memgraph/src/raft/raft_server.cpp

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#include "raft/raft_server.hpp"
#include <kj/std/iostream.h>
#include <chrono>
#include <iostream>
#include <memory>
#include <fmt/format.h>
#include <gflags/gflags.h>
#include <glog/logging.h>
#include "database/graph_db_accessor.hpp"
#include "durability/single_node_ha/paths.hpp"
#include "durability/single_node_ha/recovery.hpp"
#include "durability/single_node_ha/snapshooter.hpp"
#include "raft/exceptions.hpp"
#include "utils/exceptions.hpp"
#include "utils/on_scope_exit.hpp"
#include "utils/serialization.hpp"
#include "utils/thread.hpp"
namespace raft {
using namespace std::literals::chrono_literals;
namespace fs = std::experimental::filesystem;
const std::string kCurrentTermKey = "current_term";
const std::string kVotedForKey = "voted_for";
const std::string kLogSizeKey = "log_size";
const std::string kLogEntryPrefix = "log_entry_";
const std::string kSnapshotMetadataKey = "snapshot_metadata";
const std::string kRaftDir = "raft";
const std::chrono::duration<int64_t> kSnapshotPeriod = 1s;
RaftServer::RaftServer(uint16_t server_id, const std::string &durability_dir,
bool db_recover_on_startup, const Config &config,
Coordination *coordination,
database::StateDeltaApplier *delta_applier,
database::GraphDb *db)
: config_(config),
coordination_(coordination),
delta_applier_(delta_applier),
db_(db),
rlog_(std::make_unique<ReplicationLog>()),
mode_(Mode::FOLLOWER),
server_id_(server_id),
durability_dir_(fs::path(durability_dir)),
db_recover_on_startup_(db_recover_on_startup),
commit_index_(0),
last_applied_(0),
disk_storage_(fs::path(durability_dir) / kRaftDir) {}
void RaftServer::Start() {
if (db_recover_on_startup_) {
auto snapshot_metadata = GetSnapshotMetadata();
if (snapshot_metadata) {
RecoverSnapshot(snapshot_metadata->snapshot_filename);
last_applied_ = snapshot_metadata->last_included_index;
commit_index_ = snapshot_metadata->last_included_index;
}
} else {
// We need to clear persisted data if we don't want any recovery.
disk_storage_.DeletePrefix("");
disk_storage_.Put(kLogSizeKey, "0");
durability::RemoveAllSnapshots(durability_dir_);
}
// Persistent storage initialization
if (LogSize() == 0) {
UpdateTerm(0);
LogEntry empty_log_entry(0, {});
AppendLogEntries(0, 0, {empty_log_entry});
}
// Peer state initialization
int cluster_size = coordination_->WorkerCount() + 1;
next_index_.resize(cluster_size);
match_index_.resize(cluster_size);
next_heartbeat_.resize(cluster_size);
backoff_until_.resize(cluster_size);
// RPC registration
coordination_->Register<RequestVoteRpc>(
[this](const auto &req_reader, auto *res_builder) {
std::lock_guard<std::mutex> guard(lock_);
RequestVoteReq req;
Load(&req, req_reader);
// [Raft paper 5.1]
// "If a server recieves a request with a stale term,
// it rejects the request"
uint64_t current_term = CurrentTerm();
if (exiting_ || req.term < current_term) {
RequestVoteRes res(false, current_term);
Save(res, res_builder);
return;
}
// [Raft paper figure 2]
// If RPC request or response contains term T > currentTerm,
// set currentTerm = T and convert to follower.
if (req.term > current_term) {
UpdateTerm(req.term);
if (mode_ != Mode::FOLLOWER) Transition(Mode::FOLLOWER);
}
// [Raft paper 5.2, 5.4]
// "Each server will vote for at most one candidate in a given
// term, on a first-come-first-serve basis with an additional
// restriction on votes"
// Restriction: "The voter denies its vote if its own log is more
// up-to-date than that of the candidate"
std::experimental::optional<uint16_t> voted_for = VotedFor();
auto last_entry_data = LastEntryData();
bool grant_vote =
(!voted_for || voted_for.value() == req.candidate_id) &&
AtLeastUpToDate(req.last_log_index, req.last_log_term,
last_entry_data.first, last_entry_data.second);
RequestVoteRes res(grant_vote, current_term);
if (grant_vote) SetNextElectionTimePoint();
Save(res, res_builder);
});
coordination_->Register<AppendEntriesRpc>([this](const auto &req_reader,
auto *res_builder) {
std::lock_guard<std::mutex> guard(lock_);
AppendEntriesReq req;
Load(&req, req_reader);
// [Raft paper 5.1]
// "If a server receives a request with a stale term, it rejects the
// request"
uint64_t current_term = CurrentTerm();
if (exiting_ || req.term < current_term) {
AppendEntriesRes res(false, current_term);
Save(res, res_builder);
return;
}
// Everything below is considered to be a valid RPC. This will ensure that
// after we finish processing the current request, the election timeout will
// be extended.
utils::OnScopeExit extend_election_timeout([this] {
// [Raft thesis 3.4]
// A server remains in follower state as long as it receives valid RPCs
// from a leader or candidate.
SetNextElectionTimePoint();
election_change_.notify_all();
});
// [Raft paper 5.3]
// "If a follower's log is inconsistent with the leader's, the
// consistency check will fail in the AppendEntries RPC."
//
// Consistency checking assures the Log Matching Property:
// - If two entries in different logs have the same index and
// term, then they store the same command.
// - If two entries in different logs have the same index and term,
// then the logs are identical in all preceding entries.
auto snapshot_metadata = GetSnapshotMetadata();
if (snapshot_metadata &&
snapshot_metadata->last_included_index == req.prev_log_index) {
if (req.prev_log_term != snapshot_metadata->last_included_term) {
AppendEntriesRes res(false, current_term);
Save(res, res_builder);
return;
}
} else if (snapshot_metadata &&
snapshot_metadata->last_included_index > req.prev_log_index) {
LOG(ERROR) << "Received entries that are already commited and have been "
"compacted";
AppendEntriesRes res(false, current_term);
Save(res, res_builder);
return;
} else {
if (LogSize() <= req.prev_log_index ||
GetLogEntry(req.prev_log_index).term != req.prev_log_term) {
AppendEntriesRes res(false, current_term);
Save(res, res_builder);
return;
}
}
// [Raft paper figure 2]
// If RPC request or response contains term T > currentTerm,
// set currentTerm = T and convert to follower.
if (req.term > current_term) {
UpdateTerm(req.term);
if (mode_ != Mode::FOLLOWER) Transition(Mode::FOLLOWER);
}
AppendLogEntries(req.leader_commit, req.prev_log_index + 1, req.entries);
// [Raft paper 5.3]
// "Once a follower learns that a log entry is committed, it applies
// the entry to its state machine (in log order)
while (req.leader_commit > last_applied_ && last_applied_ + 1 < LogSize()) {
++last_applied_;
delta_applier_->Apply(GetLogEntry(last_applied_).deltas);
}
// Respond positively to a heartbeat.
if (req.entries.empty()) {
AppendEntriesRes res(true, current_term);
Save(res, res_builder);
if (mode_ != Mode::FOLLOWER) Transition(Mode::FOLLOWER);
return;
}
AppendEntriesRes res(true, current_term);
Save(res, res_builder);
});
coordination_->Register<InstallSnapshotRpc>(
[this](const auto &req_reader, auto *res_builder) {
// Acquire snapshot lock.
std::lock_guard<std::mutex> snapshot_guard(snapshot_lock_);
std::lock_guard<std::mutex> guard(lock_);
InstallSnapshotReq req;
Load(&req, req_reader);
uint64_t current_term = CurrentTerm();
if (exiting_ || req.term < current_term) {
InstallSnapshotRes res(current_term);
Save(res, res_builder);
return;
}
// Check if the current state matches the one in snapshot
if (req.snapshot_metadata.last_included_index == last_applied_ &&
req.snapshot_metadata.last_included_term == current_term) {
InstallSnapshotRes res(current_term);
Save(res, res_builder);
return;
}
VLOG(40) << "[InstallSnapshotRpc] Starting.";
if (req.term > current_term) {
VLOG(40) << "[InstallSnapshotRpc] Updating term.";
UpdateTerm(req.term);
if (mode_ != Mode::FOLLOWER) Transition(Mode::FOLLOWER);
}
utils::OnScopeExit extend_election_timeout([this] {
SetNextElectionTimePoint();
election_change_.notify_all();
});
// Temporary freeze election timer while we handle the snapshot.
next_election_ = TimePoint::max();
VLOG(40) << "[InstallSnapshotRpc] Remove all snapshots.";
// Remove all previous snapshots
durability::RemoveAllSnapshots(durability_dir_);
const auto snapshot_path = durability::MakeSnapshotPath(
durability_dir_, req.snapshot_metadata.snapshot_filename);
// Save snapshot file
{
VLOG(40) << "[InstallSnapshotRpc] Saving received snapshot.";
std::ofstream output_stream;
output_stream.open(snapshot_path, std::ios::out | std::ios::binary);
output_stream.write(req.data.get(), req.size);
output_stream.flush();
output_stream.close();
}
// Discard the all logs. We keep the one at index 0.
VLOG(40) << "[InstallSnapshotRpc] Discarding logs.";
uint64_t log_size = LogSize();
for (uint64_t i = 1; i < log_size; ++i)
disk_storage_.Delete(LogEntryKey(i));
// Reset the database.
VLOG(40) << "[InstallSnapshotRpc] Reset database.";
db_->Reset();
// Apply the snapshot.
VLOG(40) << "[InstallSnapshotRpc] Recover from received snapshot.";
RecoverSnapshot(req.snapshot_metadata.snapshot_filename);
VLOG(40) << "[InstallSnapshotRpc] Persist snapshot metadata.";
PersistSnapshotMetadata(req.snapshot_metadata);
// Update the state to match the one from snapshot.
VLOG(40) << "[InstallSnapshotRpc] Update Raft state.";
last_applied_ = req.snapshot_metadata.last_included_index;
commit_index_ = req.snapshot_metadata.last_included_index;
disk_storage_.Put(
kLogSizeKey,
std::to_string(req.snapshot_metadata.last_included_index + 1));
InstallSnapshotRes res(CurrentTerm());
Save(res, res_builder);
});
// start threads
SetNextElectionTimePoint();
election_thread_ = std::thread(&RaftServer::ElectionThreadMain, this);
for (const auto &peer_id : coordination_->GetWorkerIds()) {
if (peer_id == server_id_) continue;
peer_threads_.emplace_back(&RaftServer::PeerThreadMain, this, peer_id);
}
no_op_issuer_thread_ = std::thread(&RaftServer::NoOpIssuerThreadMain, this);
snapshot_thread_ = std::thread(&RaftServer::SnapshotThread, this);
}
void RaftServer::Shutdown() {
{
std::lock_guard<std::mutex> guard(lock_);
exiting_ = true;
state_changed_.notify_all();
election_change_.notify_all();
leader_changed_.notify_all();
}
for (auto &peer_thread : peer_threads_) {
if (peer_thread.joinable()) peer_thread.join();
}
if (election_thread_.joinable()) election_thread_.join();
if (no_op_issuer_thread_.joinable()) no_op_issuer_thread_.join();
if (snapshot_thread_.joinable()) snapshot_thread_.join();
}
uint64_t RaftServer::CurrentTerm() {
auto opt_value = disk_storage_.Get(kCurrentTermKey);
if (opt_value == std::experimental::nullopt)
throw MissingPersistentDataException(kCurrentTermKey);
return std::stoull(opt_value.value());
}
std::experimental::optional<uint16_t> RaftServer::VotedFor() {
auto opt_value = disk_storage_.Get(kVotedForKey);
if (opt_value == std::experimental::nullopt)
return std::experimental::nullopt;
return {std::stoul(opt_value.value())};
}
uint64_t RaftServer::LogSize() {
auto opt_value = disk_storage_.Get(kLogSizeKey);
if (opt_value == std::experimental::nullopt) {
disk_storage_.Put(kLogSizeKey, "0");
return 0;
}
return std::stoull(opt_value.value());
}
std::experimental::optional<SnapshotMetadata>
RaftServer::GetSnapshotMetadata() {
auto opt_value = disk_storage_.Get(kSnapshotMetadataKey);
if (opt_value == std::experimental::nullopt) {
return std::experimental::nullopt;
}
::capnp::MallocMessageBuilder message;
std::stringstream stream(std::ios_base::in | std::ios_base::out |
std::ios_base::binary);
kj::std::StdInputStream std_stream(stream);
kj::BufferedInputStreamWrapper buffered_stream(std_stream);
stream << *opt_value;
readMessageCopy(buffered_stream, message);
capnp::SnapshotMetadata::Reader reader =
message.getRoot<capnp::SnapshotMetadata>().asReader();
SnapshotMetadata deserialized;
Load(&deserialized, reader);
return std::experimental::make_optional(deserialized);
}
void RaftServer::PersistSnapshotMetadata(
const SnapshotMetadata &snapshot_metadata) {
std::stringstream stream(std::ios_base::in | std::ios_base::out |
std::ios_base::binary);
{
::capnp::MallocMessageBuilder message;
capnp::SnapshotMetadata::Builder builder =
message.initRoot<capnp::SnapshotMetadata>();
Save(snapshot_metadata, &builder);
kj::std::StdOutputStream std_stream(stream);
kj::BufferedOutputStreamWrapper buffered_stream(std_stream);
writeMessage(buffered_stream, message);
}
disk_storage_.Put(kSnapshotMetadataKey, stream.str());
}
void RaftServer::AppendToLog(const tx::TransactionId &tx_id,
const std::vector<database::StateDelta> &deltas) {
std::unique_lock<std::mutex> lock(lock_);
DCHECK(mode_ == Mode::LEADER)
<< "`AppendToLog` should only be called in LEADER mode";
if (deltas.size() == 2) {
DCHECK(deltas[0].type == database::StateDelta::Type::TRANSACTION_BEGIN &&
deltas[1].type == database::StateDelta::Type::TRANSACTION_COMMIT)
<< "Transactions with two state deltas must be reads (start with BEGIN "
"and end with COMMIT)";
rlog_->set_replicated(tx_id);
return;
}
uint64_t log_size = LogSize();
rlog_->set_active(tx_id);
LogEntry new_entry(CurrentTerm(), deltas);
disk_storage_.Put(LogEntryKey(log_size), SerializeLogEntry(new_entry));
disk_storage_.Put(kLogSizeKey, std::to_string(log_size + 1));
// Force issuing heartbeats
TimePoint now = Clock::now();
for (auto &peer_heartbeat : next_heartbeat_) peer_heartbeat = now;
state_changed_.notify_all();
}
void RaftServer::Emplace(const database::StateDelta &delta) {
log_entry_buffer_.Emplace(delta);
}
bool RaftServer::SafeToCommit(const tx::TransactionId &tx_id) {
switch (mode_) {
case Mode::CANDIDATE:
// When Memgraph first starts, the Raft is initialized in candidate
// mode and we try to perform recovery. Since everything for recovery
// needs to be able to commit, we return true.
return true;
case Mode::FOLLOWER:
// When in follower mode, we will only try to apply a Raft Log when we
// receive a commit index greater or equal from the Log index from the
// leader. At that moment we don't have to check the replication log
// because the leader won't commit the Log locally if it's not replicated
// on the majority of the peers in the cluster. This is why we can short
// circut the check to always return true if in follower mode.
return true;
case Mode::LEADER:
if (rlog_->is_active(tx_id)) return false;
if (rlog_->is_replicated(tx_id)) return true;
// The only possibility left is that our ReplicationLog doesn't contain
// information about that tx.
throw InvalidReplicationLogLookup();
break;
}
}
void RaftServer::GarbageCollectReplicationLog(const tx::TransactionId &tx_id) {
rlog_->garbage_collect_older(tx_id);
}
bool RaftServer::IsLeader() { return mode_ == Mode::LEADER; }
RaftServer::LogEntryBuffer::LogEntryBuffer(RaftServer *raft_server)
: raft_server_(raft_server) {
CHECK(raft_server_) << "RaftServer can't be nullptr";
}
void RaftServer::LogEntryBuffer::Enable() {
std::lock_guard<std::mutex> guard(buffer_lock_);
enabled_ = true;
}
void RaftServer::LogEntryBuffer::Disable() {
std::lock_guard<std::mutex> guard(buffer_lock_);
enabled_ = false;
// Clear all existing logs from buffers.
logs_.clear();
}
void RaftServer::LogEntryBuffer::Emplace(const database::StateDelta &delta) {
std::unique_lock<std::mutex> lock(buffer_lock_);
if (!enabled_) return;
tx::TransactionId tx_id = delta.transaction_id;
if (delta.type == database::StateDelta::Type::TRANSACTION_COMMIT) {
auto it = logs_.find(tx_id);
CHECK(it != logs_.end()) << "Missing StateDeltas for transaction " << tx_id;
std::vector<database::StateDelta> log(std::move(it->second));
log.emplace_back(std::move(delta));
logs_.erase(it);
lock.unlock();
raft_server_->AppendToLog(tx_id, log);
} else if (delta.type == database::StateDelta::Type::TRANSACTION_ABORT) {
auto it = logs_.find(tx_id);
CHECK(it != logs_.end()) << "Missing StateDeltas for transaction " << tx_id;
logs_.erase(it);
} else {
logs_[tx_id].emplace_back(std::move(delta));
}
}
void RaftServer::Transition(const Mode &new_mode) {
switch (new_mode) {
case Mode::FOLLOWER: {
VLOG(40) << "Server " << server_id_
<< ": Transition to FOLLOWER (Term: " << CurrentTerm() << ")";
bool reset = mode_ == Mode::LEADER;
mode_ = Mode::FOLLOWER;
log_entry_buffer_.Disable();
if (reset) {
VLOG(40) << "Resetting internal state";
// Temporary freeze election timer while we do the reset.
next_election_ = TimePoint::max();
db_->Reset();
ResetReplicationLog();
// Re-apply raft log.
uint64_t starting_index = 1;
auto snapshot_metadata = GetSnapshotMetadata();
if (snapshot_metadata) {
RecoverSnapshot(snapshot_metadata->snapshot_filename);
starting_index = snapshot_metadata->last_included_index + 1;
}
for (uint64_t i = starting_index; i <= commit_index_; ++i) {
delta_applier_->Apply(GetLogEntry(i).deltas);
}
last_applied_ = commit_index_;
}
SetNextElectionTimePoint();
election_change_.notify_all();
break;
}
case Mode::CANDIDATE: {
VLOG(40) << "Server " << server_id_
<< ": Transition to CANDIDATE (Term: " << CurrentTerm() << ")";
// [Raft thesis, section 3.4]
// "Each candidate restarts its randomized election timeout at the start
// of an election, and it waits for that timeout to elapse before
// starting the next election; this reduces the likelihood of another
// split vote in the new election."
SetNextElectionTimePoint();
election_change_.notify_all();
// [Raft thesis, section 3.4]
// "To begin an election, a follower increments its current term and
// transitions to candidate state. It then votes for itself and issues
// RequestVote RPCs in parallel to each of the other servers in the
// cluster."
disk_storage_.Put(kCurrentTermKey, std::to_string(CurrentTerm() + 1));
disk_storage_.Put(kVotedForKey, std::to_string(server_id_));
granted_votes_ = 1;
vote_requested_.assign(coordination_->WorkerCount(), false);
mode_ = Mode::CANDIDATE;
if (HasMajortyVote()) {
Transition(Mode::LEADER);
state_changed_.notify_all();
return;
}
break;
}
case Mode::LEADER: {
VLOG(40) << "Server " << server_id_
<< ": Transition to LEADER (Term: " << CurrentTerm() << ")";
// Freeze election timer
next_election_ = TimePoint::max();
election_change_.notify_all();
uint64_t log_size = LogSize();
// Set next heartbeat to correct values
TimePoint now = Clock::now();
for (auto &peer_heartbeat : next_heartbeat_)
peer_heartbeat = now + config_.heartbeat_interval;
// [Raft paper figure 2]
// "For each server, index of the next log entry to send to that server
// is initialized to leader's last log index + 1"
for (int i = 1; i < coordination_->WorkerCount() + 1; ++i) {
next_index_[i] = log_size;
match_index_[i] = 0;
}
// Raft guarantees the Leader Append-Only property [Raft paper 5.2]
// so its safe to apply everything from our log into our state machine
for (int i = last_applied_ + 1; i < log_size; ++i)
delta_applier_->Apply(GetLogEntry(i).deltas);
last_applied_ = log_size - 1;
mode_ = Mode::LEADER;
log_entry_buffer_.Enable();
leader_changed_.notify_all();
break;
}
}
}
void RaftServer::UpdateTerm(uint64_t new_term) {
disk_storage_.Put(kCurrentTermKey, std::to_string(new_term));
disk_storage_.Delete(kVotedForKey);
}
void RaftServer::AdvanceCommitIndex() {
DCHECK(mode_ == Mode::LEADER)
<< "Commit index can only be advanced by the leader";
std::vector<uint64_t> known_replication_indices;
uint64_t log_size = LogSize();
for (int i = 1; i < coordination_->WorkerCount() + 1; ++i) {
if (i != server_id_)
known_replication_indices.push_back(match_index_[i]);
else
known_replication_indices.push_back(log_size - 1);
}
std::sort(known_replication_indices.begin(), known_replication_indices.end());
uint64_t new_commit_index =
known_replication_indices[(coordination_->WorkerCount() - 1) / 2];
// This can happen because we reset `match_index` vector to 0 after a
// new leader has been elected.
if (commit_index_ >= new_commit_index) return;
// [Raft thesis, section 3.6.2]
// "(...) Raft never commits log entries from previous terms by counting
// replicas. Only log entries from the leader's current term are committed by
// counting replicas; once an entry from the current term has been committed
// in this way, then all prior entries are committed indirectly because of the
// Log Matching Property."
if (GetLogEntry(new_commit_index).term != CurrentTerm()) {
VLOG(40) << "Server " << server_id_
<< ": cannot commit log entry from "
"previous term based on "
"replication count.";
return;
}
VLOG(40) << "Begin applying commited transactions";
for (int i = commit_index_ + 1; i <= new_commit_index; ++i) {
auto deltas = GetLogEntry(i).deltas;
DCHECK(deltas.size() > 2)
<< "Log entry should consist of at least two state deltas.";
rlog_->set_replicated(deltas[0].transaction_id);
}
commit_index_ = new_commit_index;
last_applied_ = new_commit_index;
}
void RaftServer::SendEntries(uint16_t peer_id,
std::unique_lock<std::mutex> *lock) {
auto snapshot_metadata = GetSnapshotMetadata();
if (snapshot_metadata &&
snapshot_metadata->last_included_index >= next_index_[peer_id]) {
SendSnapshot(peer_id, *snapshot_metadata, lock);
} else {
SendLogEntries(peer_id, snapshot_metadata, lock);
}
}
void RaftServer::SendLogEntries(
uint16_t peer_id,
const std::experimental::optional<SnapshotMetadata> &snapshot_metadata,
std::unique_lock<std::mutex> *lock) {
uint64_t request_term = CurrentTerm();
uint64_t request_prev_log_index = next_index_[peer_id] - 1;
uint64_t request_prev_log_term;
if (snapshot_metadata &&
snapshot_metadata->last_included_index == next_index_[peer_id] - 1) {
request_prev_log_term = snapshot_metadata->last_included_term;
} else {
request_prev_log_term = GetLogEntry(next_index_[peer_id] - 1).term;
}
std::vector<LogEntry> request_entries;
if (next_index_[peer_id] <= LogSize() - 1)
GetLogSuffix(next_index_[peer_id], request_entries);
bool unreachable_peer = false;
auto peer_future = coordination_->ExecuteOnWorker<AppendEntriesRes>(
peer_id, [&](int worker_id, auto &client) {
try {
auto res = client.template Call<AppendEntriesRpc>(
server_id_, commit_index_, request_term, request_prev_log_index,
request_prev_log_term, request_entries);
return res;
} catch (...) {
// not being able to connect to peer means we need to retry.
// TODO(ipaljak): Consider backoff.
unreachable_peer = true;
return AppendEntriesRes(false, request_term);
}
});
VLOG(40) << "Entries size: " << request_entries.size();
lock->unlock(); // Release lock while waiting for response.
auto reply = peer_future.get();
lock->lock();
if (unreachable_peer) {
next_heartbeat_[peer_id] = Clock::now() + config_.heartbeat_interval;
return;
}
if (CurrentTerm() != request_term || exiting_) {
return;
}
if (OutOfSync(reply.term)) {
state_changed_.notify_all();
return;
}
DCHECK(mode_ == Mode::LEADER)
<< "Elected leader for term should never change.";
if (reply.term != CurrentTerm()) {
VLOG(40) << "Server " << server_id_
<< ": Ignoring stale AppendEntriesRPC reply from " << peer_id;
return;
}
if (!reply.success) {
DCHECK(next_index_[peer_id] > 1)
<< "Log replication should not fail for first log entry";
--next_index_[peer_id];
} else {
uint64_t new_match_index = request_prev_log_index + request_entries.size();
DCHECK(match_index_[peer_id] <= new_match_index)
<< "`match_index` should increase monotonically within a term";
match_index_[peer_id] = new_match_index;
if (request_entries.size() > 0) AdvanceCommitIndex();
next_index_[peer_id] = match_index_[peer_id] + 1;
next_heartbeat_[peer_id] = Clock::now() + config_.heartbeat_interval;
}
state_changed_.notify_all();
}
void RaftServer::SendSnapshot(uint16_t peer_id,
const SnapshotMetadata &snapshot_metadata,
std::unique_lock<std::mutex> *lock) {
uint64_t request_term = CurrentTerm();
uint32_t snapshot_size = 0;
std::unique_ptr<char[]> snapshot;
{
const auto snapshot_path = durability::MakeSnapshotPath(
durability_dir_, snapshot_metadata.snapshot_filename);
std::ifstream input_stream;
input_stream.open(snapshot_path, std::ios::in | std::ios::binary);
input_stream.seekg(0, std::ios::end);
snapshot_size = input_stream.tellg();
snapshot.reset(new char[snapshot_size]);
input_stream.seekg(0, std::ios::beg);
input_stream.read(snapshot.get(), snapshot_size);
input_stream.close();
}
VLOG(40) << "Snapshot size: " << snapshot_size << " bytes.";
bool unreachable_peer = false;
auto peer_future = coordination_->ExecuteOnWorker<InstallSnapshotRes>(
peer_id, [&](int worker_id, auto &client) {
try {
auto res = client.template Call<InstallSnapshotRpc>(
server_id_, request_term, snapshot_metadata, std::move(snapshot),
snapshot_size);
return res;
} catch (...) {
unreachable_peer = true;
return InstallSnapshotRes(request_term);
}
});
lock->unlock();
auto reply = peer_future.get();
lock->lock();
if (unreachable_peer) {
next_heartbeat_[peer_id] = Clock::now() + config_.heartbeat_interval;
return;
}
if (CurrentTerm() != request_term || exiting_) {
return;
}
if (OutOfSync(reply.term)) {
state_changed_.notify_all();
return;
}
if (reply.term != CurrentTerm()) {
VLOG(40) << "Server " << server_id_
<< ": Ignoring stale InstallSnapshotRpc reply from " << peer_id;
return;
}
match_index_[peer_id] = snapshot_metadata.last_included_index;
next_index_[peer_id] = snapshot_metadata.last_included_index + 1;
next_heartbeat_[peer_id] = Clock::now() + config_.heartbeat_interval;
state_changed_.notify_all();
}
void RaftServer::ElectionThreadMain() {
std::unique_lock<std::mutex> lock(lock_);
while (!exiting_) {
if (Clock::now() >= next_election_) {
VLOG(40) << "Server " << server_id_
<< ": Election timeout exceeded (Term: " << CurrentTerm() << ")";
Transition(Mode::CANDIDATE);
state_changed_.notify_all();
}
election_change_.wait_until(lock, next_election_);
}
}
void RaftServer::PeerThreadMain(uint16_t peer_id) {
utils::ThreadSetName(fmt::format("RaftPeer{}", peer_id));
std::unique_lock<std::mutex> lock(lock_);
/* This loop will either call a function that issues an RPC or wait on the
* condition variable. It must not do both! Lock on `mutex_` is released
* while waiting for RPC response, which might cause us to miss a
* notification on `state_changed_` conditional variable and wait
* indefinitely. The safest thing to do is to assume some important part of
* state was modified while we were waiting for the response and loop around
* to check. */
while (!exiting_) {
TimePoint now = Clock::now();
TimePoint wait_until;
if (mode_ != Mode::FOLLOWER && backoff_until_[peer_id] > now) {
wait_until = backoff_until_[peer_id];
} else {
switch (mode_) {
case Mode::FOLLOWER: {
wait_until = TimePoint::max();
break;
}
case Mode::CANDIDATE: {
if (vote_requested_[peer_id]) break;
// TODO(ipaljak): Consider backoff.
wait_until = TimePoint::max();
auto request_term = CurrentTerm();
auto peer_future = coordination_->ExecuteOnWorker<RequestVoteRes>(
peer_id, [&](int worker_id, auto &client) {
auto last_entry_data = LastEntryData();
try {
auto res = client.template Call<RequestVoteRpc>(
server_id_, request_term, last_entry_data.first,
last_entry_data.second);
return res;
} catch (...) {
// not being able to connect to peer defaults to a vote
// being denied from that peer. This is correct but not
// optimal.
//
// TODO(ipaljak): reconsider this decision :)
return RequestVoteRes(false, request_term);
}
});
lock.unlock(); // Release lock while waiting for response
auto reply = peer_future.get();
lock.lock();
if (CurrentTerm() != request_term || mode_ != Mode::CANDIDATE ||
exiting_) {
VLOG(40) << "Server " << server_id_
<< ": Ignoring RequestVoteRPC reply from " << peer_id;
break;
}
if (OutOfSync(reply.term)) {
state_changed_.notify_all();
continue;
}
vote_requested_[peer_id] = true;
if (reply.vote_granted) {
VLOG(40) << "Server " << server_id_ << ": Got vote from "
<< peer_id;
++granted_votes_;
if (HasMajortyVote()) Transition(Mode::LEADER);
} else {
VLOG(40) << "Server " << server_id_ << ": Denied vote from "
<< peer_id;
}
state_changed_.notify_all();
continue;
}
case Mode::LEADER: {
if (now >= next_heartbeat_[peer_id]) {
VLOG(40) << "Server " << server_id_
<< ": Sending Entries RPC to server " << peer_id
<< " (Term: " << CurrentTerm() << ")";
SendEntries(peer_id, &lock);
continue;
}
wait_until = next_heartbeat_[peer_id];
break;
}
}
}
state_changed_.wait_until(lock, wait_until);
}
}
void RaftServer::NoOpIssuerThreadMain() {
utils::ThreadSetName(fmt::format("NoOpIssuer"));
std::mutex m;
auto lock = std::unique_lock<std::mutex>(m);
while (!exiting_) {
leader_changed_.wait(lock);
// no_op_create_callback_ will create a new transaction that has a NO_OP
// StateDelta. This will trigger the whole procedure of replicating logs
// in our implementation of Raft.
if (!exiting_) NoOpCreate();
}
}
void RaftServer::SnapshotThread() {
utils::ThreadSetName(fmt::format("RaftSnapshot"));
if (config_.log_size_snapshot_threshold == -1) return;
while (true) {
{
// Acquire snapshot lock before we acquire the Raft lock. This should
// avoid the situation where we release the lock to start writing a
// snapshot but the `InstallSnapshotRpc` deletes it and we write wrong
// metadata.
std::lock_guard<std::mutex> snapshot_guard(snapshot_lock_);
std::unique_lock<std::mutex> lock(lock_);
if (exiting_) break;
uint64_t committed_log_size = last_applied_;
auto snapshot_metadata = GetSnapshotMetadata();
if (snapshot_metadata) {
committed_log_size -= snapshot_metadata->last_included_index;
}
// Compare the log size to the config
if (config_.log_size_snapshot_threshold < committed_log_size) {
VLOG(40) << "[LogCompaction] Starting log compaction.";
// Create a DB accessor for snapshot creation
std::unique_ptr<database::GraphDbAccessor> dba = db_->Access();
uint64_t last_included_term = GetLogEntry(last_applied_).term;
uint64_t last_included_index = last_applied_;
std::string snapshot_filename =
durability::GetSnapshotFilename(dba->transaction_id());
lock.unlock();
VLOG(40) << "[LogCompaction] Creating snapshot.";
bool status = durability::MakeSnapshot(*db_, *dba, durability_dir_,
snapshot_filename);
lock.lock();
if (status) {
uint64_t log_compaction_start_index = 1;
if (snapshot_metadata) {
log_compaction_start_index =
snapshot_metadata->last_included_index + 1;
}
VLOG(40) << "[LogCompaction] Persisting snapshot metadata";
PersistSnapshotMetadata(
{last_included_term, last_included_index, snapshot_filename});
// Log compaction.
VLOG(40) << "[LogCompaction] Compacting log from "
<< log_compaction_start_index << " to "
<< last_included_index;
for (int i = log_compaction_start_index; i <= last_included_index;
++i) {
disk_storage_.Delete(LogEntryKey(i));
}
}
lock.unlock();
// Raft lock must be released when destroying dba object.
dba = nullptr;
}
}
std::this_thread::sleep_for(kSnapshotPeriod);
}
}
void RaftServer::SetNextElectionTimePoint() {
// [Raft thesis, section 3.4]
// "Raft uses randomized election timeouts to ensure that split votes are
// rare and that they are resolved quickly. To prevent split votes in the
// first place, election timeouts are chosen randomly from a fixed interval
// (e.g., 150-300 ms)."
std::uniform_int_distribution<uint64_t> distribution(
config_.election_timeout_min.count(),
config_.election_timeout_max.count());
Clock::duration wait_interval = std::chrono::milliseconds(distribution(rng_));
next_election_ = Clock::now() + wait_interval;
}
bool RaftServer::HasMajortyVote() {
if (2 * granted_votes_ > coordination_->WorkerCount()) {
VLOG(40) << "Server " << server_id_
<< ": Obtained majority vote (Term: " << CurrentTerm() << ")";
return true;
}
return false;
}
std::pair<uint64_t, uint64_t> RaftServer::LastEntryData() {
uint64_t log_size = LogSize();
if (log_size == 0) return {0, 0};
auto snapshot_metadata = GetSnapshotMetadata();
if (snapshot_metadata &&
snapshot_metadata->last_included_index == log_size - 1) {
return {log_size, snapshot_metadata->last_included_term};
}
return {log_size, GetLogEntry(log_size - 1).term};
}
bool RaftServer::AtLeastUpToDate(uint64_t last_log_index_a,
uint64_t last_log_term_a,
uint64_t last_log_index_b,
uint64_t last_log_term_b) {
if (last_log_term_a == last_log_term_b)
return last_log_index_a >= last_log_index_b;
return last_log_term_a > last_log_term_b;
}
bool RaftServer::OutOfSync(uint64_t reply_term) {
DCHECK(mode_ != Mode::FOLLOWER) << "`OutOfSync` called from FOLLOWER mode";
// [Raft thesis, Section 3.3]
// "Current terms are exchanged whenever servers communicate; if one
// server's current term is smaller than the other's, then it updates
// its current term to the larger value. If a candidate or leader
// discovers that its term is out of date, it immediately reverts to
// follower state."
if (CurrentTerm() < reply_term) {
disk_storage_.Put(kCurrentTermKey, std::to_string(reply_term));
disk_storage_.Delete(kVotedForKey);
granted_votes_ = 0;
Transition(Mode::FOLLOWER);
return true;
}
return false;
}
LogEntry RaftServer::GetLogEntry(int index) {
auto opt_value = disk_storage_.Get(LogEntryKey(index));
DCHECK(opt_value != std::experimental::nullopt)
<< "Log index (" << index << ") out of bounds.";
return DeserializeLogEntry(opt_value.value());
}
void RaftServer::DeleteLogSuffix(int starting_index) {
uint64_t log_size = LogSize();
DCHECK(0 <= starting_index && starting_index < log_size)
<< "Log index out of bounds.";
for (int i = starting_index; i < log_size; ++i)
disk_storage_.Delete(LogEntryKey(i));
disk_storage_.Put(kLogSizeKey, std::to_string(starting_index));
}
void RaftServer::GetLogSuffix(int starting_index,
std::vector<raft::LogEntry> &entries) {
uint64_t log_size = LogSize();
DCHECK(0 <= starting_index && starting_index < log_size)
<< "Log index out of bounds.";
for (int i = starting_index; i < log_size; ++i)
entries.push_back(GetLogEntry(i));
}
void RaftServer::AppendLogEntries(uint64_t leader_commit_index,
uint64_t starting_index,
const std::vector<LogEntry> &new_entries) {
uint64_t log_size = LogSize();
for (int i = 0; i < new_entries.size(); ++i) {
// If existing entry conflicts with new one, we need to delete the
// existing entry and all that follow it.
int current_index = i + starting_index;
if (log_size > current_index &&
GetLogEntry(current_index).term != new_entries[i].term) {
DeleteLogSuffix(current_index);
log_size = LogSize();
}
DCHECK(log_size >= current_index) << "Current Log index out of bounds.";
if (log_size == current_index) {
disk_storage_.Put(LogEntryKey(log_size),
SerializeLogEntry(new_entries[i]));
disk_storage_.Put(kLogSizeKey, std::to_string(log_size + 1));
log_size += 1;
}
}
// See Raft paper 5.3
if (leader_commit_index > commit_index_) {
commit_index_ = std::min(leader_commit_index, log_size - 1);
}
}
std::string RaftServer::LogEntryKey(uint64_t index) {
return kLogEntryPrefix + std::to_string(index);
}
std::string RaftServer::SerializeLogEntry(const LogEntry &log_entry) {
std::stringstream stream(std::ios_base::in | std::ios_base::out |
std::ios_base::binary);
{
::capnp::MallocMessageBuilder message;
capnp::LogEntry::Builder log_builder = message.initRoot<capnp::LogEntry>();
Save(log_entry, &log_builder);
kj::std::StdOutputStream std_stream(stream);
kj::BufferedOutputStreamWrapper buffered_stream(std_stream);
writeMessage(buffered_stream, message);
}
return stream.str();
}
LogEntry RaftServer::DeserializeLogEntry(
const std::string &serialized_log_entry) {
::capnp::MallocMessageBuilder message;
std::stringstream stream(std::ios_base::in | std::ios_base::out |
std::ios_base::binary);
kj::std::StdInputStream std_stream(stream);
kj::BufferedInputStreamWrapper buffered_stream(std_stream);
stream << serialized_log_entry;
readMessageCopy(buffered_stream, message);
capnp::LogEntry::Reader log_reader =
message.getRoot<capnp::LogEntry>().asReader();
LogEntry deserialized_log;
Load(&deserialized_log, log_reader);
return deserialized_log;
}
void RaftServer::ResetReplicationLog() {
rlog_ = nullptr;
rlog_ = std::make_unique<ReplicationLog>();
}
void RaftServer::RecoverSnapshot(const std::string &snapshot_filename) {
durability::RecoveryData recovery_data;
bool recovery = durability::RecoverSnapshot(
db_, &recovery_data, durability_dir_, snapshot_filename);
CHECK(recovery);
durability::RecoverIndexes(db_, recovery_data.indexes);
}
void RaftServer::NoOpCreate() {
auto dba = db_->Access();
Emplace(database::StateDelta::NoOp(dba->transaction_id()));
dba->Commit();
}
} // namespace raft