dispatching votes, do not take priority into
     // account when we transfer leadership
     if (current_priority_to_low && !leadership_transfer) {
+        const bool is_only_voter = latest_config.unique_voter_count() == 1
+                                   && latest_config.is_voter(_self);
+        if (!is_only_voter) {
+            vlog(
+              _ctxlog.trace,
+              "current node priority {} is lower than target {} (next vote {})",
+              self_priority,
+              cur_target_priority,
+              _target_priority);
+            arm_vote_timeout();
+            return;
+        }
         vlog(
-          _ctxlog.trace,
-          "current node priority {} is lower than target {} (next vote {})",
+          _ctxlog.info,
+          "current node priority {} is lower than target {}, however the node "
+          "is the only voter, continue with dispatching vote",
           self_priority,
-          cur_target_priority,
-          _target_priority);
-        arm_vote_timeout();
-        return;
+          cur_target_priority);
     }
     // background, acquire lock, transition state
     ssx::background

diff --git a/src/v/raft/prevote_stm.cc b/src/v/raft/prevote_stm.cc
@@ -183,7 +183,11 @@ ss::future<bool> prevote_stm::do_prevote() {
 
     // process results
     return process_replies().then([this]() {
-        if (_success && _ptr->_node_priority_override == zero_voter_priority) {
+        const auto only_voter = _config->unique_voter_count() == 1
+                                && _config->is_voter(_ptr->self());
+        if (
+          _success && !only_voter
+          && _ptr->_node_priority_override == zero_voter_priority) {
             vlog(
               _ctxlog.debug,
               "Ignoring successful pre-vote. Node priority too low: {}",

diff --git a/src/v/raft/vote_stm.cc b/src/v/raft/vote_stm.cc
@@ -235,8 +235,9 @@ ss::future<> vote_stm::update_vote_state(ssx::semaphore_units u) {
         _ptr->_vstate = consensus::vote_state::follower;
         co_return;
     }
-
-    if (_ptr->_node_priority_override == zero_voter_priority) {
+    const auto only_voter = _config->unique_voter_count() == 1
+                            && _config->is_voter(_ptr->self());
+    if (!only_voter && _ptr->_node_priority_override == zero_voter_priority) {
         vlog(
           _ctxlog.debug,
           "Ignoring successful vote. Node priority too low: {}",