raft.h

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// Copyright (c) Microsoft Corporation. All rights reserved.
// Licensed under the Apache 2.0 License.
#pragma once
 
#include "ccf/ds/ccf_exception.h"
#include "ccf/ds/logger.h"
#include "ccf/pal/locking.h"
#include "ccf/tx_id.h"
#include "ccf/tx_status.h"
#include "ds/serialized.h"
#include "enclave/reconfiguration_type.h"
#include "impl/state.h"
#include "kv/kv_types.h"
#include "node/node_client.h"
#include "node/node_to_node.h"
#include "node/node_types.h"
#include "node/retired_nodes_cleanup.h"
#include "raft_types.h"
#include "service/tables/signatures.h"
 
#include <algorithm>
#include <list>
#include <random>
#include <unordered_map>
#include <vector>
 
#ifdef VERBOSE_RAFT_LOGGING
#  define RAFT_TRACE_FMT(s, ...) \
    CCF_LOG_FMT(TRACE, "raft") \
    ("{} | {} | {} | " s, \
     state->node_id, \
     state->leadership_state, \
     state->membership_state, \
     ##__VA_ARGS__)
#  define RAFT_DEBUG_FMT(s, ...) \
    CCF_LOG_FMT(DEBUG, "raft") \
    ("{} | {} | {} | " s, \
     state->node_id, \
     state->leadership_state, \
     state->membership_state, \
     ##__VA_ARGS__)
#  define RAFT_INFO_FMT(s, ...) \
    CCF_LOG_FMT(INFO, "raft") \
    ("{} | {} | {} | " s, \
     state->node_id, \
     state->leadership_state, \
     state->membership_state, \
     ##__VA_ARGS__)
#  define RAFT_FAIL_FMT(s, ...) \
    CCF_LOG_FMT(FAIL, "raft") \
    ("{} | {} | {} | " s, \
     state->node_id, \
     state->leadership_state, \
     state->membership_state, \
     ##__VA_ARGS__)
#else
#  define RAFT_TRACE_FMT LOG_TRACE_FMT
#  define RAFT_DEBUG_FMT LOG_DEBUG_FMT
#  define RAFT_INFO_FMT LOG_INFO_FMT
#  define RAFT_FAIL_FMT LOG_FAIL_FMT
#endif
 
#define RAFT_TRACE_JSON_OUT(json_object) \
  CCF_LOG_OUT(DEBUG, "raft_trace") << json_object
 
#ifdef CCF_RAFT_TRACING
 
static inline void add_committable_indices_start_and_end(
  nlohmann::json& j, const std::shared_ptr<aft::State>& state)
{
  std::vector<aft::Index> committable_indices;
  if (!state->committable_indices.empty())
  {
    committable_indices.push_back(state->committable_indices.front());
    if (state->committable_indices.size() > 1)
    {
      committable_indices.push_back(state->committable_indices.back());
    }
  }
  j["committable_indices"] = committable_indices;
}
 
#  define COMMITTABLE_INDICES(event_state, state) \
    add_committable_indices_start_and_end(event_state, state);
 
#endif
 
#define LOG_ROLLBACK_INFO_FMT CCF_LOG_FMT(INFO, "rollback")
 
namespace aft
{
  using Configuration = ccf::kv::Configuration;
 
  template <class LedgerProxy>
  class Aft : public ccf::kv::Consensus
  {
  private:
    struct NodeState
    {
      Configuration::NodeInfo node_info;
 
      // the highest index sent to the node
      Index sent_idx;
 
      // the highest matching index with the node that was confirmed
      Index match_idx;
 
      // timeout tracking the last time an ack was received from the node
      std::chrono::milliseconds last_ack_timeout;
 
      NodeState() = default;
 
      NodeState(
        const Configuration::NodeInfo& node_info_,
        Index sent_idx_,
        Index match_idx_ = 0) :
        node_info(node_info_),
        sent_idx(sent_idx_),
        match_idx(match_idx_),
        last_ack_timeout(0)
      {}
    };
 
    // Persistent
    std::unique_ptr<Store> store;
 
    // Volatile
    std::optional<ccf::NodeId> voted_for = std::nullopt;
    std::optional<ccf::NodeId> leader_id = std::nullopt;
 
    // Keep track of votes in each active configuration
    struct Votes
    {
      std::unordered_set<ccf::NodeId> votes;
      size_t quorum;
    };
    std::map<Index, Votes> votes_for_me;
 
    std::chrono::milliseconds timeout_elapsed;
 
    // When this node receives append entries from a new primary, it may need to
    // roll back a committable but uncommitted suffix it holds. The
    // new primary dictates the index where this suffix begins, which
    // following the Raft election rules must be at least as high as the highest
    // commit index reported by the previous primary. The window in which this
    // rollback could be accepted is minimised to avoid unnecessary
    // retransmissions - this node only executes this rollback instruction on
    // the first append entries after it became a follower. As with any append
    // entries, the initial index will not advance until this node acks.
    bool is_new_follower = false;
 
    // When this node becomes primary, they should produce a new signature in
    // the current view. This signature is the first thing they may commit, as
    // they cannot confirm commit of anything from a previous view (Raft paper
    // section 5.4.2). This bool is true from the point this node becomes
    // primary, until it sees a committable entry
    bool should_sign = false;
 
    std::shared_ptr<aft::State> state;
 
    // Timeouts
    std::chrono::milliseconds request_timeout;
    std::chrono::milliseconds election_timeout;
    size_t max_uncommitted_tx_count;
    bool ticking = false;
 
    // Configurations
    std::list<Configuration> configurations;
    // Union of other nodes (i.e. all nodes but us) in each active
    // configuration, plus those that are retired, but for which
    // the persistence of retirement knowledge is not yet established,
    // i.e. Completed but not RetiredCommitted
    // This should be used for diagnostic or broadcasting
    // messages but _not_ for counting quorums, which should be done for each
    // active configuration.
    std::unordered_map<ccf::NodeId, NodeState> all_other_nodes;
    std::unordered_map<ccf::NodeId, ccf::SeqNo> retired_nodes;
    ReconfigurationType reconfiguration_type;
 
    // Node client to trigger submission of RPC requests
    std::shared_ptr<ccf::NodeClient> node_client;
 
    // Used to remove retired nodes from store
    std::unique_ptr<ccf::RetiredNodeCleanup> retired_node_cleanup;
 
    size_t entry_size_not_limited = 0;
    size_t entry_count = 0;
    Index entries_batch_size = 20;
    static constexpr int batch_window_size = 100;
    int batch_window_sum = 0;
 
    // When this is set, only public domain is deserialised when receiving
    // append entries
    bool public_only = false;
 
    // Randomness
    std::uniform_int_distribution<int> distrib;
    std::default_random_engine rand;
 
    // AppendEntries messages are currently constrained to only contain entries
    // from a single term, so that the receiver can know the term of each entry
    // pre-deserialisation, without an additional header.
    static constexpr size_t max_terms_per_append_entries = 1;
 
  public:
    static constexpr size_t append_entries_size_limit = 20000;
    std::unique_ptr<LedgerProxy> ledger;
    std::shared_ptr<ccf::NodeToNode> channels;
 
  public:
    Aft(
      const ccf::consensus::Configuration& settings_,
      std::unique_ptr<Store> store_,
      std::unique_ptr<LedgerProxy> ledger_,
      std::shared_ptr<ccf::NodeToNode> channels_,
      std::shared_ptr<aft::State> state_,
      std::shared_ptr<ccf::NodeClient> rpc_request_context_,
      bool public_only_ = false,
      ccf::kv::MembershipState initial_membership_state_ =
        ccf::kv::MembershipState::Active,
      ReconfigurationType reconfiguration_type_ =
        ReconfigurationType::ONE_TRANSACTION) :
      store(std::move(store_)),
 
      timeout_elapsed(0),
 
      state(state_),
 
      request_timeout(settings_.message_timeout),
      election_timeout(settings_.election_timeout),
      max_uncommitted_tx_count(settings_.max_uncommitted_tx_count),
 
      reconfiguration_type(reconfiguration_type_),
      node_client(rpc_request_context_),
      retired_node_cleanup(
        std::make_unique<ccf::RetiredNodeCleanup>(node_client)),
 
      public_only(public_only_),
 
      distrib(0, (int)election_timeout.count() / 2),
      rand((int)(uintptr_t)this),
 
      ledger(std::move(ledger_)),
      channels(channels_)
    {}
 
    virtual ~Aft() = default;
 
    std::optional<ccf::NodeId> primary() override
    {
      return leader_id;
    }
 
    ccf::NodeId id() override
    {
      return state->node_id;
    }
 
    bool is_primary() override
    {
      return state->leadership_state == ccf::kv::LeadershipState::Leader;
    }
 
    bool is_candidate() override
    {
      return state->leadership_state == ccf::kv::LeadershipState::Candidate;
    }
 
    bool can_replicate() override
    {
      std::unique_lock<ccf::pal::Mutex> guard(state->lock);
      return can_replicate_unsafe();
    }
 
    bool is_at_max_capacity() override
    {
      if (max_uncommitted_tx_count == 0)
      {
        return false;
      }
      std::unique_lock<ccf::pal::Mutex> guard(state->lock);
      return state->leadership_state == ccf::kv::LeadershipState::Leader &&
        (state->last_idx - state->commit_idx >= max_uncommitted_tx_count);
    }
 
    Consensus::SignatureDisposition get_signature_disposition() override
    {
      std::unique_lock<ccf::pal::Mutex> guard(state->lock);
      if (can_sign_unsafe())
      {
        if (should_sign)
        {
          return Consensus::SignatureDisposition::SHOULD_SIGN;
        }
        else
        {
          return Consensus::SignatureDisposition::CAN_SIGN;
        }
      }
      else
      {
        return Consensus::SignatureDisposition::CANT_REPLICATE;
      }
    }
 
    bool is_backup() override
    {
      return state->leadership_state == ccf::kv::LeadershipState::Follower;
    }
 
    bool is_active() const
    {
      return state->membership_state == ccf::kv::MembershipState::Active;
    }
 
    bool is_retired() const
    {
      return state->membership_state == ccf::kv::MembershipState::Retired;
    }
 
    bool is_retired_committed() const
    {
      return state->membership_state == ccf::kv::MembershipState::Retired &&
        state->retirement_phase == ccf::kv::RetirementPhase::RetiredCommitted;
    }
 
    bool is_retired_completed() const
    {
      return state->membership_state == ccf::kv::MembershipState::Retired &&
        state->retirement_phase == ccf::kv::RetirementPhase::Completed;
    }
 
    void set_retired_committed(
      ccf::SeqNo seqno, const std::vector<ccf::kv::NodeId>& node_ids) override
    {
      for (auto& node_id : node_ids)
      {
        if (id() == node_id)
        {
          CCF_ASSERT(
            state->membership_state == ccf::kv::MembershipState::Retired,
            "Node is not retired, cannot become retired committed");
          CCF_ASSERT(
            state->retirement_phase == ccf::kv::RetirementPhase::Completed,
            "Node is not retired, cannot become retired committed");
          state->retired_committed_idx = seqno;
          become_retired(seqno, ccf::kv::RetirementPhase::RetiredCommitted);
        }
        else
        {
          // Once a node's retired_committed status is itself committed, all
          // future primaries in the network must be aware its retirement is
          // committed, and so no longer need any communication with it to
          // advance commit. No further communication with this node is needed.
          all_other_nodes.erase(node_id);
          RAFT_INFO_FMT("Removed {} from nodes known to consensus", node_id);
        }
      }
    }
 
    Index last_committable_index() const
    {
      return state->committable_indices.empty() ?
        state->commit_idx :
        state->committable_indices.back();
    }
 
    // Returns the highest committable index which is not greater than the
    // given idx.
    std::optional<Index> find_highest_possible_committable_index(
      Index idx) const
    {
      const auto it = std::upper_bound(
        state->committable_indices.rbegin(),
        state->committable_indices.rend(),
        idx,
        [](const auto& l, const auto& r) { return l >= r; });
      if (it == state->committable_indices.rend())
      {
        return std::nullopt;
      }
 
      return *it;
    }
 
    void compact_committable_indices(Index idx)
    {
      while (!state->committable_indices.empty() &&
             (state->committable_indices.front() <= idx))
      {
        state->committable_indices.pop_front();
      }
    }
 
    void enable_all_domains() override
    {
      // When receiving append entries as a follower, all security domains will
      // be deserialised
      std::lock_guard<ccf::pal::Mutex> guard(state->lock);
      public_only = false;
    }
 
    void force_become_primary() override
    {
      // This is unsafe and should only be called when the node is certain
      // there is no leader and no other node will attempt to force leadership.
      if (leader_id.has_value())
      {
        throw std::logic_error(
          "Can't force leadership if there is already a leader");
      }
 
      std::lock_guard<ccf::pal::Mutex> guard(state->lock);
      state->current_view += starting_view_change;
      become_leader(true);
    }
 
    void force_become_primary(
      Index index,
      Term term,
      const std::vector<Index>& terms,
      Index commit_idx_) override
    {
      // This is unsafe and should only be called when the node is certain
      // there is no leader and no other node will attempt to force leadership.
      if (leader_id.has_value())
      {
        throw std::logic_error(
          "Can't force leadership if there is already a leader");
      }
 
      std::lock_guard<ccf::pal::Mutex> guard(state->lock);
      state->current_view = term;
      state->last_idx = index;
      state->commit_idx = commit_idx_;
      state->view_history.initialise(terms);
      state->view_history.update(index, term);
      state->current_view += starting_view_change;
      become_leader(true);
    }
 
    void init_as_backup(
      Index index,
      Term term,
      const std::vector<Index>& term_history,
      Index recovery_start_index = 0) override
    {
      // This should only be called when the node resumes from a snapshot and
      // before it has received any append entries.
      std::lock_guard<ccf::pal::Mutex> guard(state->lock);
 
      state->last_idx = index;
      state->commit_idx = index;
 
      state->view_history.initialise(term_history);
 
      ledger->init(index, recovery_start_index);
 
      become_aware_of_new_term(term);
    }
 
    Index get_last_idx()
    {
      return state->last_idx;
    }
 
    Index get_committed_seqno() override
    {
      std::lock_guard<ccf::pal::Mutex> guard(state->lock);
      return get_commit_idx_unsafe();
    }
 
    Term get_view() override
    {
      std::lock_guard<ccf::pal::Mutex> guard(state->lock);
      return state->current_view;
    }
 
    std::pair<Term, Index> get_committed_txid() override
    {
      std::lock_guard<ccf::pal::Mutex> guard(state->lock);
      ccf::SeqNo commit_idx = get_commit_idx_unsafe();
      return {get_term_internal(commit_idx), commit_idx};
    }
 
    Term get_view(Index idx) override
    {
      std::lock_guard<ccf::pal::Mutex> guard(state->lock);
      return get_term_internal(idx);
    }
 
    std::vector<Index> get_view_history(Index idx) override
    {
      // This should only be called when the spin lock is held.
      return state->view_history.get_history_until(idx);
    }
 
    std::vector<Index> get_view_history_since(Index idx) override
    {
      // This should only be called when the spin lock is held.
      return state->view_history.get_history_since(idx);
    }
 
    // Same as ccfraft.tla GetServerSet/IsInServerSet
    // Not to be confused with all_other_nodes, which includes retired completed
    // nodes. Used to restrict sending vote requests, and when becoming a
    // leader, to decide whether to advance commit.
    std::set<ccf::NodeId> other_nodes_in_active_configs() const
    {
      std::set<ccf::NodeId> nodes;
 
      for (auto const& conf : configurations)
      {
        for (auto const& [node_id, _] : conf.nodes)
        {
          if (node_id != state->node_id)
          {
            nodes.insert(node_id);
          }
        }
      }
 
      return nodes;
    }
 
  public:
    void add_configuration(
      Index idx,
      const ccf::kv::Configuration::Nodes& conf,
      const std::unordered_set<ccf::NodeId>& new_learner_nodes = {},
      const std::unordered_set<ccf::NodeId>& new_retired_nodes = {}) override
    {
      RAFT_DEBUG_FMT(
        "Configurations: add new configuration at {}: {{{}}}", idx, conf);
 
      assert(new_learner_nodes.empty());
 
#ifdef CCF_RAFT_TRACING
      nlohmann::json j = {};
      j["function"] = "add_configuration";
      j["state"] = *state;
      COMMITTABLE_INDICES(j["state"], state);
      j["configurations"] = configurations;
      j["args"] = nlohmann::json::object();
      j["args"]["configuration"] = Configuration{idx, conf, idx};
      RAFT_TRACE_JSON_OUT(j);
#endif
 
      // Detect when we are retired by observing a configuration
      // from which we are absent following a configuration in which
      // we were included. Note that this relies on retirement being
      // a final state, and node identities never being re-used.
      if (
        !configurations.empty() &&
        configurations.back().nodes.find(state->node_id) !=
          configurations.back().nodes.end() &&
        conf.find(state->node_id) == conf.end())
      {
        become_retired(idx, ccf::kv::RetirementPhase::Ordered);
      }
 
      if (conf != configurations.back().nodes)
      {
        Configuration new_config = {idx, std::move(conf), idx};
        configurations.push_back(new_config);
 
        create_and_remove_node_state();
      }
    }
 
    void start_ticking()
    {
      ticking = true;
      using namespace std::chrono_literals;
      timeout_elapsed = 0ms;
      RAFT_INFO_FMT("Election timer has become active");
    }
 
    void reset_last_ack_timeouts()
    {
      for (auto& node : all_other_nodes)
      {
        using namespace std::chrono_literals;
        node.second.last_ack_timeout = 0ms;
      }
    }
 
    Configuration::Nodes get_latest_configuration_unsafe() const override
    {
      if (configurations.empty())
      {
        return {};
      }
 
      return configurations.back().nodes;
    }
 
    Configuration::Nodes get_latest_configuration() override
    {
      std::lock_guard<ccf::pal::Mutex> guard(state->lock);
      return get_latest_configuration_unsafe();
    }
 
    ccf::kv::ConsensusDetails get_details() override
    {
      ccf::kv::ConsensusDetails details;
      std::lock_guard<ccf::pal::Mutex> guard(state->lock);
      details.primary_id = leader_id;
      details.current_view = state->current_view;
      details.ticking = ticking;
      details.leadership_state = state->leadership_state;
      details.membership_state = state->membership_state;
      if (is_retired())
      {
        details.retirement_phase = state->retirement_phase;
      }
      for (auto const& conf : configurations)
      {
        details.configs.push_back(conf);
      }
      for (auto& [k, v] : all_other_nodes)
      {
        details.acks[k] = {
          v.match_idx, static_cast<size_t>(v.last_ack_timeout.count())};
      }
      details.reconfiguration_type = reconfiguration_type;
      return details;
    }
 
    bool replicate(const ccf::kv::BatchVector& entries, Term term) override
    {
      std::lock_guard<ccf::pal::Mutex> guard(state->lock);
 
      if (state->leadership_state != ccf::kv::LeadershipState::Leader)
      {
        RAFT_DEBUG_FMT(
          "Failed to replicate {} items: not leader", entries.size());
        rollback(state->last_idx);
        return false;
      }
 
      if (term != state->current_view)
      {
        RAFT_DEBUG_FMT(
          "Failed to replicate {} items at term {}, current term is {}",
          entries.size(),
          term,
          state->current_view);
        return false;
      }
 
      if (is_retired_committed())
      {
        RAFT_DEBUG_FMT(
          "Failed to replicate {} items: node retirement is complete",
          entries.size());
        rollback(state->last_idx);
        return false;
      }
 
      RAFT_DEBUG_FMT("Replicating {} entries", entries.size());
 
      for (auto& [index, data, is_globally_committable, hooks] : entries)
      {
        bool globally_committable = is_globally_committable;
 
        if (index != state->last_idx + 1)
          return false;
 
        RAFT_DEBUG_FMT(
          "Replicated on leader {}: {}{} ({} hooks)",
          state->node_id,
          index,
          (globally_committable ? " committable" : ""),
          hooks->size());
 
#ifdef CCF_RAFT_TRACING
        nlohmann::json j = {};
        j["function"] = "replicate";
        j["state"] = *state;
        COMMITTABLE_INDICES(j["state"], state);
        j["view"] = term;
        j["seqno"] = index;
        j["globally_committable"] = globally_committable;
        RAFT_TRACE_JSON_OUT(j);
#endif
 
        for (auto& hook : *hooks)
        {
          hook->call(this);
        }
 
        if (globally_committable)
        {
          RAFT_DEBUG_FMT(
            "membership: {} leadership: {}",
            state->membership_state,
            state->leadership_state);
          if (
            state->membership_state == ccf::kv::MembershipState::Retired &&
            state->retirement_phase == ccf::kv::RetirementPhase::Ordered)
          {
            become_retired(index, ccf::kv::RetirementPhase::Signed);
          }
          state->committable_indices.push_back(index);
          start_ticking_if_necessary();
 
          // Reset should_sign here - whenever we see a committable entry we
          // don't need to produce _another_ signature
          should_sign = false;
        }
 
        state->last_idx = index;
        ledger->put_entry(
          *data, globally_committable, state->current_view, index);
        entry_size_not_limited += data->size();
        entry_count++;
 
        state->view_history.update(index, state->current_view);
        if (entry_size_not_limited >= append_entries_size_limit)
        {
          update_batch_size();
          entry_count = 0;
          entry_size_not_limited = 0;
          for (const auto& it : all_other_nodes)
          {
            RAFT_DEBUG_FMT("Sending updates to follower {}", it.first);
            send_append_entries(it.first, it.second.sent_idx + 1);
          }
        }
      }
 
      // Try to advance commit at once if there are no other nodes.
      if (other_nodes_in_active_configs().size() == 0)
      {
        update_commit();
      }
 
      return true;
    }
 
    void recv_message(
      const ccf::NodeId& from, const uint8_t* data, size_t size) override
    {
      RaftMsgType type = serialized::peek<RaftMsgType>(data, size);
 
      try
      {
        switch (type)
        {
          case raft_append_entries:
          {
            AppendEntries r =
              channels->template recv_authenticated<AppendEntries>(
                from, data, size);
            recv_append_entries(from, r, data, size);
            break;
          }
 
          case raft_append_entries_response:
          {
            AppendEntriesResponse r =
              channels->template recv_authenticated<AppendEntriesResponse>(
                from, data, size);
            recv_append_entries_response(from, r);
            break;
          }
 
          case raft_request_vote:
          {
            RequestVote r = channels->template recv_authenticated<RequestVote>(
              from, data, size);
            recv_request_vote(from, r);
            break;
          }
 
          case raft_request_vote_response:
          {
            RequestVoteResponse r =
              channels->template recv_authenticated<RequestVoteResponse>(
                from, data, size);
            recv_request_vote_response(from, r);
            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);
          }
        }
      }
      catch (const ccf::NodeToNode::DroppedMessageException& e)
      {
        RAFT_INFO_FMT("Dropped invalid message from {}", e.from);
        return;
      }
      catch (const serialized::InsufficientSpaceException& ise)
      {
        RAFT_FAIL_FMT("Failed to parse message: {}", ise.what());
        return;
      }
      catch (const std::exception& e)
      {
        LOG_FAIL_EXC(e.what());
        return;
      }
    }
 
    void periodic(std::chrono::milliseconds elapsed) override
    {
      std::unique_lock<ccf::pal::Mutex> guard(state->lock);
      timeout_elapsed += elapsed;
 
      if (state->leadership_state == ccf::kv::LeadershipState::Leader)
      {
        if (timeout_elapsed >= request_timeout)
        {
          using namespace std::chrono_literals;
          timeout_elapsed = 0ms;
 
          update_batch_size();
          // Send newly available entries to all other nodes.
          for (const auto& node : all_other_nodes)
          {
            send_append_entries(node.first, node.second.sent_idx + 1);
          }
        }
 
        for (auto& node : all_other_nodes)
        {
          node.second.last_ack_timeout += elapsed;
        }
 
        bool has_quorum_of_backups = false;
        for (auto const& conf : configurations)
        {
          size_t backup_ack_timeout_count = 0;
          for (auto const& node : conf.nodes)
          {
            auto search = all_other_nodes.find(node.first);
            if (search == all_other_nodes.end())
            {
              // Ignore ourselves as primary
              continue;
            }
            if (search->second.last_ack_timeout >= election_timeout)
            {
              RAFT_DEBUG_FMT(
                "No ack received from {} in last {}",
                node.first,
                election_timeout);
              backup_ack_timeout_count++;
            }
          }
 
          if (backup_ack_timeout_count < get_quorum(conf.nodes.size() - 1))
          {
            // If primary has quorum of active backups in _any_ configuration,
            // it should remain primary
            has_quorum_of_backups = true;
            break;
          }
        }
 
        if (!has_quorum_of_backups)
        {
          // CheckQuorum: The primary automatically steps down if there are no
          // active configuration in which it has heard back from a majority of
          // backups within an election timeout.
          // Also see CheckQuorum action in tla/ccfraft.tla.
          RAFT_INFO_FMT(
            "Stepping down as leader {}: No ack received from a majority of "
            "backups in last {}",
            state->node_id,
            election_timeout);
          become_follower();
        }
      }
      else
      {
        if (
          !is_retired_committed() && ticking &&
          timeout_elapsed >= election_timeout)
        {
          // Start an election.
          become_candidate();
        }
      }
    }
 
  private:
    Index find_highest_possible_match(const ccf::TxID& tx_id)
    {
      // Find the highest TxID this node thinks exists, which is still
      // compatible with the given tx_id. That is, given T.n, find largest n'
      // such that n' <= n && term_of(n') == T' && T' <= T. This may be T.n
      // itself, if this node holds that index. Otherwise, examine the final
      // entry in each term, counting backwards, until we find one which is
      // still possible.
      Index probe_index = std::min(tx_id.seqno, state->last_idx);
      Term term_of_probe = state->view_history.view_at(probe_index);
      while (term_of_probe > tx_id.view)
      {
        // Next possible match is the end of the previous term, which is
        // 1-before the start of the currently considered term. Anything after
        // that must have a term which is still too high.
        probe_index = state->view_history.start_of_view(term_of_probe);
        if (probe_index > 0)
        {
          --probe_index;
        }
        term_of_probe = state->view_history.view_at(probe_index);
      }
 
      RAFT_TRACE_FMT(
        "Looking for match with {}.{}, from {}.{}, best answer is {}",
        tx_id.view,
        tx_id.seqno,
        state->view_history.view_at(state->last_idx),
        state->last_idx,
        probe_index);
      return probe_index;
    }
 
    inline void update_batch_size()
    {
      auto avg_entry_size = (entry_count == 0) ?
        append_entries_size_limit :
        entry_size_not_limited / entry_count;
 
      auto batch_size = (avg_entry_size == 0) ?
        append_entries_size_limit / 2 :
        append_entries_size_limit / avg_entry_size;
 
      auto batch_avg = batch_window_sum / batch_window_size;
      // balance out total batch size across batch window
      batch_window_sum += (batch_size - batch_avg);
      entries_batch_size = std::max((batch_window_sum / batch_window_size), 1);
    }
 
    Term get_term_internal(Index idx)
    {
      if (idx > state->last_idx)
        return ccf::VIEW_UNKNOWN;
 
      return state->view_history.view_at(idx);
    }
 
    bool can_replicate_unsafe()
    {
      return state->leadership_state == ccf::kv::LeadershipState::Leader &&
        !is_retired_committed();
    }
 
    bool can_sign_unsafe()
    {
      return state->leadership_state == ccf::kv::LeadershipState::Leader &&
        !is_retired_committed();
    }
 
    Index get_commit_idx_unsafe()
    {
      return state->commit_idx;
    }
 
    void send_append_entries(const ccf::NodeId& to, Index start_idx)
    {
      RAFT_TRACE_FMT(
        "Sending append entries to node {} in batches of {}, covering the "
        "range {} -> {}",
        to,
        entries_batch_size,
        start_idx,
        state->last_idx);
 
      auto calculate_end_index = [this](Index start) {
        // Cap the end index in 2 ways:
        // - Must contain no more than entries_batch_size entries
        // - Must contain entries from a single term
        static_assert(
          max_terms_per_append_entries == 1,
          "AppendEntries construction logic enforces single term");
        auto max_idx = state->last_idx;
        const auto term_of_ae = state->view_history.view_at(start);
        const auto index_at_end_of_term =
          state->view_history.end_of_view(term_of_ae);
        if (index_at_end_of_term != ccf::kv::NoVersion)
        {
          max_idx = index_at_end_of_term;
        }
        return std::min(start + entries_batch_size - 1, max_idx);
      };
 
      Index end_idx;
 
      // We break _after_ sending, so that in the case where this is called
      // with start==last, we send a single empty heartbeat
      do
      {
        end_idx = calculate_end_index(start_idx);
        RAFT_TRACE_FMT("Sending sub range {} -> {}", start_idx, end_idx);
        send_append_entries_range(to, start_idx, end_idx);
        start_idx = std::min(end_idx + 1, state->last_idx);
      } while (end_idx != state->last_idx);
    }
 
    void send_append_entries_range(
      const ccf::NodeId& to, Index start_idx, Index end_idx)
    {
      const auto prev_idx = start_idx - 1;
 
      if (is_retired_committed() && start_idx >= end_idx)
      {
        // Continue to replicate, but do not send heartbeats if we know our
        // retirement is committed
        return;
      }
 
      const auto prev_term = get_term_internal(prev_idx);
      const auto term_of_idx = get_term_internal(end_idx);
 
      RAFT_DEBUG_FMT(
        "Send append entries from {} to {}: ({}.{}, {}.{}] ({})",
        state->node_id,
        to,
        prev_term,
        prev_idx,
        term_of_idx,
        end_idx,
        state->commit_idx);
 
#pragma clang diagnostic push
#pragma clang diagnostic ignored "-Wc99-designator"
      AppendEntries ae{
        {},
        {.idx = end_idx, .prev_idx = prev_idx},
        .term = state->current_view,
        .prev_term = prev_term,
        .leader_commit_idx = state->commit_idx,
        .term_of_idx = term_of_idx,
      };
#pragma clang diagnostic pop
 
      auto& node = all_other_nodes.at(to);
 
#ifdef CCF_RAFT_TRACING
      nlohmann::json j = {};
      j["function"] = "send_append_entries";
      j["packet"] = ae;
      j["state"] = *state;
      COMMITTABLE_INDICES(j["state"], state);
      j["to_node_id"] = to;
      j["match_idx"] = node.match_idx;
      j["sent_idx"] = node.sent_idx;
      RAFT_TRACE_JSON_OUT(j);
#endif
 
      // The host will append log entries to this message when it is
      // sent to the destination node.
      if (!channels->send_authenticated(
            to, ccf::NodeMsgType::consensus_msg, ae))
      {
        return;
      }
 
      // Record the most recent index we have sent to this node.
      node.sent_idx = end_idx;
    }
 
    void recv_append_entries(
      const ccf::NodeId& from,
      AppendEntries r,
      const uint8_t* data,
      size_t size)
    {
      std::unique_lock<ccf::pal::Mutex> guard(state->lock);
 
      RAFT_DEBUG_FMT(
        "Received append entries: {}.{} to {}.{} (from {} in term {})",
        r.prev_term,
        r.prev_idx,
        r.term_of_idx,
        r.idx,
        from,
        r.term);
 
#ifdef CCF_RAFT_TRACING
      nlohmann::json j = {};
      j["function"] = "recv_append_entries";
      j["packet"] = r;
      j["state"] = *state;
      COMMITTABLE_INDICES(j["state"], state);
      j["from_node_id"] = from;
      RAFT_TRACE_JSON_OUT(j);
#endif
 
      // Don't check that the sender node ID is valid. Accept anything that
      // passes the integrity check. This way, entries containing dynamic
      // topology changes that include adding this new leader can be accepted.
 
      // First, check append entries term against our own term, becoming
      // follower if necessary
      if (
        state->current_view == r.term &&
        state->leadership_state == ccf::kv::LeadershipState::Candidate)
      {
        become_aware_of_new_term(r.term);
      }
      else if (state->current_view < r.term)
      {
        become_aware_of_new_term(r.term);
      }
      else if (state->current_view > r.term)
      {
        // Reply false, since our term is later than the received term.
        RAFT_INFO_FMT(
          "Recv append entries to {} from {} but our term is later ({} > {})",
          state->node_id,
          from,
          state->current_view,
          r.term);
        send_append_entries_response_nack(from);
        return;
      }
 
      // Second, check term consistency with the entries we have so far
      const auto prev_term = get_term_internal(r.prev_idx);
      if (prev_term != r.prev_term)
      {
        RAFT_DEBUG_FMT(
          "Previous term for {} should be {}", r.prev_idx, prev_term);
 
        // Reply false if the log doesn't contain an entry at r.prev_idx
        // whose term is r.prev_term. Rejects "future" entries.
        if (prev_term == 0)
        {
          RAFT_DEBUG_FMT(
            "Recv append entries to {} from {} but our log does not yet "
            "contain index {}",
            state->node_id,
            from,
            r.prev_idx);
          send_append_entries_response_nack(from);
        }
        else
        {
          RAFT_DEBUG_FMT(
            "Recv append entries to {} from {} but our log at {} has the wrong "
            "previous term (ours: {}, theirs: {})",
            state->node_id,
            from,
            r.prev_idx,
            prev_term,
            r.prev_term);
          const ccf::TxID rejected_tx{r.prev_term, r.prev_idx};
          send_append_entries_response_nack(from, rejected_tx);
        }
        return;
      }
 
      // If the terms match up, it is sufficient to convince us that the sender
      // is leader in our term
      restart_election_timeout();
      if (!leader_id.has_value() || leader_id.value() != from)
      {
        leader_id = from;
        RAFT_DEBUG_FMT(
          "Node {} thinks leader is {}", state->node_id, leader_id.value());
      }
 
      // Third, check index consistency, making sure entries are not in the past
      if (r.prev_idx < state->commit_idx)
      {
        RAFT_DEBUG_FMT(
          "Recv append entries to {} from {} but prev_idx ({}) < commit_idx "
          "({})",
          state->node_id,
          from,
          r.prev_idx,
          state->commit_idx);
        return;
      }
      // Redundant with check on get_term_internal() at line 1149
      // Which captures this case in every situation, except r.prev_term == 0.
      // That only happens if r.prev_idx == 0 however, see line 1033,
      // in which case this path should not be taken either.
      else if (r.prev_idx > state->last_idx)
      {
        RAFT_FAIL_FMT(
          "Recv append entries to {} from {} but prev_idx ({}) > last_idx ({})",
          state->node_id,
          from,
          r.prev_idx,
          state->last_idx);
        return;
      }
 
      RAFT_DEBUG_FMT(
        "Recv append entries to {} from {} for index {} and previous index {}",
        state->node_id,
        from,
        r.idx,
        r.prev_idx);
 
      std::vector<std::tuple<
        std::unique_ptr<ccf::kv::AbstractExecutionWrapper>,
        ccf::kv::Version>>
        append_entries;
      // Finally, deserialise each entry in the batch
      for (Index i = r.prev_idx + 1; i <= r.idx; i++)
      {
        if (i <= state->last_idx)
        {
          // NB: This is only safe as long as AppendEntries only contain a
          // single term. If they cover multiple terms, then we would need to at
          // least partially deserialise each entry to establish what term it is
          // in (or report the terms in the header)
          static_assert(
            max_terms_per_append_entries == 1,
            "AppendEntries rollback logic assumes single term");
          const auto incoming_term = r.term_of_idx;
          const auto local_term = state->view_history.view_at(i);
          if (incoming_term != local_term)
          {
            if (is_new_follower)
            {
              auto rollback_level = i - 1;
              RAFT_DEBUG_FMT(
                "New follower received AppendEntries with conflict. Incoming "
                "entry {}.{} conflicts with local {}.{}. Rolling back to {}.",
                incoming_term,
                i,
                local_term,
                i,
                rollback_level);
              LOG_ROLLBACK_INFO_FMT(
                "Dropping conflicting branch. Rolling back {} entries, "
                "beginning with {}.{}.",
                state->last_idx - rollback_level,
                local_term,
                i);
              rollback(rollback_level);
              is_new_follower = false;
              // Then continue to process this AE as normal
            }
            else
            {
              // We have a node retaining a conflicting suffix, and refusing to
              // roll it back. It will remain divergent (not contributing to
              // commit) this term, and can only be brought in-sync in a future
              // term.
              // This log is emitted as a canary, for what we hope is an
              // unreachable branch. If it is ever seen we should revisit this.
              LOG_ROLLBACK_INFO_FMT(
                "Ignoring conflicting AppendEntries. Retaining {} entries, "
                "beginning with {}.{}.",
                state->last_idx - (i - 1),
                local_term,
                i);
              return;
            }
          }
          else
          {
            // If the current entry has already been deserialised, skip the
            // payload for that entry
            ledger->skip_entry(data, size);
            continue;
          }
        }
 
        std::vector<uint8_t> entry;
        try
        {
          entry = LedgerProxy::get_entry(data, size);
        }
        catch (const std::logic_error& e)
        {
          // This should only fail if there is malformed data.
          RAFT_FAIL_FMT(
            "Recv append entries to {} from {} but the data is malformed: {}",
            state->node_id,
            from,
            e.what());
          send_append_entries_response_nack(from);
          return;
        }
 
        ccf::kv::TxID expected{r.term_of_idx, i};
        auto ds = store->deserialize(entry, public_only, expected);
        if (ds == nullptr)
        {
          RAFT_FAIL_FMT(
            "Recv append entries to {} from {} but the entry could not be "
            "deserialised",
            state->node_id,
            from);
          send_append_entries_response_nack(from);
          return;
        }
 
        append_entries.push_back(std::make_tuple(std::move(ds), i));
      }
 
      execute_append_entries_sync(
        std::move(append_entries), from, std::move(r));
    }
 
    void execute_append_entries_sync(
      std::vector<std::tuple<
        std::unique_ptr<ccf::kv::AbstractExecutionWrapper>,
        ccf::kv::Version>>&& append_entries,
      const ccf::NodeId& from,
      AppendEntries&& r)
    {
      for (auto& ae : append_entries)
      {
        auto& [ds, i] = ae;
        RAFT_DEBUG_FMT("Replicating on follower {}: {}", state->node_id, i);
 
#ifdef CCF_RAFT_TRACING
        nlohmann::json j = {};
        j["function"] = "execute_append_entries_sync";
        j["state"] = *state;
        COMMITTABLE_INDICES(j["state"], state);
        j["from_node_id"] = from;
        RAFT_TRACE_JSON_OUT(j);
#endif
 
        bool track_deletes_on_missing_keys = false;
        ccf::kv::ApplyResult apply_success =
          ds->apply(track_deletes_on_missing_keys);
        if (apply_success == ccf::kv::ApplyResult::FAIL)
        {
          ledger->truncate(i - 1);
          send_append_entries_response_nack(from);
          return;
        }
        state->last_idx = i;
 
        for (auto& hook : ds->get_hooks())
        {
          hook->call(this);
        }
 
        bool globally_committable =
          (apply_success == ccf::kv::ApplyResult::PASS_SIGNATURE);
        if (globally_committable)
        {
          start_ticking_if_necessary();
        }
 
        const auto& entry = ds->get_entry();
 
        ledger->put_entry(
          entry, globally_committable, ds->get_term(), ds->get_index());
 
        switch (apply_success)
        {
          case ccf::kv::ApplyResult::FAIL:
          {
            RAFT_FAIL_FMT("Follower failed to apply log entry: {}", i);
            state->last_idx--;
            ledger->truncate(state->last_idx);
            send_append_entries_response_nack(from);
            break;
          }
 
          case ccf::kv::ApplyResult::PASS_SIGNATURE:
          {
            RAFT_DEBUG_FMT("Deserialising signature at {}", i);
            if (
              state->membership_state == ccf::kv::MembershipState::Retired &&
              state->retirement_phase == ccf::kv::RetirementPhase::Ordered)
            {
              become_retired(i, ccf::kv::RetirementPhase::Signed);
            }
            state->committable_indices.push_back(i);
 
            if (ds->get_term())
            {
              // A signature for sig_term tells us that all transactions from
              // the previous signature onwards (at least, if not further back)
              // happened in sig_term. We reflect this in the history.
              if (r.term_of_idx == aft::ViewHistory::InvalidView)
              {
                state->view_history.update(1, r.term);
              }
              else
              {
                // NB: This is only safe as long as AppendEntries only contain a
                // single term. If they cover multiple terms, then we need to
                // know our previous signature locally.
                static_assert(
                  max_terms_per_append_entries == 1,
                  "AppendEntries processing for term updates assumes single "
                  "term");
                state->view_history.update(r.prev_idx + 1, ds->get_term());
              }
 
              commit_if_possible(r.leader_commit_idx);
            }
            break;
          }
 
          case ccf::kv::ApplyResult::PASS:
          {
            break;
          }
 
          case ccf::kv::ApplyResult::PASS_ENCRYPTED_PAST_LEDGER_SECRET:
          {
            break;
          }
 
          default:
          {
            throw std::logic_error("Unknown ApplyResult value");
          }
        }
      }
 
      execute_append_entries_finish(r, from);
    }
 
    void execute_append_entries_finish(
      AppendEntries& r, const ccf::NodeId& from)
    {
      // After entries have been deserialised, try to commit the leader's
      // commit index and update our term history accordingly
      commit_if_possible(r.leader_commit_idx);
 
      // The term may have changed, and we have not have seen a signature yet.
      auto lci = last_committable_index();
      if (r.term_of_idx == aft::ViewHistory::InvalidView)
      {
        // If we don't yet have a term history, then this must be happening in
        // the current term. This can only happen before _any_ transactions have
        // occurred, when processing a heartbeat at index 0, which does not
        // happen in a real node (due to the genesis transaction executing
        // before ticks start), but may happen in tests.
        state->view_history.update(1, r.term);
      }
      else
      {
        // The end of this append entries (r.idx) was not a signature, but may
        // be in a new term. If it's a new term, this term started immediately
        // after the previous signature we saw (lci, last committable index).
        if (r.idx > lci)
        {
          state->view_history.update(lci + 1, r.term_of_idx);
        }
      }
 
      send_append_entries_response_ack(from, r);
    }
 
    void send_append_entries_response_ack(
      ccf::NodeId to, const AppendEntries& ae)
    {
      // If we get to here, we have applied up to r.idx in this AppendEntries.
      // We must only ACK this far, as we know nothing about the agreement of a
      // suffix we may still hold _after_ r.idx with the leader's log
      const auto response_idx = ae.idx;
      send_append_entries_response(
        to, AppendEntriesResponseType::OK, state->current_view, response_idx);
    }
 
    void send_append_entries_response_nack(
      ccf::NodeId to, const ccf::TxID& rejected)
    {
      const auto response_idx = find_highest_possible_match(rejected);
      const auto response_term = get_term_internal(response_idx);
 
      send_append_entries_response(
        to, AppendEntriesResponseType::FAIL, response_term, response_idx);
    }
 
    void send_append_entries_response_nack(ccf::NodeId to)
    {
      send_append_entries_response(
        to,
        AppendEntriesResponseType::FAIL,
        state->current_view,
        state->last_idx);
    }
 
    void send_append_entries_response(
      ccf::NodeId to,
      AppendEntriesResponseType answer,
      aft::Term response_term,
      aft::Index response_idx)
    {
      RAFT_DEBUG_FMT(
        "Send append entries response from {} to {} for index {}: {}",
        state->node_id,
        to,
        response_idx,
        (answer == AppendEntriesResponseType::OK ? "ACK" : "NACK"));
 
      AppendEntriesResponse response{
        .term = response_term,
        .last_log_idx = response_idx,
        .success = answer,
      };
 
#ifdef CCF_RAFT_TRACING
      nlohmann::json j = {};
      j["function"] = "send_append_entries_response";
      j["packet"] = response;
      j["state"] = *state;
      COMMITTABLE_INDICES(j["state"], state);
      j["to_node_id"] = to;
      RAFT_TRACE_JSON_OUT(j);
#endif
 
      channels->send_authenticated(
        to, ccf::NodeMsgType::consensus_msg, response);
    }
 
    void recv_append_entries_response(
      const ccf::NodeId& from, AppendEntriesResponse r)
    {
      std::lock_guard<ccf::pal::Mutex> guard(state->lock);
 
      auto node = all_other_nodes.find(from);
      if (node == all_other_nodes.end())
      {
        // Ignore if we don't recognise the node.
        RAFT_FAIL_FMT(
          "Recv append entries response to {} from {}: unknown node",
          state->node_id,
          from);
        return;
      }
 
#ifdef CCF_RAFT_TRACING
      nlohmann::json j = {};
      j["function"] = "recv_append_entries_response";
      j["packet"] = r;
      j["state"] = *state;
      COMMITTABLE_INDICES(j["state"], state);
      j["from_node_id"] = from;
      j["match_idx"] = node->second.match_idx;
      j["sent_idx"] = node->second.sent_idx;
      RAFT_TRACE_JSON_OUT(j);
#endif
 
      // Ignore if we're not the leader.
      if (state->leadership_state != ccf::kv::LeadershipState::Leader)
      {
        RAFT_FAIL_FMT(
          "Recv append entries response to {} from {}: no longer leader",
          state->node_id,
          from);
        return;
      }
 
      using namespace std::chrono_literals;
      node->second.last_ack_timeout = 0ms;
 
      if (state->current_view < r.term)
      {
        // We are behind, update our state.
        RAFT_DEBUG_FMT(
          "Recv append entries response to {} from {}: more recent term ({} "
          "> {})",
          state->node_id,
          from,
          r.term,
          state->current_view);
        become_aware_of_new_term(r.term);
        return;
      }
      else if (state->current_view != r.term)
      {
        // Stale response, discard if success.
        // Otherwise reset sent_idx and try again.
        // NB: In NACKs the term may be that of an estimated matching index
        // in the log, rather than the current term, so it is correct for it to
        // be older in this case.
        if (r.success == AppendEntriesResponseType::OK)
        {
          RAFT_DEBUG_FMT(
            "Recv append entries response to {} from {}: stale term ({} != {})",
            state->node_id,
            from,
            r.term,
            state->current_view);
          return;
        }
      }
      else if (r.last_log_idx < node->second.match_idx)
      {
        // Response about past indices, discard if success.
        // Otherwise reset sent_idx and try again.
        // NB: It is correct for this index to move backwards during NACKs
        // which iteratively discover the last matching index of divergent logs
        // after an election.
        if (r.success == AppendEntriesResponseType::OK)
        {
          RAFT_DEBUG_FMT(
            "Recv append entries response to {} from {}: stale idx",
            state->node_id,
            from);
          return;
        }
      }
 
      // Update next or match for the responding node.
      if (r.success == AppendEntriesResponseType::FAIL)
      {
        // Failed due to log inconsistency. Reset sent_idx, and try again soon.
        RAFT_DEBUG_FMT(
          "Recv append entries response to {} from {}: failed",
          state->node_id,
          from);
        const auto this_match =
          find_highest_possible_match({r.term, r.last_log_idx});
        node->second.sent_idx = std::max(
          std::min(this_match, node->second.sent_idx), node->second.match_idx);
        return;
      }
      else
      {
        // max(...) because why would we ever want to go backwards on a success
        // response?!
        node->second.match_idx =
          std::max(node->second.match_idx, r.last_log_idx);
      }
 
      RAFT_DEBUG_FMT(
        "Recv append entries response to {} from {} for index {}: success",
        state->node_id,
        from,
        r.last_log_idx);
      update_commit();
    }
 
    void send_request_vote(const ccf::NodeId& to)
    {
      auto last_committable_idx = last_committable_index();
      RAFT_INFO_FMT(
        "Send request vote from {} to {} at {}",
        state->node_id,
        to,
        last_committable_idx);
      CCF_ASSERT(last_committable_idx >= state->commit_idx, "lci < ci");
 
      RequestVote rv{
        .term = state->current_view,
        .last_committable_idx = last_committable_idx,
        .term_of_last_committable_idx = get_term_internal(last_committable_idx),
      };
 
#ifdef CCF_RAFT_TRACING
      nlohmann::json j = {};
      j["function"] = "send_request_vote";
      j["packet"] = rv;
      j["state"] = *state;
      COMMITTABLE_INDICES(j["state"], state);
      j["to_node_id"] = to;
      RAFT_TRACE_JSON_OUT(j);
#endif
 
      channels->send_authenticated(to, ccf::NodeMsgType::consensus_msg, rv);
    }
 
    void recv_request_vote(const ccf::NodeId& from, RequestVote r)
    {
      std::lock_guard<ccf::pal::Mutex> guard(state->lock);
 
      // Do not check that from is a known node. It is possible to receive
      // RequestVotes from nodes that this node doesn't yet know, just as it
      // receives AppendEntries from those nodes. These should be obeyed just
      // like any other RequestVote - it is possible that this node is needed to
      // produce a primary in the new term, who will then help this node catch
      // up.
 
#ifdef CCF_RAFT_TRACING
      nlohmann::json j = {};
      j["function"] = "recv_request_vote";
      j["packet"] = r;
      j["state"] = *state;
      COMMITTABLE_INDICES(j["state"], state);
      j["from_node_id"] = from;
      RAFT_TRACE_JSON_OUT(j);
#endif
 
      if (state->current_view > r.term)
      {
        // Reply false, since our term is later than the received term.
        RAFT_DEBUG_FMT(
          "Recv request vote to {} from {}: our term is later ({} > {})",
          state->node_id,
          from,
          state->current_view,
          r.term);
        send_request_vote_response(from, false);
        return;
      }
      else if (state->current_view < r.term)
      {
        RAFT_DEBUG_FMT(
          "Recv request vote to {} from {}: their term is later ({} < {})",
          state->node_id,
          from,
          state->current_view,
          r.term);
        become_aware_of_new_term(r.term);
      }
 
      if (leader_id.has_value())
      {
        // Reply false, since we already know the leader in the current term.
        RAFT_DEBUG_FMT(
          "Recv request vote to {} from {}: leader {} already known in term {}",
          state->node_id,
          from,
          leader_id.value(),
          state->current_view);
        send_request_vote_response(from, false);
        return;
      }
 
      if ((voted_for.has_value()) && (voted_for.value() != from))
      {
        // Reply false, since we already voted for someone else.
        RAFT_DEBUG_FMT(
          "Recv request vote to {} from {}: already voted for {}",
          state->node_id,
          from,
          voted_for.value());
        send_request_vote_response(from, false);
        return;
      }
 
      // If the candidate's committable log is at least as up-to-date as ours,
      // vote yes
 
      const auto last_committable_idx = last_committable_index();
      const auto term_of_last_committable_idx =
        get_term_internal(last_committable_idx);
 
      const auto answer =
        (r.term_of_last_committable_idx > term_of_last_committable_idx) ||
        ((r.term_of_last_committable_idx == term_of_last_committable_idx) &&
         (r.last_committable_idx >= last_committable_idx));
 
      if (answer)
      {
        // If we grant our vote, we also acknowledge that an election is in
        // progress.
        restart_election_timeout();
        leader_id.reset();
        voted_for = from;
      }
      else
      {
        RAFT_INFO_FMT(
          "Voting against candidate at {}.{} because local state is at {}.{}",
          r.term_of_last_committable_idx,
          r.last_committable_idx,
          term_of_last_committable_idx,
          last_committable_idx);
      }
 
      send_request_vote_response(from, answer);
    }
 
    void send_request_vote_response(const ccf::NodeId& to, bool answer)
    {
      RAFT_INFO_FMT(
        "Send request vote response from {} to {}: {}",
        state->node_id,
        to,
        answer);
 
      RequestVoteResponse response{
        .term = state->current_view, .vote_granted = answer};
 
      channels->send_authenticated(
        to, ccf::NodeMsgType::consensus_msg, response);
    }
 
    void recv_request_vote_response(
      const ccf::NodeId& from, RequestVoteResponse r)
    {
      std::lock_guard<ccf::pal::Mutex> guard(state->lock);
 
#ifdef CCF_RAFT_TRACING
      nlohmann::json j = {};
      j["function"] = "recv_request_vote_response";
      j["packet"] = r;
      j["state"] = *state;
      COMMITTABLE_INDICES(j["state"], state);
      j["from_node_id"] = from;
      RAFT_TRACE_JSON_OUT(j);
#endif
 
      if (state->leadership_state != ccf::kv::LeadershipState::Candidate)
      {
        RAFT_INFO_FMT(
          "Recv request vote response to {} from: {}: we aren't a candidate",
          state->node_id,
          from);
        return;
      }
 
      // Ignore if we don't recognise the node.
      auto node = all_other_nodes.find(from);
      if (node == all_other_nodes.end())
      {
        RAFT_INFO_FMT(
          "Recv request vote response to {} from {}: unknown node",
          state->node_id,
          from);
        return;
      }
 
      if (state->current_view < r.term)
      {
        RAFT_INFO_FMT(
          "Recv request vote response to {} from {}: their term is more recent "
          "({} < {})",
          state->node_id,
          from,
          state->current_view,
          r.term);
        become_aware_of_new_term(r.term);
        return;
      }
      else if (state->current_view != r.term)
      {
        // Ignore as it is stale.
        RAFT_INFO_FMT(
          "Recv request vote response to {} from {}: stale ({} != {})",
          state->node_id,
          from,
          state->current_view,
          r.term);
        return;
      }
      else if (!r.vote_granted)
      {
        // Do nothing.
        RAFT_INFO_FMT(
          "Recv request vote response to {} from {}: they voted no",
          state->node_id,
          from);
        return;
      }
 
      RAFT_INFO_FMT(
        "Recv request vote response to {} from {}: they voted yes",
        state->node_id,
        from);
 
      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;
      COMMITTABLE_INDICES(j["state"], state);
      j["from_node_id"] = from;
      RAFT_TRACE_JSON_OUT(j);
#endif
      if (!is_retired_committed() && 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
      // between 0.5x and 1x the configured election timeout.
      timeout_elapsed = std::chrono::milliseconds(distrib(rand));
    }
 
    void reset_votes_for_me()
    {
      votes_for_me.clear();
      for (auto const& conf : configurations)
      {
        votes_for_me[conf.idx].quorum = get_quorum(conf.nodes.size());
        votes_for_me[conf.idx].votes.clear();
      }
    }
 
    // ccfraft!Timeout
    void become_candidate()
    {
      if (configurations.empty())
      {
        // ccfraft!Timeout:
        //  /\ \E c \in DOMAIN configurations[i] :
        //     /\ i \in configurations[i][c]
        LOG_INFO_FMT(
          "Not becoming candidate {} due to lack of a configuration.",
          state->node_id);
        return;
      }
 
      state->leadership_state = ccf::kv::LeadershipState::Candidate;
      leader_id.reset();
 
      voted_for = state->node_id;
      reset_votes_for_me();
      state->current_view++;
 
      restart_election_timeout();
      reset_last_ack_timeouts();
 
      RAFT_INFO_FMT(
        "Becoming candidate {}: {}", state->node_id, state->current_view);
 
#ifdef CCF_RAFT_TRACING
      nlohmann::json j = {};
      j["function"] = "become_candidate";
      j["state"] = *state;
      COMMITTABLE_INDICES(j["state"], state);
      j["configurations"] = configurations;
      RAFT_TRACE_JSON_OUT(j);
#endif
 
      add_vote_for_me(state->node_id);
 
      // Request votes only go to nodes in configurations, since only
      // their votes can be tallied towards an election quorum.
      for (auto const& node_id : other_nodes_in_active_configs())
      {
        // ccfraft!RequestVote
        send_request_vote(node_id);
      }
    }
 
    void become_leader(bool force_become_leader = false)
    {
      if (is_retired_committed())
      {
        return;
      }
 
      const auto election_index = last_committable_index();
 
      RAFT_DEBUG_FMT(
        "Election index is {} in term {}", election_index, state->current_view);
      // Discard any un-committable updates we may hold,
      // since we have no signature for them. Except at startup,
      // where we do not want to roll back the genesis transaction.
      if (state->commit_idx > 0)
      {
        rollback(election_index);
      }
      else
      {
        // but we still want the KV to know which term we're in
        store->initialise_term(state->current_view);
      }
 
      state->leadership_state = ccf::kv::LeadershipState::Leader;
      leader_id = state->node_id;
      should_sign = true;
 
      using namespace std::chrono_literals;
      timeout_elapsed = 0ms;
 
      reset_last_ack_timeouts();
 
      RAFT_INFO_FMT(
        "Becoming leader {}: {}", state->node_id, state->current_view);
 
#ifdef CCF_RAFT_TRACING
      nlohmann::json j = {};
      j["function"] = "become_leader";
      j["state"] = *state;
      COMMITTABLE_INDICES(j["state"], state);
      j["configurations"] = configurations;
      RAFT_TRACE_JSON_OUT(j);
#endif
 
      // Try to advance commit at once if there are no other nodes.
      if (other_nodes_in_active_configs().size() == 0)
      {
        update_commit();
      }
 
      // Reset next, match, and sent indices for all nodes.
      auto next = state->last_idx + 1;
 
      for (auto& node : all_other_nodes)
      {
        node.second.match_idx = 0;
        node.second.sent_idx = next - 1;
 
        // Send an empty append_entries to all nodes.
        send_append_entries(node.first, next);
      }
 
      if (retired_node_cleanup)
      {
        retired_node_cleanup->cleanup();
      }
    }
 
  public:
    // Called when a replica becomes follower in the same term, e.g. when the
    // primary node has not received a majority of acks (CheckQuorum)
    void become_follower()
    {
      leader_id.reset();
      restart_election_timeout();
      reset_last_ack_timeouts();
 
      // Drop anything unsigned here, but retain all signed entries. Only do a
      // more aggressive rollback, potentially including signatures, when
      // receiving a conflicting AppendEntries
      rollback(last_committable_index());
 
      state->leadership_state = ccf::kv::LeadershipState::Follower;
      RAFT_INFO_FMT(
        "Becoming follower {}: {}.{}",
        state->node_id,
        state->current_view,
        state->commit_idx);
 
#ifdef CCF_RAFT_TRACING
      nlohmann::json j = {};
      j["function"] = "become_follower";
      j["state"] = *state;
      COMMITTABLE_INDICES(j["state"], state);
      j["configurations"] = configurations;
      RAFT_TRACE_JSON_OUT(j);
#endif
    }
 
    // Called when a replica becomes aware of the existence of a new term
    // If retired already, state remains unchanged, but the replica otherwise
    // becomes a follower in the new term.
    void become_aware_of_new_term(Term term)
    {
      RAFT_DEBUG_FMT("Becoming aware of new term {}", term);
 
      state->current_view = term;
      voted_for.reset();
      reset_votes_for_me();
      become_follower();
      is_new_follower = true;
    }
 
  private:
    void become_retired(Index idx, ccf::kv::RetirementPhase phase)
    {
      RAFT_INFO_FMT(
        "Becoming retired, phase {} (leadership {}): {}: {} at {}",
        phase,
        state->leadership_state,
        state->node_id,
        state->current_view,
        idx);
 
      if (phase == ccf::kv::RetirementPhase::Ordered)
      {
        CCF_ASSERT_FMT(
          !state->retirement_idx.has_value(),
          "retirement_idx already set to {}",
          state->retirement_idx.value());
        state->retirement_idx = idx;
        RAFT_INFO_FMT("Node retiring at {}", idx);
      }
      else if (phase == ccf::kv::RetirementPhase::Signed)
      {
        assert(state->retirement_idx.has_value());
        CCF_ASSERT_FMT(
          idx >= state->retirement_idx.value(),
          "Index {} unexpectedly lower than retirement_idx {}",
          idx,
          state->retirement_idx.value());
        state->retirement_committable_idx = idx;
        RAFT_INFO_FMT("Node retirement committable at {}", idx);
      }
      else if (phase == ccf::kv::RetirementPhase::RetiredCommitted)
      {
        if (state->leadership_state == ccf::kv::LeadershipState::Leader)
        {
          ProposeRequestVote prv{.term = state->current_view};
 
          std::optional<ccf::NodeId> successor = std::nullopt;
          Index max_match_idx = 0;
          ccf::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)
            {
              ccf::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);
          }
        }
 
        leader_id.reset();
        state->leadership_state = ccf::kv::LeadershipState::None;
      }
 
      state->membership_state = ccf::kv::MembershipState::Retired;
      state->retirement_phase = phase;
    }
 
    void add_vote_for_me(const ccf::NodeId& from)
    {
      if (configurations.empty())
      {
        LOG_INFO_FMT(
          "Not voting for myself {} due to lack of a configuration.",
          state->node_id);
        return;
      }
 
      // Add vote for from node in each configuration where it is present
      for (auto const& conf : configurations)
      {
        auto const& nodes = conf.nodes;
        if (nodes.find(from) == nodes.end())
        {
          // from node is no longer in any active configuration.
          continue;
        }
 
        votes_for_me[conf.idx].votes.insert(from);
        RAFT_DEBUG_FMT(
          "Node {} voted for {} in configuration {} with quorum {}",
          from,
          state->node_id,
          conf.idx,
          votes_for_me[conf.idx].quorum);
      }
 
      // We need a quorum of votes in _all_ configurations to become leader
      bool is_elected = true;
      for (auto const& v : votes_for_me)
      {
        auto const& quorum = v.second.quorum;
        auto const& votes = v.second.votes;
 
        if (votes.size() < quorum)
        {
          is_elected = false;
          break;
        }
      }
 
      if (is_elected)
      {
        become_leader();
      }
    }
 
    // If there exists some committable idx in the current term such that idx >
    // commit_idx and a majority of nodes have replicated it, commit to that
    // idx.
    void update_commit()
    {
      if (state->leadership_state != ccf::kv::LeadershipState::Leader)
      {
        throw std::logic_error(
          "update_commit() must only be called while this node is leader");
      }
 
      std::optional<Index> new_agreement_index = std::nullopt;
 
      // Obtain CFT watermarks
      for (auto const& c : configurations)
      {
        // The majority must be checked separately for each active
        // configuration.
        std::vector<Index> match;
        match.reserve(c.nodes.size());
 
        for (auto node : c.nodes)
        {
          if (node.first == state->node_id)
          {
            match.push_back(state->last_idx);
          }
          else
          {
            match.push_back(all_other_nodes.at(node.first).match_idx);
          }
        }
 
        sort(match.begin(), match.end());
        auto confirmed = match.at((match.size() - 1) / 2);
 
        if (
          !new_agreement_index.has_value() ||
          confirmed < new_agreement_index.value())
        {
          new_agreement_index = confirmed;
        }
      }
 
      if (new_agreement_index.has_value())
      {
        if (new_agreement_index.value() > state->last_idx)
        {
          throw std::logic_error(
            "Followers appear to have later match indices than leader");
        }
 
        const auto new_commit_idx =
          find_highest_possible_committable_index(new_agreement_index.value());
 
        if (new_commit_idx.has_value())
        {
          RAFT_DEBUG_FMT(
            "In update_commit, new_commit_idx: {}, "
            "last_idx: {}",
            new_commit_idx.value(),
            state->last_idx);
 
          const auto term_of_new = get_term_internal(new_commit_idx.value());
          if (term_of_new == state->current_view)
          {
            commit(new_commit_idx.value());
          }
          else
          {
            RAFT_DEBUG_FMT(
              "Ack quorum at {} resulted in proposed commit index {}, which "
              "is in term {}. Waiting for agreement on committable entry in "
              "current term {} to update commit",
              new_agreement_index.value(),
              new_commit_idx.value(),
              term_of_new,
              state->current_view);
          }
        }
      }
    }
 
    // Commits at the highest committable index which is not greater than the
    // given idx.
    void commit_if_possible(Index idx)
    {
      RAFT_DEBUG_FMT(
        "Commit if possible {} (ci: {}) (ti {})",
        idx,
        state->commit_idx,
        get_term_internal(idx));
      if (
        (idx > state->commit_idx) &&
        (get_term_internal(idx) <= state->current_view))
      {
        const auto highest_committable =
          find_highest_possible_committable_index(idx);
        if (highest_committable.has_value())
        {
          commit(highest_committable.value());
        }
      }
    }
 
    size_t get_quorum(size_t n) const
    {
      return (n / 2) + 1;
    }
 
    void commit(Index idx)
    {
      if (idx > state->last_idx)
      {
        throw std::logic_error(fmt::format(
          "Tried to commit {} but last_idx is {}", idx, state->last_idx));
      }
 
      RAFT_DEBUG_FMT("Starting commit");
 
      // This could happen if a follower becomes the leader when it
      // has committed fewer log entries, although it has them available.
      if (idx <= state->commit_idx)
        return;
 
#ifdef CCF_RAFT_TRACING
      nlohmann::json j = {};
      j["function"] = "commit";
      j["args"] = nlohmann::json::object();
      j["args"]["idx"] = idx;
      j["state"] = *state;
      COMMITTABLE_INDICES(j["state"], state);
      j["configurations"] = configurations;
      RAFT_TRACE_JSON_OUT(j);
#endif
 
      compact_committable_indices(idx);
 
      state->commit_idx = idx;
      if (
        is_retired() &&
        state->retirement_phase == ccf::kv::RetirementPhase::Signed &&
        state->retirement_committable_idx.has_value() &&
        idx >= state->retirement_committable_idx.value())
      {
        become_retired(idx, ccf::kv::RetirementPhase::Completed);
      }
 
      RAFT_DEBUG_FMT("Compacting...");
      store->compact(idx);
      ledger->commit(idx);
 
      RAFT_DEBUG_FMT("Commit on {}: {}", state->node_id, idx);
 
      // Examine each configuration that is followed by a globally committed
      // configuration.
      bool changed = false;
 
      while (true)
      {
        auto conf = configurations.begin();
        if (conf == configurations.end())
        {
          break;
        }
 
        auto next = std::next(conf);
        if (next == configurations.end())
        {
          break;
        }
 
        if (idx < next->idx)
        {
          break;
        }
 
        RAFT_DEBUG_FMT(
          "Configurations: discard committed configuration at {}", conf->idx);
        configurations.pop_front();
        changed = true;
      }
 
      if (changed)
      {
        create_and_remove_node_state();
        if (retired_node_cleanup && is_primary())
        {
          retired_node_cleanup->cleanup();
        }
      }
    }
 
    bool is_self_in_latest_config()
    {
      bool present = false;
      if (!configurations.empty())
      {
        auto current_nodes = configurations.back().nodes;
        present = current_nodes.find(state->node_id) != current_nodes.end();
      }
      return present;
    }
 
    void start_ticking_if_necessary()
    {
      if (!ticking && is_self_in_latest_config())
      {
        start_ticking();
      }
    }
 
  public:
    void rollback(Index idx)
    {
      if (idx < state->commit_idx)
      {
        RAFT_FAIL_FMT(
          "Asked to rollback to idx:{} but committed to commit_idx:{} - "
          "ignoring rollback request",
          idx,
          state->commit_idx);
        return;
      }
 
      store->rollback({get_term_internal(idx), idx}, state->current_view);
 
      RAFT_DEBUG_FMT("Setting term in store to: {}", state->current_view);
      ledger->truncate(idx);
      state->last_idx = idx;
      RAFT_DEBUG_FMT("Rolled back at {}", idx);
 
      state->view_history.rollback(idx);
 
      while (!state->committable_indices.empty() &&
             (state->committable_indices.back() > idx))
      {
        state->committable_indices.pop_back();
      }
 
      if (
        state->membership_state == ccf::kv::MembershipState::Retired &&
        state->retirement_phase == ccf::kv::RetirementPhase::Signed)
      {
        assert(state->retirement_committable_idx.has_value());
        if (state->retirement_committable_idx.value() > idx)
        {
          state->retirement_committable_idx = std::nullopt;
          state->retirement_phase = ccf::kv::RetirementPhase::Ordered;
        }
      }
 
      if (
        state->membership_state == ccf::kv::MembershipState::Retired &&
        state->retirement_phase == ccf::kv::RetirementPhase::Ordered)
      {
        assert(state->retirement_idx.has_value());
        if (state->retirement_idx.value() > idx)
        {
          state->retirement_idx = std::nullopt;
          state->retirement_phase = std::nullopt;
          state->membership_state = ccf::kv::MembershipState::Active;
          RAFT_DEBUG_FMT("Becoming Active after rollback");
        }
      }
 
      // Rollback configurations.
      bool changed = false;
 
      while (!configurations.empty() && (configurations.back().idx > idx))
      {
        RAFT_DEBUG_FMT(
          "Configurations: rollback configuration at {}",
          configurations.back().idx);
        configurations.pop_back();
        changed = true;
      }
 
      if (changed)
      {
        create_and_remove_node_state();
      }
    }
 
    nlohmann::json get_state_representation() const
    {
      return *state;
    }
 
  private:
    void create_and_remove_node_state()
    {
      // Find all nodes present in any active configuration.
      Configuration::Nodes active_nodes;
 
      for (auto const& conf : configurations)
      {
        for (auto const& node : conf.nodes)
        {
          active_nodes.emplace(node.first, node.second);
        }
      }
 
      // Remove all nodes in the node state that are not present in any active
      // configuration.
      std::vector<ccf::NodeId> to_remove;
 
      for (const auto& node : all_other_nodes)
      {
        if (active_nodes.find(node.first) == active_nodes.end())
        {
          to_remove.push_back(node.first);
        }
      }
 
      // Add all active nodes that are not already present in the node state.
      for (auto node_info : active_nodes)
      {
        if (node_info.first == state->node_id)
        {
          continue;
        }
 
        if (all_other_nodes.find(node_info.first) == all_other_nodes.end())
        {
          if (!channels->have_channel(node_info.first))
          {
            RAFT_DEBUG_FMT(
              "Configurations: create node channel with {}", node_info.first);
 
            channels->associate_node_address(
              node_info.first,
              node_info.second.hostname,
              node_info.second.port);
          }
 
          // A new node is sent only future entries initially. If it does not
          // have prior data, it will communicate that back to the leader.
          auto index = state->last_idx + 1;
          all_other_nodes.try_emplace(
            node_info.first, node_info.second, index, 0);
 
          if (state->leadership_state == ccf::kv::LeadershipState::Leader)
          {
            send_append_entries(node_info.first, index);
          }
 
          RAFT_INFO_FMT(
            "Added raft node {} ({}:{})",
            node_info.first,
            node_info.second.hostname,
            node_info.second.port);
        }
      }
    }
  };
}