Merge pull request #38200 from ClickHouse/keeper-linearizable-reads

Keeper faster linearizable reads

contrib/NuRaft

@@ -1 +1 @@
-Subproject commit 1be805e7cb2494aa8170015493474379b0362dfc
+Subproject commit e15858f8ad0ce8aba85cf74e3763874c76bf927c

src/Coordination/CoordinationSettings.cpp

@@ -189,6 +189,9 @@ KeeperConfigurationAndSettings::loadFromConfig(const Poco::Util::AbstractConfigu
 
     ret->coordination_settings->loadFromConfig("keeper_server.coordination_settings", config);
 
+    if (ret->coordination_settings->quorum_reads)
+        LOG_WARNING(&Poco::Logger::get("KeeperConfigurationAndSettings"), "Setting 'quorum_reads' is deprecated. Please use 'read_mode'");
+
     return ret;
 }
 

src/Coordination/CoordinationSettings.h

@@ -26,6 +26,7 @@ struct Settings;
     M(Milliseconds, heart_beat_interval_ms, 500, "Heartbeat interval between quorum nodes", 0) \
     M(Milliseconds, election_timeout_lower_bound_ms, 1000, "Lower bound of election timer (avoid too often leader elections)", 0) \
     M(Milliseconds, election_timeout_upper_bound_ms, 2000, "Upper bound of election timer (avoid too often leader elections)", 0) \
+    M(Milliseconds, leadership_expiry, 0, "How often will leader node check if it still has majority. Set it lower or equal to election_timeout_lower_bound_ms to have linearizable reads.", 0) \
     M(UInt64, reserved_log_items, 100000, "How many log items to store (don't remove during compaction)", 0) \
     M(UInt64, snapshot_distance, 100000, "How many log items we have to collect to write new snapshot", 0) \
     M(Bool, auto_forwarding, true, "Allow to forward write requests from followers to leader", 0) \
@@ -38,11 +39,12 @@ struct Settings;
     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, quorum_reads, false, "Deprecated - use read_mode. 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) \
     M(Bool, compress_logs, true, "Write compressed coordination logs in ZSTD format", 0) \
     M(Bool, compress_snapshots_with_zstd_format, true, "Write compressed snapshots in ZSTD format (instead of custom LZ4)", 0) \
-    M(UInt64, configuration_change_tries_count, 20, "How many times we will try to apply configuration change (add/remove server) to the cluster", 0)
+    M(UInt64, configuration_change_tries_count, 20, "How many times we will try to apply configuration change (add/remove server) to the cluster", 0) \
+    M(String, read_mode, "nonlinear", "How should reads be processed. Valid values: 'nonlinear', 'fastlinear', 'quorum'. 'nonlinear' is the fastest option because there are no consistency requirements", 0)
 
 DECLARE_SETTINGS_TRAITS(CoordinationSettingsTraits, LIST_OF_COORDINATION_SETTINGS)
 

src/Coordination/KeeperDispatcher.cpp

@@ -1,4 +1,5 @@
 #include <Coordination/KeeperDispatcher.h>
+#include <libnuraft/async.hxx>
 #include <Common/setThreadName.h>
 #include <Common/ZooKeeper/KeeperException.h>
 #include <future>
@@ -6,6 +7,8 @@
 #include <Poco/Path.h>
 #include <Common/hex.h>
 #include <filesystem>
+#include <iterator>
+#include <limits>
 #include <Common/checkStackSize.h>
 #include <Common/CurrentMetrics.h>
 
@@ -30,22 +33,83 @@ namespace ErrorCodes
 
 KeeperDispatcher::KeeperDispatcher()
     : responses_queue(std::numeric_limits<size_t>::max())
+    , read_requests_queue(std::numeric_limits<size_t>::max())
+    , finalize_requests_queue(std::numeric_limits<size_t>::max())
     , configuration_and_settings(std::make_shared<KeeperConfigurationAndSettings>())
     , log(&Poco::Logger::get("KeeperDispatcher"))
 {
 }
 
 
+/// ZooKeepers has 2 requirements:
+/// - writes need to be linearizable
+/// - all requests from single session need to be processed in the order of their arrival
+///
+/// Because of that, we cannot process read and write requests from SAME session at the same time.
+/// To be able to process read and write requests in parallel we need to make sure that only 1 type
+/// of request is being processed from a single session.
+/// Multiple types from different sessions can be processed at the same time.
+///
+/// We do some in-session housekeeping to make sure that the multithreaded request processing is correct.
+/// When a request is received from a client, we check if there are requests being processed from that same
+/// session, and if yes, of what type. If the types are the same, and there are no requests of different
+/// type inbetetween, we can instanly add it to active request queue. Otherwise, we need to wait until
+/// all requests of the other type are processed.
+///
+/// There are multiple threads used for processing the request, each of them communicating with a queue.
+/// Assumption: only one type of request is being processed from a same session at any point in time (read or write).
+///
+/// requestThread -> requests currently being processed
+/// readRequestThread -> thread for processing read requests
+/// finalizeRequestThread -> thread for finalizing requests:
+///                          - in-session housekeeping, add requests to the active request queue if there are any
+///
+/// If reads are linearizable without quorum, a request can possibly wait for a certain log to be committed.
+/// In that case we add it to the waiting queue for that log.
+/// When that log is committed, the committing thread will send that read request to readRequestThread so it can be processed.
+///
 void KeeperDispatcher::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;
+    RaftResult prev_result = nullptr;
+    const auto previous_quorum_done = [&] { return !prev_result || prev_result->has_result() || prev_result->get_result_code() != nuraft::cmd_result_code::OK; };
 
+    const auto needs_quorum = [](const auto & coordination_settings, const auto & request)
+    {
+        return coordination_settings->quorum_reads || coordination_settings->read_mode.toString() == "quorum" || !request.request->isReadRequest();
+    };
+
+    KeeperStorage::RequestsForSessions quorum_requests;
+    KeeperStorage::RequestsForSessions read_requests;
+
+    auto process_quorum_requests = [&, this]() mutable
+    {
+        /// Forcefully process all previous pending requests
+        if (prev_result)
+            forceWaitAndProcessResult(prev_result);
+
+        prev_result = server->putRequestBatch(quorum_requests);
+
+        if (prev_result)
+        {
+            prev_result->when_ready([&, requests_for_sessions = std::move(quorum_requests)](nuraft::cmd_result<nuraft::ptr<nuraft::buffer>> & result, nuraft::ptr<std::exception> &) mutable
+            {
+                if (!result.get_accepted() || result.get_result_code() == nuraft::cmd_result_code::TIMEOUT)
+                    addErrorResponses(requests_for_sessions, Coordination::Error::ZOPERATIONTIMEOUT);
+                else if (result.get_result_code() != nuraft::cmd_result_code::OK)
+                    addErrorResponses(requests_for_sessions, Coordination::Error::ZCONNECTIONLOSS);
+            });
+        }
+
+        quorum_requests.clear();
+    };
+
+    /// ZooKeeper requires that the requests inside a single session are processed in a strict order
+    /// (we cannot process later requests before all the previous once are processed)
+    /// By making sure that at this point we can either have just read requests or just write requests
+    /// from a single session, we can process them independently
     while (!shutdown_called)
     {
         KeeperStorage::RequestForSession request;
@@ -54,94 +118,67 @@ void KeeperDispatcher::requestThread()
         uint64_t max_wait = coordination_settings->operation_timeout_ms.totalMilliseconds();
         uint64_t max_batch_size = coordination_settings->max_requests_batch_size;
 
-        /// The code below 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 (active_requests_queue->tryPop(request, max_wait))
             {
                 CurrentMetrics::sub(CurrentMetrics::KeeperOutstandingRequets);
                 if (shutdown_called)
                     break;
 
-                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())
-                {
-                    current_batch.emplace_back(request);
+                if (needs_quorum(coordination_settings, request))
+                    quorum_requests.emplace_back(request);
+                else
+                    read_requests.emplace_back(request);
 
                 /// Waiting until previous append will be successful, or batch is big enough
                 /// has_result == false && get_result_code == OK means that our request still not processed.
                 /// Sometimes NuRaft set errorcode without setting result, so we check both here.
-                    while (prev_result && (!prev_result->has_result() && prev_result->get_result_code() == nuraft::cmd_result_code::OK) && current_batch.size() <= max_batch_size)
+                while (true)
                 {
+                    if (quorum_requests.size() > max_batch_size)
+                        break;
+
+                    if (read_requests.size() > max_batch_size)
+                    {
+                        processReadRequests(coordination_settings, read_requests);
+
+                        if (previous_quorum_done())
+                            break;
+                    }
+
                     /// Trying to get batch requests as fast as possible
-                        if (requests_queue->tryPop(request, 1))
+                    if (active_requests_queue->tryPop(request, 1))
                     {
                         CurrentMetrics::sub(CurrentMetrics::KeeperOutstandingRequets);
-                            /// Don't append read request into batch, we have to process them separately
-                            if (!coordination_settings->quorum_reads && request.request->isReadRequest())
-                            {
-                                has_read_request = true;
-                                break;
+                        if (needs_quorum(coordination_settings, request))
+                            quorum_requests.emplace_back(request);
+                        else
+                            read_requests.emplace_back(request);
                     }
                     else
                     {
+                        /// batch of read requests can send at most one request
+                        /// so we don't care if the previous batch hasn't received response
+                        if (!read_requests.empty())
+                            processReadRequests(coordination_settings, read_requests);
 
-                                current_batch.emplace_back(request);
-                            }
+                        /// if we still didn't process previous batch we can
+                        /// increase are current batch even more
+                        if (previous_quorum_done())
+                            break;
                     }
 
                     if (shutdown_called)
                         break;
                 }
-                }
-                else
-                    has_read_request = true;
 
                 if (shutdown_called)
                     break;
 
-                /// Forcefully process all previous pending requests
-                if (prev_result)
-                    forceWaitAndProcessResult(prev_result, prev_batch);
+                if (!quorum_requests.empty())
+                    process_quorum_requests();
 
-                /// Process collected write requests batch
-                if (!current_batch.empty())
-                {
-                    auto result = server->putRequestBatch(current_batch);
-
-                    if (result)
-                    {
-                        if (has_read_request) /// If we will execute read request next, than we have to process result now
-                            forceWaitAndProcessResult(result, current_batch);
-                    }
-                    else
-                    {
-                        addErrorResponses(current_batch, Coordination::Error::ZCONNECTIONLOSS);
-                        current_batch.clear();
-                    }
-
-                    prev_batch = std::move(current_batch);
-                    prev_result = result;
-                }
-
-                /// Read request always goes after write batch (last request)
-                if (has_read_request)
-                {
-                    if (server->isLeaderAlive())
-                        server->putLocalReadRequest(request);
-                    else
-                        addErrorResponses({request}, Coordination::Error::ZCONNECTIONLOSS);
-                }
             }
         }
         catch (...)
@@ -151,6 +188,72 @@ void KeeperDispatcher::requestThread()
     }
 }
 
+void KeeperDispatcher::processReadRequests(const CoordinationSettingsPtr & coordination_settings, KeeperStorage::RequestsForSessions & read_requests)
+{
+    if (coordination_settings->read_mode.toString() == "fastlinear")
+    {
+        // we just want to know what's the current latest committed log on Leader node
+        auto leader_info_result = server->getLeaderInfo();
+        if (leader_info_result)
+        {
+            leader_info_result->when_ready([&, requests_for_sessions = std::move(read_requests)](nuraft::cmd_result<nuraft::ptr<nuraft::buffer>> & result, nuraft::ptr<std::exception> & exception) mutable
+            {
+                if (!result.get_accepted() || result.get_result_code() == nuraft::cmd_result_code::TIMEOUT)
+                {
+                    addErrorResponses(requests_for_sessions, Coordination::Error::ZOPERATIONTIMEOUT);
+                    return;
+                }
+
+                if (result.get_result_code() != nuraft::cmd_result_code::OK)
+                {
+                    addErrorResponses(requests_for_sessions, Coordination::Error::ZCONNECTIONLOSS);
+                    return;
+                }
+
+                if (exception)
+                {
+                    LOG_INFO(log, "Got exception while waiting for read results {}", exception->what());
+                    addErrorResponses(requests_for_sessions, Coordination::Error::ZCONNECTIONLOSS);
+                    return;
+                }
+
+                auto & leader_info_ctx = result.get();
+
+                if (!leader_info_ctx)
+                {
+                    addErrorResponses(requests_for_sessions, Coordination::Error::ZCONNECTIONLOSS);
+                    return;
+                }
+
+                KeeperServer::NodeInfo leader_info;
+                leader_info.term = leader_info_ctx->get_ulong();
+                leader_info.last_committed_index = leader_info_ctx->get_ulong();
+                std::lock_guard lock(leader_waiter_mutex);
+                auto node_info = server->getNodeInfo();
+
+                /// we're behind, we need to wait
+                if (node_info.term < leader_info.term || node_info.last_committed_index < leader_info.last_committed_index)
+                {
+                    auto & leader_waiter = leader_waiters[leader_info];
+                    leader_waiter.insert(leader_waiter.end(), requests_for_sessions.begin(), requests_for_sessions.end());
+                    LOG_TRACE(log, "waiting for term {}, idx {}", leader_info.term, leader_info.last_committed_index);
+                }
+                /// process it in background thread
+                else if (!read_requests_queue.push(std::move(requests_for_sessions)))
+                    throw Exception(ErrorCodes::SYSTEM_ERROR, "Cannot push read requests to queue");
+            });
+        }
+    }
+    else
+    {
+        assert(coordination_settings->read_mode.toString() == "nonlinear");
+        if (!read_requests_queue.push(std::move(read_requests)))
+            throw Exception(ErrorCodes::SYSTEM_ERROR, "Cannot push read requests to queue");
+    }
+
+    read_requests.clear();
+}
+
 void KeeperDispatcher::responseThread()
 {
     setThreadName("KeeperRspT");
@@ -200,6 +303,65 @@ void KeeperDispatcher::snapshotThread()
     }
 }
 
+/// Background thread for processing read requests
+void KeeperDispatcher::readRequestThread()
+{
+    setThreadName("KeeperReadT");
+    while (!shutdown_called)
+    {
+        KeeperStorage::RequestsForSessions requests;
+        if (!read_requests_queue.pop(requests))
+            break;
+
+        if (shutdown_called)
+            break;
+
+        try
+        {
+            for (const auto & request_info : requests)
+            {
+                if (server->isLeaderAlive())
+                    server->putLocalReadRequest(request_info);
+                else
+                    addErrorResponses({request_info}, Coordination::Error::ZCONNECTIONLOSS);
+            }
+
+            if (!finalize_requests_queue.push(std::move(requests)))
+                throw Exception(ErrorCodes::SYSTEM_ERROR, "Cannot push read requests to queue");
+        }
+        catch (...)
+        {
+            tryLogCurrentException(__PRETTY_FUNCTION__);
+        }
+    }
+}
+
+/// We finalize requests every time we commit a single log with request
+/// or process a batch of read requests.
+/// Because it can get heavy, we do it in background thread.
+void KeeperDispatcher::finalizeRequestsThread()
+{
+    setThreadName("KeeperFinalT");
+    while (!shutdown_called)
+    {
+        KeeperStorage::RequestsForSessions requests;
+        if (!finalize_requests_queue.pop(requests))
+            break;
+
+        if (shutdown_called)
+            break;
+
+        try
+        {
+            finalizeRequests(requests);
+        }
+        catch (...)
+        {
+            tryLogCurrentException(__PRETTY_FUNCTION__);
+        }
+    }
+}
+
 void KeeperDispatcher::setResponse(int64_t session_id, const Coordination::ZooKeeperResponsePtr & response)
 {
     std::lock_guard lock(session_to_response_callback_mutex);
@@ -255,6 +417,30 @@ bool KeeperDispatcher::putRequest(const Coordination::ZooKeeperRequestPtr & requ
     request_info.time = duration_cast<milliseconds>(system_clock::now().time_since_epoch()).count();
     request_info.session_id = session_id;
 
+    {
+        std::lock_guard lock{unprocessed_request_mutex};
+        auto unprocessed_requests_it = unprocessed_requests_for_session.find(session_id);
+        if (unprocessed_requests_it == unprocessed_requests_for_session.end())
+        {
+            auto & unprocessed_requests = unprocessed_requests_for_session[session_id];
+            unprocessed_requests.unprocessed_num = 1;
+            unprocessed_requests.is_read = request->isReadRequest();
+        }
+        else
+        {
+            auto & unprocessed_requests = unprocessed_requests_it->second;
+
+            /// queue is not empty, or the request types don't match, put it in the waiting queue
+            if (!unprocessed_requests.request_queue.empty() || unprocessed_requests.is_read != request->isReadRequest())
+            {
+                unprocessed_requests.request_queue.push_back(std::move(request_info));
+                return true;
+            }
+
+            ++unprocessed_requests.unprocessed_num;
+        }
+    }
+
     std::lock_guard lock(push_request_mutex);
 
     if (shutdown_called)
@@ -263,10 +449,10 @@ bool KeeperDispatcher::putRequest(const Coordination::ZooKeeperRequestPtr & requ
     /// Put close requests without timeouts
     if (request->getOpNum() == Coordination::OpNum::Close)
     {
-        if (!requests_queue->push(std::move(request_info)))
+        if (!active_requests_queue->push(std::move(request_info)))
             throw Exception("Cannot push request to queue", ErrorCodes::SYSTEM_ERROR);
     }
-    else if (!requests_queue->tryPush(std::move(request_info), configuration_and_settings->coordination_settings->operation_timeout_ms.totalMilliseconds()))
+    else if (!active_requests_queue->tryPush(std::move(request_info), configuration_and_settings->coordination_settings->operation_timeout_ms.totalMilliseconds()))
     {
         throw Exception("Cannot push request to queue within operation timeout", ErrorCodes::TIMEOUT_EXCEEDED);
     }
@@ -279,13 +465,23 @@ void KeeperDispatcher::initialize(const Poco::Util::AbstractConfiguration & conf
     LOG_DEBUG(log, "Initializing storage dispatcher");
 
     configuration_and_settings = KeeperConfigurationAndSettings::loadFromConfig(config, standalone_keeper);
-    requests_queue = std::make_unique<RequestsQueue>(configuration_and_settings->coordination_settings->max_requests_batch_size);
+    active_requests_queue = std::make_unique<RequestsQueue>(configuration_and_settings->coordination_settings->max_requests_batch_size);
 
     request_thread = ThreadFromGlobalPool([this] { requestThread(); });
     responses_thread = ThreadFromGlobalPool([this] { responseThread(); });
     snapshot_thread = ThreadFromGlobalPool([this] { snapshotThread(); });
+    read_request_thread = ThreadFromGlobalPool([this] { readRequestThread(); });
+    finalize_requests_thread = ThreadFromGlobalPool([this] { finalizeRequestsThread(); });
 
-    server = std::make_unique<KeeperServer>(configuration_and_settings, config, responses_queue, snapshots_queue);
+    server = std::make_unique<KeeperServer>(
+        configuration_and_settings,
+        config,
+        responses_queue,
+        snapshots_queue,
+        [this](const KeeperStorage::RequestForSession & request_for_session, uint64_t log_term, uint64_t log_idx)
+        { onRequestCommit(request_for_session, log_term, log_idx); },
+        [this](uint64_t term, uint64_t last_idx)
+        { onApplySnapshot(term, last_idx); });
 
     try
     {
@@ -333,9 +529,9 @@ void KeeperDispatcher::shutdown()
             if (session_cleaner_thread.joinable())
                 session_cleaner_thread.join();
 
-            if (requests_queue)
+            if (active_requests_queue)
             {
-                requests_queue->finish();
+                active_requests_queue->finish();
 
                 if (request_thread.joinable())
                     request_thread.join();
@@ -349,6 +545,14 @@ void KeeperDispatcher::shutdown()
             if (snapshot_thread.joinable())
                 snapshot_thread.join();
 
+            read_requests_queue.finish();
+            if (read_request_thread.joinable())
+                read_request_thread.join();
+
+            finalize_requests_queue.finish();
+            if (finalize_requests_thread.joinable())
+                finalize_requests_thread.join();
+
             update_configuration_queue.finish();
             if (update_configuration_thread.joinable())
                 update_configuration_thread.join();
@@ -357,7 +561,7 @@ void KeeperDispatcher::shutdown()
         KeeperStorage::RequestForSession request_for_session;
 
         /// Set session expired for all pending requests
-        while (requests_queue && requests_queue->tryPop(request_for_session))
+        while (active_requests_queue && active_requests_queue->tryPop(request_for_session))
         {
             CurrentMetrics::sub(CurrentMetrics::KeeperOutstandingRequets);
             auto response = request_for_session.request->makeResponse();
@@ -474,7 +678,7 @@ void KeeperDispatcher::sessionCleanerTask()
                     };
                     {
                         std::lock_guard lock(push_request_mutex);
-                        if (!requests_queue->push(std::move(request_info)))
+                        if (!active_requests_queue->push(std::move(request_info)))
                             LOG_INFO(log, "Cannot push close request to queue while cleaning outdated sessions");
                         CurrentMetrics::add(CurrentMetrics::KeeperOutstandingRequets);
                     }
@@ -524,19 +728,12 @@ void KeeperDispatcher::addErrorResponses(const KeeperStorage::RequestsForSession
     }
 }
 
-void KeeperDispatcher::forceWaitAndProcessResult(RaftAppendResult & result, KeeperStorage::RequestsForSessions & requests_for_sessions)
+void KeeperDispatcher::forceWaitAndProcessResult(RaftResult & result)
 {
     if (!result->has_result())
         result->get();
 
-    /// If we get some errors, than send them to clients
-    if (!result->get_accepted() || result->get_result_code() == nuraft::cmd_result_code::TIMEOUT)
-        addErrorResponses(requests_for_sessions, Coordination::Error::ZOPERATIONTIMEOUT);
-    else if (result->get_result_code() != nuraft::cmd_result_code::OK)
-        addErrorResponses(requests_for_sessions, Coordination::Error::ZCONNECTIONLOSS);
-
     result = nullptr;
-    requests_for_sessions.clear();
 }
 
 int64_t KeeperDispatcher::getSessionID(int64_t session_timeout_ms)
@@ -584,7 +781,7 @@ int64_t KeeperDispatcher::getSessionID(int64_t session_timeout_ms)
     /// Push new session request to queue
     {
         std::lock_guard lock(push_request_mutex);
-        if (!requests_queue->tryPush(std::move(request_info), session_timeout_ms))
+        if (!active_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);
         CurrentMetrics::add(CurrentMetrics::KeeperOutstandingRequets);
     }
@@ -657,6 +854,122 @@ void KeeperDispatcher::updateConfigurationThread()
     }
 }
 
+// Used to update the state for a session based on the requests
+// - update the number of current unprocessed requests for the session
+// - if the number of unprocessed requests is 0, we can start adding next type of requests
+//   from unprocessed requests queue to the active queue
+void KeeperDispatcher::finalizeRequests(const KeeperStorage::RequestsForSessions & requests_for_sessions)
+{
+    std::unordered_map<int64_t, size_t> counts_for_session;
+
+    for (const auto & request_for_session : requests_for_sessions)
+    {
+        ++counts_for_session[request_for_session.session_id];
+    }
+
+    std::lock_guard lock{unprocessed_request_mutex};
+    for (const auto [session_id, count] : counts_for_session)
+    {
+        auto unprocessed_requests_it = unprocessed_requests_for_session.find(session_id);
+        if (unprocessed_requests_it == unprocessed_requests_for_session.end())
+            continue;
+
+        auto & unprocessed_requests = unprocessed_requests_it->second;
+        unprocessed_requests.unprocessed_num -= count;
+
+        if (unprocessed_requests.unprocessed_num == 0)
+        {
+            if (!unprocessed_requests.request_queue.empty())
+            {
+                auto & unprocessed_requests_queue = unprocessed_requests.request_queue;
+                unprocessed_requests.is_read = !unprocessed_requests.is_read;
+                // start adding next type of requests
+                while (!unprocessed_requests_queue.empty() && unprocessed_requests_queue.front().request->isReadRequest() == unprocessed_requests.is_read)
+                {
+                    auto & front_request = unprocessed_requests_queue.front();
+
+                    /// Put close requests without timeouts
+                    if (front_request.request->getOpNum() == Coordination::OpNum::Close)
+                    {
+                        if (!active_requests_queue->push(std::move(front_request)))
+                            throw Exception("Cannot push request to queue", ErrorCodes::SYSTEM_ERROR);
+                    }
+                    else if (!active_requests_queue->tryPush(std::move(front_request), configuration_and_settings->coordination_settings->operation_timeout_ms.totalMilliseconds()))
+                    {
+                        throw Exception("Cannot push request to queue within operation timeout", ErrorCodes::TIMEOUT_EXCEEDED);
+                    }
+
+                    ++unprocessed_requests.unprocessed_num;
+                    unprocessed_requests_queue.pop_front();
+                }
+            }
+            else
+            {
+                unprocessed_requests_for_session.erase(unprocessed_requests_it);
+            }
+        }
+    }
+}
+
+// Finalize request
+// Process read requests that were waiting for this commit
+void KeeperDispatcher::onRequestCommit(const KeeperStorage::RequestForSession & request_for_session, uint64_t log_term, uint64_t log_idx)
+{
+    if (!finalize_requests_queue.push({request_for_session}))
+        throw Exception(ErrorCodes::SYSTEM_ERROR, "Cannot push read requests to queue");
+
+    KeeperStorage::RequestsForSessions requests;
+    {
+        std::lock_guard lock(leader_waiter_mutex);
+        auto request_queue_it = leader_waiters.find(KeeperServer::NodeInfo{.term = log_term, .last_committed_index = log_idx});
+        if (request_queue_it != leader_waiters.end())
+        {
+            requests = std::move(request_queue_it->second);
+            leader_waiters.erase(request_queue_it);
+        }
+    }
+
+    if (requests.empty())
+        return;
+
+    if (!read_requests_queue.push(std::move(requests)))
+        throw Exception(ErrorCodes::SYSTEM_ERROR, "Cannot push read requests to queue");
+}
+
+/// Process all read request that are waiting for lower or currently last processed log index
+void KeeperDispatcher::onApplySnapshot(uint64_t term, uint64_t last_idx)
+{
+    KeeperServer::NodeInfo current_node_info{term, last_idx};
+    KeeperStorage::RequestsForSessions requests;
+    {
+        std::lock_guard lock(leader_waiter_mutex);
+        for (auto leader_waiter_it = leader_waiters.begin(); leader_waiter_it != leader_waiters.end();)
+        {
+            auto waiting_node_info = leader_waiter_it->first;
+            if (waiting_node_info.term <= current_node_info.term
+                && waiting_node_info.last_committed_index <= current_node_info.last_committed_index)
+            {
+                for (auto & request : leader_waiter_it->second)
+                {
+                    requests.push_back(std::move(request));
+                }
+
+                leader_waiter_it = leader_waiters.erase(leader_waiter_it);
+            }
+            else
+            {
+                ++leader_waiter_it;
+            }
+        }
+    }
+
+    if (requests.empty())
+        return;
+
+    if (!read_requests_queue.push(std::move(requests)))
+        throw Exception(ErrorCodes::SYSTEM_ERROR, "Cannot push read requests to queue");
+}
+
 bool KeeperDispatcher::isServerActive() const
 {
     return checkInit() && hasLeader() && !server->isRecovering();
@@ -721,7 +1034,7 @@ Keeper4LWInfo KeeperDispatcher::getKeeper4LWInfo() const
     Keeper4LWInfo result = server->getPartiallyFilled4LWInfo();
     {
         std::lock_guard lock(push_request_mutex);
-        result.outstanding_requests_count = requests_queue->size();
+        result.outstanding_requests_count = active_requests_queue->size();
     }
     {
         std::lock_guard lock(session_to_response_callback_mutex);

src/Coordination/KeeperDispatcher.h

@@ -32,9 +32,12 @@ private:
     using UpdateConfigurationQueue = ConcurrentBoundedQueue<ConfigUpdateAction>;
 
     /// Size depends on coordination settings
-    std::unique_ptr<RequestsQueue> requests_queue;
+    /// Request currently being processed
+    std::unique_ptr<RequestsQueue> active_requests_queue;
     ResponsesQueue responses_queue;
     SnapshotsQueue snapshots_queue{1};
+    ConcurrentBoundedQueue<KeeperStorage::RequestsForSessions> read_requests_queue;
+    ConcurrentBoundedQueue<KeeperStorage::RequestsForSessions> finalize_requests_queue;
 
     /// More than 1k updates is definitely misconfiguration.
     UpdateConfigurationQueue update_configuration_queue{1000};
@@ -64,6 +67,8 @@ private:
     ThreadFromGlobalPool snapshot_thread;
     /// Apply or wait for configuration changes
     ThreadFromGlobalPool update_configuration_thread;
+    ThreadFromGlobalPool read_request_thread;
+    ThreadFromGlobalPool finalize_requests_thread;
 
     /// RAFT wrapper.
     std::unique_ptr<KeeperServer> server;
@@ -77,6 +82,34 @@ private:
     /// Counter for new session_id requests.
     std::atomic<int64_t> internal_session_id_counter{0};
 
+    /// A read request needs to have at least the log it was the last committed log on the leader
+    /// at the time the request was being made.
+    /// If the node is stale, we need to wait to commit that log before doing local read requests to achieve
+    /// linearizability.
+    std::unordered_map<KeeperServer::NodeInfo, KeeperStorage::RequestsForSessions> leader_waiters;
+    std::mutex leader_waiter_mutex;
+
+    /// We can be actively processing one type of requests (either read or write) from a single session.
+    /// If we receive a request of a type that is not currently being processed, we put it in the waiting queue.
+    /// Also, we want to process them in ariving order, so if we have a different type in the queue, we cannot process that request
+    /// but wait for all the previous requests to finish.
+    /// E.g. READ -> WRITE -> READ, the last READ will go to the waiting queue even though we are currently processing the first READ
+    /// because we have WRITE request before it that needs to be processed.
+    struct UnprocessedRequests
+    {
+        /// how many requests are currently in the active request queue
+        size_t unprocessed_num{0};
+        /// is_read currently being processed
+        bool is_read{false};
+        std::list<KeeperStorage::RequestForSession> request_queue;
+    };
+
+    // Called every time a batch of requests are processed.
+    void finalizeRequests(const KeeperStorage::RequestsForSessions & requests_for_sessions);
+
+    std::unordered_map<int64_t, UnprocessedRequests> unprocessed_requests_for_session;
+    std::mutex unprocessed_request_mutex;
+
     /// Thread put requests to raft
     void requestThread();
     /// Thread put responses for subscribed sessions
@@ -88,6 +121,12 @@ private:
     /// Thread apply or wait configuration changes from leader
     void updateConfigurationThread();
 
+    void readRequestThread();
+
+    void finalizeRequestsThread();
+
+    void processReadRequests(const CoordinationSettingsPtr & coordination_settings, KeeperStorage::RequestsForSessions & read_requests);
+
     void setResponse(int64_t session_id, const Coordination::ZooKeeperResponsePtr & response);
 
     /// Add error responses for requests to responses queue.
@@ -96,7 +135,7 @@ private:
 
     /// Forcefully wait for result and sets errors if something when wrong.
     /// Clears both arguments
-    void forceWaitAndProcessResult(RaftAppendResult & result, KeeperStorage::RequestsForSessions & requests_for_sessions);
+    static void forceWaitAndProcessResult(RaftResult & result);
 
 public:
     /// Just allocate some objects, real initialization is done by `intialize method`
@@ -116,6 +155,12 @@ public:
         return server && server->checkInit();
     }
 
+    /// Called when a single log with request is committed.
+    void onRequestCommit(const KeeperStorage::RequestForSession & request_for_session, uint64_t log_term, uint64_t log_idx);
+
+    /// Called when a snapshot is applied
+    void onApplySnapshot(uint64_t term, uint64_t last_idx);
+
     /// Is server accepting requests, i.e. connected to the cluster
     /// and achieved quorum
     bool isServerActive() const;

src/Coordination/KeeperServer.cpp

@@ -105,7 +105,9 @@ KeeperServer::KeeperServer(
     const KeeperConfigurationAndSettingsPtr & configuration_and_settings_,
     const Poco::Util::AbstractConfiguration & config,
     ResponsesQueue & responses_queue_,
-    SnapshotsQueue & snapshots_queue_)
+    SnapshotsQueue & snapshots_queue_,
+    KeeperStateMachine::CommitCallback commit_callback,
+    KeeperStateMachine::ApplySnapshotCallback apply_snapshot_callback)
     : server_id(configuration_and_settings_->server_id)
     , coordination_settings(configuration_and_settings_->coordination_settings)
     , log(&Poco::Logger::get("KeeperServer"))
@@ -113,7 +115,7 @@ KeeperServer::KeeperServer(
     , keeper_context{std::make_shared<KeeperContext>()}
     , create_snapshot_on_exit(config.getBool("keeper_server.create_snapshot_on_exit", true))
 {
-    if (coordination_settings->quorum_reads)
+    if (coordination_settings->quorum_reads || coordination_settings->read_mode.toString() == "quorum")
         LOG_WARNING(log, "Quorum reads enabled, Keeper will work slower.");
 
     keeper_context->digest_enabled = config.getBool("keeper_server.digest_enabled", false);
@@ -125,7 +127,9 @@ KeeperServer::KeeperServer(
         configuration_and_settings_->snapshot_storage_path,
         coordination_settings,
         keeper_context,
-        checkAndGetSuperdigest(configuration_and_settings_->super_digest));
+        checkAndGetSuperdigest(configuration_and_settings_->super_digest),
+        std::move(commit_callback),
+        std::move(apply_snapshot_callback));
 
     state_manager = nuraft::cs_new<KeeperStateManager>(
         server_id,
@@ -176,6 +180,13 @@ struct KeeperServer::KeeperRaftServer : public nuraft::raft_server
         reconfigure(new_config);
     }
 
+    RaftResult getLeaderInfo()
+    {
+        nuraft::ptr<nuraft::req_msg> req
+            = nuraft::cs_new<nuraft::req_msg>(0ull, nuraft::msg_type::leader_status_request, 0, 0, 0ull, 0ull, 0ull);
+        return send_msg_to_leader(req);
+    }
+
     void commit_in_bg() override
     {
         // For NuRaft, if any commit fails (uncaught exception) the whole server aborts as a safety
@@ -269,6 +280,20 @@ void KeeperServer::launchRaftServer(const Poco::Util::AbstractConfiguration & co
         coordination_settings->election_timeout_lower_bound_ms.totalMilliseconds(), "election_timeout_lower_bound_ms", log);
     params.election_timeout_upper_bound_ = getValueOrMaxInt32AndLogWarning(
         coordination_settings->election_timeout_upper_bound_ms.totalMilliseconds(), "election_timeout_upper_bound_ms", log);
+
+    params.leadership_expiry_ = getValueOrMaxInt32AndLogWarning(coordination_settings->leadership_expiry.totalMilliseconds(), "leadership_expiry", log);
+
+    if (coordination_settings->read_mode.toString() == "fastlinear")
+    {
+        if (params.leadership_expiry_ == 0)
+            params.leadership_expiry_ = params.election_timeout_lower_bound_;
+        else if (params.leadership_expiry_ > params.election_timeout_lower_bound_)
+        {
+            LOG_WARNING(log, "To use fast linearizable reads, leadership_expiry should be set to a value that is less or equal to the election_timeout_upper_bound_ms. "
+                    "Based on current settings, there are no guarantees for linearizability of reads.");
+        }
+    }
+
     params.reserved_log_items_ = getValueOrMaxInt32AndLogWarning(coordination_settings->reserved_log_items, "reserved_log_items", log);
     params.snapshot_distance_ = getValueOrMaxInt32AndLogWarning(coordination_settings->snapshot_distance, "snapshot_distance", log);
 
@@ -487,7 +512,7 @@ void KeeperServer::putLocalReadRequest(const KeeperStorage::RequestForSession &
     state_machine->processReadRequest(request_for_session);
 }
 
-RaftAppendResult KeeperServer::putRequestBatch(const KeeperStorage::RequestsForSessions & requests_for_sessions)
+RaftResult KeeperServer::putRequestBatch(const KeeperStorage::RequestsForSessions & requests_for_sessions)
 {
     std::vector<nuraft::ptr<nuraft::buffer>> entries;
     for (const auto & request_for_session : requests_for_sessions)
@@ -713,6 +738,20 @@ std::vector<int64_t> KeeperServer::getDeadSessions()
     return state_machine->getDeadSessions();
 }
 
+RaftResult KeeperServer::getLeaderInfo()
+{
+    std::lock_guard lock{server_write_mutex};
+    if (is_recovering)
+        return nullptr;
+
+    return raft_instance->getLeaderInfo();
+}
+
+KeeperServer::NodeInfo KeeperServer::getNodeInfo()
+{
+    return { .term = raft_instance->get_term(), .last_committed_index = state_machine->last_commit_index() };
+}
+
 ConfigUpdateActions KeeperServer::getConfigurationDiff(const Poco::Util::AbstractConfiguration & config)
 {
     auto diff = state_manager->getConfigurationDiff(config);

src/Coordination/KeeperServer.h

@@ -14,7 +14,7 @@
 namespace DB
 {
 
-using RaftAppendResult = nuraft::ptr<nuraft::cmd_result<nuraft::ptr<nuraft::buffer>>>;
+using RaftResult = nuraft::ptr<nuraft::cmd_result<nuraft::ptr<nuraft::buffer>>>;
 
 class KeeperServer
 {
@@ -71,7 +71,9 @@ public:
         const KeeperConfigurationAndSettingsPtr & settings_,
         const Poco::Util::AbstractConfiguration & config_,
         ResponsesQueue & responses_queue_,
-        SnapshotsQueue & snapshots_queue_);
+        SnapshotsQueue & snapshots_queue_,
+        KeeperStateMachine::CommitCallback commit_callback,
+        KeeperStateMachine::ApplySnapshotCallback apply_snapshot_callback);
 
     /// Load state machine from the latest snapshot and load log storage. Start NuRaft with required settings.
     void startup(const Poco::Util::AbstractConfiguration & config, bool enable_ipv6 = true);
@@ -84,7 +86,7 @@ public:
 
     /// Put batch of requests into Raft and get result of put. Responses will be set separately into
     /// responses_queue.
-    RaftAppendResult putRequestBatch(const KeeperStorage::RequestsForSessions & requests);
+    RaftResult putRequestBatch(const KeeperStorage::RequestsForSessions & requests);
 
     /// Return set of the non-active sessions
     std::vector<int64_t> getDeadSessions();
@@ -119,6 +121,17 @@ public:
 
     int getServerID() const { return server_id; }
 
+    struct NodeInfo
+    {
+        uint64_t term;
+        uint64_t last_committed_index;
+
+        bool operator==(const NodeInfo &) const = default;
+    };
+
+    RaftResult getLeaderInfo();
+    NodeInfo getNodeInfo();
+
     /// Get configuration diff between current configuration in RAFT and in XML file
     ConfigUpdateActions getConfigurationDiff(const Poco::Util::AbstractConfiguration & config);
 
@@ -126,10 +139,23 @@ public:
     /// Synchronously check for update results with retries.
     void applyConfigurationUpdate(const ConfigUpdateAction & task);
 
-
     /// Wait configuration update for action. Used by followers.
     /// Return true if update was successfully received.
     bool waitConfigurationUpdate(const ConfigUpdateAction & task);
 };
 
 }
+namespace std
+{
+    template <>
+    struct hash<DB::KeeperServer::NodeInfo>
+    {
+        size_t operator()(const DB::KeeperServer::NodeInfo & info) const
+        {
+            SipHash hash_state;
+            hash_state.update(info.term);
+            hash_state.update(info.last_committed_index);
+            return hash_state.get64();
+        }
+    };
+}

src/Coordination/KeeperStateMachine.cpp

@@ -44,7 +44,9 @@ KeeperStateMachine::KeeperStateMachine(
     const std::string & snapshots_path_,
     const CoordinationSettingsPtr & coordination_settings_,
     const KeeperContextPtr & keeper_context_,
-    const std::string & superdigest_)
+    const std::string & superdigest_,
+    CommitCallback commit_callback_,
+    ApplySnapshotCallback apply_snapshot_callback_)
     : coordination_settings(coordination_settings_)
     , snapshot_manager(
           snapshots_path_,
@@ -58,6 +60,8 @@ KeeperStateMachine::KeeperStateMachine(
     , last_committed_idx(0)
     , log(&Poco::Logger::get("KeeperStateMachine"))
     , superdigest(superdigest_)
+    , commit_callback(std::move(commit_callback_))
+    , apply_snapshot_callback(std::move(apply_snapshot_callback_))
     , keeper_context(keeper_context_)
 {
 }
@@ -223,11 +227,11 @@ bool KeeperStateMachine::preprocess(const KeeperStorage::RequestForSession & req
     return true;
 }
 
-nuraft::ptr<nuraft::buffer> KeeperStateMachine::commit(const uint64_t log_idx, nuraft::buffer & data)
+nuraft::ptr<nuraft::buffer> KeeperStateMachine::commit_ext(const ext_op_params & params)
 {
-    auto request_for_session = parseRequest(data);
+    auto request_for_session = parseRequest(*params.data);
     if (!request_for_session.zxid)
-        request_for_session.zxid = log_idx;
+        request_for_session.zxid = params.log_idx;
 
     /// Special processing of session_id request
     if (request_for_session.request->getOpNum() == Coordination::OpNum::SessionID)
@@ -272,8 +276,9 @@ nuraft::ptr<nuraft::buffer> KeeperStateMachine::commit(const uint64_t log_idx, n
             assertDigest(*request_for_session.digest, storage->getNodesDigest(true), *request_for_session.request, true);
     }
 
+    last_committed_idx = params.log_idx;
+    commit_callback(request_for_session, params.log_term, params.log_idx);
     ProfileEvents::increment(ProfileEvents::KeeperCommits);
-    last_committed_idx = log_idx;
     return nullptr;
 }
 
@@ -306,6 +311,7 @@ bool KeeperStateMachine::apply_snapshot(nuraft::snapshot & s)
 
     ProfileEvents::increment(ProfileEvents::KeeperSnapshotApplys);
     last_committed_idx = s.get_last_log_idx();
+    apply_snapshot_callback(s.get_last_log_term(), s.get_last_log_idx());
     return true;
 }
 
@@ -320,6 +326,10 @@ void KeeperStateMachine::commit_config(const uint64_t /* log_idx */, nuraft::ptr
 void KeeperStateMachine::rollback(uint64_t log_idx, nuraft::buffer & data)
 {
     auto request_for_session = parseRequest(data);
+
+    if (request_for_session.request->getOpNum() == Coordination::OpNum::SessionID)
+        return;
+
     // If we received a log from an older node, use the log_idx as the zxid
     // log_idx will always be larger or equal to the zxid so we can safely do this
     // (log_idx is increased for all logs, while zxid is only increased for requests)

src/Coordination/KeeperStateMachine.h

@@ -20,13 +20,18 @@ using SnapshotsQueue = ConcurrentBoundedQueue<CreateSnapshotTask>;
 class KeeperStateMachine : public nuraft::state_machine
 {
 public:
+    using CommitCallback = std::function<void(const KeeperStorage::RequestForSession &, uint64_t, uint64_t)>;
+    using ApplySnapshotCallback = std::function<void(uint64_t, uint64_t)>;
+
     KeeperStateMachine(
         ResponsesQueue & responses_queue_,
         SnapshotsQueue & snapshots_queue_,
         const std::string & snapshots_path_,
         const CoordinationSettingsPtr & coordination_settings_,
         const KeeperContextPtr & keeper_context_,
-        const std::string & superdigest_ = "");
+        const std::string & superdigest_ = "",
+        CommitCallback commit_callback_ = [](const KeeperStorage::RequestForSession &, uint64_t, uint64_t){},
+        ApplySnapshotCallback apply_snapshot_callback_ = [](uint64_t, uint64_t){});
 
     /// Read state from the latest snapshot
     void init();
@@ -37,7 +42,7 @@ public:
 
     nuraft::ptr<nuraft::buffer> pre_commit(uint64_t log_idx, nuraft::buffer & data) override;
 
-    nuraft::ptr<nuraft::buffer> commit(const uint64_t log_idx, nuraft::buffer & data) override; /// NOLINT
+    nuraft::ptr<nuraft::buffer> commit_ext(const ext_op_params & params) override; /// NOLINT
 
     /// Save new cluster config to our snapshot (copy of the config stored in StateManager)
     void commit_config(const uint64_t log_idx, nuraft::ptr<nuraft::cluster_config> & new_conf) override; /// NOLINT
@@ -145,6 +150,11 @@ private:
     /// Special part of ACL system -- superdigest specified in server config.
     const std::string superdigest;
 
+    /// call when a request is committed
+    const CommitCallback commit_callback;
+    /// call when snapshot is applied
+    const ApplySnapshotCallback apply_snapshot_callback;
+
     KeeperContextPtr keeper_context;
 };
 

src/Coordination/tests/gtest_coordination.cpp

@@ -1330,8 +1330,9 @@ void testLogAndStateMachine(Coordination::CoordinationSettingsPtr settings, uint
         changelog.append(entry);
         changelog.end_of_append_batch(0, 0);
 
-        state_machine->pre_commit(i, changelog.entry_at(i)->get_buf());
-        state_machine->commit(i, changelog.entry_at(i)->get_buf());
+        auto entry_buf = changelog.entry_at(i)->get_buf_ptr();
+        state_machine->pre_commit(i, *entry_buf);
+        state_machine->commit_ext(nuraft::state_machine::ext_op_params{i, entry_buf});
         bool snapshot_created = false;
         if (i % settings->snapshot_distance == 0)
         {
@@ -1375,8 +1376,9 @@ void testLogAndStateMachine(Coordination::CoordinationSettingsPtr settings, uint
 
     for (size_t i = restore_machine->last_commit_index() + 1; i < restore_changelog.next_slot(); ++i)
     {
-        restore_machine->pre_commit(i, changelog.entry_at(i)->get_buf());
-        restore_machine->commit(i, changelog.entry_at(i)->get_buf());
+        auto entry = changelog.entry_at(i)->get_buf_ptr();
+        restore_machine->pre_commit(i, *entry);
+        restore_machine->commit_ext(nuraft::state_machine::ext_op_params{i, entry});
     }
 
     auto & source_storage = state_machine->getStorage();
@@ -1477,18 +1479,18 @@ TEST_P(CoordinationTest, TestEphemeralNodeRemove)
     std::shared_ptr<ZooKeeperCreateRequest> request_c = std::make_shared<ZooKeeperCreateRequest>();
     request_c->path = "/hello";
     request_c->is_ephemeral = true;
-    auto entry_c = getLogEntryFromZKRequest(0, 1, state_machine->getNextZxid(), request_c);
-    state_machine->pre_commit(1, entry_c->get_buf());
-    state_machine->commit(1, entry_c->get_buf());
+    auto entry_c = getLogEntryFromZKRequest(0, 1, state_machine->getNextZxid(), request_c)->get_buf_ptr();
+    state_machine->pre_commit(1, *entry_c);
+    state_machine->commit_ext(nuraft::state_machine::ext_op_params{1, entry_c});
     const auto & storage = state_machine->getStorage();
 
     EXPECT_EQ(storage.ephemerals.size(), 1);
     std::shared_ptr<ZooKeeperRemoveRequest> request_d = std::make_shared<ZooKeeperRemoveRequest>();
     request_d->path = "/hello";
     /// Delete from other session
-    auto entry_d = getLogEntryFromZKRequest(0, 2, state_machine->getNextZxid(), request_d);
-    state_machine->pre_commit(2, entry_d->get_buf());
-    state_machine->commit(2, entry_d->get_buf());
+    auto entry_d = getLogEntryFromZKRequest(0, 2, state_machine->getNextZxid(), request_d)->get_buf_ptr();
+    state_machine->pre_commit(2, *entry_d);
+    state_machine->commit_ext(nuraft::state_machine::ext_op_params{2, entry_d});
 
     EXPECT_EQ(storage.ephemerals.size(), 0);
 }

tests/jepsen.clickhouse-keeper/resources/keeper_config.xml

@@ -21,7 +21,8 @@
             <heart_beat_interval_ms>1000</heart_beat_interval_ms>
             <election_timeout_lower_bound_ms>2000</election_timeout_lower_bound_ms>
             <election_timeout_upper_bound_ms>4000</election_timeout_upper_bound_ms>
-            <quorum_reads>{quorum_reads}</quorum_reads>
+            <quorum_reads>0</quorum_reads>
+            <read_mode>fastlinear</read_mode>
             <snapshot_distance>{snapshot_distance}</snapshot_distance>
             <stale_log_gap>{stale_log_gap}</stale_log_gap>
             <reserved_log_items>{reserved_log_items}</reserved_log_items>

tests/jepsen.clickhouse-keeper/src/jepsen/clickhouse_keeper/counter.clj

@@ -27,7 +27,12 @@
 
   (invoke! [this test op]
     (case (:f op)
-      :read (exec-with-retries 30 (fn []
+      :read (try
+             (assoc op
+               :type :ok
+               :value (count (zk-list conn root-path)))
+             (catch Exception _ (assoc op :type :info, :error :connect-error)))
+      :final-read (exec-with-retries 30 (fn []
                                     (assoc op
                                            :type :ok
                                            :value (count (zk-list conn root-path)))))
@@ -49,7 +54,5 @@
    :checker   (checker/compose
                 {:counter (checker/counter)
                  :perf    (checker/perf)})
-   :generator (->> (range)
-                   (map (fn [x]
-                          (->> (gen/mix [r add])))))
-   :final-generator (gen/once {:type :invoke, :f :read, :value nil})})
+   :generator (gen/mix [r add])
+   :final-generator (gen/once {:type :invoke, :f :final-read, :value nil})})

tests/jepsen.clickhouse-keeper/src/jepsen/clickhouse_keeper/db.clj

@@ -98,7 +98,6 @@
                          #"\{srv2\}" (get nodes 1)
                          #"\{srv3\}" (get nodes 2)
                          #"\{id\}" (str (inc (.indexOf nodes node)))
-                         #"\{quorum_reads\}" (str (boolean (:quorum test)))
                          #"\{snapshot_distance\}" (str (:snapshot-distance test))
                          #"\{stale_log_gap\}" (str (:stale-log-gap test))
                          #"\{reserved_log_items\}" (str (:reserved-log-items test))}]

tests/jepsen.clickhouse-keeper/src/jepsen/clickhouse_keeper/main.clj

@@ -103,7 +103,7 @@
         current-nemesis (get custom-nemesis/custom-nemesises (:nemesis opts))]
     (merge tests/noop-test
            opts
-           {:name (str "clickhouse-keeper-quorum=" quorum "-"  (name (:workload opts)) "-" (name (:nemesis opts)))
+           {:name (str "clickhouse-keeper-" (name (:workload opts)) "-" (name (:nemesis opts)))
             :os ubuntu/os
             :db (get-db opts)
             :pure-generators true

tests/jepsen.clickhouse-keeper/src/jepsen/clickhouse_keeper/register.clj

@@ -20,7 +20,8 @@
     (assoc this :conn (zk-connect node 9181 30000)))
 
   (setup! [this test]
-    (zk-create-range conn 300)) ; 300 nodes to be sure
+    (exec-with-retries 30 (fn []
+                            (zk-create-range conn 300))))
 
   (invoke! [_ test op]
     (let [[k v] (:value op)

tests/jepsen.clickhouse-keeper/src/jepsen/clickhouse_keeper/utils.clj

@@ -45,7 +45,7 @@
 
 (defn zk-connect
   [host port timeout]
-  (exec-with-retries 30 (fn [] (zk/connect (str host ":" port) :timeout-msec timeout))))
+  (zk/connect (str host ":" port) :timeout-msec timeout))
 
 (defn zk-create-range
   [conn n]