More optimal

src/Common/ZooKeeper/ZooKeeperCommon.cpp

@@ -455,6 +455,35 @@ ZooKeeperResponsePtr ZooKeeperCheckRequest::makeResponse() const { return std::m
 ZooKeeperResponsePtr ZooKeeperMultiRequest::makeResponse() const { return std::make_shared<ZooKeeperMultiResponse>(requests); }
 ZooKeeperResponsePtr ZooKeeperCloseRequest::makeResponse() const { return std::make_shared<ZooKeeperCloseResponse>(); }
 
+void ZooKeeperSessionIDRequest::writeImpl(WriteBuffer & out) const
+{
+    Coordination::write(internal_id, out);
+    Coordination::write(session_timeout_ms, out);
+}
+
+void ZooKeeperSessionIDRequest::readImpl(ReadBuffer & in)
+{
+    Coordination::read(internal_id, in);
+    Coordination::read(session_timeout_ms, in);
+}
+
+Coordination::ZooKeeperResponsePtr ZooKeeperSessionIDRequest::makeResponse() const
+{
+    return std::make_shared<ZooKeeperSessionIDResponse>();
+}
+
+void ZooKeeperSessionIDResponse::readImpl(ReadBuffer & in)
+{
+    Coordination::read(internal_id, in);
+    Coordination::read(session_id, in);
+}
+
+void ZooKeeperSessionIDResponse::writeImpl(WriteBuffer & out) const
+{
+    Coordination::write(internal_id, out);
+    Coordination::write(session_id, out);
+}
+
 void ZooKeeperRequestFactory::registerRequest(OpNum op_num, Creator creator)
 {
     if (!op_num_to_request.try_emplace(op_num, creator).second)
@@ -511,6 +540,7 @@ ZooKeeperRequestFactory::ZooKeeperRequestFactory()
     registerZooKeeperRequest<OpNum::List, ZooKeeperListRequest>(*this);
     registerZooKeeperRequest<OpNum::Check, ZooKeeperCheckRequest>(*this);
     registerZooKeeperRequest<OpNum::Multi, ZooKeeperMultiRequest>(*this);
+    registerZooKeeperRequest<OpNum::SessionID, ZooKeeperSessionIDRequest>(*this);
 }
 
 }

src/Common/ZooKeeper/ZooKeeperCommon.h

@@ -390,6 +390,36 @@ struct ZooKeeperMultiResponse final : MultiResponse, ZooKeeperResponse
     size_t bytesSize() const override { return MultiResponse::bytesSize() + sizeof(xid) + sizeof(zxid); }
 };
 
+/// Fake internal coordination (keeper) response. Never received from client
+/// and never send to client.
+struct ZooKeeperSessionIDRequest final : ZooKeeperRequest
+{
+    int64_t internal_id;
+    int64_t session_timeout_ms;
+
+    Coordination::OpNum getOpNum() const override { return OpNum::SessionID; }
+    String getPath() const override { return {}; }
+    void writeImpl(WriteBuffer & out) const override;
+    void readImpl(ReadBuffer & in) override;
+
+    Coordination::ZooKeeperResponsePtr makeResponse() const override;
+    bool isReadRequest() const override { return false; }
+};
+
+/// Fake internal coordination (keeper) response. Never received from client
+/// and never send to client.
+struct ZooKeeperSessionIDResponse final : ZooKeeperResponse
+{
+    int64_t internal_id;
+    int64_t session_id;
+
+    void readImpl(ReadBuffer & in) override;
+
+    void writeImpl(WriteBuffer & out) const override;
+
+    Coordination::OpNum getOpNum() const override { return OpNum::SessionID; }
+};
+
 class ZooKeeperRequestFactory final : private boost::noncopyable
 {
 

src/Common/ZooKeeper/ZooKeeperConstants.cpp

@@ -21,6 +21,7 @@ static const std::unordered_set<int32_t> VALID_OPERATIONS =
     static_cast<int32_t>(OpNum::Check),
     static_cast<int32_t>(OpNum::Multi),
     static_cast<int32_t>(OpNum::Auth),
+    static_cast<int32_t>(OpNum::SessionID),
 };
 
 std::string toString(OpNum op_num)
@@ -55,6 +56,8 @@ std::string toString(OpNum op_num)
             return "Heartbeat";
         case OpNum::Auth:
             return "Auth";
+        case OpNum::SessionID:
+            return "SessionID";
     }
     int32_t raw_op = static_cast<int32_t>(op_num);
     throw Exception("Operation " + std::to_string(raw_op) + " is unknown", Error::ZUNIMPLEMENTED);

src/Common/ZooKeeper/ZooKeeperConstants.h

@@ -30,6 +30,7 @@ enum class OpNum : int32_t
     Check = 13,
     Multi = 14,
     Auth = 100,
+    SessionID = 997, /// Special internal request
 };
 
 std::string toString(OpNum op_num);

src/Coordination/Changelog.cpp

@@ -80,7 +80,7 @@ public:
     {}
 
 
-    off_t appendRecord(ChangelogRecord && record, bool sync)
+    off_t appendRecord(ChangelogRecord && record)
     {
         off_t result = plain_buf.count();
         writeIntBinary(computeRecordChecksum(record), plain_buf);
@@ -96,22 +96,20 @@ public:
 
         entries_written++;
 
-        if (sync)
-            plain_buf.sync();
-        else
-            plain_buf.next();
         return result;
     }
 
     void truncateToLength(off_t new_length)
     {
-        flush();
+        plain_buf.next();
         plain_buf.truncate(new_length);
         plain_buf.seek(new_length, SEEK_SET);
     }
 
-    void flush()
+    void flush(bool force_fsync)
     {
+        plain_buf.next();
+        if (force_fsync)
             plain_buf.sync();
     }
 
@@ -247,9 +245,14 @@ private:
     ReadBufferFromFile read_buf;
 };
 
-Changelog::Changelog(const std::string & changelogs_dir_, uint64_t rotate_interval_, Poco::Logger * log_)
+Changelog::Changelog(
+    const std::string & changelogs_dir_,
+    uint64_t rotate_interval_,
+    bool force_sync_,
+    Poco::Logger * log_)
     : changelogs_dir(changelogs_dir_)
     , rotate_interval(rotate_interval_)
+    , force_sync(force_sync_)
     , log(log_)
 {
     namespace fs = std::filesystem;
@@ -357,6 +360,9 @@ void Changelog::readChangelogAndInitWriter(uint64_t last_commited_log_index, uin
 
 void Changelog::rotate(uint64_t new_start_log_index)
 {
+    /// Flush previous log
+    flush();
+
     ChangelogFileDescription new_description;
     new_description.prefix = DEFAULT_PREFIX;
     new_description.from_log_index = new_start_log_index;
@@ -387,7 +393,7 @@ ChangelogRecord Changelog::buildRecord(uint64_t index, const LogEntryPtr & log_e
     return record;
 }
 
-void Changelog::appendEntry(uint64_t index, const LogEntryPtr & log_entry, bool force_sync)
+void Changelog::appendEntry(uint64_t index, const LogEntryPtr & log_entry)
 {
     if (!current_writer)
         throw Exception(ErrorCodes::LOGICAL_ERROR, "Changelog must be initialized before appending records");
@@ -398,14 +404,14 @@ void Changelog::appendEntry(uint64_t index, const LogEntryPtr & log_entry, bool
     if (current_writer->getEntriesWritten() == rotate_interval)
         rotate(index);
 
-    auto offset = current_writer->appendRecord(buildRecord(index, log_entry), force_sync);
+    auto offset = current_writer->appendRecord(buildRecord(index, log_entry));
     if (!index_to_start_pos.try_emplace(index, offset).second)
         throw Exception(ErrorCodes::LOGICAL_ERROR, "Record with index {} already exists", index);
 
     logs[index] = makeClone(log_entry);
 }
 
-void Changelog::writeAt(uint64_t index, const LogEntryPtr & log_entry, bool force_sync)
+void Changelog::writeAt(uint64_t index, const LogEntryPtr & log_entry)
 {
     if (index_to_start_pos.count(index) == 0)
         throw Exception(ErrorCodes::LOGICAL_ERROR, "Cannot write at index {} because changelog doesn't contain it", index);
@@ -451,7 +457,7 @@ void Changelog::writeAt(uint64_t index, const LogEntryPtr & log_entry, bool forc
 
     current_writer->setEntriesWritten(entries_written);
 
-    appendEntry(index, log_entry, force_sync);
+    appendEntry(index, log_entry);
 }
 
 void Changelog::compact(uint64_t up_to_log_index)
@@ -540,7 +546,7 @@ nuraft::ptr<nuraft::buffer> Changelog::serializeEntriesToBuffer(uint64_t index,
     return buf_out;
 }
 
-void Changelog::applyEntriesFromBuffer(uint64_t index, nuraft::buffer & buffer, bool force_sync)
+void Changelog::applyEntriesFromBuffer(uint64_t index, nuraft::buffer & buffer)
 {
     buffer.pos(0);
     int num_logs = buffer.get_int();
@@ -555,23 +561,23 @@ void Changelog::applyEntriesFromBuffer(uint64_t index, nuraft::buffer & buffer,
 
         LogEntryPtr log_entry = nuraft::log_entry::deserialize(*buf_local);
         if (i == 0 && logs.count(cur_index))
-            writeAt(cur_index, log_entry, force_sync);
+            writeAt(cur_index, log_entry);
         else
-            appendEntry(cur_index, log_entry, force_sync);
+            appendEntry(cur_index, log_entry);
     }
 }
 
 void Changelog::flush()
 {
-    current_writer->flush();
+    if (current_writer)
+        current_writer->flush(force_sync);
 }
 
 Changelog::~Changelog()
 {
     try
     {
-        if (current_writer)
-            current_writer->flush();
+        flush();
     }
     catch (...)
     {

src/Coordination/Changelog.h

@@ -63,17 +63,17 @@ class Changelog
 {
 
 public:
-    Changelog(const std::string & changelogs_dir_, uint64_t rotate_interval_, Poco::Logger * log_);
+    Changelog(const std::string & changelogs_dir_, uint64_t rotate_interval_, bool force_sync_, Poco::Logger * log_);
 
     /// Read changelog from files on changelogs_dir_ skipping all entries before from_log_index
     /// Truncate broken entries, remove files after broken entries.
     void readChangelogAndInitWriter(uint64_t last_commited_log_index, uint64_t logs_to_keep);
 
-    /// Add entry to log with index. Call fsync if force_sync true.
-    void appendEntry(uint64_t index, const LogEntryPtr & log_entry, bool force_sync);
+    /// Add entry to log with index.
+    void appendEntry(uint64_t index, const LogEntryPtr & log_entry);
 
     /// Write entry at index and truncate all subsequent entries.
-    void writeAt(uint64_t index, const LogEntryPtr & log_entry, bool force_sync);
+    void writeAt(uint64_t index, const LogEntryPtr & log_entry);
 
     /// Remove log files with to_log_index <= up_to_log_index.
     void compact(uint64_t up_to_log_index);
@@ -101,9 +101,9 @@ public:
     BufferPtr serializeEntriesToBuffer(uint64_t index, int32_t count);
 
     /// Apply entries from buffer overriding existing entries
-    void applyEntriesFromBuffer(uint64_t index, nuraft::buffer & buffer, bool force_sync);
+    void applyEntriesFromBuffer(uint64_t index, nuraft::buffer & buffer);
 
-    /// Fsync log to disk
+    /// Fsync latest log to disk and flush buffer
     void flush();
 
     uint64_t size() const
@@ -124,6 +124,7 @@ private:
 private:
     const std::string changelogs_dir;
     const uint64_t rotate_interval;
+    const bool force_sync;
     Poco::Logger * log;
 
     std::map<uint64_t, ChangelogFileDescription> existing_changelogs;

src/Coordination/CoordinationSettings.h

@@ -32,6 +32,7 @@ struct Settings;
     M(UInt64, snapshots_to_keep, 3, "How many compressed snapshots to keep on disk", 0) \
     M(UInt64, stale_log_gap, 10000, "When node became stale and should receive snapshots from leader", 0) \
     M(UInt64, fresh_log_gap, 200, "When node became fresh", 0) \
+    M(UInt64, max_requests_batch_size, 100, "Max size of batch in requests count before it will be sent to RAFT", 0) \
     M(Bool, quorum_reads, false, "Execute read requests as writes through whole RAFT consesus with similar speed", 0) \
     M(Bool, force_sync, true, "Call fsync on each change in RAFT changelog", 0)
 

src/Coordination/KeeperLogStore.cpp

@@ -5,9 +5,12 @@ namespace DB
 
 KeeperLogStore::KeeperLogStore(const std::string & changelogs_path, uint64_t rotate_interval_, bool force_sync_)
     : log(&Poco::Logger::get("KeeperLogStore"))
-    , changelog(changelogs_path, rotate_interval_, log)
-    , force_sync(force_sync_)
+    , changelog(changelogs_path, rotate_interval_, force_sync_, log)
 {
+    if (force_sync_)
+        LOG_INFO(log, "force_sync enabled");
+    else
+        LOG_INFO(log, "force_sync disabled");
 }
 
 uint64_t KeeperLogStore::start_index() const
@@ -38,7 +41,7 @@ uint64_t KeeperLogStore::append(nuraft::ptr<nuraft::log_entry> & entry)
 {
     std::lock_guard lock(changelog_lock);
     uint64_t idx = changelog.getNextEntryIndex();
-    changelog.appendEntry(idx, entry, force_sync);
+    changelog.appendEntry(idx, entry);
     return idx;
 }
 
@@ -46,7 +49,7 @@ uint64_t KeeperLogStore::append(nuraft::ptr<nuraft::log_entry> & entry)
 void KeeperLogStore::write_at(uint64_t index, nuraft::ptr<nuraft::log_entry> & entry)
 {
     std::lock_guard lock(changelog_lock);
-    changelog.writeAt(index, entry, force_sync);
+    changelog.writeAt(index, entry);
 }
 
 nuraft::ptr<std::vector<nuraft::ptr<nuraft::log_entry>>> KeeperLogStore::log_entries(uint64_t start, uint64_t end)
@@ -93,7 +96,7 @@ bool KeeperLogStore::flush()
 void KeeperLogStore::apply_pack(uint64_t index, nuraft::buffer & pack)
 {
     std::lock_guard lock(changelog_lock);
-    changelog.applyEntriesFromBuffer(index, pack, force_sync);
+    changelog.applyEntriesFromBuffer(index, pack);
 }
 
 uint64_t KeeperLogStore::size() const
@@ -102,4 +105,10 @@ uint64_t KeeperLogStore::size() const
     return changelog.size();
 }
 
+void KeeperLogStore::end_of_append_batch(uint64_t /*start_index*/, uint64_t /*count*/)
+{
+    std::lock_guard lock(changelog_lock);
+    changelog.flush();
+}
+
 }

src/Coordination/KeeperLogStore.h

@@ -42,11 +42,12 @@ public:
 
     uint64_t size() const;
 
+    void end_of_append_batch(uint64_t start_index, uint64_t count) override;
+
 private:
     mutable std::mutex changelog_lock;
     Poco::Logger * log;
     Changelog changelog;
-    bool force_sync;
 };
 
 }

src/Coordination/KeeperServer.cpp

@@ -73,7 +73,6 @@ KeeperServer::KeeperServer(
                         config.getString("keeper_server.snapshot_storage_path", config.getString("path", DBMS_DEFAULT_PATH) + "coordination/snapshots"),
                         coordination_settings))
     , state_manager(nuraft::cs_new<KeeperStateManager>(server_id, "keeper_server", config, coordination_settings))
-    , responses_queue(responses_queue_)
     , log(&Poco::Logger::get("KeeperServer"))
 {
     if (coordination_settings->quorum_reads)
@@ -111,7 +110,7 @@ void KeeperServer::startup()
     params.auto_forwarding_ = coordination_settings->auto_forwarding;
     params.auto_forwarding_req_timeout_ = coordination_settings->operation_timeout_ms.totalMilliseconds() * 2;
 
-    params.return_method_ = nuraft::raft_params::blocking;
+    params.return_method_ = nuraft::raft_params::async_handler;
 
     nuraft::asio_service::options asio_opts{};
     if (state_manager->isSecure())
@@ -222,75 +221,23 @@ nuraft::ptr<nuraft::buffer> getZooKeeperLogEntry(int64_t session_id, const Coord
 
 }
 
-void KeeperServer::putRequest(const KeeperStorage::RequestForSession & request_for_session)
-{
-    auto [session_id, request] = request_for_session;
-    if (!coordination_settings->quorum_reads && isLeaderAlive() && request->isReadRequest())
+
+void KeeperServer::putLocalReadRequest(const KeeperStorage::RequestForSession & request_for_session)
 {
     state_machine->processReadRequest(request_for_session);
 }
-    else
+
+RaftAppendResult KeeperServer::putRequestBatch(const KeeperStorage::RequestsForSessions & requests_for_sessions)
 {
+
     std::vector<nuraft::ptr<nuraft::buffer>> entries;
+    for (const auto & [session_id, request] : requests_for_sessions)
         entries.push_back(getZooKeeperLogEntry(session_id, request));
 
+    {
         std::lock_guard lock(append_entries_mutex);
-
-        auto result = raft_instance->append_entries(entries);
-        if (!result->get_accepted())
-        {
-            KeeperStorage::ResponsesForSessions responses;
-            auto response = request->makeResponse();
-            response->xid = request->xid;
-            response->zxid = 0;
-            response->error = Coordination::Error::ZOPERATIONTIMEOUT;
-            responses_queue.push(DB::KeeperStorage::ResponseForSession{session_id, response});
+        return raft_instance->append_entries(entries);
     }
-
-        if (result->get_result_code() == nuraft::cmd_result_code::TIMEOUT)
-        {
-            KeeperStorage::ResponsesForSessions responses;
-            auto response = request->makeResponse();
-            response->xid = request->xid;
-            response->zxid = 0;
-            response->error = Coordination::Error::ZOPERATIONTIMEOUT;
-            responses_queue.push(DB::KeeperStorage::ResponseForSession{session_id, response});
-        }
-        else if (result->get_result_code() != nuraft::cmd_result_code::OK)
-            throw Exception(ErrorCodes::RAFT_ERROR, "Requests result failed with code {} and message: '{}'", result->get_result_code(), result->get_result_str());
-    }
-}
-
-int64_t KeeperServer::getSessionID(int64_t session_timeout_ms)
-{
-    /// Just some sanity check. We don't want to make a lot of clients wait with lock.
-    if (active_session_id_requests > 10)
-        throw Exception(ErrorCodes::RAFT_ERROR, "Too many concurrent SessionID requests already in flight");
-
-    ++active_session_id_requests;
-    SCOPE_EXIT({ --active_session_id_requests; });
-
-    auto entry = nuraft::buffer::alloc(sizeof(int64_t));
-    /// Just special session request
-    nuraft::buffer_serializer bs(entry);
-    bs.put_i64(session_timeout_ms);
-
-    std::lock_guard lock(append_entries_mutex);
-
-    auto result = raft_instance->append_entries({entry});
-
-    if (!result->get_accepted())
-        throw Exception(ErrorCodes::RAFT_ERROR, "Cannot send session_id request to RAFT");
-
-    if (result->get_result_code() != nuraft::cmd_result_code::OK)
-        throw Exception(ErrorCodes::RAFT_ERROR, "session_id request failed to RAFT");
-
-    auto resp = result->get();
-    if (resp == nullptr)
-        throw Exception(ErrorCodes::RAFT_ERROR, "Received nullptr as session_id");
-
-    nuraft::buffer_serializer bs_resp(resp);
-    return bs_resp.get_i64();
 }
 
 bool KeeperServer::isLeader() const

src/Coordination/KeeperServer.h

@@ -12,6 +12,8 @@
 namespace DB
 {
 
+using RaftAppendResult = nuraft::ptr<nuraft::cmd_result<nuraft::ptr<nuraft::buffer>>>;
+
 class KeeperServer
 {
 private:
@@ -29,13 +31,10 @@ private:
 
     std::mutex append_entries_mutex;
 
-    ResponsesQueue & responses_queue;
-
     std::mutex initialized_mutex;
     std::atomic<bool> initialized_flag = false;
     std::condition_variable initialized_cv;
     std::atomic<bool> initial_batch_committed = false;
-    std::atomic<size_t> active_session_id_requests = 0;
 
     Poco::Logger * log;
 
@@ -60,9 +59,9 @@ public:
 
     void startup();
 
-    void putRequest(const KeeperStorage::RequestForSession & request);
+    void putLocalReadRequest(const KeeperStorage::RequestForSession & request);
 
-    int64_t getSessionID(int64_t session_timeout_ms);
+    RaftAppendResult putRequestBatch(const KeeperStorage::RequestsForSessions & requests);
 
     std::unordered_set<int64_t> getDeadSessions();
 

src/Coordination/KeeperStateMachine.cpp

@@ -90,25 +90,28 @@ void KeeperStateMachine::init()
 
 nuraft::ptr<nuraft::buffer> KeeperStateMachine::commit(const uint64_t log_idx, nuraft::buffer & data)
 {
-    if (data.size() == sizeof(int64_t))
+    auto request_for_session = parseRequest(data);
+    if (request_for_session.request->getOpNum() == Coordination::OpNum::SessionID)
     {
-        nuraft::buffer_serializer timeout_data(data);
-        int64_t session_timeout_ms = timeout_data.get_i64();
-        auto response = nuraft::buffer::alloc(sizeof(int64_t));
+        const Coordination::ZooKeeperSessionIDRequest & session_id_request = dynamic_cast<const Coordination::ZooKeeperSessionIDRequest &>(*request_for_session.request);
         int64_t session_id;
-        nuraft::buffer_serializer bs(response);
         {
             std::lock_guard lock(storage_lock);
-            session_id = storage->getSessionID(session_timeout_ms);
-            bs.put_i64(session_id);
+            session_id = storage->getSessionID(session_id_request.session_timeout_ms);
         }
-        LOG_DEBUG(log, "Session ID response {} with timeout {}", session_id, session_timeout_ms);
-        last_committed_idx = log_idx;
-        return response;
+        LOG_DEBUG(log, "Session ID response {} with timeout {}", session_id, session_id_request.session_timeout_ms);
+
+        std::shared_ptr<Coordination::ZooKeeperSessionIDResponse> response = std::make_shared<Coordination::ZooKeeperSessionIDResponse>();
+        response->internal_id = session_id_request.internal_id;
+        response->session_id = session_id;
+
+        KeeperStorage::ResponseForSession response_for_session;
+        response_for_session.session_id = -1;
+        response_for_session.response = response;
+        responses_queue.push(response_for_session);
     }
     else
     {
-        auto request_for_session = parseRequest(data);
         KeeperStorage::ResponsesForSessions responses_for_sessions;
         {
             std::lock_guard lock(storage_lock);
@@ -116,11 +119,12 @@ nuraft::ptr<nuraft::buffer> KeeperStateMachine::commit(const uint64_t log_idx, n
             for (auto & response_for_session : responses_for_sessions)
                 responses_queue.push(response_for_session);
         }
+    }
+
 
     last_committed_idx = log_idx;
     return nullptr;
 }
-}
 
 bool KeeperStateMachine::apply_snapshot(nuraft::snapshot & s)
 {

src/Coordination/KeeperStorage.h

@@ -8,7 +8,6 @@
 #include <Coordination/SnapshotableHashTable.h>
 #include <unordered_map>
 #include <unordered_set>
-#include <set>
 #include <vector>
 
 namespace DB
@@ -18,7 +17,7 @@ using namespace DB;
 struct KeeperStorageRequest;
 using KeeperStorageRequestPtr = std::shared_ptr<KeeperStorageRequest>;
 using ResponseCallback = std::function<void(const Coordination::ZooKeeperResponsePtr &)>;
-using ChildrenSet = std::set<std::string>;
+using ChildrenSet = std::unordered_set<std::string>;
 using SessionAndTimeout = std::unordered_map<int64_t, int64_t>;
 
 struct KeeperStorageSnapshot;

src/Coordination/KeeperStorageDispatcher.cpp

@@ -1,5 +1,9 @@
 #include <Coordination/KeeperStorageDispatcher.h>
 #include <Common/setThreadName.h>
+#include <Common/Stopwatch.h>
+#include <Common/ZooKeeper/KeeperException.h>
+#include <future>
+#include <chrono>
 
 namespace DB
 {
@@ -17,23 +21,101 @@ KeeperStorageDispatcher::KeeperStorageDispatcher()
 {
 }
 
+
 void KeeperStorageDispatcher::requestThread()
 {
     setThreadName("KeeperReqT");
+
+    /// Result of requests batch from previous iteration
+    RaftAppendResult prev_result = nullptr;
+    /// Requests from previous iteration. We store them to be able
+    /// to send errors to the client.
+    KeeperStorage::RequestsForSessions prev_batch;
+
     while (!shutdown_called)
     {
         KeeperStorage::RequestForSession request;
 
         UInt64 max_wait = UInt64(coordination_settings->operation_timeout_ms.totalMilliseconds());
+        uint64_t max_batch_size = coordination_settings->max_requests_batch_size;
 
-        if (requests_queue.tryPop(request, max_wait))
+        /// The code bellow do a very simple thing: batch all write (quorum) requests into vector until
+        /// previous write batch is not finished or max_batch size achieved. The main complexity goes from
+        /// the ability to process read requests without quorum (from local state). So when we are collecting
+        /// requests into a batch we must check that the new request is not read request. Otherwise we have to
+        /// process all already accumulated write requests, wait them synchronously and only after that process
+        /// read request. So reads are some kind of "separator" for writes.
+        try
+        {
+            if (requests_queue->tryPop(request, max_wait))
             {
                 if (shutdown_called)
                     break;
 
-            try
+                KeeperStorage::RequestsForSessions current_batch;
+
+                bool has_read_request = false;
+
+                /// If new request is not read request or we must to process it through quorum.
+                /// Otherwise we will process it locally.
+                if (coordination_settings->quorum_reads || !request.request->isReadRequest() || !server->isLeaderAlive())
                 {
-                server->putRequest(request);
+                    current_batch.emplace_back(request);
+
+                    /// Waiting until previous append will be successful, or batch is big enough
+                    while (prev_result && (!prev_result->has_result() && prev_result->get_result_code() == nuraft::cmd_result_code::OK) && current_batch.size() <= max_batch_size)
+                    {
+                        /// Trying to get batch requests as fast as possible
+                        if (requests_queue->tryPop(request, 1))
+                        {
+                            /// Don't append read request into batch, we have to process them separately
+                            if (!coordination_settings->quorum_reads && request.request->isReadRequest() && server->isLeaderAlive())
+                            {
+                                has_read_request = true;
+                                break;
+                            }
+
+                            current_batch.emplace_back(request);
+                        }
+
+                        if (shutdown_called)
+                            break;
+                    }
+                }
+                else
+                    has_read_request = true;
+
+                if (shutdown_called)
+                    break;
+
+                /// Forcefully process all previous pending requests
+                if (prev_result)
+                    forceWaitAndProcessResult(std::move(prev_result), std::move(prev_batch));
+
+                /// Process collected write requests batch
+                if (!current_batch.empty())
+                {
+                    prev_result = server->putRequestBatch(current_batch);
+
+                    if (prev_result)
+                    {
+                        if (has_read_request) /// If we will execute read request next, than we have to process result now
+                            forceWaitAndProcessResult(std::move(prev_result), std::move(current_batch));
+                    }
+                    else
+                    {
+                        addErrorResponses(std::move(current_batch), Coordination::Error::ZRUNTIMEINCONSISTENCY);
+                    }
+
+                    prev_batch = current_batch;
+                }
+
+                /// Read request always goes after write batch (last request)
+                /// We don't check leader aliveness, because it was alive when this request happen
+                /// so our state is consistent.
+                if (has_read_request)
+                    server->putLocalReadRequest(request);
+            }
         }
         catch (...)
         {
@@ -41,7 +123,6 @@ void KeeperStorageDispatcher::requestThread()
         }
     }
 }
-}
 
 void KeeperStorageDispatcher::responseThread()
 {
@@ -94,6 +175,19 @@ void KeeperStorageDispatcher::snapshotThread()
 void KeeperStorageDispatcher::setResponse(int64_t session_id, const Coordination::ZooKeeperResponsePtr & response)
 {
     std::lock_guard lock(session_to_response_callback_mutex);
+    if (response->xid != Coordination::WATCH_XID && response->getOpNum() == Coordination::OpNum::SessionID)
+    {
+        const Coordination::ZooKeeperSessionIDResponse & session_id_resp = dynamic_cast<const Coordination::ZooKeeperSessionIDResponse &>(*response);
+        /// Nobody waits for this session id
+        if (!new_session_id_response_callback.count(session_id_resp.internal_id))
+            return;
+
+        auto callback = new_session_id_response_callback[session_id_resp.internal_id];
+        callback(response);
+        new_session_id_response_callback.erase(session_id_resp.internal_id);
+    }
+    else
+    {
         auto session_writer = session_to_response_callback.find(session_id);
         if (session_writer == session_to_response_callback.end())
             return;
@@ -101,8 +195,11 @@ void KeeperStorageDispatcher::setResponse(int64_t session_id, const Coordination
         session_writer->second(response);
         /// Session closed, no more writes
         if (response->xid != Coordination::WATCH_XID && response->getOpNum() == Coordination::OpNum::Close)
+        {
             session_to_response_callback.erase(session_writer);
         }
+    }
+}
 
 bool KeeperStorageDispatcher::putRequest(const Coordination::ZooKeeperRequestPtr & request, int64_t session_id)
 {
@@ -119,8 +216,8 @@ bool KeeperStorageDispatcher::putRequest(const Coordination::ZooKeeperRequestPtr
     std::lock_guard lock(push_request_mutex);
     /// Put close requests without timeouts
     if (request->getOpNum() == Coordination::OpNum::Close)
-        requests_queue.push(std::move(request_info));
-    else if (!requests_queue.tryPush(std::move(request_info), coordination_settings->operation_timeout_ms.totalMilliseconds()))
+        requests_queue->push(std::move(request_info));
+    else if (!requests_queue->tryPush(std::move(request_info), coordination_settings->operation_timeout_ms.totalMilliseconds()))
         throw Exception("Cannot push request to queue within operation timeout", ErrorCodes::TIMEOUT_EXCEEDED);
     return true;
 }
@@ -131,6 +228,7 @@ void KeeperStorageDispatcher::initialize(const Poco::Util::AbstractConfiguration
     int myid = config.getInt("keeper_server.server_id");
 
     coordination_settings->loadFromConfig("keeper_server.coordination_settings", config);
+    requests_queue = std::make_unique<RequestsQueue>(coordination_settings->max_requests_batch_size);
 
     request_thread = ThreadFromGlobalPool([this] { requestThread(); });
     responses_thread = ThreadFromGlobalPool([this] { responseThread(); });
@@ -175,7 +273,7 @@ void KeeperStorageDispatcher::shutdown()
                 session_cleaner_thread.join();
 
             /// FIXME not the best way to notify
-            requests_queue.push({});
+            requests_queue->push({});
             if (request_thread.joinable())
                 request_thread.join();
 
@@ -192,7 +290,7 @@ void KeeperStorageDispatcher::shutdown()
             server->shutdown();
 
         KeeperStorage::RequestForSession request_for_session;
-        while (requests_queue.tryPop(request_for_session))
+        while (requests_queue->tryPop(request_for_session))
         {
             if (request_for_session.request)
             {
@@ -249,7 +347,7 @@ void KeeperStorageDispatcher::sessionCleanerTask()
                     request_info.session_id = dead_session;
                     {
                         std::lock_guard lock(push_request_mutex);
-                        requests_queue.push(std::move(request_info));
+                        requests_queue->push(std::move(request_info));
                     }
                     finishSession(dead_session);
                     LOG_INFO(log, "Dead session close request pushed");
@@ -273,4 +371,70 @@ void KeeperStorageDispatcher::finishSession(int64_t session_id)
         session_to_response_callback.erase(session_it);
 }
 
+void KeeperStorageDispatcher::addErrorResponses(KeeperStorage::RequestsForSessions && requests_for_sessions, Coordination::Error error)
+{
+    for (const auto & [session_id, request] : requests_for_sessions)
+    {
+        KeeperStorage::ResponsesForSessions responses;
+        auto response = request->makeResponse();
+        response->xid = request->xid;
+        response->zxid = 0;
+        response->error = error;
+        responses_queue.push(DB::KeeperStorage::ResponseForSession{session_id, response});
+    }
+    requests_for_sessions.clear();
+}
+
+void KeeperStorageDispatcher::forceWaitAndProcessResult(RaftAppendResult && result, KeeperStorage::RequestsForSessions && requests_for_sessions)
+{
+    if (!result->has_result())
+        result->get();
+
+    /// If we get some errors, than send them to clients
+    if (result->get_result_code() == nuraft::cmd_result_code::TIMEOUT)
+        addErrorResponses(std::move(requests_for_sessions), Coordination::Error::ZOPERATIONTIMEOUT);
+    else if (result->get_result_code() != nuraft::cmd_result_code::OK)
+        addErrorResponses(std::move(requests_for_sessions), Coordination::Error::ZRUNTIMEINCONSISTENCY);
+
+    result = nullptr;
+}
+
+int64_t KeeperStorageDispatcher::getSessionID(long session_timeout_ms)
+{
+    KeeperStorage::RequestForSession request_info;
+    std::shared_ptr<Coordination::ZooKeeperSessionIDRequest> request = std::make_shared<Coordination::ZooKeeperSessionIDRequest>();
+    request->internal_id = internal_session_id_counter.fetch_add(1);
+    request->session_timeout_ms = session_timeout_ms;
+    request_info.request = request;
+    request_info.session_id = -1;
+
+    auto promise = std::make_shared<std::promise<int64_t>>();
+    auto future = promise->get_future();
+    {
+        std::lock_guard lock(session_to_response_callback_mutex);
+        new_session_id_response_callback[request->internal_id] = [promise] (const Coordination::ZooKeeperResponsePtr & response)
+        {
+            if (response->getOpNum() != Coordination::OpNum::SessionID)
+                promise->set_exception(std::make_exception_ptr(Exception(ErrorCodes::LOGICAL_ERROR,
+                            "Incorrect response of type {} instead of SessionID response", Coordination::toString(response->getOpNum()))));
+
+            if (response->error != Coordination::Error::ZOK)
+                promise->set_exception(std::make_exception_ptr(zkutil::KeeperException("SessionID request failed with error", response->error)));
+
+            promise->set_value(dynamic_cast<const Coordination::ZooKeeperSessionIDResponse &>(*response).session_id);
+        };
+    }
+
+    {
+        std::lock_guard lock(push_request_mutex);
+        if (!requests_queue->tryPush(std::move(request_info), session_timeout_ms))
+            throw Exception("Cannot push session id request to queue within session timeout", ErrorCodes::TIMEOUT_EXCEEDED);
+    }
+
+    if (future.wait_for(std::chrono::milliseconds(session_timeout_ms)) != std::future_status::ready)
+        throw Exception("Cannot receive session id within session timeout", ErrorCodes::TIMEOUT_EXCEEDED);
+
+    return future.get();
+}
+
 }

src/Coordination/KeeperStorageDispatcher.h

@@ -32,24 +32,42 @@ private:
     using RequestsQueue = ConcurrentBoundedQueue<KeeperStorage::RequestForSession>;
     using SessionToResponseCallback = std::unordered_map<int64_t, ZooKeeperResponseCallback>;
 
-    RequestsQueue requests_queue{1};
+    /// Size depends on coordination settings
+    std::unique_ptr<RequestsQueue> requests_queue;
     ResponsesQueue responses_queue;
     SnapshotsQueue snapshots_queue{1};
 
     std::atomic<bool> shutdown_called{false};
 
     std::mutex session_to_response_callback_mutex;
+    /// These two maps looks similar, but serves different purposes.
+    /// The first map is subscription map for normal responses like
+    /// (get, set, list, etc.). Dispatcher determines callback for each response
+    /// using session id from this map.
     SessionToResponseCallback session_to_response_callback;
+    /// But when client connects to the server for the first time it doesn't
+    /// have session_id. It request it from server. We give temporary
+    /// internal id for such requests just to much client with its response.
+    SessionToResponseCallback new_session_id_response_callback;
 
+    /// Reading and batching new requests from client handlers
     ThreadFromGlobalPool request_thread;
+    /// Pushing responses to clients client handlers
+    /// using session_id.
     ThreadFromGlobalPool responses_thread;
+    /// Cleaning old dead sessions
     ThreadFromGlobalPool session_cleaner_thread;
+    /// Dumping new snapshots to disk
     ThreadFromGlobalPool snapshot_thread;
 
+    /// RAFT wrapper. Most important class.
     std::unique_ptr<KeeperServer> server;
 
     Poco::Logger * log;
 
+    /// Counter for new session_id requests.
+    std::atomic<int64_t> internal_session_id_counter{0};
+
 private:
     void requestThread();
     void responseThread();
@@ -57,6 +75,14 @@ private:
     void snapshotThread();
     void setResponse(int64_t session_id, const Coordination::ZooKeeperResponsePtr & response);
 
+    /// Add error responses for requests to responses queue.
+    /// Clears requests.
+    void addErrorResponses(KeeperStorage::RequestsForSessions && requests_for_sessions, Coordination::Error error);
+
+    /// Forcefully wait for result and sets errors if something when wrong.
+    /// Clears both arguments
+    void forceWaitAndProcessResult(RaftAppendResult && result, KeeperStorage::RequestsForSessions && requests_for_sessions);
+
 public:
     KeeperStorageDispatcher();
 
@@ -78,10 +104,7 @@ public:
         return server->isLeaderAlive();
     }
 
-    int64_t getSessionID(long session_timeout_ms)
-    {
-        return server->getSessionID(session_timeout_ms);
-    }
+    int64_t getSessionID(long session_timeout_ms);
 
     void registerSession(int64_t session_id, ZooKeeperResponseCallback callback);
     /// Call if we don't need any responses for this session no more (session was expired)