Monitor command (#427)

feat(server): support monitor command - allowing user to debug commands
from all connections by using a connection as monitors for the this
(#344)

Signed-off-by: Boaz Sade <boaz@dragonflydb.io>

docs/api_status.md

@@ -104,7 +104,7 @@ with respect to Memcached and Redis APIs.
   - [X] ZSCORE
 - [ ] Other
   - [ ] BGREWRITEAOF
-  - [ ] MONITOR
+  - [x] MONITOR
   - [ ] RANDOMKEY
 
 ### API 2

src/facade/conn_context.h

@@ -19,7 +19,8 @@ class ConnectionContext {
 
   // We won't have any virtual methods, probably. However, since we allocate a derived class,
   // we need to declare a virtual d-tor, so we could properly delete it from Connection code.
-  virtual ~ConnectionContext() {}
+  virtual ~ConnectionContext() {
+  }
 
   Connection* owner() {
     return owner_;
@@ -44,12 +45,12 @@ class ConnectionContext {
   }
 
   // connection state / properties.
-  bool async_dispatch: 1;  // whether this connection is currently handled by dispatch fiber.
-  bool conn_closing: 1;
-  bool req_auth: 1;
-  bool replica_conn: 1;
-  bool authenticated: 1;
-  bool force_dispatch: 1;   // whether we should route all requests to the dispatch fiber.
+  bool async_dispatch : 1;  // whether this connection is currently handled by dispatch fiber.
+  bool conn_closing : 1;
+  bool req_auth : 1;
+  bool replica_conn : 1;
+  bool authenticated : 1;
+  bool force_dispatch : 1;  // whether we should route all requests to the dispatch fiber.
 
  private:
   Connection* owner_;

src/facade/dragonfly_connection.cc

@@ -109,6 +109,8 @@ struct Connection::RequestDeleter {
 // Please note: The call to the Dtor is mandatory for this!!
 // This class contain types that don't have trivial destructed objects
 struct Connection::Request {
+  using MonitorMessage = std::string;
+
   struct PipelineMsg {
     absl::FixedArray<MutableSlice, 6> args;
 
@@ -122,7 +124,7 @@ struct Connection::Request {
   };
 
  private:
-  using MessagePayload = std::variant<PipelineMsg, PubMsgRecord>;
+  using MessagePayload = std::variant<PipelineMsg, PubMsgRecord, MonitorMessage>;
 
   Request(size_t nargs, size_t capacity) : payload(PipelineMsg{nargs, capacity}) {
   }
@@ -130,18 +132,30 @@ struct Connection::Request {
   Request(PubMsgRecord msg) : payload(std::move(msg)) {
   }
 
+  Request(MonitorMessage msg) : payload(std::move(msg)) {
+  }
+
   Request(const Request&) = delete;
 
  public:
   // Overload to create the a new pipeline message
   static RequestPtr New(mi_heap_t* heap, RespVec args, size_t capacity);
 
-  // overload to create a new pubsub message
+  // Overload to create a new pubsub message
   static RequestPtr New(const PubMessage& pub_msg, fibers_ext::BlockingCounter bc);
 
+  // Overload to create a new the monitor message
+  static RequestPtr New(MonitorMessage msg);
+
   MessagePayload payload;
 };
 
+Connection::RequestPtr Connection::Request::New(std::string msg) {
+  void* ptr = mi_malloc(sizeof(Request));
+  Request* req = new (ptr) Request(std::move(msg));
+  return Connection::RequestPtr{req, Connection::RequestDeleter{}};
+}
+
 Connection::RequestPtr Connection::Request::New(mi_heap_t* heap, RespVec args, size_t capacity) {
   constexpr auto kReqSz = sizeof(Request);
   void* ptr = mi_heap_malloc_small(heap, kReqSz);
@@ -159,6 +173,7 @@ Connection::RequestPtr Connection::Request::New(mi_heap_t* heap, RespVec args, s
     pipeline_msg.args[i] = MutableSlice(next, s);
     next += s;
   }
+
   return Connection::RequestPtr{req, Connection::RequestDeleter{}};
 }
 
@@ -491,8 +506,6 @@ auto Connection::ParseRedis() -> ParserStatus {
         service_->DispatchCommand(cmd_list, cc_.get());
         last_interaction_ = time(nullptr);
       } else {
-        VLOG(2) << "Dispatch async";
-
         // Dispatch via queue to speedup input reading.
         RequestPtr req = FromArgs(std::move(parse_args_), tlh);
 
@@ -655,19 +668,23 @@ auto Connection::IoLoop(util::FiberSocketBase* peer) -> variant<error_code, Pars
 
 struct Connection::DispatchOperations {
   DispatchOperations(SinkReplyBuilder* b, Connection* me)
-      : stats{me->service_->GetThreadLocalConnectionStats()}, builder{b},
-        empty{me->dispatch_q_.empty()}, self(me) {
+      : stats{me->service_->GetThreadLocalConnectionStats()}, builder{b}, self(me) {
   }
 
   void operator()(PubMsgRecord& msg);
   void operator()(Request::PipelineMsg& msg);
+  void operator()(const Request::MonitorMessage& msg);
 
   ConnectionStats* stats = nullptr;
   SinkReplyBuilder* builder = nullptr;
-  bool empty = false;
   Connection* self = nullptr;
 };
 
+void Connection::DispatchOperations::operator()(const Request::MonitorMessage& msg) {
+  RedisReplyBuilder* rbuilder = (RedisReplyBuilder*)builder;
+  rbuilder->SendSimpleString(msg);
+}
+
 void Connection::DispatchOperations::operator()(PubMsgRecord& msg) {
   RedisReplyBuilder* rbuilder = (RedisReplyBuilder*)builder;
   ++stats->async_writes_cnt;
@@ -690,7 +707,7 @@ void Connection::DispatchOperations::operator()(PubMsgRecord& msg) {
 
 void Connection::DispatchOperations::operator()(Request::PipelineMsg& msg) {
   ++stats->pipelined_cmd_cnt;
-
+  bool empty = self->dispatch_q_.empty();
   builder->SetBatchMode(!empty);
   self->cc_->async_dispatch = true;
   self->service_->DispatchCommand(CmdArgList{msg.args.data(), msg.args.size()}, self->cc_.get());
@@ -703,6 +720,9 @@ struct Connection::DispatchCleanup {
     msg.bc.Dec();
   }
 
+  void operator()(const Connection::Request::MonitorMessage&) const {
+  }
+
   void operator()(const Connection::Request::PipelineMsg&) const {
   }
 };
@@ -769,4 +789,26 @@ void RespToArgList(const RespVec& src, CmdArgVec* dest) {
   }
 }
 
+void Connection::SendMonitorMsg(std::string monitor_msg) {
+  DCHECK(cc_);
+
+  if (!cc_->conn_closing) {
+    RequestPtr req = Request::New(std::move(monitor_msg));
+    dispatch_q_.push_back(std::move(req));
+    if (dispatch_q_.size() == 1) {
+      evc_.notify();
+    }
+  }
+}
+
+std::string Connection::RemoteEndpointStr() const {
+  LinuxSocketBase* lsb = static_cast<LinuxSocketBase*>(socket_.get());
+  bool unix_socket = lsb->IsUDS();
+  std::string connection_str = unix_socket ? "unix:" : std::string{};
+
+  auto re = lsb->RemoteEndpoint();
+  absl::StrAppend(&connection_str, re.address().to_string(), ":", re.port());
+  return connection_str;
+}
+
 }  // namespace facade

src/facade/dragonfly_connection.h

@@ -59,8 +59,15 @@ class Connection : public util::Connection {
     std::string_view message;
   };
 
+  // this function is overriden at test_utils TestConnection
   virtual void SendMsgVecAsync(const PubMessage& pub_msg, util::fibers_ext::BlockingCounter bc);
 
+  // Please note, this accept the message by value, since we really want to
+  // create a new copy here, so that we would not need to "worry" about memory
+  // management, we are assuming that we would not have many copy for this, and that
+  // we would not need in this way to sync on the lifetime of the message
+  void SendMonitorMsg(std::string monitor_msg);
+
   void SetName(std::string_view name) {
     CopyCharBuf(name, sizeof(name_), name_);
   }
@@ -70,6 +77,7 @@ class Connection : public util::Connection {
   }
 
   std::string GetClientInfo() const;
+  std::string RemoteEndpointStr() const;
   uint32 GetClientId() const;
 
   void ShutdownSelf();

src/facade/facade.cc

@@ -63,7 +63,6 @@ string UnknownSubCmd(string_view subcmd, string_view cmd) {
                       cmd, " HELP.");
 }
 
-
 const char kSyntaxErr[] = "syntax error";
 const char kWrongTypeErr[] = "-WRONGTYPE Operation against a key holding the wrong kind of value";
 const char kKeyNotFoundErr[] = "no such key";

src/server/CMakeLists.txt

@@ -13,7 +13,7 @@ add_library(dfly_transaction db_slice.cc engine_shard_set.cc blocking_controller
 cxx_link(dfly_transaction uring_fiber_lib dfly_core strings_lib)
 
 add_library(dragonfly_lib  channel_slice.cc command_registry.cc
-            config_flags.cc conn_context.cc debugcmd.cc dflycmd.cc
+            config_flags.cc conn_context.cc debugcmd.cc server_state.cc dflycmd.cc
             generic_family.cc hset_family.cc json_family.cc
             list_family.cc main_service.cc memory_cmd.cc rdb_load.cc rdb_save.cc replica.cc
             snapshot.cc script_mgr.cc server_family.cc malloc_stats.cc

src/server/conn_context.cc

@@ -6,12 +6,40 @@
 
 #include "base/logging.h"
 #include "server/engine_shard_set.h"
+#include "server/server_family.h"
+#include "server/server_state.h"
+#include "src/facade/dragonfly_connection.h"
 #include "util/proactor_base.h"
 
 namespace dfly {
 
 using namespace std;
 
+void ConnectionContext::SendMonitorMsg(std::string msg) {
+  CHECK(owner());
+
+  owner()->SendMonitorMsg(std::move(msg));
+}
+
+void ConnectionContext::ChangeMonitor(bool start) {
+  // This will either remove or register a new connection
+  // at the "top level" thread --> ServerState context
+  // note that we are registering/removing this connection to the thread at which at run
+  // then notify all other threads that there is a change in the number of monitors
+  auto& my_monitors = ServerState::tlocal()->Monitors();
+  if (start) {
+    my_monitors.Add(this);
+  } else {
+    VLOG(1) << "connection " << owner()->GetClientInfo()
+            << " no longer needs to be monitored - removing 0x" << std::hex << (const void*)this;
+    my_monitors.Remove(this);
+  }
+  // Tell other threads that about the change in the number of connection that we monitor
+  shard_set->pool()->Await(
+      [start](auto*) { ServerState::tlocal()->Monitors().NotifyChangeCount(start); });
+  EnableMonitoring(start);
+}
+
 void ConnectionContext::ChangeSubscription(bool to_add, bool to_reply, CmdArgList args) {
   vector<unsigned> result(to_reply ? args.size() : 0, 0);
 

src/server/conn_context.h

@@ -32,7 +32,9 @@ struct ConnectionState {
     ExecInfo(ExecInfo&&) = delete;
 
     // Return true if ExecInfo is active (after MULTI)
-    bool IsActive() { return state != EXEC_INACTIVE; }
+    bool IsActive() {
+      return state != EXEC_INACTIVE;
+    }
 
     // Resets to blank state after EXEC or DISCARD
     void Clear();
@@ -117,17 +119,29 @@ class ConnectionContext : public facade::ConnectionContext {
     return conn_state.db_index;
   }
 
+  // Note that this is accepted by value for lifetime reasons
+  // we want to have our own copy since we are assuming that
+  // 1. there will be not to many connections that we in monitor state
+  // 2. we need to have for each of them each own copy for thread safe reasons
+  void SendMonitorMsg(std::string msg);
+
   void ChangeSubscription(bool to_add, bool to_reply, CmdArgList args);
   void ChangePSub(bool to_add, bool to_reply, CmdArgList args);
   void UnsubscribeAll(bool to_reply);
   void PUnsubscribeAll(bool to_reply);
+  void ChangeMonitor(bool start);  // either start or stop monitor on a given connection
 
   bool is_replicating = false;
+  bool monitor = false;  // when a monitor command is sent over a given connection, we need to aware
+                         // of it as a state for the connection
 
  private:
+  void EnableMonitoring(bool enable) {
+    force_dispatch = enable;  // required to support the monitoring
+    monitor = enable;
+  }
   void SendSubscriptionChangedResponse(std::string_view action,
-                                       std::optional<std::string_view> topic,
-                                       unsigned count);
+                                       std::optional<std::string_view> topic, unsigned count);
 };
 
 }  // namespace dfly

src/server/main_service.cc

@@ -68,6 +68,105 @@ std::optional<VarzFunction> engine_varz;
 
 constexpr size_t kMaxThreadSize = 1024;
 
+void DeactivateMonitoring(ConnectionContext* server_ctx) {
+  if (server_ctx->monitor) {
+    // remove monitor on this connection
+    server_ctx->ChangeMonitor(false /*start*/);
+  }
+}
+
+// The format of the message that are sending is
+// +"time of day" [db-number <lua|unix:path|connection info] "command" "arg1" .. "argM"
+std::string CreateMonitorTimestamp() {
+  timeval tv;
+
+  gettimeofday(&tv, nullptr);
+  return absl::StrCat(tv.tv_sec, ".", tv.tv_usec, absl::kZeroPad6);
+}
+
+auto CmdEntryToMonitorFormat(std::string_view str) -> std::string {
+  // This code is based on Redis impl for it at sdscatrepr@sds.c
+  std::string result = absl::StrCat("\"");
+
+  for (auto c : str) {
+    switch (c) {
+      case '\\':
+        absl::StrAppend(&result, "\\\\");
+        break;
+      case '"':
+        absl::StrAppend(&result, "\\\"");
+        break;
+      case '\n':
+        absl::StrAppend(&result, "\\n");
+        break;
+      case '\r':
+        absl::StrAppend(&result, "\\r");
+        break;
+      case '\t':
+        absl::StrAppend(&result, "\\t");
+        break;
+      case '\a':
+        absl::StrAppend(&result, "\\a");
+        break;
+      case '\b':
+        absl::StrAppend(&result, "\\b");
+        break;
+      default:
+        if (isprint(c)) {
+          result += c;
+        } else {
+          absl::StrAppend(&result, "\\x", absl::Hex((unsigned char)c, absl::kZeroPad2));
+        }
+        break;
+    }
+  }
+  absl::StrAppend(&result, "\"");
+  return result;
+}
+
+std::string MakeMonitorMessage(const ConnectionState& conn_state,
+                               const facade::Connection* connection, CmdArgList args) {
+  std::string message = CreateMonitorTimestamp();
+
+  if (conn_state.script_info.has_value()) {
+    absl::StrAppend(&message, "lua] ");
+  } else {
+    absl::StrAppend(&message, connection->RemoteEndpointStr());
+  }
+  if (args.empty()) {
+    absl::StrAppend(&message, "error - empty cmd list!");
+  } else if (auto cmd_name = std::string_view(args[0].data(), args[0].size());
+             cmd_name == "AUTH") {  // we cannot just send auth details in this case
+    absl::StrAppend(&message, "\"", cmd_name, "\"");
+  } else {
+    message = std::accumulate(args.begin(), args.end(), message, [](auto str, const auto& cmd) {
+      absl::StrAppend(&str, " ", CmdEntryToMonitorFormat(std::string_view(cmd.data(), cmd.size())));
+      return str;
+    });
+  }
+  return message;
+}
+
+void DispatchMonitorIfNeeded(bool admin_cmd, ConnectionContext* connection, CmdArgList args) {
+  // We are not sending any admin command in the monitor, and we do not want to
+  // do any processing if we don't have any waiting connections with monitor
+  // enabled on them - see https://redis.io/commands/monitor/
+  const auto& my_monitors = ServerState::tlocal()->Monitors();
+  if (!(my_monitors.Empty() || admin_cmd)) {
+    //  We have connections waiting to get the info on the last command, send it to them
+    auto monitor_msg = MakeMonitorMessage(connection->conn_state, connection->owner(), args);
+    // Note that this is accepted by value for lifetime reasons
+    // we want to have our own copy since we are assuming that
+    // 1. there will be not to many connections that we in monitor state
+    // 2. we need to have for each of them each own copy for thread safe reasons
+    VLOG(1) << "sending command '" << monitor_msg << "' to the clients that registered on it";
+    shard_set->pool()->DispatchBrief(
+        [msg = std::move(monitor_msg)](unsigned idx, util::ProactorBase*) {
+          ServerState::tlocal()->Monitors().Send(msg);
+        });
+  }
+}
+
 class InterpreterReplier : public RedisReplyBuilder {
  public:
   InterpreterReplier(ObjectExplorer* explr) : RedisReplyBuilder(nullptr), explr_(explr) {
@@ -376,7 +475,7 @@ void Service::Init(util::AcceptServer* acceptor, util::ListenerInterface* main_i
 void Service::Shutdown() {
   VLOG(1) << "Service::Shutdown";
 
-  // We mark that we are shuttind down. After this incoming requests will be
+  // We mark that we are shutting down. After this incoming requests will be
   // rejected
   pp_.AwaitFiberOnAll([](ProactorBase* pb) { ServerState::tlocal()->Shutdown(); });
 
@@ -442,6 +541,11 @@ void Service::DispatchCommand(CmdArgList args, facade::ConnectionContext* cntx)
     }
   }
 
+  // only reset and quit are allow if this connection is used for monitoring
+  if (dfly_cntx->monitor && (cmd_name != "RESET" && cmd_name != "QUIT")) {
+    return (*cntx)->SendError("Replica can't interact with the keyspace");
+  }
+
   bool under_script = dfly_cntx->conn_state.script_info.has_value();
 
   if (under_script && (cid->opt_mask() & CO::NOSCRIPT)) {
@@ -555,6 +659,8 @@ void Service::DispatchCommand(CmdArgList args, facade::ConnectionContext* cntx)
     dfly_cntx->reply_builder()->CloseConnection();
   }
 
+  DispatchMonitorIfNeeded(cid->opt_mask() & CO::ADMIN, dfly_cntx, args);
+
   end_usec = ProactorBase::GetMonotonicTimeNs();
 
   request_latency_usec.IncBy(cmd_str, (end_usec - start_usec) / 1000);
@@ -726,6 +832,8 @@ void Service::Quit(CmdArgList args, ConnectionContext* cntx) {
 
   SinkReplyBuilder* builder = cntx->reply_builder();
   builder->CloseConnection();
+
+  DeactivateMonitoring(static_cast<ConnectionContext*>(cntx));
   cntx->owner()->ShutdownSelf();
 }
 
@@ -973,7 +1081,8 @@ bool CheckWatchedKeyExpiry(ConnectionContext* cntx, const CommandRegistry& regis
 
   // The comparison can still be true even if a key expired due to another one being created.
   // So we have to check the watched_dirty flag, which is set if a key expired.
-  return watch_exist_count.load() == exec_info.watched_existed && !exec_info.watched_dirty.load(memory_order_relaxed);
+  return watch_exist_count.load() == exec_info.watched_existed &&
+         !exec_info.watched_dirty.load(memory_order_relaxed);
 }
 
 // Check if exec_info watches keys on dbs other than db_indx.
@@ -1180,6 +1289,13 @@ void Service::PubsubPatterns(ConnectionContext* cntx) {
   (*cntx)->SendLong(pattern_count);
 }
 
+void Service::Monitor(CmdArgList args, ConnectionContext* cntx) {
+  VLOG(1) << "starting monitor on this connection: " << cntx->owner()->GetClientInfo();
+  // we are registering the current connection for all threads so they will be aware of
+  // this connection, to send to it any command
+  cntx->ChangeMonitor(true /* start */);
+}
+
 void Service::Pubsub(CmdArgList args, ConnectionContext* cntx) {
   if (args.size() < 2) {
     (*cntx)->SendError(WrongNumArgsError(ArgS(args, 0)));
@@ -1273,6 +1389,8 @@ void Service::OnClose(facade::ConnectionContext* cntx) {
     }
   }
 
+  DeactivateMonitoring(server_cntx);
+
   server_family_.OnClose(server_cntx);
 }
 
@@ -1316,6 +1434,7 @@ void Service::RegisterCommands() {
       << CI{"PSUBSCRIBE", CO::NOSCRIPT | CO::LOADING, -2, 0, 0, 0}.MFUNC(PSubscribe)
       << CI{"PUNSUBSCRIBE", CO::NOSCRIPT | CO::LOADING, -1, 0, 0, 0}.MFUNC(PUnsubscribe)
       << CI{"FUNCTION", CO::NOSCRIPT, 2, 0, 0, 0}.MFUNC(Function)
+      << CI{"MONITOR", CO::ADMIN, 1, 0, 0, 0}.MFUNC(Monitor)
       << CI{"PUBSUB", CO::LOADING | CO::FAST, -1, 0, 0, 0}.MFUNC(Pubsub);
 
   StreamFamily::Register(&registry_);

src/server/main_service.h

@@ -102,7 +102,7 @@ class Service : public facade::ServiceInterface {
   void PSubscribe(CmdArgList args, ConnectionContext* cntx);
   void PUnsubscribe(CmdArgList args, ConnectionContext* cntx);
   void Function(CmdArgList args, ConnectionContext* cntx);
-
+  void Monitor(CmdArgList args, ConnectionContext* cntx);
   void Pubsub(CmdArgList args, ConnectionContext* cntx);
   void PubsubChannels(std::string_view pattern, ConnectionContext* cntx);
   void PubsubPatterns(ConnectionContext* cntx);

src/server/server_state.cc

@@ -0,0 +1,51 @@
+// Copyright 2022, DragonflyDB authors.  All rights reserved.
+// See LICENSE for licensing terms.
+//
+
+#include "server/server_state.h"
+
+#include "base/logging.h"
+#include "server/conn_context.h"
+
+namespace dfly {
+
+void MonitorsRepo::Add(ConnectionContext* connection) {
+  VLOG(1) << "register connection "
+          << " at address 0x" << std::hex << (const void*)connection << " for thread "
+          << util::ProactorBase::GetIndex();
+
+  monitors_.push_back(connection);
+}
+
+void MonitorsRepo::Send(const std::string& msg) {
+  if (!monitors_.empty()) {
+    VLOG(1) << "thread " << util::ProactorBase::GetIndex() << " sending monitor message '" << msg
+            << "' for " << monitors_.size();
+    for (auto monitor_conn : monitors_) {
+      monitor_conn->SendMonitorMsg(msg);
+    }
+  }
+}
+
+void MonitorsRepo::Remove(const ConnectionContext* conn) {
+  auto it = std::find_if(monitors_.begin(), monitors_.end(),
+                         [&conn](const auto& val) { return val == conn; });
+  if (it != monitors_.end()) {
+    VLOG(1) << "removing connection 0x" << std::hex << (const void*)conn << " releasing token";
+    monitors_.erase(it);
+  } else {
+    VLOG(1) << "no connection 0x" << std::hex << (const void*)conn
+            << " found in the registered list here";
+  }
+}
+
+void MonitorsRepo::NotifyChangeCount(bool added) {
+  if (added) {
+    ++global_count_;
+  } else {
+    DCHECK(global_count_ > 0);
+    --global_count_;
+  }
+}
+
+}  // end of namespace dfly

src/server/server_state.h

@@ -15,10 +15,64 @@ typedef struct mi_heap_s mi_heap_t;
 
 namespace dfly {
 
+class ConnectionContext;
 namespace journal {
 class Journal;
 }  // namespace journal
 
+// This would be used as a thread local storage of sending
+// monitor messages.
+// Each thread will have its own list of all the connections that are
+// used for monitoring. When a connection is set to monitor it would register
+// itself to this list on all i/o threads. When a new command is dispatched,
+// and this list is not empty, it would send in the same thread context as then
+// thread that registered here the command.
+// Note about performance: we are assuming that we would not have many connections
+// that are registered here. This is not pub sub where it must be high performance
+// and may support many to many with tens or more of connections. It is assumed that
+// since monitoring is for debugging only, we would have less than 1 in most cases.
+// Also note that we holding this list on the thread level since this is the context
+// at which this would run. It also minimized the number of copied for this list.
+class MonitorsRepo {
+ public:
+  // This function adds a new connection to be monitored. This function only add
+  // new connection that belong to this thread! Must not be called outside of this
+  // thread context
+  void Add(ConnectionContext* info);
+
+  // Note that this is accepted by value for lifetime reasons
+  // we want to have our own copy since we are assuming that
+  // 1. there will be not to many connections that we in monitor state
+  // 2. we need to have for each of them each own copy for thread safe reasons
+  void Send(const std::string& msg);
+
+  // This function remove a connection what was monitored. This function only removes
+  // a connection that belong to this thread! Must not be called outside of this
+  // thread context
+  void Remove(const ConnectionContext* conn);
+
+  // We have for each thread the total number of monitors in the application.
+  // So this call is thread safe since we hold a copy of this for each thread.
+  // If this return true, then we don't need to run the monitor operation at all.
+  bool Empty() const {
+    return global_count_ == 0u;
+  }
+
+  // This function is run on all threads to either increment or decrement the "shared" counter
+  // of the monitors - it must be called as part of removing a monitor (for example
+  // when a connection is closed).
+  void NotifyChangeCount(bool added);
+
+  std::size_t Size() const {
+    return monitors_.size();
+  }
+
+ private:
+  using MonitorVec = std::vector<ConnectionContext*>;
+  MonitorVec monitors_;            // save connections belonging to this thread only!
+  unsigned int global_count_ = 0;  // by global its means that we count the monitor for all threads
+};
+
 // Present in every server thread. This class differs from EngineShard. The latter manages
 // state around engine shards while the former represents coordinator/connection state.
 // There may be threads that handle engine shards but not IO, there may be threads that handle IO
@@ -94,6 +148,10 @@ class ServerState {  // public struct - to allow initialization.
     journal_ = j;
   }
 
+  constexpr MonitorsRepo& Monitors() {
+    return monitors_;
+  }
+
  private:
   int64_t live_transactions_ = 0;
   mi_heap_t* data_heap_;
@@ -105,6 +163,8 @@ class ServerState {  // public struct - to allow initialization.
   using Counter = util::SlidingCounter<7>;
   Counter qps_;
 
+  MonitorsRepo monitors_;
+
   static thread_local ServerState state_;
 };