src/kudu/consensus/raft_consensus_state.cc

// Licensed to the Apache Software Foundation (ASF) under one
// or more contributor license agreements.  See the NOTICE file
// distributed with this work for additional information
// regarding copyright ownership.  The ASF licenses this file
// to you under the Apache License, Version 2.0 (the
// "License"); you may not use this file except in compliance
// with the License.  You may obtain a copy of the License at
//
//   http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing,
// software distributed under the License is distributed on an
// "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
// KIND, either express or implied.  See the License for the
// specific language governing permissions and limitations
// under the License.

#include <algorithm>

#include "kudu/consensus/log_util.h"
#include "kudu/consensus/quorum_util.h"
#include "kudu/consensus/raft_consensus_state.h"
#include "kudu/gutil/map-util.h"
#include "kudu/gutil/strings/join.h"
#include "kudu/gutil/strings/strcat.h"
#include "kudu/gutil/strings/substitute.h"
#include "kudu/util/debug/trace_event.h"
#include "kudu/util/logging.h"
#include "kudu/util/status.h"
#include "kudu/util/trace.h"

namespace kudu {
namespace consensus {

using std::string;
using strings::Substitute;
using strings::SubstituteAndAppend;

//////////////////////////////////////////////////
// ReplicaState
//////////////////////////////////////////////////

ReplicaState::ReplicaState(ConsensusOptions options, string peer_uuid,
                           gscoped_ptr<ConsensusMetadata> cmeta,
                           ReplicaTransactionFactory* txn_factory)
    : options_(std::move(options)),
      peer_uuid_(std::move(peer_uuid)),
      cmeta_(std::move(cmeta)),
      next_index_(0),
      txn_factory_(txn_factory),
      last_received_op_id_(MinimumOpId()),
      last_received_op_id_current_leader_(MinimumOpId()),
      last_committed_index_(MinimumOpId()),
      state_(kInitialized) {
  CHECK(cmeta_) << "ConsensusMeta passed as NULL";
}

Status ReplicaState::StartUnlocked(const OpId& last_id_in_wal) {
  DCHECK(update_lock_.is_locked());

  // Our last persisted term can be higher than the last persisted operation
  // (i.e. if we called an election) but reverse should never happen.
  CHECK_LE(last_id_in_wal.term(), GetCurrentTermUnlocked()) << LogPrefixUnlocked()
      << "The last op in the WAL with id " << OpIdToString(last_id_in_wal)
      << " has a term (" << last_id_in_wal.term() << ") that is greater "
      << "than the latest recorded term, which is " << GetCurrentTermUnlocked();

  next_index_ = last_id_in_wal.index() + 1;
  last_received_op_id_.CopyFrom(last_id_in_wal);

  state_ = kRunning;
  return Status::OK();
}

Status ReplicaState::LockForStart(UniqueLock* lock) const {
  ThreadRestrictions::AssertWaitAllowed();
  UniqueLock l(&update_lock_);
  CHECK_EQ(state_, kInitialized) << "Illegal state for Start()."
      << " Replica is not in kInitialized state";
  lock->swap(&l);
  return Status::OK();
}

Status ReplicaState::LockForRead(UniqueLock* lock) const {
  ThreadRestrictions::AssertWaitAllowed();
  UniqueLock l(&update_lock_);
  lock->swap(&l);
  return Status::OK();
}

Status ReplicaState::LockForReplicate(UniqueLock* lock, const ReplicateMsg& msg) const {
  ThreadRestrictions::AssertWaitAllowed();
  DCHECK(!msg.has_id()) << "Should not have an ID yet: " << msg.ShortDebugString();
  UniqueLock l(&update_lock_);
  if (PREDICT_FALSE(state_ != kRunning)) {
    return Status::IllegalState("Replica not in running state");
  }

  RETURN_NOT_OK(CheckActiveLeaderUnlocked());
  lock->swap(&l);
  return Status::OK();
}

Status ReplicaState::LockForCommit(UniqueLock* lock) const {
  TRACE_EVENT0("consensus", "ReplicaState::LockForCommit");
  ThreadRestrictions::AssertWaitAllowed();
  UniqueLock l(&update_lock_);
  if (PREDICT_FALSE(state_ != kRunning && state_ != kShuttingDown)) {
    return Status::IllegalState("Replica not in running state");
  }
  lock->swap(&l);
  return Status::OK();
}

Status ReplicaState::LockForMajorityReplicatedIndexUpdate(UniqueLock* lock) const {
  TRACE_EVENT0("consensus", "ReplicaState::LockForMajorityReplicatedIndexUpdate");
  ThreadRestrictions::AssertWaitAllowed();
  UniqueLock l(&update_lock_);

  if (PREDICT_FALSE(state_ != kRunning)) {
    return Status::IllegalState("Replica not in running state");
  }

  if (PREDICT_FALSE(GetActiveRoleUnlocked() != RaftPeerPB::LEADER)) {
    return Status::IllegalState("Replica not LEADER");
  }
  lock->swap(&l);
  return Status::OK();
}

Status ReplicaState::CheckActiveLeaderUnlocked() const {
  RaftPeerPB::Role role = GetActiveRoleUnlocked();
  switch (role) {
    case RaftPeerPB::LEADER:
      return Status::OK();
    default:
      ConsensusStatePB cstate = ConsensusStateUnlocked(CONSENSUS_CONFIG_ACTIVE);
      return Status::IllegalState(Substitute("Replica $0 is not leader of this config. Role: $1. "
                                             "Consensus state: $2",
                                             peer_uuid_,
                                             RaftPeerPB::Role_Name(role),
                                             cstate.ShortDebugString()));
  }
}

Status ReplicaState::LockForConfigChange(UniqueLock* lock) const {
  TRACE_EVENT0("consensus", "ReplicaState::LockForConfigChange");

  ThreadRestrictions::AssertWaitAllowed();
  UniqueLock l(&update_lock_);
  // Can only change the config on running replicas.
  if (PREDICT_FALSE(state_ != kRunning)) {
    return Status::IllegalState("Unable to lock ReplicaState for config change",
                                Substitute("State = $0", state_));
  }
  lock->swap(&l);
  return Status::OK();
}

Status ReplicaState::LockForUpdate(UniqueLock* lock) const {
  TRACE_EVENT0("consensus", "ReplicaState::LockForUpdate");
  ThreadRestrictions::AssertWaitAllowed();
  UniqueLock l(&update_lock_);
  if (PREDICT_FALSE(state_ != kRunning)) {
    return Status::IllegalState("Replica not in running state");
  }
  if (!IsRaftConfigVoter(peer_uuid_, ConsensusStateUnlocked(CONSENSUS_CONFIG_ACTIVE).config())) {
    LOG_WITH_PREFIX_UNLOCKED(INFO) << "Allowing update even though not a member of the config";
  }
  lock->swap(&l);
  return Status::OK();
}

Status ReplicaState::LockForShutdown(UniqueLock* lock) {
  TRACE_EVENT0("consensus", "ReplicaState::LockForShutdown");
  ThreadRestrictions::AssertWaitAllowed();
  UniqueLock l(&update_lock_);
  if (state_ != kShuttingDown && state_ != kShutDown) {
    state_ = kShuttingDown;
  }
  lock->swap(&l);
  return Status::OK();
}

Status ReplicaState::ShutdownUnlocked() {
  DCHECK(update_lock_.is_locked());
  CHECK_EQ(state_, kShuttingDown);
  state_ = kShutDown;
  return Status::OK();
}

RaftPeerPB::Role ReplicaState::GetActiveRoleUnlocked() const {
  DCHECK(update_lock_.is_locked());
  return cmeta_->active_role();
}

bool ReplicaState::IsConfigChangePendingUnlocked() const {
  DCHECK(update_lock_.is_locked());
  return cmeta_->has_pending_config();
}

Status ReplicaState::CheckNoConfigChangePendingUnlocked() const {
  DCHECK(update_lock_.is_locked());
  if (IsConfigChangePendingUnlocked()) {
    return Status::IllegalState(
        Substitute("RaftConfig change currently pending. Only one is allowed at a time.\n"
                   "  Committed config: $0.\n  Pending config: $1",
                   GetCommittedConfigUnlocked().ShortDebugString(),
                   GetPendingConfigUnlocked().ShortDebugString()));
  }
  return Status::OK();
}

Status ReplicaState::SetPendingConfigUnlocked(const RaftConfigPB& new_config) {
  DCHECK(update_lock_.is_locked());
  RETURN_NOT_OK_PREPEND(VerifyRaftConfig(new_config, UNCOMMITTED_QUORUM),
                        "Invalid config to set as pending");
  CHECK(!cmeta_->has_pending_config())
      << "Attempt to set pending config while another is already pending! "
      << "Existing pending config: " << cmeta_->pending_config().ShortDebugString() << "; "
      << "Attempted new pending config: " << new_config.ShortDebugString();
  cmeta_->set_pending_config(new_config);
  return Status::OK();
}

void ReplicaState::ClearPendingConfigUnlocked() {
  cmeta_->clear_pending_config();
}

const RaftConfigPB& ReplicaState::GetPendingConfigUnlocked() const {
  DCHECK(update_lock_.is_locked());
  CHECK(IsConfigChangePendingUnlocked()) << "No pending config";
  return cmeta_->pending_config();
}

Status ReplicaState::SetCommittedConfigUnlocked(const RaftConfigPB& committed_config) {
  TRACE_EVENT0("consensus", "ReplicaState::SetCommittedConfigUnlocked");
  DCHECK(update_lock_.is_locked());
  DCHECK(committed_config.IsInitialized());
  RETURN_NOT_OK_PREPEND(VerifyRaftConfig(committed_config, COMMITTED_QUORUM),
                        "Invalid config to set as committed");

  // Compare committed with pending configuration, ensure they are the same.
  // Pending will not have an opid_index, so ignore that field.
  DCHECK(cmeta_->has_pending_config());
  RaftConfigPB config_no_opid = committed_config;
  config_no_opid.clear_opid_index();
  const RaftConfigPB& pending_config = GetPendingConfigUnlocked();
  // Quorums must be exactly equal, even w.r.t. peer ordering.
  CHECK_EQ(GetPendingConfigUnlocked().SerializeAsString(), config_no_opid.SerializeAsString())
      << Substitute("New committed config must equal pending config, but does not. "
                    "Pending config: $0, committed config: $1",
                    pending_config.ShortDebugString(), committed_config.ShortDebugString());

  cmeta_->set_committed_config(committed_config);
  cmeta_->clear_pending_config();
  CHECK_OK(cmeta_->Flush());
  return Status::OK();
}

const RaftConfigPB& ReplicaState::GetCommittedConfigUnlocked() const {
  DCHECK(update_lock_.is_locked());
  return cmeta_->committed_config();
}

const RaftConfigPB& ReplicaState::GetActiveConfigUnlocked() const {
  DCHECK(update_lock_.is_locked());
  return cmeta_->active_config();
}

bool ReplicaState::IsOpCommittedOrPending(const OpId& op_id, bool* term_mismatch) {

  *term_mismatch = false;

  if (op_id.index() <= GetCommittedOpIdUnlocked().index()) {
    return true;
  }

  if (op_id.index() > GetLastReceivedOpIdUnlocked().index()) {
    return false;
  }

  scoped_refptr<ConsensusRound> round = GetPendingOpByIndexOrNullUnlocked(op_id.index());
  DCHECK(round);

  if (round->id().term() != op_id.term()) {
    *term_mismatch = true;
    return false;
  }
  return true;
}

Status ReplicaState::SetCurrentTermUnlocked(int64_t new_term) {
  TRACE_EVENT1("consensus", "ReplicaState::SetCurrentTermUnlocked",
               "term", new_term);
  DCHECK(update_lock_.is_locked());
  if (PREDICT_FALSE(new_term <= GetCurrentTermUnlocked())) {
    return Status::IllegalState(
        Substitute("Cannot change term to a term that is lower than or equal to the current one. "
                   "Current: $0, Proposed: $1", GetCurrentTermUnlocked(), new_term));
  }
  cmeta_->set_current_term(new_term);
  cmeta_->clear_voted_for();
  CHECK_OK(cmeta_->Flush());
  ClearLeaderUnlocked();
  last_received_op_id_current_leader_ = MinimumOpId();
  return Status::OK();
}

const int64_t ReplicaState::GetCurrentTermUnlocked() const {
  DCHECK(update_lock_.is_locked());
  return cmeta_->current_term();
}

void ReplicaState::SetLeaderUuidUnlocked(const std::string& uuid) {
  DCHECK(update_lock_.is_locked());
  cmeta_->set_leader_uuid(uuid);
}

const string& ReplicaState::GetLeaderUuidUnlocked() const {
  DCHECK(update_lock_.is_locked());
  return cmeta_->leader_uuid();
}

const bool ReplicaState::HasVotedCurrentTermUnlocked() const {
  DCHECK(update_lock_.is_locked());
  return cmeta_->has_voted_for();
}

Status ReplicaState::SetVotedForCurrentTermUnlocked(const std::string& uuid) {
  TRACE_EVENT1("consensus", "ReplicaState::SetVotedForCurrentTermUnlocked",
               "uuid", uuid);
  DCHECK(update_lock_.is_locked());
  cmeta_->set_voted_for(uuid);
  CHECK_OK(cmeta_->Flush());
  return Status::OK();
}

const std::string& ReplicaState::GetVotedForCurrentTermUnlocked() const {
  DCHECK(update_lock_.is_locked());
  DCHECK(cmeta_->has_voted_for());
  return cmeta_->voted_for();
}

ReplicaTransactionFactory* ReplicaState::GetReplicaTransactionFactoryUnlocked() const {
  return txn_factory_;
}

const string& ReplicaState::GetPeerUuid() const {
  return peer_uuid_;
}

const ConsensusOptions& ReplicaState::GetOptions() const {
  return options_;
}

int ReplicaState::GetNumPendingTxnsUnlocked() const {
  DCHECK(update_lock_.is_locked());
  return pending_txns_.size();
}

Status ReplicaState::CancelPendingTransactions() {
  {
    ThreadRestrictions::AssertWaitAllowed();
    UniqueLock lock(&update_lock_);
    if (state_ != kShuttingDown) {
      return Status::IllegalState("Can only wait for pending commits on kShuttingDown state.");
    }
    if (pending_txns_.empty()) {
      return Status::OK();
    }

    LOG_WITH_PREFIX_UNLOCKED(INFO) << "Trying to abort " << pending_txns_.size()
                                   << " pending transactions.";
    for (const auto& txn : pending_txns_) {
      const scoped_refptr<ConsensusRound>& round = txn.second;
      // We cancel only transactions whose applies have not yet been triggered.
      LOG_WITH_PREFIX_UNLOCKED(INFO) << "Aborting transaction as it isn't in flight: "
                            << txn.second->replicate_msg()->ShortDebugString();
      round->NotifyReplicationFinished(Status::Aborted("Transaction aborted"));
    }
  }
  return Status::OK();
}

void ReplicaState::GetUncommittedPendingOperationsUnlocked(
    vector<scoped_refptr<ConsensusRound> >* ops) {
  for (const IndexToRoundMap::value_type& entry : pending_txns_) {
    if (entry.first > last_committed_index_.index()) {
      ops->push_back(entry.second);
    }
  }
}

Status ReplicaState::AbortOpsAfterUnlocked(int64_t new_preceding_idx) {
  DCHECK(update_lock_.is_locked());
  LOG_WITH_PREFIX_UNLOCKED(INFO) << "Aborting all transactions after (but not including): "
      << new_preceding_idx << ". Current State: " << ToStringUnlocked();

  DCHECK_GE(new_preceding_idx, 0);
  OpId new_preceding;

  auto iter = pending_txns_.lower_bound(new_preceding_idx);

  // Either the new preceding id is in the pendings set or it must be equal to the
  // committed index since we can't truncate already committed operations.
  if (iter != pending_txns_.end() && (*iter).first == new_preceding_idx) {
    new_preceding = (*iter).second->replicate_msg()->id();
    ++iter;
  } else {
    CHECK_EQ(new_preceding_idx, last_committed_index_.index());
    new_preceding = last_committed_index_;
  }

  // This is the same as UpdateLastReceivedOpIdUnlocked() but we do it
  // here to avoid the bounds check, since we're breaking monotonicity.
  last_received_op_id_ = new_preceding;
  last_received_op_id_current_leader_ = last_received_op_id_;
  next_index_ = new_preceding.index() + 1;

  for (; iter != pending_txns_.end();) {
    const scoped_refptr<ConsensusRound>& round = (*iter).second;
    auto op_type = round->replicate_msg()->op_type();
    LOG_WITH_PREFIX_UNLOCKED(INFO)
        << "Aborting uncommitted " << OperationType_Name(op_type)
        << " operation due to leader change: " << round->replicate_msg()->id();

    // When aborting a config-change operation, go back to using the committed
    // configuration.
    if (PREDICT_FALSE(op_type == CHANGE_CONFIG_OP)) {
      CHECK(IsConfigChangePendingUnlocked())
          << LogPrefixUnlocked() << "Aborting CHANGE_CONFIG_OP but "
          << "there was no pending config set. Op: "
          << round->replicate_msg()->ShortDebugString();
      ClearPendingConfigUnlocked();
    }

    round->NotifyReplicationFinished(Status::Aborted("Transaction aborted by new leader"));
    // Erase the entry from pendings.
    pending_txns_.erase(iter++);
  }

  return Status::OK();
}

Status ReplicaState::AddPendingOperation(const scoped_refptr<ConsensusRound>& round) {
  DCHECK(update_lock_.is_locked());
  if (PREDICT_FALSE(state_ != kRunning)) {
    // Special case when we're configuring and this is a config change, refuse
    // everything else.
    // TODO: Don't require a NO_OP to get to kRunning state
    if (round->replicate_msg()->op_type() != NO_OP) {
      return Status::IllegalState("Cannot trigger prepare. Replica is not in kRunning state.");
    }
  }

  // Mark pending configuration.
  if (PREDICT_FALSE(round->replicate_msg()->op_type() == CHANGE_CONFIG_OP)) {
    DCHECK(round->replicate_msg()->change_config_record().has_old_config());
    DCHECK(round->replicate_msg()->change_config_record().old_config().has_opid_index());
    DCHECK(round->replicate_msg()->change_config_record().has_new_config());
    DCHECK(!round->replicate_msg()->change_config_record().new_config().has_opid_index());
    const RaftConfigPB& old_config = round->replicate_msg()->change_config_record().old_config();
    const RaftConfigPB& new_config = round->replicate_msg()->change_config_record().new_config();
    if (GetActiveRoleUnlocked() != RaftPeerPB::LEADER) {
      // The leader has to mark the configuration as pending before it gets here
      // because the active configuration affects the replication queue.
      // Do one last sanity check.
      Status s = CheckNoConfigChangePendingUnlocked();
      if (PREDICT_FALSE(!s.ok())) {
        s = s.CloneAndAppend(Substitute("\n  New config: $0", new_config.ShortDebugString()));
        LOG_WITH_PREFIX_UNLOCKED(INFO) << s.ToString();
        return s;
      }
      // Check if the pending Raft config has an OpId less than the committed
      // config. If so, this is a replay at startup in which the COMMIT
      // messages were delayed.
      const RaftConfigPB& committed_config = GetCommittedConfigUnlocked();
      if (round->replicate_msg()->id().index() > committed_config.opid_index()) {
        CHECK_OK(SetPendingConfigUnlocked(new_config));
      } else {
        LOG_WITH_PREFIX_UNLOCKED(INFO) << "Ignoring setting pending config change with OpId "
            << round->replicate_msg()->id() << " because the committed config has OpId index "
            << committed_config.opid_index() << ". The config change we are ignoring is: "
            << "Old config: { " << old_config.ShortDebugString() << " }. "
            << "New config: { " << new_config.ShortDebugString() << " }";
      }
    }
  }

  InsertOrDie(&pending_txns_, round->replicate_msg()->id().index(), round);
  return Status::OK();
}

scoped_refptr<ConsensusRound> ReplicaState::GetPendingOpByIndexOrNullUnlocked(int64_t index) {
  DCHECK(update_lock_.is_locked());
  return FindPtrOrNull(pending_txns_, index);
}

Status ReplicaState::UpdateMajorityReplicatedUnlocked(const OpId& majority_replicated,
                                                      OpId* committed_index,
                                                      bool* committed_index_changed) {
  DCHECK(update_lock_.is_locked());
  DCHECK(majority_replicated.IsInitialized());
  DCHECK(last_committed_index_.IsInitialized());
  if (PREDICT_FALSE(state_ == kShuttingDown || state_ == kShutDown)) {
    return Status::ServiceUnavailable("Cannot trigger apply. Replica is shutting down.");
  }
  if (PREDICT_FALSE(state_ != kRunning)) {
    return Status::IllegalState("Cannot trigger apply. Replica is not in kRunning state.");
  }

  // If the last committed operation was in the current term (the normal case)
  // then 'committed_index' is simply equal to majority replicated.
  if (last_committed_index_.term() == GetCurrentTermUnlocked()) {
    RETURN_NOT_OK(AdvanceCommittedIndexUnlocked(majority_replicated,
                                                committed_index_changed));
    committed_index->CopyFrom(last_committed_index_);
    return Status::OK();
  }

  // If the last committed operation is not in the current term (such as when
  // we change leaders) but 'majority_replicated' is then we can advance the
  // 'committed_index' too.
  if (majority_replicated.term() == GetCurrentTermUnlocked()) {
    OpId previous = last_committed_index_;
    RETURN_NOT_OK(AdvanceCommittedIndexUnlocked(majority_replicated,
                                                committed_index_changed));
    committed_index->CopyFrom(last_committed_index_);
    LOG_WITH_PREFIX_UNLOCKED(INFO) << "Advanced the committed_index across terms."
        << " Last committed operation was: " << previous.ShortDebugString()
        << " New committed index is: " << last_committed_index_.ShortDebugString();
    return Status::OK();
  }

  committed_index->CopyFrom(last_committed_index_);
  KLOG_EVERY_N_SECS(WARNING, 1) << LogPrefixUnlocked()
          << "Can't advance the committed index across term boundaries"
          << " until operations from the current term are replicated."
          << " Last committed operation was: " << last_committed_index_.ShortDebugString() << ","
          << " New majority replicated is: " << majority_replicated.ShortDebugString() << ","
          << " Current term is: " << GetCurrentTermUnlocked();

  return Status::OK();
}

Status ReplicaState::AdvanceCommittedIndexUnlocked(const OpId& committed_index,
                                                   bool *committed_index_changed) {
  *committed_index_changed = false;
  // If we already committed up to (or past) 'id' return.
  // This can happen in the case that multiple UpdateConsensus() calls end
  // up in the RPC queue at the same time, and then might get interleaved out
  // of order.
  if (last_committed_index_.index() >= committed_index.index()) {
    VLOG_WITH_PREFIX_UNLOCKED(1)
      << "Already marked ops through " << last_committed_index_ << " as committed. "
      << "Now trying to mark " << committed_index << " which would be a no-op.";
    return Status::OK();
  }

  if (pending_txns_.empty()) {
    last_committed_index_.CopyFrom(committed_index);
    VLOG_WITH_PREFIX_UNLOCKED(1) << "No transactions to mark as committed up to: "
        << committed_index.ShortDebugString();
    return Status::OK();
  }

  // Start at the operation after the last committed one.
  auto iter = pending_txns_.upper_bound(last_committed_index_.index());
  // Stop at the operation after the last one we must commit.
  auto end_iter = pending_txns_.upper_bound(committed_index.index());
  CHECK(iter != pending_txns_.end());

  VLOG_WITH_PREFIX_UNLOCKED(1) << "Last triggered apply was: "
      <<  last_committed_index_.ShortDebugString()
      << " Starting to apply from log index: " << (*iter).first;

  OpId prev_id = last_committed_index_;

  while (iter != end_iter) {
    scoped_refptr<ConsensusRound> round = (*iter).second; // Make a copy.
    DCHECK(round);
    const OpId& current_id = round->id();

    if (PREDICT_TRUE(!OpIdEquals(prev_id, MinimumOpId()))) {
      CHECK_OK(CheckOpInSequence(prev_id, current_id));
    }

    pending_txns_.erase(iter++);

    // Set committed configuration.
    if (PREDICT_FALSE(round->replicate_msg()->op_type() == CHANGE_CONFIG_OP)) {
      DCHECK(round->replicate_msg()->change_config_record().has_old_config());
      DCHECK(round->replicate_msg()->change_config_record().has_new_config());
      RaftConfigPB old_config = round->replicate_msg()->change_config_record().old_config();
      RaftConfigPB new_config = round->replicate_msg()->change_config_record().new_config();
      DCHECK(old_config.has_opid_index());
      DCHECK(!new_config.has_opid_index());
      new_config.set_opid_index(current_id.index());
      // Check if the pending Raft config has an OpId less than the committed
      // config. If so, this is a replay at startup in which the COMMIT
      // messages were delayed.
      const RaftConfigPB& committed_config = GetCommittedConfigUnlocked();
      if (new_config.opid_index() > committed_config.opid_index()) {
        LOG_WITH_PREFIX_UNLOCKED(INFO) << "Committing config change with OpId "
            << current_id << ". "
            << "Old config: { " << old_config.ShortDebugString() << " }. "
            << "New config: { " << new_config.ShortDebugString() << " }";
        CHECK_OK(SetCommittedConfigUnlocked(new_config));
      } else {
        LOG_WITH_PREFIX_UNLOCKED(INFO) << "Ignoring commit of config change with OpId "
            << current_id << " because the committed config has OpId index "
            << committed_config.opid_index() << ". The config change we are ignoring is: "
            << "Old config: { " << old_config.ShortDebugString() << " }. "
            << "New config: { " << new_config.ShortDebugString() << " }";
      }
    }

    prev_id.CopyFrom(round->id());
    round->NotifyReplicationFinished(Status::OK());
  }

  last_committed_index_.CopyFrom(committed_index);
  *committed_index_changed = true;
  return Status::OK();
}

const OpId& ReplicaState::GetCommittedOpIdUnlocked() const {
  DCHECK(update_lock_.is_locked());
  return last_committed_index_;
}

Status ReplicaState::CheckHasCommittedOpInCurrentTermUnlocked() const {
  int64_t term = GetCurrentTermUnlocked();
  const OpId& opid = GetCommittedOpIdUnlocked();
  if (opid.term() != term) {
    return Status::IllegalState("Latest committed op is not from this term", OpIdToString(opid));
  }
  return Status::OK();
}

void ReplicaState::UpdateLastReceivedOpIdUnlocked(const OpId& op_id) {
  DCHECK(update_lock_.is_locked());
  DCHECK_LE(OpIdCompare(last_received_op_id_, op_id), 0)
    << "Previously received OpId: " << last_received_op_id_.ShortDebugString()
    << ", updated OpId: " << op_id.ShortDebugString()
    << ", Trace:" << std::endl << Trace::CurrentTrace()->DumpToString(true);
  last_received_op_id_ = op_id;
  last_received_op_id_current_leader_ = last_received_op_id_;
  next_index_ = op_id.index() + 1;
}

const OpId& ReplicaState::GetLastReceivedOpIdUnlocked() const {
  DCHECK(update_lock_.is_locked());
  return last_received_op_id_;
}

const OpId& ReplicaState::GetLastReceivedOpIdCurLeaderUnlocked() const {
  DCHECK(update_lock_.is_locked());
  return last_received_op_id_current_leader_;
}

OpId ReplicaState::GetLastPendingTransactionOpIdUnlocked() const {
  DCHECK(update_lock_.is_locked());
  return pending_txns_.empty()
      ? MinimumOpId() : (--pending_txns_.end())->second->id();
}

void ReplicaState::NewIdUnlocked(OpId* id) {
  DCHECK(update_lock_.is_locked());
  id->set_term(GetCurrentTermUnlocked());
  id->set_index(next_index_++);
}

void ReplicaState::CancelPendingOperation(const OpId& id) {
  OpId previous = id;
  previous.set_index(previous.index() - 1);
  DCHECK(update_lock_.is_locked());
  CHECK_EQ(GetCurrentTermUnlocked(), id.term());
  CHECK_EQ(next_index_, id.index() + 1);
  next_index_ = id.index();

  // We don't use UpdateLastReceivedOpIdUnlocked because we're actually
  // updating it back to a lower value and we need to avoid the checks
  // that method has.

  // This is only ok if we do _not_ release the lock after calling
  // NewIdUnlocked() (which we don't in RaftConsensus::Replicate()).
  last_received_op_id_ = previous;
  scoped_refptr<ConsensusRound> round = EraseKeyReturnValuePtr(&pending_txns_, id.index());
  DCHECK(round);
}

string ReplicaState::LogPrefix() {
  ReplicaState::UniqueLock lock;
  CHECK_OK(LockForRead(&lock));
  return LogPrefixUnlocked();
}

string ReplicaState::LogPrefixUnlocked() const {
  DCHECK(update_lock_.is_locked());
  return Substitute("T $0 P $1 [term $2 $3]: ",
                    options_.tablet_id,
                    peer_uuid_,
                    GetCurrentTermUnlocked(),
                    RaftPeerPB::Role_Name(GetActiveRoleUnlocked()));
}

string ReplicaState::LogPrefixThreadSafe() const {
  return Substitute("T $0 P $1: ",
                    options_.tablet_id,
                    peer_uuid_);
}

ReplicaState::State ReplicaState::state() const {
  DCHECK(update_lock_.is_locked());
  return state_;
}

string ReplicaState::ToString() const {
  ThreadRestrictions::AssertWaitAllowed();
  ReplicaState::UniqueLock lock(&update_lock_);
  return ToStringUnlocked();
}

string ReplicaState::ToStringUnlocked() const {
  DCHECK(update_lock_.is_locked());
  string ret;
  SubstituteAndAppend(&ret, "Replica: $0, State: $1, Role: $2\n",
                      peer_uuid_, state_,
                      RaftPeerPB::Role_Name(GetActiveRoleUnlocked()));

  SubstituteAndAppend(&ret, "Watermarks: {Received: $0 Committed: $1}\n",
                      last_received_op_id_.ShortDebugString(),
                      last_committed_index_.ShortDebugString());
  return ret;
}

Status ReplicaState::CheckOpInSequence(const OpId& previous, const OpId& current) {
  if (current.term() < previous.term()) {
    return Status::Corruption(Substitute("New operation's term is not >= than the previous "
        "op's term. Current: $0. Previous: $1", OpIdToString(current), OpIdToString(previous)));
  }
  if (current.index() != previous.index() + 1) {
    return Status::Corruption(Substitute("New operation's index does not follow the previous"
        " op's index. Current: $0. Previous: $1", OpIdToString(current), OpIdToString(previous)));
  }
  return Status::OK();
}

}  // namespace consensus
}  // namespace kudu