Transport prototype (#466)

.github/workflows/diff.yaml

@@ -171,7 +171,7 @@ jobs:
 
           # Run leftover CTest tests (all except unit and benchmark tests).
           cd build
-          ctest -E "(memgraph__unit|memgraph__benchmark)" --output-on-failure
+          ctest -E "(memgraph__unit|memgraph__benchmark|memgraph__simulation)" --output-on-failure
 
       - name: Run drivers tests
         run: |
@@ -262,6 +262,15 @@ jobs:
           cd build
           ctest -R memgraph__unit --output-on-failure -j$THREADS
 
+      - name: Run simulation tests
+        run: |
+          # Activate toolchain.
+          source /opt/toolchain-v4/activate
+
+          # Run unit tests.
+          cd build
+          ctest -R memgraph__simulation --output-on-failure -j$THREADS
+
       - name: Run e2e tests
         run: |
           # TODO(gitbuda): Setup mgclient and pymgclient properly.

src/CMakeLists.txt

@@ -5,6 +5,7 @@ add_subdirectory(lisp)
 add_subdirectory(utils)
 add_subdirectory(requests)
 add_subdirectory(io)
+add_subdirectory(io/simulator)
 add_subdirectory(kvstore)
 add_subdirectory(telemetry)
 add_subdirectory(communication)

src/io/address.hpp

@@ -0,0 +1,60 @@
+// Copyright 2022 Memgraph Ltd.
+//
+// Use of this software is governed by the Business Source License
+// included in the file licenses/BSL.txt; by using this file, you agree to be bound by the terms of the Business Source
+// License, and you may not use this file except in compliance with the Business Source License.
+//
+// As of the Change Date specified in that file, in accordance with
+// the Business Source License, use of this software will be governed
+// by the Apache License, Version 2.0, included in the file
+// licenses/APL.txt.
+
+#pragma once
+
+#include <compare>
+
+#include <fmt/format.h>
+#include <boost/asio/ip/tcp.hpp>
+#include <boost/uuid/uuid.hpp>
+#include <boost/uuid/uuid_io.hpp>
+
+namespace memgraph::io {
+struct Address {
+  // It's important for all participants to have a
+  // unique identifier - IP and port alone are not
+  // enough, and may change over the lifecycle of
+  // the nodes. Particularly storage nodes may change
+  // their IP addresses over time, and the system
+  // should gracefully update its information
+  // about them.
+  boost::uuids::uuid unique_id;
+  boost::asio::ip::address last_known_ip;
+  uint16_t last_known_port;
+
+  static Address TestAddress(uint16_t port) {
+    Address ret;
+    ret.last_known_port = port;
+    return ret;
+  }
+
+  friend bool operator==(const Address &lhs, const Address &rhs) = default;
+
+  /// unique_id is most dominant for ordering, then last_known_ip, then last_known_port
+  friend bool operator<(const Address &lhs, const Address &rhs) {
+    if (lhs.unique_id != rhs.unique_id) {
+      return lhs.unique_id < rhs.unique_id;
+    }
+
+    if (lhs.last_known_ip != rhs.last_known_ip) {
+      return lhs.last_known_ip < rhs.last_known_ip;
+    }
+
+    return lhs.last_known_port < rhs.last_known_port;
+  }
+
+  std::string ToString() const {
+    return fmt::format("Address {{ unique_id: {}, last_known_ip: {}, last_known_port: {} }}",
+                       boost::uuids::to_string(unique_id), last_known_ip.to_string(), last_known_port);
+  }
+};
+};  // namespace memgraph::io

src/io/errors.hpp

@@ -0,0 +1,26 @@
+// Copyright 2022 Memgraph Ltd.
+//
+// Use of this software is governed by the Business Source License
+// included in the file licenses/BSL.txt; by using this file, you agree to be bound by the terms of the Business Source
+// License, and you may not use this file except in compliance with the Business Source License.
+//
+// As of the Change Date specified in that file, in accordance with
+// the Business Source License, use of this software will be governed
+// by the Apache License, Version 2.0, included in the file
+// licenses/APL.txt.
+
+#pragma once
+
+namespace memgraph::io {
+// Signifies that a retriable operation was unable to
+// complete after a configured number of retries.
+struct RetriesExhausted {};
+
+// Signifies that a request was unable to receive a response
+// within some configured timeout duration. It is important
+// to remember that in distributed systems, a timeout does
+// not signify that a request was not received or processed.
+// It may be the case that the request was fully processed
+// but that the response was not received.
+struct TimedOut {};
+};  // namespace memgraph::io

src/io/future.hpp

@@ -0,0 +1,262 @@
+// Copyright 2022 Memgraph Ltd.
+//
+// Use of this software is governed by the Business Source License
+// included in the file licenses/BSL.txt; by using this file, you agree to be bound by the terms of the Business Source
+// License, and you may not use this file except in compliance with the Business Source License.
+//
+// As of the Change Date specified in that file, in accordance with
+// the Business Source License, use of this software will be governed
+// by the Apache License, Version 2.0, included in the file
+// licenses/APL.txt.
+
+#pragma once
+
+#include <condition_variable>
+#include <memory>
+#include <mutex>
+#include <optional>
+#include <thread>
+#include <utility>
+
+#include "io/errors.hpp"
+#include "utils/logging.hpp"
+
+namespace memgraph::io {
+
+// Shared is in an anonymous namespace, and the only way to
+// construct a Promise or Future is to pass a Shared in. This
+// ensures that Promises and Futures can only be constructed
+// in this translation unit.
+namespace details {
+template <typename T>
+class Shared {
+  mutable std::condition_variable cv_;
+  mutable std::mutex mu_;
+  std::optional<T> item_;
+  bool consumed_ = false;
+  bool waiting_ = false;
+  std::function<bool()> simulator_notifier_ = nullptr;
+
+ public:
+  explicit Shared(std::function<bool()> simulator_notifier) : simulator_notifier_(simulator_notifier) {}
+  Shared() = default;
+  Shared(Shared &&) = delete;
+  Shared &operator=(Shared &&) = delete;
+  Shared(const Shared &) = delete;
+  Shared &operator=(const Shared &) = delete;
+  ~Shared() = default;
+
+  /// Takes the item out of our optional item_ and returns it.
+  T Take() {
+    MG_ASSERT(item_, "Take called without item_ being present");
+    MG_ASSERT(!consumed_, "Take called on already-consumed Future");
+
+    T ret = std::move(item_).value();
+    item_.reset();
+
+    consumed_ = true;
+
+    return ret;
+  }
+
+  T Wait() {
+    std::unique_lock<std::mutex> lock(mu_);
+    waiting_ = true;
+
+    while (!item_) {
+      bool simulator_progressed = false;
+      if (simulator_notifier_) [[unlikely]] {
+        // We can't hold our own lock while notifying
+        // the simulator because notifying the simulator
+        // involves acquiring the simulator's mutex
+        // to guarantee that our notification linearizes
+        // with the simulator's condition variable.
+        // However, the simulator may acquire our
+        // mutex to check if we are being awaited,
+        // while determining system quiescence,
+        // so we have to get out of its way to avoid
+        // a cyclical deadlock.
+        lock.unlock();
+        simulator_progressed = std::invoke(simulator_notifier_);
+        lock.lock();
+        if (item_) {
+          // item may have been filled while we
+          // had dropped our mutex while notifying
+          // the simulator of our waiting_ status.
+          break;
+        }
+      }
+      if (!simulator_progressed) [[likely]] {
+        cv_.wait(lock);
+      }
+      MG_ASSERT(!consumed_, "Future consumed twice!");
+    }
+
+    waiting_ = false;
+
+    return Take();
+  }
+
+  bool IsReady() const {
+    std::unique_lock<std::mutex> lock(mu_);
+    return item_;
+  }
+
+  std::optional<T> TryGet() {
+    std::unique_lock<std::mutex> lock(mu_);
+
+    if (item_) {
+      return Take();
+    }
+
+    return std::nullopt;
+  }
+
+  void Fill(T item) {
+    {
+      std::unique_lock<std::mutex> lock(mu_);
+
+      MG_ASSERT(!consumed_, "Promise filled after it was already consumed!");
+      MG_ASSERT(!item_, "Promise filled twice!");
+
+      item_ = item;
+    }  // lock released before condition variable notification
+
+    cv_.notify_all();
+  }
+
+  bool IsAwaited() const {
+    std::unique_lock<std::mutex> lock(mu_);
+    return waiting_;
+  }
+};
+}  // namespace details
+
+template <typename T>
+class Future {
+  bool consumed_or_moved_ = false;
+  std::shared_ptr<details::Shared<T>> shared_;
+
+ public:
+  explicit Future(std::shared_ptr<details::Shared<T>> shared) : shared_(shared) {}
+
+  Future() = delete;
+  Future(Future &&old) noexcept {
+    MG_ASSERT(!old.consumed_or_moved_, "Future moved from after already being moved from or consumed.");
+    shared_ = std::move(old.shared_);
+    consumed_or_moved_ = old.consumed_or_moved_;
+    old.consumed_or_moved_ = true;
+  }
+
+  Future &operator=(Future &&old) noexcept {
+    MG_ASSERT(!old.consumed_or_moved_, "Future moved from after already being moved from or consumed.");
+    shared_ = std::move(old.shared_);
+    old.consumed_or_moved_ = true;
+  }
+
+  Future(const Future &) = delete;
+  Future &operator=(const Future &) = delete;
+  ~Future() = default;
+
+  /// Returns true if the Future is ready to
+  /// be consumed using TryGet or Wait (prefer Wait
+  /// if you know it's ready, because it doesn't
+  /// return an optional.
+  bool IsReady() {
+    MG_ASSERT(!consumed_or_moved_, "Called IsReady after Future already consumed!");
+    return shared_->IsReady();
+  }
+
+  /// Non-blocking method that returns the inner
+  /// item if it's already ready, or std::nullopt
+  /// if it is not ready yet.
+  std::optional<T> TryGet() {
+    MG_ASSERT(!consumed_or_moved_, "Called TryGet after Future already consumed!");
+    std::optional<T> ret = shared_->TryGet();
+    if (ret) {
+      consumed_or_moved_ = true;
+    }
+    return ret;
+  }
+
+  /// Block on the corresponding promise to be filled,
+  /// returning the inner item when ready.
+  T Wait() && {
+    MG_ASSERT(!consumed_or_moved_, "Future should only be consumed with Wait once!");
+    T ret = shared_->Wait();
+    consumed_or_moved_ = true;
+    return ret;
+  }
+
+  /// Marks this Future as canceled.
+  void Cancel() {
+    MG_ASSERT(!consumed_or_moved_, "Future::Cancel called on a future that was already moved or consumed!");
+    consumed_or_moved_ = true;
+  }
+};
+
+template <typename T>
+class Promise {
+  std::shared_ptr<details::Shared<T>> shared_;
+  bool filled_or_moved_ = false;
+
+ public:
+  explicit Promise(std::shared_ptr<details::Shared<T>> shared) : shared_(shared) {}
+
+  Promise() = delete;
+  Promise(Promise &&old) noexcept {
+    MG_ASSERT(!old.filled_or_moved_, "Promise moved from after already being moved from or filled.");
+    shared_ = std::move(old.shared_);
+    old.filled_or_moved_ = true;
+  }
+
+  Promise &operator=(Promise &&old) noexcept {
+    MG_ASSERT(!old.filled_or_moved_, "Promise moved from after already being moved from or filled.");
+    shared_ = std::move(old.shared_);
+    old.filled_or_moved_ = true;
+  }
+  Promise(const Promise &) = delete;
+  Promise &operator=(const Promise &) = delete;
+
+  ~Promise() { MG_ASSERT(filled_or_moved_, "Promise destroyed before its associated Future was filled!"); }
+
+  // Fill the expected item into the Future.
+  void Fill(T item) {
+    MG_ASSERT(!filled_or_moved_, "Promise::Fill called on a promise that is already filled or moved!");
+    shared_->Fill(item);
+    filled_or_moved_ = true;
+  }
+
+  bool IsAwaited() { return shared_->IsAwaited(); }
+
+  /// Moves this Promise into a unique_ptr.
+  std::unique_ptr<Promise<T>> ToUnique() && {
+    std::unique_ptr<Promise<T>> up = std::make_unique<Promise<T>>(std::move(shared_));
+
+    filled_or_moved_ = true;
+
+    return up;
+  }
+};
+
+template <typename T>
+std::pair<Future<T>, Promise<T>> FuturePromisePair() {
+  std::shared_ptr<details::Shared<T>> shared = std::make_shared<details::Shared<T>>();
+
+  Future<T> future = Future<T>(shared);
+  Promise<T> promise = Promise<T>(shared);
+
+  return std::make_pair(std::move(future), std::move(promise));
+}
+
+template <typename T>
+std::pair<Future<T>, Promise<T>> FuturePromisePairWithNotifier(std::function<bool()> simulator_notifier) {
+  std::shared_ptr<details::Shared<T>> shared = std::make_shared<details::Shared<T>>(simulator_notifier);
+
+  Future<T> future = Future<T>(shared);
+  Promise<T> promise = Promise<T>(shared);
+
+  return std::make_pair(std::move(future), std::move(promise));
+}
+
+};  // namespace memgraph::io

src/io/simulator/CMakeLists.txt

@@ -0,0 +1,8 @@
+set(io_simulator_sources
+    simulator_handle.cpp)
+
+find_package(fmt REQUIRED)
+find_package(Threads REQUIRED)
+
+add_library(mg-io-simulator STATIC ${io_simulator_sources})
+target_link_libraries(mg-io-simulator stdc++fs Threads::Threads fmt::fmt mg-utils)

src/io/simulator/message_conversion.hpp

@@ -0,0 +1,177 @@
+// Copyright 2022 Memgraph Ltd.
+//
+// Use of this software is governed by the Business Source License
+// included in the file licenses/BSL.txt; by using this file, you agree to be bound by the terms of the Business Source
+// License, and you may not use this file except in compliance with the Business Source License.
+//
+// As of the Change Date specified in that file, in accordance with
+// the Business Source License, use of this software will be governed
+// by the Apache License, Version 2.0, included in the file
+// licenses/APL.txt.
+
+#pragma once
+
+#include "io/transport.hpp"
+
+namespace memgraph::io::simulator {
+
+using memgraph::io::Duration;
+using memgraph::io::Message;
+using memgraph::io::Time;
+
+struct OpaqueMessage {
+  Address from_address;
+  uint64_t request_id;
+  std::any message;
+
+  /// Recursively tries to match a specific type from the outer
+  /// variant's parameter pack against the type of the std::any,
+  /// and if it matches, make it concrete and return it. Otherwise,
+  /// move on and compare the any with the next type from the
+  /// parameter pack.
+  ///
+  /// Return is the full std::variant<Ts...> type that holds the
+  /// full parameter pack without interfering with recursive
+  /// narrowing expansion.
+  template <typename Return, Message Head, Message... Rest>
+  std::optional<Return> Unpack(std::any &&a) {
+    if (typeid(Head) == a.type()) {
+      Head concrete = std::any_cast<Head>(std::move(a));
+      return concrete;
+    }
+
+    if constexpr (sizeof...(Rest) > 0) {
+      return Unpack<Return, Rest...>(std::move(a));
+    } else {
+      return std::nullopt;
+    }
+  }
+
+  /// High level "user-facing" conversion function that lets
+  /// people interested in conversion only supply a single
+  /// parameter pack for the types that they want to compare
+  /// with the any and potentially include in the returned
+  /// variant.
+  template <Message... Ms>
+  requires(sizeof...(Ms) > 0) std::optional<std::variant<Ms...>> VariantFromAny(std::any &&a) {
+    return Unpack<std::variant<Ms...>, Ms...>(std::move(a));
+  }
+
+  template <Message... Ms>
+  requires(sizeof...(Ms) > 0) std::optional<RequestEnvelope<Ms...>> Take() && {
+    std::optional<std::variant<Ms...>> m_opt = VariantFromAny<Ms...>(std::move(message));
+
+    if (m_opt) {
+      return RequestEnvelope<Ms...>{
+          .message = std::move(*m_opt),
+          .request_id = request_id,
+          .from_address = from_address,
+      };
+    }
+
+    return std::nullopt;
+  }
+};
+
+class OpaquePromiseTraitBase {
+ public:
+  virtual const std::type_info *TypeInfo() const = 0;
+  virtual bool IsAwaited(void *ptr) const = 0;
+  virtual void Fill(void *ptr, OpaqueMessage &&) const = 0;
+  virtual void TimeOut(void *ptr) const = 0;
+
+  virtual ~OpaquePromiseTraitBase() = default;
+  OpaquePromiseTraitBase() = default;
+  OpaquePromiseTraitBase(const OpaquePromiseTraitBase &) = delete;
+  OpaquePromiseTraitBase &operator=(const OpaquePromiseTraitBase &) = delete;
+  OpaquePromiseTraitBase(OpaquePromiseTraitBase &&old) = delete;
+  OpaquePromiseTraitBase &operator=(OpaquePromiseTraitBase &&) = delete;
+};
+
+template <typename T>
+class OpaquePromiseTrait : public OpaquePromiseTraitBase {
+ public:
+  const std::type_info *TypeInfo() const override { return &typeid(T); };
+
+  bool IsAwaited(void *ptr) const override { return static_cast<ResponsePromise<T> *>(ptr)->IsAwaited(); };
+
+  void Fill(void *ptr, OpaqueMessage &&opaque_message) const override {
+    T message = std::any_cast<T>(std::move(opaque_message.message));
+    auto response_envelope = ResponseEnvelope<T>{.message = std::move(message),
+                                                 .request_id = opaque_message.request_id,
+                                                 .from_address = opaque_message.from_address};
+    auto promise = static_cast<ResponsePromise<T> *>(ptr);
+    auto unique_promise = std::unique_ptr<ResponsePromise<T>>(promise);
+    unique_promise->Fill(std::move(response_envelope));
+  };
+
+  void TimeOut(void *ptr) const override {
+    auto promise = static_cast<ResponsePromise<T> *>(ptr);
+    auto unique_promise = std::unique_ptr<ResponsePromise<T>>(promise);
+    ResponseResult<T> result = TimedOut{};
+    unique_promise->Fill(std::move(result));
+  }
+};
+
+class OpaquePromise {
+  void *ptr_;
+  std::unique_ptr<OpaquePromiseTraitBase> trait_;
+
+ public:
+  OpaquePromise(OpaquePromise &&old) noexcept : ptr_(old.ptr_), trait_(std::move(old.trait_)) {
+    MG_ASSERT(old.ptr_ != nullptr);
+    old.ptr_ = nullptr;
+  }
+
+  OpaquePromise &operator=(OpaquePromise &&old) noexcept {
+    MG_ASSERT(ptr_ == nullptr);
+    MG_ASSERT(old.ptr_ != nullptr);
+    MG_ASSERT(this != &old);
+    ptr_ = old.ptr_;
+    trait_ = std::move(old.trait_);
+    old.ptr_ = nullptr;
+    return *this;
+  }
+
+  OpaquePromise(const OpaquePromise &) = delete;
+  OpaquePromise &operator=(const OpaquePromise &) = delete;
+
+  template <typename T>
+  std::unique_ptr<ResponsePromise<T>> Take() && {
+    MG_ASSERT(typeid(T) == *trait_->TypeInfo());
+    MG_ASSERT(ptr_ != nullptr);
+
+    auto ptr = static_cast<ResponsePromise<T> *>(ptr_);
+
+    ptr_ = nullptr;
+
+    return std::unique_ptr<T>(ptr);
+  }
+
+  template <typename T>
+  explicit OpaquePromise(std::unique_ptr<ResponsePromise<T>> promise)
+      : ptr_(static_cast<void *>(promise.release())), trait_(std::make_unique<OpaquePromiseTrait<T>>()) {}
+
+  bool IsAwaited() {
+    MG_ASSERT(ptr_ != nullptr);
+    return trait_->IsAwaited(ptr_);
+  }
+
+  void TimeOut() {
+    MG_ASSERT(ptr_ != nullptr);
+    trait_->TimeOut(ptr_);
+    ptr_ = nullptr;
+  }
+
+  void Fill(OpaqueMessage &&opaque_message) {
+    MG_ASSERT(ptr_ != nullptr);
+    trait_->Fill(ptr_, std::move(opaque_message));
+    ptr_ = nullptr;
+  }
+
+  ~OpaquePromise() {
+    MG_ASSERT(ptr_ == nullptr, "OpaquePromise destroyed without being explicitly timed out or filled");
+  }
+};
+
+}  // namespace memgraph::io::simulator

src/io/simulator/simulator.hpp

@@ -0,0 +1,45 @@
+// Copyright 2022 Memgraph Ltd.
+//
+// Use of this software is governed by the Business Source License
+// included in the file licenses/BSL.txt; by using this file, you agree to be bound by the terms of the Business Source
+// License, and you may not use this file except in compliance with the Business Source License.
+//
+// As of the Change Date specified in that file, in accordance with
+// the Business Source License, use of this software will be governed
+// by the Apache License, Version 2.0, included in the file
+// licenses/APL.txt.
+
+#pragma once
+
+#include <memory>
+#include <random>
+
+#include "io/address.hpp"
+#include "io/simulator/simulator_config.hpp"
+#include "io/simulator/simulator_handle.hpp"
+#include "io/simulator/simulator_transport.hpp"
+
+namespace memgraph::io::simulator {
+class Simulator {
+  std::mt19937 rng_;
+  std::shared_ptr<SimulatorHandle> simulator_handle_;
+
+ public:
+  explicit Simulator(SimulatorConfig config)
+      : rng_(std::mt19937{config.rng_seed}), simulator_handle_{std::make_shared<SimulatorHandle>(config)} {}
+
+  void ShutDown() { simulator_handle_->ShutDown(); }
+
+  Io<SimulatorTransport> Register(Address address) {
+    std::uniform_int_distribution<uint64_t> seed_distrib;
+    uint64_t seed = seed_distrib(rng_);
+    return Io{SimulatorTransport{simulator_handle_, address, seed}, address};
+  }
+
+  void IncrementServerCountAndWaitForQuiescentState(Address address) {
+    simulator_handle_->IncrementServerCountAndWaitForQuiescentState(address);
+  }
+
+  SimulatorStats Stats() { return simulator_handle_->Stats(); }
+};
+};  // namespace memgraph::io::simulator

src/io/simulator/simulator_config.hpp

@@ -0,0 +1,30 @@
+// Copyright 2022 Memgraph Ltd.
+//
+// Use of this software is governed by the Business Source License
+// included in the file licenses/BSL.txt; by using this file, you agree to be bound by the terms of the Business Source
+// License, and you may not use this file except in compliance with the Business Source License.
+//
+// As of the Change Date specified in that file, in accordance with
+// the Business Source License, use of this software will be governed
+// by the Apache License, Version 2.0, included in the file
+// licenses/APL.txt.
+
+#pragma once
+
+#include <chrono>
+
+#include "io/time.hpp"
+
+namespace memgraph::io::simulator {
+
+using memgraph::io::Time;
+
+struct SimulatorConfig {
+  uint8_t drop_percent = 0;
+  bool perform_timeouts = false;
+  bool scramble_messages = true;
+  uint64_t rng_seed = 0;
+  Time start_time = Time::min();
+  Time abort_time = Time::max();
+};
+};  // namespace memgraph::io::simulator

src/io/simulator/simulator_handle.cpp

@@ -0,0 +1,154 @@
+// Copyright 2022 Memgraph Ltd.
+//
+// Use of this software is governed by the Business Source License
+// included in the file licenses/BSL.txt; by using this file, you agree to be bound by the terms of the Business Source
+// License, and you may not use this file except in compliance with the Business Source License.
+//
+// As of the Change Date specified in that file, in accordance with
+// the Business Source License, use of this software will be governed
+// by the Apache License, Version 2.0, included in the file
+// licenses/APL.txt.
+
+#include "io/simulator/simulator_handle.hpp"
+#include "io/address.hpp"
+#include "io/errors.hpp"
+#include "io/simulator/simulator_config.hpp"
+#include "io/simulator/simulator_stats.hpp"
+#include "io/time.hpp"
+#include "io/transport.hpp"
+
+namespace memgraph::io::simulator {
+
+using memgraph::io::Duration;
+using memgraph::io::Time;
+
+void SimulatorHandle::ShutDown() {
+  std::unique_lock<std::mutex> lock(mu_);
+  should_shut_down_ = true;
+  cv_.notify_all();
+}
+
+bool SimulatorHandle::ShouldShutDown() const {
+  std::unique_lock<std::mutex> lock(mu_);
+  return should_shut_down_;
+}
+
+void SimulatorHandle::IncrementServerCountAndWaitForQuiescentState(Address address) {
+  std::unique_lock<std::mutex> lock(mu_);
+  server_addresses_.insert(address);
+
+  while (true) {
+    const size_t blocked_servers = BlockedServers();
+
+    const bool all_servers_blocked = blocked_servers == server_addresses_.size();
+
+    if (all_servers_blocked) {
+      return;
+    }
+
+    cv_.wait(lock);
+  }
+}
+
+size_t SimulatorHandle::BlockedServers() {
+  size_t blocked_servers = blocked_on_receive_;
+
+  for (auto &[promise_key, opaque_promise] : promises_) {
+    if (opaque_promise.promise.IsAwaited() && server_addresses_.contains(promise_key.requester_address)) {
+      blocked_servers++;
+    }
+  }
+
+  return blocked_servers;
+}
+
+bool SimulatorHandle::MaybeTickSimulator() {
+  std::unique_lock<std::mutex> lock(mu_);
+
+  const size_t blocked_servers = BlockedServers();
+
+  if (blocked_servers < server_addresses_.size()) {
+    // we only need to advance the simulator when all
+    // servers have reached a quiescent state, blocked
+    // on their own futures or receive methods.
+    return false;
+  }
+
+  stats_.simulator_ticks++;
+
+  cv_.notify_all();
+
+  TimeoutPromisesPastDeadline();
+
+  if (in_flight_.empty()) {
+    // return early here because there are no messages to schedule
+
+    // We tick the clock forward when all servers are blocked but
+    // there are no in-flight messages to schedule delivery of.
+    std::poisson_distribution<> time_distrib(50);
+    Duration clock_advance = std::chrono::microseconds{time_distrib(rng_)};
+    cluster_wide_time_microseconds_ += clock_advance;
+
+    MG_ASSERT(cluster_wide_time_microseconds_ < config_.abort_time,
+              "Cluster has executed beyond its configured abort_time, and something may be failing to make progress "
+              "in an expected amount of time.");
+    return true;
+  }
+
+  if (config_.scramble_messages) {
+    // scramble messages
+    std::uniform_int_distribution<size_t> swap_distrib(0, in_flight_.size() - 1);
+    const size_t swap_index = swap_distrib(rng_);
+    std::swap(in_flight_[swap_index], in_flight_.back());
+  }
+
+  auto [to_address, opaque_message] = std::move(in_flight_.back());
+  in_flight_.pop_back();
+
+  std::uniform_int_distribution<int> drop_distrib(0, 99);
+  const int drop_threshold = drop_distrib(rng_);
+  const bool should_drop = drop_threshold < config_.drop_percent;
+
+  if (should_drop) {
+    stats_.dropped_messages++;
+  }
+
+  PromiseKey promise_key{.requester_address = to_address,
+                         .request_id = opaque_message.request_id,
+                         .replier_address = opaque_message.from_address};
+
+  if (promises_.contains(promise_key)) {
+    // complete waiting promise if it's there
+    DeadlineAndOpaquePromise dop = std::move(promises_.at(promise_key));
+    promises_.erase(promise_key);
+
+    const bool normal_timeout = config_.perform_timeouts && (dop.deadline < cluster_wide_time_microseconds_);
+
+    if (should_drop || normal_timeout) {
+      stats_.timed_out_requests++;
+      dop.promise.TimeOut();
+    } else {
+      stats_.total_responses++;
+      dop.promise.Fill(std::move(opaque_message));
+    }
+  } else if (should_drop) {
+    // don't add it anywhere, let it drop
+  } else {
+    // add to can_receive_ if not
+    const auto &[om_vec, inserted] = can_receive_.try_emplace(to_address, std::vector<OpaqueMessage>());
+    om_vec->second.emplace_back(std::move(opaque_message));
+  }
+
+  return true;
+}
+
+Time SimulatorHandle::Now() const {
+  std::unique_lock<std::mutex> lock(mu_);
+  return cluster_wide_time_microseconds_;
+}
+
+SimulatorStats SimulatorHandle::Stats() {
+  std::unique_lock<std::mutex> lock(mu_);
+  return stats_;
+}
+}  // namespace memgraph::io::simulator

src/io/simulator/simulator_handle.hpp

@@ -0,0 +1,206 @@
+// Copyright 2022 Memgraph Ltd.
+//
+// Use of this software is governed by the Business Source License
+// included in the file licenses/BSL.txt; by using this file, you agree to be bound by the terms of the Business Source
+// License, and you may not use this file except in compliance with the Business Source License.
+//
+// As of the Change Date specified in that file, in accordance with
+// the Business Source License, use of this software will be governed
+// by the Apache License, Version 2.0, included in the file
+// licenses/APL.txt.
+
+#pragma once
+
+#include <any>
+#include <compare>
+#include <iostream>
+#include <map>
+#include <memory>
+#include <optional>
+#include <set>
+#include <utility>
+#include <variant>
+#include <vector>
+
+#include "io/address.hpp"
+#include "io/errors.hpp"
+#include "io/simulator/message_conversion.hpp"
+#include "io/simulator/simulator_config.hpp"
+#include "io/simulator/simulator_stats.hpp"
+#include "io/time.hpp"
+#include "io/transport.hpp"
+
+namespace memgraph::io::simulator {
+
+using memgraph::io::Duration;
+using memgraph::io::Time;
+
+struct PromiseKey {
+  Address requester_address;
+  uint64_t request_id;
+  // TODO(tyler) possibly remove replier_address from promise key
+  // once we want to support DSR.
+  Address replier_address;
+
+ public:
+  friend bool operator<(const PromiseKey &lhs, const PromiseKey &rhs) {
+    if (lhs.requester_address != rhs.requester_address) {
+      return lhs.requester_address < rhs.requester_address;
+    }
+
+    if (lhs.request_id != rhs.request_id) {
+      return lhs.request_id < rhs.request_id;
+    }
+
+    return lhs.replier_address < rhs.replier_address;
+  }
+};
+
+struct DeadlineAndOpaquePromise {
+  Time deadline;
+  OpaquePromise promise;
+};
+
+class SimulatorHandle {
+  mutable std::mutex mu_{};
+  mutable std::condition_variable cv_;
+
+  // messages that have not yet been scheduled or dropped
+  std::vector<std::pair<Address, OpaqueMessage>> in_flight_;
+
+  // the responses to requests that are being waited on
+  std::map<PromiseKey, DeadlineAndOpaquePromise> promises_;
+
+  // messages that are sent to servers that may later receive them
+  std::map<Address, std::vector<OpaqueMessage>> can_receive_;
+
+  Time cluster_wide_time_microseconds_;
+  bool should_shut_down_ = false;
+  SimulatorStats stats_;
+  size_t blocked_on_receive_ = 0;
+  std::set<Address> server_addresses_;
+  std::mt19937 rng_;
+  SimulatorConfig config_;
+
+  /// Returns the number of servers currently blocked on Receive, plus
+  /// the servers that are blocked on Futures that were created through
+  /// SimulatorTransport::Request.
+  ///
+  /// TODO(tyler) investigate whether avoiding consideration of Futures
+  /// increases determinism.
+  size_t BlockedServers();
+
+  void TimeoutPromisesPastDeadline() {
+    const Time now = cluster_wide_time_microseconds_;
+
+    for (auto &[promise_key, dop] : promises_) {
+      if (dop.deadline < now) {
+        spdlog::debug("timing out request from requester {} to replier {}.", promise_key.requester_address.ToString(),
+                      promise_key.replier_address.ToString());
+        std::move(dop).promise.TimeOut();
+        promises_.erase(promise_key);
+
+        stats_.timed_out_requests++;
+      }
+    }
+  }
+
+ public:
+  explicit SimulatorHandle(SimulatorConfig config)
+      : cluster_wide_time_microseconds_(config.start_time), rng_(config.rng_seed), config_(config) {}
+
+  void IncrementServerCountAndWaitForQuiescentState(Address address);
+
+  /// This method causes most of the interesting simulation logic to happen, wrt network behavior.
+  /// It checks to see if all background "server" threads are blocked on new messages, and if so,
+  /// it will decide whether to drop, reorder, or deliver in-flight messages based on the SimulatorConfig
+  /// that was used to create the Simulator.
+  bool MaybeTickSimulator();
+
+  void ShutDown();
+
+  bool ShouldShutDown() const;
+
+  template <Message Request, Message Response>
+  void SubmitRequest(Address to_address, Address from_address, uint64_t request_id, Request &&request, Duration timeout,
+                     ResponsePromise<Response> &&promise) {
+    std::unique_lock<std::mutex> lock(mu_);
+
+    const Time deadline = cluster_wide_time_microseconds_ + timeout;
+
+    std::any message(request);
+    OpaqueMessage om{.from_address = from_address, .request_id = request_id, .message = std::move(message)};
+    in_flight_.emplace_back(std::make_pair(to_address, std::move(om)));
+
+    PromiseKey promise_key{.requester_address = from_address, .request_id = request_id, .replier_address = to_address};
+    OpaquePromise opaque_promise(std::move(promise).ToUnique());
+    DeadlineAndOpaquePromise dop{.deadline = deadline, .promise = std::move(opaque_promise)};
+    promises_.emplace(std::move(promise_key), std::move(dop));
+
+    stats_.total_messages++;
+    stats_.total_requests++;
+
+    cv_.notify_all();
+  }
+
+  template <Message... Ms>
+  requires(sizeof...(Ms) > 0) RequestResult<Ms...> Receive(const Address &receiver, Duration timeout) {
+    std::unique_lock<std::mutex> lock(mu_);
+
+    blocked_on_receive_ += 1;
+
+    const Time deadline = cluster_wide_time_microseconds_ + timeout;
+
+    while (!should_shut_down_ && (cluster_wide_time_microseconds_ < deadline)) {
+      if (can_receive_.contains(receiver)) {
+        std::vector<OpaqueMessage> &can_rx = can_receive_.at(receiver);
+        if (!can_rx.empty()) {
+          OpaqueMessage message = std::move(can_rx.back());
+          can_rx.pop_back();
+
+          // TODO(tyler) search for item in can_receive_ that matches the desired types, rather
+          // than asserting that the last item in can_rx matches.
+          auto m_opt = std::move(message).Take<Ms...>();
+
+          blocked_on_receive_ -= 1;
+
+          return std::move(m_opt).value();
+        }
+      }
+
+      lock.unlock();
+      bool made_progress = MaybeTickSimulator();
+      lock.lock();
+      if (!should_shut_down_ && !made_progress) {
+        cv_.wait(lock);
+      }
+    }
+
+    blocked_on_receive_ -= 1;
+
+    return TimedOut{};
+  }
+
+  template <Message M>
+  void Send(Address to_address, Address from_address, uint64_t request_id, M message) {
+    std::unique_lock<std::mutex> lock(mu_);
+    std::any message_any(std::move(message));
+    OpaqueMessage om{.from_address = from_address, .request_id = request_id, .message = std::move(message_any)};
+    in_flight_.emplace_back(std::make_pair(std::move(to_address), std::move(om)));
+
+    stats_.total_messages++;
+
+    cv_.notify_all();
+  }
+
+  Time Now() const;
+
+  template <class D = std::poisson_distribution<>, class Return = uint64_t>
+  Return Rand(D distrib) {
+    std::unique_lock<std::mutex> lock(mu_);
+    return distrib(rng_);
+  }
+
+  SimulatorStats Stats();
+};
+};  // namespace memgraph::io::simulator

src/io/simulator/simulator_stats.hpp

@@ -0,0 +1,25 @@
+// Copyright 2022 Memgraph Ltd.
+//
+// Use of this software is governed by the Business Source License
+// included in the file licenses/BSL.txt; by using this file, you agree to be bound by the terms of the Business Source
+// License, and you may not use this file except in compliance with the Business Source License.
+//
+// As of the Change Date specified in that file, in accordance with
+// the Business Source License, use of this software will be governed
+// by the Apache License, Version 2.0, included in the file
+// licenses/APL.txt.
+
+#pragma once
+
+#include <cstdint>
+
+namespace memgraph::io::simulator {
+struct SimulatorStats {
+  uint64_t total_messages = 0;
+  uint64_t dropped_messages = 0;
+  uint64_t timed_out_requests = 0;
+  uint64_t total_requests = 0;
+  uint64_t total_responses = 0;
+  uint64_t simulator_ticks = 0;
+};
+};  // namespace memgraph::io::simulator

src/io/simulator/simulator_transport.hpp

@@ -0,0 +1,65 @@
+// Copyright 2022 Memgraph Ltd.
+//
+// Use of this software is governed by the Business Source License
+// included in the file licenses/BSL.txt; by using this file, you agree to be bound by the terms of the Business Source
+// License, and you may not use this file except in compliance with the Business Source License.
+//
+// As of the Change Date specified in that file, in accordance with
+// the Business Source License, use of this software will be governed
+// by the Apache License, Version 2.0, included in the file
+// licenses/APL.txt.
+
+#pragma once
+
+#include <memory>
+#include <utility>
+
+#include "io/address.hpp"
+#include "io/simulator/simulator_handle.hpp"
+#include "io/time.hpp"
+
+namespace memgraph::io::simulator {
+
+using memgraph::io::Duration;
+using memgraph::io::Time;
+
+class SimulatorTransport {
+  std::shared_ptr<SimulatorHandle> simulator_handle_;
+  const Address address_;
+  std::mt19937 rng_;
+
+ public:
+  SimulatorTransport(std::shared_ptr<SimulatorHandle> simulator_handle, Address address, uint64_t seed)
+      : simulator_handle_(simulator_handle), address_(address), rng_(std::mt19937{seed}) {}
+
+  template <Message Request, Message Response>
+  ResponseFuture<Response> Request(Address address, uint64_t request_id, Request request, Duration timeout) {
+    std::function<bool()> maybe_tick_simulator = [this] { return simulator_handle_->MaybeTickSimulator(); };
+    auto [future, promise] =
+        memgraph::io::FuturePromisePairWithNotifier<ResponseResult<Response>>(maybe_tick_simulator);
+
+    simulator_handle_->SubmitRequest(address, address_, request_id, std::move(request), timeout, std::move(promise));
+
+    return std::move(future);
+  }
+
+  template <Message... Ms>
+  requires(sizeof...(Ms) > 0) RequestResult<Ms...> Receive(Duration timeout) {
+    return simulator_handle_->template Receive<Ms...>(address_, timeout);
+  }
+
+  template <Message M>
+  void Send(Address address, uint64_t request_id, M message) {
+    return simulator_handle_->template Send<M>(address, address_, request_id, message);
+  }
+
+  Time Now() const { return simulator_handle_->Now(); }
+
+  bool ShouldShutDown() const { return simulator_handle_->ShouldShutDown(); }
+
+  template <class D = std::poisson_distribution<>, class Return = uint64_t>
+  Return Rand(D distrib) {
+    return distrib(rng_);
+  }
+};
+};  // namespace memgraph::io::simulator

src/io/time.hpp

@@ -0,0 +1,21 @@
+// Copyright 2022 Memgraph Ltd.
+//
+// Use of this software is governed by the Business Source License
+// included in the file licenses/BSL.txt; by using this file, you agree to be bound by the terms of the Business Source
+// License, and you may not use this file except in compliance with the Business Source License.
+//
+// As of the Change Date specified in that file, in accordance with
+// the Business Source License, use of this software will be governed
+// by the Apache License, Version 2.0, included in the file
+// licenses/APL.txt.
+
+#pragma once
+
+#include <chrono>
+
+namespace memgraph::io {
+
+using Duration = std::chrono::microseconds;
+using Time = std::chrono::time_point<std::chrono::local_t, Duration>;
+
+}  // namespace memgraph::io

src/io/transport.hpp

@@ -0,0 +1,130 @@
+// Copyright 2022 Memgraph Ltd.
+//
+// Use of this software is governed by the Business Source License
+// included in the file licenses/BSL.txt; by using this file, you agree to be bound by the terms of the Business Source
+// License, and you may not use this file except in compliance with the Business Source License.
+//
+// As of the Change Date specified in that file, in accordance with
+// the Business Source License, use of this software will be governed
+// by the Apache License, Version 2.0, included in the file
+// licenses/APL.txt.
+
+#pragma once
+
+#include <chrono>
+#include <concepts>
+#include <random>
+#include <variant>
+
+#include "io/address.hpp"
+#include "io/errors.hpp"
+#include "io/future.hpp"
+#include "io/time.hpp"
+#include "utils/result.hpp"
+
+namespace memgraph::io {
+
+using memgraph::utils::BasicResult;
+
+// TODO(tyler) ensure that Message continues to represent
+// reasonable constraints around message types over time,
+// as we adapt things to use Thrift-generated message types.
+template <typename T>
+concept Message = std::same_as<T, std::decay_t<T>>;
+
+template <Message M>
+struct ResponseEnvelope {
+  M message;
+  uint64_t request_id;
+  Address from_address;
+};
+
+template <Message M>
+using ResponseResult = BasicResult<TimedOut, ResponseEnvelope<M>>;
+
+template <Message M>
+using ResponseFuture = memgraph::io::Future<ResponseResult<M>>;
+
+template <Message M>
+using ResponsePromise = memgraph::io::Promise<ResponseResult<M>>;
+
+template <Message... Ms>
+struct RequestEnvelope {
+  std::variant<Ms...> message;
+  uint64_t request_id;
+  Address from_address;
+};
+
+template <Message... Ms>
+using RequestResult = BasicResult<TimedOut, RequestEnvelope<Ms...>>;
+
+template <typename I>
+class Io {
+  I implementation_;
+  Address address_;
+  uint64_t request_id_counter_ = 0;
+  Duration default_timeout_ = std::chrono::microseconds{50000};
+
+ public:
+  Io(I io, Address address) : implementation_(io), address_(address) {}
+
+  /// Set the default timeout for all requests that are issued
+  /// without an explicit timeout set.
+  void SetDefaultTimeout(Duration timeout) { default_timeout_ = timeout; }
+
+  /// Issue a request with an explicit timeout in microseconds provided. This tends to be used by clients.
+  template <Message Request, Message Response>
+  ResponseFuture<Response> RequestWithTimeout(Address address, Request request, Duration timeout) {
+    const uint64_t request_id = ++request_id_counter_;
+    return implementation_.template Request<Request, Response>(address, request_id, request, timeout);
+  }
+
+  /// Issue a request that times out after the default timeout. This tends
+  /// to be used by clients.
+  template <Message Request, Message Response>
+  ResponseFuture<Response> Request(Address address, Request request) {
+    const uint64_t request_id = ++request_id_counter_;
+    const Duration timeout = default_timeout_;
+    return implementation_.template Request<Request, Response>(address, request_id, std::move(request), timeout);
+  }
+
+  /// Wait for an explicit number of microseconds for a request of one of the
+  /// provided types to arrive. This tends to be used by servers.
+  template <Message... Ms>
+  RequestResult<Ms...> ReceiveWithTimeout(Duration timeout) {
+    return implementation_.template Receive<Ms...>(timeout);
+  }
+
+  /// Wait the default number of microseconds for a request of one of the
+  /// provided types to arrive. This tends to be used by servers.
+  template <Message... Ms>
+  requires(sizeof...(Ms) > 0) RequestResult<Ms...> Receive() {
+    const Duration timeout = default_timeout_;
+    return implementation_.template Receive<Ms...>(timeout);
+  }
+
+  /// Send a message in a best-effort fashion. This is used for messaging where
+  /// responses are not necessarily expected, and for servers to respond to requests.
+  /// If you need reliable delivery, this must be built on-top. TCP is not enough for most use cases.
+  template <Message M>
+  void Send(Address address, uint64_t request_id, M message) {
+    return implementation_.template Send<M>(address, request_id, std::move(message));
+  }
+
+  /// The current system time. This time source should be preferred over any other,
+  /// because it lets us deterministically control clocks from tests for making
+  /// things like timeouts deterministic.
+  Time Now() const { return implementation_.Now(); }
+
+  /// Returns true if the system should shut-down.
+  bool ShouldShutDown() const { return implementation_.ShouldShutDown(); }
+
+  /// Returns a random number within the specified distribution.
+  template <class D = std::poisson_distribution<>, class Return = uint64_t>
+  Return Rand(D distrib) {
+    return implementation_.template Rand<D, Return>(distrib);
+  }
+
+  Address GetAddress() { return address_; }
+};
+};  // namespace memgraph::io

tests/CMakeLists.txt

@@ -10,6 +10,9 @@ add_subdirectory(stress)
 # concurrent test binaries
 add_subdirectory(concurrent)
 
+# simulation test binaries
+add_subdirectory(simulation)
+
 # manual test binaries
 add_subdirectory(manual)
 

tests/benchmark/CMakeLists.txt

@@ -62,3 +62,6 @@ target_link_libraries(${test_prefix}storage_v2_gc mg-storage-v2)
 
 add_benchmark(storage_v2_property_store.cpp)
 target_link_libraries(${test_prefix}storage_v2_property_store mg-storage-v2)
+
+add_benchmark(future.cpp)
+target_link_libraries(${test_prefix}future mg-io)

tests/benchmark/future.cpp

@@ -0,0 +1,30 @@
+// Copyright 2022 Memgraph Ltd.
+//
+// Use of this software is governed by the Business Source License
+// included in the file licenses/BSL.txt; by using this file, you agree to be bound by the terms of the Business Source
+// License, and you may not use this file except in compliance with the Business Source License.
+//
+// As of the Change Date specified in that file, in accordance with
+// the Business Source License, use of this software will be governed
+// by the Apache License, Version 2.0, included in the file
+// licenses/APL.txt.
+
+#include <benchmark/benchmark.h>
+
+#include "io/future.hpp"
+
+static void FuturePairFillWait(benchmark::State &state) {
+  uint64_t counter = 0;
+  while (state.KeepRunning()) {
+    auto [future, promise] = memgraph::io::FuturePromisePair<int>();
+    promise.Fill(1);
+    std::move(future).Wait();
+
+    ++counter;
+  }
+  state.SetItemsProcessed(counter);
+}
+
+BENCHMARK(FuturePairFillWait)->Unit(benchmark::kNanosecond)->UseRealTime();
+
+BENCHMARK_MAIN();

tests/simulation/CMakeLists.txt

@@ -0,0 +1,30 @@
+set(test_prefix memgraph__simulation__)
+
+find_package(gflags)
+
+add_custom_target(memgraph__simulation)
+
+function(add_simulation_test test_cpp san)
+  # get exec name (remove extension from the abs path)
+  get_filename_component(exec_name ${test_cpp} NAME_WE)
+  set(target_name ${test_prefix}${exec_name})
+  add_executable(${target_name} ${test_cpp})
+
+  # OUTPUT_NAME sets the real name of a target when it is built and can be
+  # used to help create two targets of the same name even though CMake
+  # requires unique logical target names
+  set_target_properties(${target_name} PROPERTIES OUTPUT_NAME ${exec_name})
+  target_link_libraries(${target_name} gtest gmock mg-utils mg-io mg-io-simulator)
+
+  # sanitize
+  target_compile_options(${target_name} PRIVATE -fsanitize=${san})
+  target_link_options(${target_name} PRIVATE -fsanitize=${san})
+
+  # register test
+  add_test(${target_name} ${exec_name})
+  add_dependencies(memgraph__simulation ${target_name})
+endfunction(add_simulation_test)
+
+add_simulation_test(basic_request.cpp address)
+
+add_simulation_test(trial_query_storage/query_storage_test.cpp address)

tests/simulation/basic_request.cpp

@@ -0,0 +1,87 @@
+// Copyright 2022 Memgraph Ltd.
+//
+// Use of this software is governed by the Business Source License
+// included in the file licenses/BSL.txt; by using this file, you agree to be bound by the terms of the Business Source
+// License, and you may not use this file except in compliance with the Business Source License.
+//
+// As of the Change Date specified in that file, in accordance with
+// the Business Source License, use of this software will be governed
+// by the Apache License, Version 2.0, included in the file
+// licenses/APL.txt.
+
+#include <thread>
+
+#include "io/simulator/simulator.hpp"
+
+using memgraph::io::Address;
+using memgraph::io::Io;
+using memgraph::io::ResponseFuture;
+using memgraph::io::ResponseResult;
+using memgraph::io::simulator::Simulator;
+using memgraph::io::simulator::SimulatorConfig;
+using memgraph::io::simulator::SimulatorTransport;
+
+struct CounterRequest {
+  uint64_t proposal;
+};
+
+struct CounterResponse {
+  uint64_t highest_seen;
+};
+
+void run_server(Io<SimulatorTransport> io) {
+  uint64_t highest_seen = 0;
+
+  while (!io.ShouldShutDown()) {
+    std::cout << "[SERVER] Is receiving..." << std::endl;
+    auto request_result = io.Receive<CounterRequest>();
+    if (request_result.HasError()) {
+      std::cout << "[SERVER] Error, continue" << std::endl;
+      continue;
+    }
+    auto request_envelope = request_result.GetValue();
+    auto req = std::get<CounterRequest>(request_envelope.message);
+
+    highest_seen = std::max(highest_seen, req.proposal);
+    auto srv_res = CounterResponse{highest_seen};
+
+    io.Send(request_envelope.from_address, request_envelope.request_id, srv_res);
+  }
+}
+
+int main() {
+  auto config = SimulatorConfig{
+      .drop_percent = 0,
+      .perform_timeouts = true,
+      .scramble_messages = true,
+      .rng_seed = 0,
+  };
+  auto simulator = Simulator(config);
+
+  auto cli_addr = Address::TestAddress(1);
+  auto srv_addr = Address::TestAddress(2);
+
+  Io<SimulatorTransport> cli_io = simulator.Register(cli_addr);
+  Io<SimulatorTransport> srv_io = simulator.Register(srv_addr);
+
+  auto srv_thread = std::jthread(run_server, std::move(srv_io));
+  simulator.IncrementServerCountAndWaitForQuiescentState(srv_addr);
+
+  for (int i = 1; i < 3; ++i) {
+    // send request
+    CounterRequest cli_req;
+    cli_req.proposal = i;
+    auto res_f = cli_io.Request<CounterRequest, CounterResponse>(srv_addr, cli_req);
+    auto res_rez = std::move(res_f).Wait();
+    if (!res_rez.HasError()) {
+      std::cout << "[CLIENT] Got a valid response" << std::endl;
+      auto env = res_rez.GetValue();
+      MG_ASSERT(env.message.highest_seen == i);
+    } else {
+      std::cout << "[CLIENT] Got an error" << std::endl;
+    }
+  }
+
+  simulator.ShutDown();
+  return 0;
+}

tests/simulation/trial_query_storage/messages.hpp

@@ -0,0 +1,34 @@
+// Copyright 2022 Memgraph Ltd.
+//
+// Use of this software is governed by the Business Source License
+// included in the file licenses/BSL.txt; by using this file, you agree to be bound by the terms of the Business Source
+// License, and you may not use this file except in compliance with the Business Source License.
+//
+// As of the Change Date specified in that file, in accordance with
+// the Business Source License, use of this software will be governed
+// by the Apache License, Version 2.0, included in the file
+// licenses/APL.txt.
+
+#pragma once
+
+#include <optional>
+#include <string>
+#include <vector>
+
+namespace memgraph::tests::simulation {
+
+struct Vertex {
+  std::string key;
+};
+
+struct ScanVerticesRequest {
+  int64_t count;
+  std::optional<int64_t> continuation;
+};
+
+struct VerticesResponse {
+  std::vector<Vertex> vertices;
+  std::optional<int64_t> continuation;
+};
+
+}  // namespace memgraph::tests::simulation

tests/simulation/trial_query_storage/query_storage_test.cpp

@@ -0,0 +1,85 @@
+// Copyright 2022 Memgraph Ltd.
+//
+// Use of this software is governed by the Business Source License
+// included in the file licenses/BSL.txt; by using this file, you agree to be bound by the terms of the Business Source
+// License, and you may not use this file except in compliance with the Business Source License.
+//
+// As of the Change Date specified in that file, in accordance with
+// the Business Source License, use of this software will be governed
+// by the Apache License, Version 2.0, included in the file
+// licenses/APL.txt.
+
+#include <iostream>
+
+#include "io/address.hpp"
+#include "io/simulator/simulator.hpp"
+#include "io/simulator/simulator_config.hpp"
+#include "io/simulator/simulator_transport.hpp"
+#include "io/transport.hpp"
+
+#include "messages.hpp"
+
+namespace memgraph::tests::simulation {
+using memgraph::io::Io;
+using memgraph::io::simulator::SimulatorTransport;
+
+void run_server(Io<SimulatorTransport> io) {
+  while (!io.ShouldShutDown()) {
+    std::cout << "[STORAGE] Is receiving..." << std::endl;
+    auto request_result = io.Receive<ScanVerticesRequest>();
+    if (request_result.HasError()) {
+      std::cout << "[STORAGE] Error, continue" << std::endl;
+      continue;
+    }
+    auto request_envelope = request_result.GetValue();
+    auto req = std::get<ScanVerticesRequest>(request_envelope.message);
+
+    VerticesResponse response{};
+    const int64_t start_index = std::invoke([&req] {
+      if (req.continuation.has_value()) {
+        return *req.continuation;
+      }
+      return 0L;
+    });
+    for (auto index = start_index; index < start_index + req.count; ++index) {
+      response.vertices.push_back({std::string("Vertex_") + std::to_string(index)});
+    }
+    io.Send(request_envelope.from_address, request_envelope.request_id, response);
+  }
+}
+
+}  // namespace memgraph::tests::simulation
+
+int main() {
+  using memgraph::io::Address;
+  using memgraph::io::Io;
+  using memgraph::io::simulator::Simulator;
+  using memgraph::io::simulator::SimulatorConfig;
+  using memgraph::io::simulator::SimulatorTransport;
+  using memgraph::tests::simulation::run_server;
+  using memgraph::tests::simulation::ScanVerticesRequest;
+  using memgraph::tests::simulation::VerticesResponse;
+  auto config = SimulatorConfig{
+      .drop_percent = 0,
+      .perform_timeouts = true,
+      .scramble_messages = true,
+      .rng_seed = 0,
+  };
+  auto simulator = Simulator(config);
+
+  auto cli_addr = Address::TestAddress(1);
+  auto srv_addr = Address::TestAddress(2);
+
+  Io<SimulatorTransport> cli_io = simulator.Register(cli_addr);
+  Io<SimulatorTransport> srv_io = simulator.Register(srv_addr);
+
+  auto srv_thread = std::jthread(run_server, std::move(srv_io));
+  simulator.IncrementServerCountAndWaitForQuiescentState(srv_addr);
+
+  auto req = ScanVerticesRequest{2, std::nullopt};
+
+  auto res_f = cli_io.Request<ScanVerticesRequest, VerticesResponse>(srv_addr, req);
+  auto res_rez = std::move(res_f).Wait();
+  simulator.ShutDown();
+  return 0;
+}

tests/unit/CMakeLists.txt

@@ -396,3 +396,7 @@ find_package(Boost REQUIRED)
 
 add_unit_test(websocket.cpp)
 target_link_libraries(${test_prefix}websocket mg-communication Boost::headers)
+
+# Test future
+add_unit_test(future.cpp)
+target_link_libraries(${test_prefix}future mg-io)

tests/unit/future.cpp

@@ -0,0 +1,55 @@
+// Copyright 2022 Memgraph Ltd.
+//
+// Use of this software is governed by the Business Source License
+// included in the file licenses/BSL.txt; by using this file, you agree to be bound by the terms of the Business Source
+// License, and you may not use this file except in compliance with the Business Source License.
+//
+// As of the Change Date specified in that file, in accordance with
+// the Business Source License, use of this software will be governed
+// by the Apache License, Version 2.0, included in the file
+// licenses/APL.txt.
+
+#include <string>
+#include <thread>
+
+#include "gtest/gtest.h"
+
+#include "io/future.hpp"
+
+using namespace memgraph::io;
+
+void Fill(Promise<std::string> promise_1) { promise_1.Fill("success"); }
+
+void Wait(Future<std::string> future_1, Promise<std::string> promise_2) {
+  std::string result_1 = std::move(future_1).Wait();
+  EXPECT_TRUE(result_1 == "success");
+  promise_2.Fill("it worked");
+}
+
+TEST(Future, BasicLifecycle) {
+  std::atomic_bool waiting = false;
+
+  std::function<bool()> notifier = [&] {
+    waiting.store(true, std::memory_order_seq_cst);
+    return false;
+  };
+
+  auto [future_1, promise_1] = FuturePromisePairWithNotifier<std::string>(notifier);
+  auto [future_2, promise_2] = FuturePromisePair<std::string>();
+
+  std::jthread t1(Wait, std::move(future_1), std::move(promise_2));
+
+  // spin in a loop until the promise signals
+  // that it is waiting
+  while (!waiting.load(std::memory_order_acquire)) {
+    std::this_thread::yield();
+  }
+
+  std::jthread t2(Fill, std::move(promise_1));
+
+  t1.join();
+  t2.join();
+
+  std::string result_2 = std::move(future_2).Wait();
+  EXPECT_TRUE(result_2 == "it worked");
+}