diff --git a/td/telegram/MessagesManager.cpp b/td/telegram/MessagesManager.cpp
index 5f5dc75a3..35b7d7afd 100644
--- a/td/telegram/MessagesManager.cpp
+++ b/td/telegram/MessagesManager.cpp
@@ -5998,7 +5998,7 @@ MessagesManager::MessagesManager(Td *td, ActorShared<> parent)
   preload_folder_dialog_list_timeout_.set_callback(on_preload_folder_dialog_list_timeout_callback);
   preload_folder_dialog_list_timeout_.set_callback_data(static_cast<void *>(this));
 
-  sequence_dispatcher_ = create_actor<MultiSequenceDispatcher>("multi sequence dispatcher");
+  sequence_dispatcher_ = MultiSequenceDispatcher::create("multi sequence dispatcher");
 }
 
 MessagesManager::~MessagesManager() = default;
diff --git a/td/telegram/MessagesManager.h b/td/telegram/MessagesManager.h
index 8a38a66f7..75077eeb1 100644
--- a/td/telegram/MessagesManager.h
+++ b/td/telegram/MessagesManager.h
@@ -53,6 +53,7 @@
 #include "td/telegram/SecretChatId.h"
 #include "td/telegram/SecretInputMedia.h"
 #include "td/telegram/ServerMessageId.h"
+#include "td/telegram/SequenceDispatcher.h"
 #include "td/telegram/td_api.h"
 #include "td/telegram/telegram_api.h"
 #include "td/telegram/UserId.h"
@@ -92,7 +93,6 @@ class DraftMessage;
 struct InputMessageContent;
 class MessageContent;
 struct MessageReactions;
-class MultiSequenceDispatcher;
 class Td;
 
 class MessagesManager final : public Actor {
diff --git a/td/telegram/SequenceDispatcher.cpp b/td/telegram/SequenceDispatcher.cpp
index 6e1cf04ca..4a129cfd1 100644
--- a/td/telegram/SequenceDispatcher.cpp
+++ b/td/telegram/SequenceDispatcher.cpp
@@ -11,6 +11,7 @@
 
 #include "td/actor/PromiseFuture.h"
 
+#include "td/utils/ChainScheduler.h"
 #include "td/utils/format.h"
 #include "td/utils/logging.h"
 #include "td/utils/misc.h"
@@ -241,7 +242,7 @@ void SequenceDispatcher::close_silent() {
 }
 
 /*** MultiSequenceDispatcher ***/
-void MultiSequenceDispatcher::send_with_callback(NetQueryPtr query, ActorShared<NetQueryCallback> callback,
+void MultiSequenceDispatcherOld::send_with_callback(NetQueryPtr query, ActorShared<NetQueryCallback> callback,
                                                  uint64 sequence_id) {
   CHECK(sequence_id != 0);
   auto it_ok = dispatchers_.emplace(sequence_id, Data{0, ActorOwn<SequenceDispatcher>()});
@@ -255,13 +256,13 @@ void MultiSequenceDispatcher::send_with_callback(NetQueryPtr query, ActorShared<
   send_closure(data.dispatcher_, &SequenceDispatcher::send_with_callback, std::move(query), std::move(callback));
 }
 
-void MultiSequenceDispatcher::on_result() {
+void MultiSequenceDispatcherOld::on_result() {
   auto it = dispatchers_.find(get_link_token());
   CHECK(it != dispatchers_.end());
   it->second.cnt_--;
 }
 
-void MultiSequenceDispatcher::ready_to_close() {
+void MultiSequenceDispatcherOld::ready_to_close() {
   auto it = dispatchers_.find(get_link_token());
   CHECK(it != dispatchers_.end());
   if (it->second.cnt_ == 0) {
@@ -270,4 +271,104 @@ void MultiSequenceDispatcher::ready_to_close() {
   }
 }
 
+class MultiSequenceDispatcherNewImpl final : public MultiSequenceDispatcherNew {
+ public:
+  void send_with_callback(NetQueryPtr query, ActorShared<NetQueryCallback> callback, uint64 sequence_id) final {
+    LOG(ERROR) << "send " << query;
+    Node node;
+    node.net_query = std::move(query);
+    node.net_query->debug("Waiting at SequenceDispatcher");
+    node.net_query_ref = node.net_query.get_weak();
+    node.callback = std::move(callback);
+    scheduler_.create_task({ChainId{sequence_id}}, std::move(node));
+    loop();
+  }
+
+ private:
+  struct Node {
+    NetQueryRef net_query_ref;
+    NetQueryPtr net_query;
+    ActorShared<NetQueryCallback> callback;
+    friend StringBuilder &operator << (StringBuilder &sb, const Node &node) {
+      return sb << node.net_query;
+    }
+  };
+  ChainScheduler<Node> scheduler_;
+  using TaskId = ChainScheduler<NetQueryPtr>::TaskId;
+  using ChainId = ChainScheduler<NetQueryPtr>::ChainId;
+
+  void on_result(NetQueryPtr query) override {
+    auto task_id = TaskId(get_link_token());
+    auto &node = *scheduler_.get_task_extra(task_id);
+
+    if (query->is_error() && (query->error().code() == NetQuery::ResendInvokeAfter ||
+                              (query->error().code() == 400 && (query->error().message() == "MSG_WAIT_FAILED" ||
+                                                                query->error().message() == "MSG_WAIT_TIMEOUT")))) {
+      VLOG(net_query) << "Resend " << query;
+      query->resend();
+      return on_resend(std::move(query));
+    }
+    auto promise = promise_send_closure(actor_shared(this, task_id), &MultiSequenceDispatcherNewImpl::on_resend);
+    send_closure(node.callback, &NetQueryCallback::on_result_resendable, std::move(query), std::move(promise));
+  }
+
+  // TODO: without td::Result?
+  void on_resend(td::Result<NetQueryPtr> query) {
+    auto task_id = TaskId(get_link_token());
+    auto &node = *scheduler_.get_task_extra(task_id);
+    if (query.is_error()) {
+      scheduler_.finish_task(task_id);
+    } else {
+      node.net_query = query.move_as_ok();
+      node.net_query->debug("Waiting at SequenceDispatcher");
+      node.net_query_ref = node.net_query.get_weak();
+      scheduler_.reset_task(task_id);
+    }
+    loop();
+  }
+
+  void loop() override {
+    flush_pending_queries();
+  }
+
+  void flush_pending_queries() {
+    while (true) {
+      auto o_task = scheduler_.start_next_task();
+      if (!o_task) {
+        LOG(ERROR) << " no more tasks " << scheduler_;
+        break;
+      }
+      auto task = o_task.unwrap();
+      LOG(ERROR) << " next task = " << task.task_id;
+      auto &node = *scheduler_.get_task_extra(task.task_id);
+      CHECK(!node.net_query.empty());
+
+      auto query = std::move(node.net_query);
+      std::vector<NetQueryRef> parents;
+      for (auto parent_id : task.parents) {
+        auto &parent_node = *scheduler_.get_task_extra(parent_id);
+        parents.push_back(parent_node.net_query_ref);
+      }
+
+      if (parents.empty()) {
+        query->set_invoke_after({});
+      } else if (parents.size() == 1) {
+        query->set_invoke_after(parents[0]);
+      } else if (parents.size() > 1){
+        LOG(FATAL) << "TODO: support invokeAfterMsgs";
+      }
+      query->last_timeout_ = 0; // TODO: flood
+      VLOG(net_query) << "Send " << query;
+      query->debug("send to Td::send_with_callback");
+      query->set_session_rand(123); // TODO: chain_rand
+      G()->net_query_dispatcher().dispatch_with_callback(std::move(query),
+                                                         actor_shared(this, task.task_id));
+    }
+  }
+};
+
+ActorOwn<MultiSequenceDispatcherNew> MultiSequenceDispatcherNew::create(Slice name) {
+  return ActorOwn<MultiSequenceDispatcherNew>(create_actor<MultiSequenceDispatcherNewImpl>(name));
+}
+
 }  // namespace td
diff --git a/td/telegram/SequenceDispatcher.h b/td/telegram/SequenceDispatcher.h
index 1018f3397..4b8e9d996 100644
--- a/td/telegram/SequenceDispatcher.h
+++ b/td/telegram/SequenceDispatcher.h
@@ -73,9 +73,12 @@ class SequenceDispatcher final : public NetQueryCallback {
   void tear_down() final;
 };
 
-class MultiSequenceDispatcher final : public SequenceDispatcher::Parent {
+class MultiSequenceDispatcherOld final : public SequenceDispatcher::Parent {
  public:
   void send_with_callback(NetQueryPtr query, ActorShared<NetQueryCallback> callback, uint64 sequence_id);
+  static ActorOwn<MultiSequenceDispatcherOld> create(td::Slice name) {
+    return create_actor<MultiSequenceDispatcherOld>(name);
+  }
 
  private:
   struct Data {
@@ -87,4 +90,12 @@ class MultiSequenceDispatcher final : public SequenceDispatcher::Parent {
   void ready_to_close() final;
 };
 
+class MultiSequenceDispatcherNew : public NetQueryCallback {
+ public:
+  virtual void send_with_callback(NetQueryPtr query, ActorShared<NetQueryCallback> callback, uint64 sequence_id) = 0;
+  static ActorOwn<MultiSequenceDispatcherNew> create(Slice name);
+};
+
+using MultiSequenceDispatcher = MultiSequenceDispatcherOld;
+
 }  // namespace td
diff --git a/td/telegram/net/NetQuery.h b/td/telegram/net/NetQuery.h
index ac34b1857..90c66190a 100644
--- a/td/telegram/net/NetQuery.h
+++ b/td/telegram/net/NetQuery.h
@@ -383,6 +383,9 @@ inline StringBuilder &operator<<(StringBuilder &stream, const NetQuery &net_quer
 }
 
 inline StringBuilder &operator<<(StringBuilder &stream, const NetQueryPtr &net_query_ptr) {
+  if (net_query_ptr.empty()) {
+    return stream << "[Query: null]";
+  }
   return stream << *net_query_ptr;
 }
 
diff --git a/tdutils/CMakeLists.txt b/tdutils/CMakeLists.txt
index 039d5802d..feabbcf23 100644
--- a/tdutils/CMakeLists.txt
+++ b/tdutils/CMakeLists.txt
@@ -289,6 +289,7 @@ endif()
 set(TDUTILS_TEST_SOURCE
   ${CMAKE_CURRENT_SOURCE_DIR}/test/bitmask.cpp
   ${CMAKE_CURRENT_SOURCE_DIR}/test/buffer.cpp
+  ${CMAKE_CURRENT_SOURCE_DIR}/test/ChainScheduler.cpp
   ${CMAKE_CURRENT_SOURCE_DIR}/test/ConcurrentHashMap.cpp
   ${CMAKE_CURRENT_SOURCE_DIR}/test/crypto.cpp
   ${CMAKE_CURRENT_SOURCE_DIR}/test/Enumerator.cpp
diff --git a/tdutils/td/utils/ChainScheduler.h b/tdutils/td/utils/ChainScheduler.h
new file mode 100644
index 000000000..44dd2bb65
--- /dev/null
+++ b/tdutils/td/utils/ChainScheduler.h
@@ -0,0 +1,291 @@
+#pragma once
+
+#include "td/utils/algorithm.h"
+#include "td/utils/Container.h"
+#include "td/utils/List.h"
+#include "td/utils/optional.h"
+#include "td/utils/Span.h"
+#include "td/utils/tests.h"
+#include "td/utils/VectorQueue.h"
+#include "td/utils/Random.h"
+
+#include <map>
+#include <vector>
+#include <set>
+#include <numeric>
+
+namespace td {
+
+template <class ExtraT = Unit>
+class ChainScheduler {
+ public:
+  using TaskId = uint64;
+  using ChainId = uint64;
+  TaskId create_task(td::Span<ChainId> chains, ExtraT extra = {});
+  ExtraT *get_task_extra(TaskId task_id);
+
+  struct TaskWithParents {
+    TaskId task_id{};
+    std::vector<TaskId> parents;
+  };
+
+  optional<TaskWithParents> start_next_task();
+  void finish_task(TaskId task_id);
+  void reset_task(TaskId task_id);
+  template <class ExtraTT>
+  friend td::StringBuilder &operator<<(StringBuilder &sb, ChainScheduler<ExtraTT> &scheduler);
+
+ private:
+  struct ChainNode : ListNode {
+    TaskId task_id{};
+  };
+
+  class Chain {
+   public:
+    void add_task(ChainNode *node) {
+      head_.put_back(node);
+    }
+
+    optional<TaskId> get_first() {
+      if (head_.empty()) {
+        return {};
+      }
+      return static_cast<ChainNode &>(*head_.get_next()).task_id;
+    }
+
+    optional<TaskId> get_child(ChainNode *chain_node) {
+      if (chain_node->get_next() == head_.end()) {
+        return {};
+      }
+      return static_cast<ChainNode &>(*chain_node->get_next()).task_id;
+    }
+    optional<TaskId> get_parent(ChainNode *chain_node) {
+      if (chain_node->get_prev() == head_.end()) {
+        return {};
+      }
+      return static_cast<ChainNode &>(*chain_node->get_prev()).task_id;
+    }
+
+    void finish_task(ChainNode *node) {
+      node->remove();
+    }
+
+    bool empty() const {
+      return head_.empty();
+    }
+
+    void foreach(std::function<void(TaskId)> f) const {
+      for (auto it = head_.begin(); it != head_.end(); it = it->get_next()) {
+        f(static_cast<const ChainNode &>(*it).task_id);
+      }
+    }
+
+   private:
+    ListNode head_;
+  };
+  struct ChainInfo {
+    Chain chain;
+    uint32 active_tasks{};
+  };
+  struct TaskChainInfo {
+    ChainNode chain_node;
+    ChainId chain_id{};
+    ChainInfo *chain_info{};
+    bool waiting_for_parent{};
+  };
+  struct Task {
+    enum class State { Pending, Active } state{State::Pending};
+    std::vector<TaskChainInfo> chains;
+    ExtraT extra;
+  };
+  std::map<ChainId, ChainInfo> chains_;
+  std::map<ChainId, TaskId> limited_tasks_;
+  Container<Task> tasks_;
+  VectorQueue<TaskId> pending_tasks_;
+
+  void on_parent_is_ready(TaskId task_id, ChainId chain_id) {
+    auto *task = tasks_.get(task_id);
+    CHECK(task);
+    for (TaskChainInfo &task_chain_info : task->chains) {
+      if (task_chain_info.chain_id == chain_id) {
+        task_chain_info.waiting_for_parent = false;
+      }
+    }
+
+    try_start_task(task_id, task);
+  }
+
+  void try_start_task(TaskId task_id, Task *task) {
+    if (task->state != Task::State::Pending) {
+      return;
+    }
+    for (TaskChainInfo &task_chain_info : task->chains) {
+      if (task_chain_info.waiting_for_parent) {
+        return;
+      }
+      ChainInfo &chain_info = chains_[task_chain_info.chain_id];
+      if (chain_info.active_tasks >= 10) {
+        limited_tasks_[task_chain_info.chain_id] = task_id;
+        return;
+      }
+    }
+
+    do_start_task(task_id, task);
+  }
+
+  void do_start_task(TaskId task_id, Task *task) {
+    for (TaskChainInfo &task_chain_info : task->chains) {
+      ChainInfo &chain_info = chains_[task_chain_info.chain_id];
+      chain_info.active_tasks++;
+    }
+    task->state = Task::State::Active;
+
+    pending_tasks_.push(task_id);
+    notify_children(task);
+  }
+
+  void notify_children(Task *task) {
+    for (TaskChainInfo &task_chain_info : task->chains) {
+      ChainInfo &chain_info = chains_[task_chain_info.chain_id];
+      auto o_child = chain_info.chain.get_child(&task_chain_info.chain_node);
+      if (o_child) {
+        on_parent_is_ready(o_child.value(), task_chain_info.chain_id);
+      }
+    }
+  }
+
+  void inactivate_task(TaskId task_id, Task *task) {
+    CHECK(task->state == Task::State::Active);
+    task->state = Task::State::Pending;
+    for (TaskChainInfo &task_chain_info : task->chains) {
+      ChainInfo &chain_info = chains_[task_chain_info.chain_id];
+      chain_info.active_tasks--;
+
+      auto it = limited_tasks_.find(task_chain_info.chain_id);
+      if (it != limited_tasks_.end()) {
+        auto limited_task_id = it->second;
+        limited_tasks_.erase(it);
+        if (limited_task_id != task_id) {
+          try_start_task(limited_task_id, tasks_.get(limited_task_id));
+        }
+      }
+      auto o_first = chain_info.chain.get_first();
+      if (o_first) {
+        auto first_task_id = o_first.unwrap();
+        if (first_task_id != task_id) {
+          try_start_task(first_task_id, tasks_.get(first_task_id));
+        }
+      }
+    }
+  }
+
+  void finish_chain_task(TaskChainInfo &task_chain_info) {
+    auto &chain = task_chain_info.chain_info->chain;
+    chain.finish_task(&task_chain_info.chain_node);
+    if (chain.empty()) {
+      chains_.erase(task_chain_info.chain_id);
+    }
+  }
+};
+template <class ExtraT>
+typename ChainScheduler<ExtraT>::TaskId ChainScheduler<ExtraT>::create_task(Span<ChainScheduler::ChainId> chains, ExtraT extra) {
+  auto task_id = tasks_.create();
+  Task &task = *tasks_.get(task_id);
+  task.extra = std::move(extra);
+  task.chains = transform(chains, [&](auto chain_id) {
+    TaskChainInfo task_chain_info;
+    ChainInfo &chain_info = chains_[chain_id];
+    task_chain_info.chain_id = chain_id;
+    task_chain_info.chain_info = &chain_info;
+    task_chain_info.chain_node.task_id = task_id;
+    return task_chain_info;
+  });
+
+  for (TaskChainInfo &task_chain_info : task.chains) {
+    auto &chain = task_chain_info.chain_info->chain;
+    chain.add_task(&task_chain_info.chain_node);
+    task_chain_info.waiting_for_parent = bool(chain.get_parent(&task_chain_info.chain_node));
+  }
+
+  try_start_task(task_id, &task);
+  return task_id;
+}
+template <class ExtraT>
+ExtraT *ChainScheduler<ExtraT>::get_task_extra(ChainScheduler::TaskId task_id) {  // may return nullptr
+  auto *task = tasks_.get(task_id);
+  if (!task) {
+    return nullptr;
+  }
+  return &task->extra;
+}
+template <class ExtraT>
+optional<typename ChainScheduler<ExtraT>::TaskWithParents> ChainScheduler<ExtraT>::start_next_task() {
+  if (pending_tasks_.empty()) {
+    return {};
+  }
+  auto task_id = pending_tasks_.pop();
+  TaskWithParents res;
+  res.task_id = task_id;
+  auto *task = tasks_.get(task_id);
+  CHECK(task);
+  for (TaskChainInfo &task_chain_info : task->chains) {
+    Chain &chain = task_chain_info.chain_info->chain;
+    auto o_parent = chain.get_parent(&task_chain_info.chain_node);
+    if (o_parent) {
+      res.parents.push_back(o_parent.value());
+    }
+  }
+  return res;
+}
+template <class ExtraT>
+void ChainScheduler<ExtraT>::finish_task(ChainScheduler::TaskId task_id) {
+  auto *task = tasks_.get(task_id);
+  CHECK(task);
+
+  inactivate_task(task_id, task);
+  notify_children(task);
+
+  for (TaskChainInfo &task_chain_info : task->chains) {
+    finish_chain_task(task_chain_info);
+  }
+  tasks_.erase(task_id);
+}
+template <class ExtraT>
+void ChainScheduler<ExtraT>::reset_task(ChainScheduler::TaskId task_id) {
+  auto *task = tasks_.get(task_id);
+  CHECK(task);
+  inactivate_task(task_id, task);
+
+  for (TaskChainInfo &task_chain_info : task->chains) {
+    ChainInfo &chain_info = chains_[task_chain_info.chain_id];
+    task_chain_info.waiting_for_parent = bool(chain_info.chain.get_parent(&task_chain_info.chain_node));
+  }
+
+  try_start_task(task_id, task);
+}
+template <class ExtraT>
+td::StringBuilder &operator<<(StringBuilder &sb, ChainScheduler<ExtraT> &scheduler) {
+  // 1 print chains
+  sb << "\n";
+  for (auto &it : scheduler.chains_) {
+    sb << "ChainId{" << it.first << "} ";
+    sb << " active_cnt=" << it.second.active_tasks;
+    sb << " : ";
+    it.second.chain.foreach([&](auto task_id) {
+      sb << *scheduler.get_task_extra(task_id);
+    });
+    sb << "\n";
+  }
+  scheduler.tasks_.for_each([&](auto id, auto &task) {
+    sb << "Task: " << task.extra;
+    sb << " state =" << static_cast<int>(task.state);
+    for (auto& task_chain_info : task.chains) {
+      if (task_chain_info.waiting_for_parent) {
+        sb << " wait " << *scheduler.get_task_extra(task_chain_info.chain_info->chain.get_parent(&task_chain_info.chain_node).value());
+      }
+    }
+    sb << "\n";
+  });
+  return sb;
+}
+}  // namespace td
diff --git a/tdutils/td/utils/List.h b/tdutils/td/utils/List.h
index 345353538..4f9bb9877 100644
--- a/tdutils/td/utils/List.h
+++ b/tdutils/td/utils/List.h
@@ -89,12 +89,24 @@ struct ListNode {
   ListNode *end() {
     return this;
   }
+  const ListNode *begin() const {
+    return next;
+  }
+  const ListNode *end() const {
+    return this;
+  }
   ListNode *get_next() {
     return next;
   }
   ListNode *get_prev() {
     return prev;
   }
+  const ListNode *get_next() const {
+    return next;
+  }
+  const ListNode *get_prev() const {
+    return prev;
+  }
 
  protected:
   void clear() {
diff --git a/tdutils/td/utils/algorithm.h b/tdutils/td/utils/algorithm.h
index f1764e313..a2b1515ea 100644
--- a/tdutils/td/utils/algorithm.h
+++ b/tdutils/td/utils/algorithm.h
@@ -117,6 +117,15 @@ bool contains(const V &v, const T &value) {
   }
   return false;
 }
+template <class V, class F>
+bool all_of(const V &v, F &&f) {
+  for (auto &x : v) {
+    if (!f(x)) {
+      return false;
+    }
+  }
+  return true;
+}
 
 template <class T>
 void reset_to_empty(T &value) {
diff --git a/tdutils/test/ChainScheduler.cpp b/tdutils/test/ChainScheduler.cpp
new file mode 100644
index 000000000..bdd5fc75c
--- /dev/null
+++ b/tdutils/test/ChainScheduler.cpp
@@ -0,0 +1,164 @@
+#include "td/utils/algorithm.h"
+#include "td/utils/optional.h"
+#include "td/utils/Span.h"
+#include "td/utils/tests.h"
+#include "td/utils/Random.h"
+
+#include "td/utils/ChainScheduler.h"
+
+#include <vector>
+#include <numeric>
+
+TEST(ChainScheduler, Basic) {
+  td::ChainScheduler<int> scheduler;
+  using ChainId = td::ChainScheduler<int>::ChainId;
+  using TaskId = td::ChainScheduler<int>::TaskId;
+  for (int i = 0; i < 100; i++) {
+    scheduler.create_task({ChainId{1}}, i);
+  }
+  int j = 0;
+  while (j != 100) {
+    std::vector<TaskId> tasks;
+    while (true) {
+      auto o_task_id = scheduler.start_next_task();
+      if (!o_task_id) {
+        break;
+      }
+      auto task_id = o_task_id.value().task_id;
+      auto extra = *scheduler.get_task_extra(task_id);
+      auto parents = td::transform(o_task_id.value().parents,
+                                   [&](auto parent) { return *scheduler.get_task_extra(parent); });
+      LOG(ERROR) << "start " << extra << parents;
+      CHECK(extra == j);
+      j++;
+      tasks.push_back(task_id);
+    }
+    for (auto &task_id : tasks) {
+      auto extra = *scheduler.get_task_extra(task_id);
+      LOG(ERROR) << "finish " << extra;
+      scheduler.finish_task(task_id);
+    }
+  }
+}
+
+struct Query;
+using QueryPtr = std::shared_ptr<Query>;
+using ChainId = td::ChainScheduler<QueryPtr>::ChainId;
+using TaskId = td::ChainScheduler<QueryPtr>::TaskId;
+struct Query {
+  int id{};
+  TaskId task_id{};
+  bool is_ok{};
+  friend td::StringBuilder &operator << (td::StringBuilder &sb, const Query &q) {
+    return sb << "Q{" << q.id << "}";
+  }
+};
+TEST(ChainScheduler, Stress) {
+  td::Random::Xorshift128plus rnd(123);
+  int max_query_id = 1000;
+  int MAX_INFLIGHT_QUERIES = 20;
+  int ChainsN = 4;
+
+  struct QueryWithParents {
+    QueryPtr id;
+    std::vector<QueryPtr> parents;
+  };
+  std::vector<QueryWithParents> active_queries;
+
+  td::ChainScheduler<QueryPtr> scheduler;
+  std::vector<std::vector<QueryPtr>> chains(ChainsN);
+  int inflight_queries{};
+  int current_query_id{};
+  bool done = false;
+
+  auto schedule_new_query = [&] {
+    if (current_query_id > max_query_id) {
+      if (inflight_queries == 0) {
+        done = true;
+      }
+      return;
+    }
+    if (inflight_queries >= MAX_INFLIGHT_QUERIES) {
+      return;
+    }
+    auto query_id = current_query_id++;
+    auto query = std::make_shared<Query>();
+    query->id = query_id;
+    int chain_n = rnd.fast(1, ChainsN);
+    std::vector<ChainId> chain_ids(ChainsN);
+    std::iota(chain_ids.begin(), chain_ids.end(), 0);
+    td::random_shuffle(td::as_mutable_span(chain_ids), rnd);
+    chain_ids.resize(chain_n);
+    for (auto chain_id : chain_ids) {
+      chains[chain_id].push_back(query);
+    }
+    auto task_id = scheduler.create_task(chain_ids, query);
+    query->task_id = task_id;
+    inflight_queries++;
+  };
+
+  auto check_parents_ok = [&] (const QueryWithParents &query_with_parents) -> bool {
+    return td::all_of(query_with_parents.parents, [](auto &parent) { return parent->is_ok; });
+  };
+
+  auto to_query_ptr = [&](TaskId task_id) {
+    return *scheduler.get_task_extra(task_id);
+  };
+  auto flush_pending_queries = [&]{
+    while (true) {
+      auto o_task_with_parents = scheduler.start_next_task();
+      if (!o_task_with_parents) {
+        break;
+      }
+      auto task_with_parents = o_task_with_parents.unwrap();
+      QueryWithParents query_with_parents;
+      query_with_parents.id = to_query_ptr(task_with_parents.task_id);
+      query_with_parents.parents = td::transform(task_with_parents.parents, to_query_ptr);
+      active_queries.push_back(query_with_parents);
+    }
+  };
+  auto execute_one_query = [&]() {
+    if (active_queries.empty()) {
+      return;
+    }
+    auto it = active_queries.begin() + rnd.fast(0, (int)active_queries.size() - 1);
+    auto query_with_parents = *it;
+    active_queries.erase(it);
+
+    auto query = query_with_parents.id;
+    if (rnd.fast(0, 20) == 0) {
+      scheduler.finish_task(query->task_id);
+      inflight_queries--;
+      LOG(ERROR) << "Fail " << query->id;
+    } else if (check_parents_ok(query_with_parents)) {
+      query->is_ok = true;
+      scheduler.finish_task(query->task_id);
+      inflight_queries--;
+      LOG(ERROR) << "OK " << query->id;
+    } else {
+      scheduler.reset_task(query->task_id);
+    }
+  };
+
+  td::RandomSteps steps({{schedule_new_query, 100}, {execute_one_query, 100}});
+  while (!done) {
+    steps.step(rnd);
+    flush_pending_queries();
+    // LOG(ERROR) << scheduler;
+  }
+  for (auto &chain : chains) {
+    int prev_ok = -1;
+    int failed_cnt = 0;
+    int ok_cnt = 0;
+    for (auto &q : chain) {
+      if (q->is_ok) {
+        CHECK(prev_ok < q->id) ;
+        prev_ok = q->id;
+        ok_cnt++;
+      } else {
+        failed_cnt++;
+      }
+    }
+    LOG(ERROR) << "Chain ok " << ok_cnt << " failed " << failed_cnt;
+  }
+}