Adding a propose request vote message to speed up some reconfigurations

src/consensus/aft/raft.h

@@ -689,6 +689,15 @@ namespace aft
             break;
           }
 
+          case raft_propose_request_vote:
+          {
+            ProposeRequestVote r =
+              channels->template recv_authenticated<ProposeRequestVote>(
+                from, data, size);
+            recv_propose_request_vote(from, r);
+            break;
+          }
+
           default:
           {
             RAFT_FAIL_FMT("Unhandled AFT message type: {}", type);
@@ -1706,6 +1715,32 @@ namespace aft
       add_vote_for_me(from);
     }
 
+    void recv_propose_request_vote(
+      const ccf::NodeId& from, ProposeRequestVote r)
+    {
+      std::lock_guard<ccf::pal::Mutex> guard(state->lock);
+
+#ifdef CCF_RAFT_TRACING
+      nlohmann::json j = {};
+      j["function"] = "recv_propose_request_vote";
+      j["packet"] = r;
+      j["state"] = *state;
+      j["from_node_id"] = from;
+      j["committable_indices"] = committable_indices;
+      RAFT_TRACE_JSON_OUT(j);
+#endif
+      if (can_endorse_primary() && ticking && r.term == state->current_view)
+      {
+        RAFT_INFO_FMT(
+          "Becoming candidate early due to propose request vote from {}", from);
+        become_candidate();
+      }
+      else
+      {
+        RAFT_INFO_FMT("Ignoring propose request vote from {}", from);
+      }
+    }
+
     void restart_election_timeout()
     {
       // Randomise timeout_elapsed to get a random election timeout
@@ -1932,6 +1967,53 @@ namespace aft
       {
         leader_id.reset();
         state->leadership_state = kv::LeadershipState::None;
+        ProposeRequestVote prv{
+          {raft_propose_request_vote}, state->current_view};
+
+        std::optional<ccf::NodeId> successor = std::nullopt;
+        Index max_match_idx = 0;
+        kv::ReconfigurationId reconf_id_of_max_match = 0;
+
+        // Pick the node that has the highest match_idx, and break
+        // ties by looking at the highest reconfiguration id they are
+        // part of. This can lead to nudging a node that is
+        // about to retire too, but that node will then nudge
+        // a successor, and that seems preferable to nudging a node that
+        // risks not being eligible if reconfiguration id is prioritised.
+        // Alternatively, we could pick the node with the higest match idx
+        // in the latest config, provided that match idx at least as high as a
+        // majority. That would make them both eligible and unlikely to retire
+        // soon.
+        for (auto& [node, node_state] : all_other_nodes)
+        {
+          if (node_state.match_idx >= max_match_idx)
+          {
+            kv::ReconfigurationId latest_reconf_id = 0;
+            auto conf = configurations.rbegin();
+            while (conf != configurations.rend())
+            {
+              if (conf->nodes.find(node) != conf->nodes.end())
+              {
+                latest_reconf_id = conf->idx;
+                break;
+              }
+              conf++;
+            }
+            if (!(node_state.match_idx == max_match_idx &&
+                  latest_reconf_id < reconf_id_of_max_match))
+            {
+              reconf_id_of_max_match = latest_reconf_id;
+              successor = node;
+              max_match_idx = node_state.match_idx;
+            }
+          }
+        }
+        if (successor.has_value())
+        {
+          RAFT_INFO_FMT("Node retired, nudging {}", successor.value());
+          channels->send_authenticated(
+            successor.value(), ccf::NodeMsgType::consensus_msg, prv);
+        }
       }
 
       state->membership_state = kv::MembershipState::Retired;

src/consensus/aft/raft_types.h

@@ -98,7 +98,8 @@ namespace aft
     raft_append_entries_response,
     raft_append_entries_signed_response,
     raft_request_vote,
-    raft_request_vote_response
+    raft_request_vote_response,
+    raft_propose_request_vote,
   };
   DECLARE_JSON_ENUM(
     RaftMsgType,
@@ -110,6 +111,7 @@ namespace aft
        "raft_append_entries_signed_response"},
       {RaftMsgType::raft_request_vote, "raft_request_vote"},
       {RaftMsgType::raft_request_vote_response, "raft_request_vote_response"},
+      {RaftMsgType::raft_propose_request_vote, "raft_propose_request_vote"},
     });
 
 #pragma pack(push, 1)
@@ -190,5 +192,14 @@ namespace aft
   DECLARE_JSON_TYPE_WITH_BASE(RequestVoteResponse, RaftHeader);
   DECLARE_JSON_REQUIRED_FIELDS(RequestVoteResponse, term, vote_granted);
 
+  struct ProposeRequestVote : RaftHeader
+  {
+    // Message sent by a leader stepping down after observing its own retirement
+    // being committed.
+    Term term;
+  };
+  DECLARE_JSON_TYPE_WITH_BASE(ProposeRequestVote, RaftHeader);
+  DECLARE_JSON_REQUIRED_FIELDS(ProposeRequestVote, term);
+
 #pragma pack(pop)
 }

src/consensus/aft/test/driver.h

@@ -334,6 +334,16 @@ class RaftDriver
     rlog(node_id, tgt_node_id, s, dropped);
   }
 
+  void log_msg_details(
+    ccf::NodeId node_id,
+    ccf::NodeId tgt_node_id,
+    aft::ProposeRequestVote prv,
+    bool dropped)
+  {
+    const auto s = fmt::format("propose_request_vote for term {}", prv.term);
+    log(node_id, tgt_node_id, s, dropped);
+  }
+
   void log_msg_details(
     ccf::NodeId node_id,
     ccf::NodeId tgt_node_id,
@@ -370,6 +380,12 @@ class RaftDriver
         log_msg_details(node_id, tgt_node_id, aer, dropped);
         break;
       }
+      case (aft::RaftMsgType::raft_propose_request_vote):
+      {
+        auto prv = *(aft::ProposeRequestVote*)data;
+        log_msg_details(node_id, tgt_node_id, prv, dropped);
+        break;
+      }
       default:
       {
         throw std::runtime_error(

tests/raft_scenarios/4582.2

@@ -60,9 +60,6 @@ assert_is_primary,0
 assert_commit_idx,0,2
 assert_commit_idx,1,0
 
-periodic_all,1000
-dispatch_all
-
 assert_is_backup,1
 state_all
 

tla/MCccfraft.cfg

@@ -26,6 +26,7 @@ CONSTANTS
     AppendEntriesRequest = AppendEntriesRequest
     AppendEntriesResponse = AppendEntriesResponse
     NotifyCommitMessage = NotifyCommitMessage
+    ProposeVoteRequest = ProposeVoteRequest
 
     TypeEntry = Entry
     TypeSignature = Signature

tla/MCccfraftWithReconfig.cfg

@@ -26,6 +26,7 @@ CONSTANTS
     AppendEntriesRequest = AppendEntriesRequest
     AppendEntriesResponse = AppendEntriesResponse
     NotifyCommitMessage = NotifyCommitMessage
+    ProposeVoteRequest = ProposeVoteRequest
 
     TypeEntry = Entry
     TypeSignature = Signature

tla/SIMccfraft.cfg

@@ -16,6 +16,7 @@ CONSTANTS
     AppendEntriesRequest = AppendEntriesRequest
     AppendEntriesResponse = AppendEntriesResponse
     NotifyCommitMessage = NotifyCommitMessage
+    ProposeVoteRequest = ProposeVoteRequest
 
     TypeEntry = Entry
     TypeSignature = Signature

tla/Traceccfraft.cfg

@@ -65,6 +65,7 @@ CONSTANTS
     AppendEntriesRequest = AppendEntriesRequest
     AppendEntriesResponse = AppendEntriesResponse
     NotifyCommitMessage = NotifyCommitMessage
+    ProposeVoteRequest = ProposeVoteRequest
 
     TypeEntry = TypeEntry
     TypeSignature = TypeSignature

tla/Traceccfraft.tla

@@ -4,7 +4,8 @@ EXTENDS ccfraft, Json, IOUtils, Sequences, Network
 \* raft_types.h enum RaftMsgType
 RaftMsgType ==
     "raft_append_entries" :> AppendEntriesRequest @@ "raft_append_entries_response" :> AppendEntriesResponse @@
-    "raft_request_vote" :> RequestVoteRequest @@ "raft_request_vote_response" :> RequestVoteResponse
+    "raft_request_vote" :> RequestVoteRequest @@ "raft_request_vote_response" :> RequestVoteResponse @@
+    "raft_propose_request_vote" :> ProposeVoteRequest
 
 LeadershipState ==
     Leader :> "Leader" @@ Follower :> "Follower" @@ Candidate :> "Candidate" @@ Pending :> "Pending"
@@ -301,6 +302,19 @@ IsCheckQuorum ==
     /\ CheckQuorum(logline.msg.state.node_id)
     /\ committableIndices[logline.msg.state.node_id] = Range(logline.msg.committable_indices)
 
+IsRcvProposeVoteRequest ==
+    /\ IsEvent("recv_propose_request_vote")
+    /\ state[logline.msg.state.node_id] = Leader
+    /\ LET i == logline.msg.state.node_id
+           j == logline.msg.to_node_id
+       IN /\ \E m \in Messages':
+                /\ m.type = ProposeVoteRequest
+                /\ m.type = RaftMsgType[logline.msg.packet.msg]
+                /\ m.term = logline.msg.packet.term
+                \* There is now one more message of this type.
+                /\ OneMoreMessage(m)
+    /\ committableIndices[logline.msg.state.node_id] = Range(logline.msg.committable_indices)
+
 TraceNext ==
     \/ IsTimeout
     \/ IsBecomeLeader
@@ -325,6 +339,8 @@ TraceNext ==
     \/ IsRcvRequestVoteResponse
     \/ IsExecuteAppendEntries
 
+    \/ IsRcvProposeVoteRequest
+
 RaftDriverQuirks ==
     \* The "nodes" command in raft scenarios causes N consecutive "add_configuration" log lines to be emitted,
      \* where N is determined by the "nodes" parameter. At this stage, the nodes are in the "Pending" state.

tla/ccfraft.tla

@@ -64,7 +64,8 @@ CONSTANTS
     RequestVoteResponse,
     AppendEntriesRequest,
     AppendEntriesResponse,
-    NotifyCommitMessage
+    NotifyCommitMessage,
+    ProposeVoteRequest
 
 \* CCF: Content types (Normal entry or a signature that signs
 \*      previous entries or a reconfiguration entry)
@@ -152,7 +153,7 @@ ReconfigurationVarsTypeInv ==
 \* A set representing requests and responses sent from one server
 \* to another. With CCF, we have message integrity and can ensure unique messages.
 \* Messages only records messages that are currently in-flight, actions should
-\* removed messages once received.
+\* remove messages once received.
 \* We model messages as a single (unsorted) set and do not assume ordered message delivery between nodes.
 \* Node-to-node channels use TCP but out-of-order delivery could be observed due to reconnection or a malicious host.
 VARIABLE messages
@@ -200,6 +201,10 @@ NotifyCommitMessageTypeOK(m) ==
     /\ m.type = NotifyCommitMessage
     /\ m.commitIndex \in Nat
 
+ProposeVoteRequestTypeOK(m) ==
+    /\ m.type = ProposeVoteRequest
+    /\ m.term \in Nat
+
 MessagesTypeInv ==
     \A m \in Messages :
         /\ m.source \in Servers
@@ -210,6 +215,7 @@ MessagesTypeInv ==
             \/ RequestVoteRequestTypeOK(m)
             \/ RequestVoteResponseTypeOK(m)
             \/ NotifyCommitMessageTypeOK(m)
+            \/ ProposeVoteRequestTypeOK(m)
 
 \* CCF: After reconfiguration, a RetiredLeader leader may need to notify servers
 \* of the current commit level to ensure that no deadlock is reached through
@@ -729,6 +735,11 @@ AdvanceCommitIndex(i) ==
               \* Retire if i is not in active configuration anymore
               /\ IF i \notin configurations[i][Min(DOMAIN new_configurations)]
                  THEN /\ state' = [state EXCEPT ![i] = RetiredLeader]
+                      /\ LET msg == [type          |-> ProposeVoteRequest,
+                                     term          |-> currentTerm[i],
+                                     source        |-> i,
+                                     dest          |-> CHOOSE n \in Servers : \A m \in Servers : matchIndex[i][n] >= matchIndex[i][m] ]
+                         IN Send(msg)
                       /\ UNCHANGED << currentTerm, votedFor, reconfigurationCount, removedFromConfiguration >>
                  \* Otherwise, states remain unchanged
                  ELSE UNCHANGED <<serverVars, reconfigurationCount, removedFromConfiguration>>
@@ -1065,6 +1076,14 @@ RcvUpdateCommitIndex(i, j) ==
         /\ UpdateCommitIndex(m.dest, m.source, m)
         /\ Discard(m)
 
+RcvProposeVoteRequest(i, j) ==
+    \E m \in MessagesTo(i) :
+        /\ j = m.source
+        /\ m.type = ProposeVoteRequest
+        /\ m.term <= currentTerm[i]
+        /\ Timeout(m.dest)
+        /\ Discard(m)
+
 Receive(i, j) ==
     \/ RcvDropIgnoredMessage(i, j)
     \/ RcvUpdateTerm(i, j)
@@ -1073,6 +1092,7 @@ Receive(i, j) ==
     \/ RcvAppendEntriesRequest(i, j)
     \/ RcvAppendEntriesResponse(i, j)
     \/ RcvUpdateCommitIndex(i, j)
+    \/ RcvProposeVoteRequest(i, j)
 
 \* End of message handlers.
 ------------------------------------------------------------------------------
@@ -1108,6 +1128,7 @@ Spec ==
     /\ \A i, j \in Servers : WF_vars(RcvAppendEntriesRequest(i, j))
     /\ \A i, j \in Servers : WF_vars(RcvAppendEntriesResponse(i, j))
     /\ \A i, j \in Servers : WF_vars(RcvUpdateCommitIndex(i, j))
+    /\ \A i, j \in Servers : WF_vars(RcvProposeVoteRequest(i, j))
     \* Node actions
     /\ \A s, t \in Servers : WF_vars(AppendEntries(s, t))
     /\ \A s, t \in Servers : WF_vars(RequestVote(s, t))