diff --git a/CMakeLists.txt b/CMakeLists.txt
index 888178a..663d7c3 100755
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -24,4 +24,5 @@ include_directories(${CMAKE_CURRENT_SOURCE_DIR})
 include_directories(helio)
 
 add_subdirectory(helio)
+add_subdirectory(core)
 add_subdirectory(server)
diff --git a/core/CMakeLists.txt b/core/CMakeLists.txt
new file mode 100644
index 0000000..3891ec4
--- /dev/null
+++ b/core/CMakeLists.txt
@@ -0,0 +1,4 @@
+add_library(dfly_core  tx_queue.cc  dragonfly_core.cc)
+cxx_link(dfly_core base absl::flat_hash_map)
+
+cxx_test(dfly_core_test dfly_core)
\ No newline at end of file
diff --git a/core/dfly_core_test.cc b/core/dfly_core_test.cc
new file mode 100644
index 0000000..c2992ad
--- /dev/null
+++ b/core/dfly_core_test.cc
@@ -0,0 +1,58 @@
+// Copyright 2021, Roman Gershman.  All rights reserved.
+// See LICENSE for licensing terms.
+//
+
+#include "core/tx_queue.h"
+
+#include "base/gtest.h"
+#include "core/intent_lock.h"
+
+namespace dfly {
+
+class TxQueueTest : public ::testing::Test {
+ protected:
+  TxQueueTest() {
+  }
+
+  uint64_t Pop() {
+    if (pq_.Empty())
+      return uint64_t(-1);
+    TxQueue::ValueType val = pq_.Front();
+    pq_.PopFront();
+
+    return std::get<uint64_t>(val);
+  }
+
+  TxQueue pq_;
+};
+
+TEST_F(TxQueueTest, Basic) {
+  pq_.Insert(4);
+  pq_.Insert(3);
+  pq_.Insert(2);
+
+  ASSERT_EQ(2, Pop());
+  ASSERT_EQ(3, Pop());
+  ASSERT_EQ(4, Pop());
+  ASSERT_TRUE(pq_.Empty());
+
+  pq_.Insert(10);
+  ASSERT_EQ(10, Pop());
+}
+
+class IntentLockTest : public ::testing::Test {
+ protected:
+  IntentLock lk_;
+};
+
+TEST_F(IntentLockTest, Basic) {
+  ASSERT_TRUE(lk_.Acquire(IntentLock::SHARED));
+  ASSERT_FALSE(lk_.Acquire(IntentLock::EXCLUSIVE));
+  lk_.Release(IntentLock::EXCLUSIVE);
+
+  ASSERT_FALSE(lk_.Check(IntentLock::EXCLUSIVE));
+  lk_.Release(IntentLock::SHARED);
+  ASSERT_TRUE(lk_.Check(IntentLock::EXCLUSIVE));
+}
+
+}  // namespace dfly
diff --git a/server/resp_expr.cc b/core/dragonfly_core.cc
similarity index 68%
rename from server/resp_expr.cc
rename to core/dragonfly_core.cc
index 7d583e0..9631197 100644
--- a/server/resp_expr.cc
+++ b/core/dragonfly_core.cc
@@ -2,9 +2,9 @@
 // See LICENSE for licensing terms.
 //
 
-#include "server/resp_expr.h"
-
 #include "base/logging.h"
+#include "core/intent_lock.h"
+#include "core/resp_expr.h"
 
 namespace dfly {
 
@@ -26,10 +26,25 @@ const char* RespExpr::TypeName(Type t) {
   ABSL_INTERNAL_UNREACHABLE;
 }
 
+const char* IntentLock::ModeName(Mode m) {
+  switch (m) {
+    case IntentLock::SHARED:
+      return "SHARED";
+    case IntentLock::EXCLUSIVE:
+      return "EXCLUSIVE";
+  }
+  ABSL_INTERNAL_UNREACHABLE;
+}
+
+void IntentLock::VerifyDebug() {
+  constexpr uint32_t kMsb = 1ULL << (sizeof(cnt_[0]) * 8 - 1);
+  DCHECK_EQ(0u, cnt_[0] & kMsb);
+  DCHECK_EQ(0u, cnt_[1] & kMsb);
+}
+
 }  // namespace dfly
 
 namespace std {
-
 ostream& operator<<(ostream& os, const dfly::RespExpr& e) {
   using dfly::RespExpr;
   using dfly::ToSV;
@@ -39,7 +54,7 @@ ostream& operator<<(ostream& os, const dfly::RespExpr& e) {
       os << "i" << get<int64_t>(e.u);
       break;
     case RespExpr::STRING:
-      os << "'" << ToSV(e.GetBuf()) << "'";
+      os << "'" << ToSV(get<RespExpr::Buffer>(e.u)) << "'";
       break;
     case RespExpr::NIL:
       os << "nil";
@@ -51,7 +66,7 @@ ostream& operator<<(ostream& os, const dfly::RespExpr& e) {
       os << dfly::RespSpan{*get<RespExpr::Vec*>(e.u)};
       break;
     case RespExpr::ERROR:
-      os << "e(" << ToSV(e.GetBuf()) << ")";
+      os << "e(" << ToSV(get<RespExpr::Buffer>(e.u)) << ")";
       break;
   }
 
@@ -71,4 +86,4 @@ ostream& operator<<(ostream& os, dfly::RespSpan ras) {
   return os;
 }
 
-}  // namespace std
\ No newline at end of file
+}  // namespace std
diff --git a/core/intent_lock.h b/core/intent_lock.h
new file mode 100644
index 0000000..1c202f2
--- /dev/null
+++ b/core/intent_lock.h
@@ -0,0 +1,54 @@
+// Copyright 2021, Roman Gershman.  All rights reserved.
+// See LICENSE for licensing terms.
+//
+#include <assert.h>
+
+#pragma once
+
+namespace dfly {
+
+// SHARED - can be acquired multiple times as long as other intents are absent.
+// EXCLUSIVE - is acquired only if it's the only lock recorded.
+// BLOCKED_READY - can not be acquired - it's recorded for intent purposes.
+// Transactions at the head of tx-queue are considered to be the ones that acquired the lock
+class IntentLock {
+ public:
+  enum Mode { SHARED = 0, EXCLUSIVE = 1 };
+
+  // Returns true if lock was acquired. In any case, the intent is recorded.
+  bool Acquire(Mode m) {
+    ++cnt_[m];
+
+    if (cnt_[1 ^ int(m)])
+      return false;
+    return m == SHARED || cnt_[EXCLUSIVE] == 1;
+  }
+
+  bool Check(Mode m) const {
+    unsigned s = cnt_[EXCLUSIVE];
+    if (s)
+      return false;
+
+    return (m == SHARED) ? true : IsFree();
+  }
+
+  void Release(Mode m, unsigned val = 1) {
+    assert(cnt_[m] >= val);
+
+    cnt_[m] -= val;
+    // return cnt_[m] == 0 ? cnt_[1 ^ int(m)] : 0;
+  }
+
+  bool IsFree() const {
+    return (cnt_[0] | cnt_[1]) == 0;
+  }
+
+  static const char* ModeName(Mode m);
+
+  void VerifyDebug();
+
+ private:
+  unsigned cnt_[2] = {0, 0};
+};
+
+}  // namespace dfly
diff --git a/server/op_status.h b/core/op_status.h
similarity index 100%
rename from server/op_status.h
rename to core/op_status.h
diff --git a/server/resp_expr.h b/core/resp_expr.h
similarity index 100%
rename from server/resp_expr.h
rename to core/resp_expr.h
diff --git a/core/tx_queue.cc b/core/tx_queue.cc
new file mode 100644
index 0000000..955eae7
--- /dev/null
+++ b/core/tx_queue.cc
@@ -0,0 +1,115 @@
+// Copyright 2021, Roman Gershman.  All rights reserved.
+// See LICENSE for licensing terms.
+//
+#include "core/tx_queue.h"
+
+#include "base/logging.h"
+
+namespace dfly {
+
+TxQueue::TxQueue(std::function<uint64_t(const Transaction*)> sf)
+    : score_fun_(sf), vec_(32) {
+  for (size_t i = 0; i < vec_.size(); ++i) {
+    vec_[i].next = i + 1;
+  }
+}
+
+auto TxQueue::Insert(Transaction* t) -> Iterator {
+  if (next_free_ >= vec_.size()) {
+    Grow();
+  }
+  DCHECK_LT(next_free_, vec_.size());
+  DCHECK_EQ(FREE_TAG, vec_[next_free_].tag);
+
+  Iterator res = next_free_;
+  vec_[next_free_].u.trans = t;
+  vec_[next_free_].tag = TRANS_TAG;
+  DVLOG(1) << "Insert " << next_free_ << " " << t;
+  LinkFree(score_fun_(t));
+  return res;
+}
+
+auto TxQueue::Insert(uint64_t val) -> Iterator {
+  if (next_free_ >= vec_.size()) {
+    Grow();
+  }
+  DCHECK_LT(next_free_, vec_.size());
+
+  Iterator res = next_free_;
+
+  vec_[next_free_].u.uval = val;
+  vec_[next_free_].tag = UINT_TAG;
+
+  LinkFree(val);
+  return res;
+}
+
+void TxQueue::LinkFree(uint64_t weight) {
+  uint32_t taken = next_free_;
+  next_free_ = vec_[taken].next;
+
+  if (size_ == 0) {
+    head_ = taken;
+    vec_[head_].next = vec_[head_].prev = head_;
+  } else {
+    uint32_t cur = vec_[head_].prev;
+    while (true) {
+      if (Rank(vec_[cur]) < weight) {
+        Link(cur, taken);
+        break;
+      }
+      if (cur == head_) {
+        Link(vec_[head_].prev, taken);
+        head_ = taken;
+        break;
+      }
+      cur = vec_[cur].prev;
+    }
+  }
+  ++size_;
+}
+
+void TxQueue::Grow() {
+  size_t start = vec_.size();
+  DVLOG(1) << "Grow from " << start << " to " << start * 2;
+
+  vec_.resize(start * 2);
+  for (size_t i = start; i < vec_.size(); ++i) {
+    vec_[i].next = i + 1;
+  }
+}
+
+void TxQueue::Remove(Iterator it) {
+  DCHECK_GT(size_, 0u);
+  DCHECK_LT(it, vec_.size());
+  DCHECK_NE(FREE_TAG, vec_[it].tag);
+
+  DVLOG(1) << "Remove " << it << " " << vec_[it].u.trans;
+  Iterator next = kEnd;
+  if (size_ > 1) {
+    Iterator prev = vec_[it].prev;
+    next = vec_[it].next;
+
+    vec_[prev].next = next;
+    vec_[next].prev = prev;
+  }
+  --size_;
+  vec_[it].next = next_free_;
+  vec_[it].tag = FREE_TAG;
+  next_free_ = it;
+  if (head_ == it) {
+    head_ = next;
+  }
+}
+
+uint64_t TxQueue::Rank(const QRecord& r) const {
+  switch (r.tag) {
+    case UINT_TAG:
+      return r.u.uval;
+    case TRANS_TAG:
+      return score_fun_(r.u.trans);
+  }
+  return 0;
+}
+
+}  // namespace dfly
diff --git a/core/tx_queue.h b/core/tx_queue.h
new file mode 100644
index 0000000..73fc138
--- /dev/null
+++ b/core/tx_queue.h
@@ -0,0 +1,111 @@
+// Copyright 2021, Roman Gershman.  All rights reserved.
+// See LICENSE for licensing terms.
+//
+#pragma once
+
+#include <cstdint>
+#include <functional>
+#include <variant>
+#include <vector>
+
+namespace dfly {
+
+class Transaction;
+
+class TxQueue {
+  void Link(uint32_t p, uint32_t n) {
+    uint32_t next = vec_[p].next;
+    vec_[n].next = next;
+    vec_[n].prev = p;
+    vec_[p].next = n;
+    vec_[next].prev = n;
+  }
+
+ public:
+  // uint64_t is used for unit-tests.
+  using ValueType = std::variant<Transaction*, uint64_t>;
+  using Iterator = uint32_t;
+  enum { kEnd = Iterator(-1) };
+
+  TxQueue(std::function<uint64_t(const Transaction*)> score_fun = nullptr);
+
+  // returns iterator to that item the list
+  Iterator Insert(Transaction* t);
+
+  Iterator Insert(uint64_t val);
+  void Remove(Iterator);
+
+  ValueType At(Iterator it) const {
+    switch (vec_[it].tag) {
+      case TRANS_TAG:
+        return vec_[it].u.trans;
+      case UINT_TAG:
+        return vec_[it].u.uval;
+    }
+    return 0u;
+  }
+
+  ValueType Front() const {
+    return At(head_);
+  }
+
+  void PopFront() {
+    Remove(head_);
+  }
+
+  size_t size() const {
+    return size_;
+  }
+
+  bool Empty() const {
+    return size_ == 0;
+  }
+
+  //! returns the score of the tail record. Can be called only if !Empty().
+  uint64_t TailScore() const {
+    return Rank(vec_[vec_[head_].prev]);
+  }
+
+  //! returns the score of the head record. Can be called only if !Empty().
+  uint64_t HeadScore() const {
+    return Rank(vec_[head_]);
+  }
+
+  //! Can be called only if !Empty().
+  Iterator Head() const {
+    return head_;
+  }
+
+ private:
+  enum { TRANS_TAG = 0, UINT_TAG = 11, FREE_TAG = 12 };
+
+  void Grow();
+  void LinkFree(uint64_t rank);
+
+  struct QRecord {
+    union {
+      Transaction* trans;
+      uint64_t uval;
+    } u;
+
+    uint32_t tag : 8;
+    uint32_t next : 24;
+    uint32_t prev;
+
+    QRecord() : tag(FREE_TAG), prev(kEnd) {
+    }
+  };
+
+  static_assert(sizeof(QRecord) == 16, "");
+
+  uint64_t Rank(const QRecord& r) const;
+
+  std::function<uint64_t(const Transaction*)> score_fun_;
+  std::vector<QRecord> vec_;
+  uint32_t next_free_ = 0, head_ = 0;
+  size_t size_ = 0;
+
+  TxQueue(const TxQueue&) = delete;
+};
+
+}  // namespace dfly
diff --git a/server/CMakeLists.txt b/server/CMakeLists.txt
index 9a656ea..c12806d 100644
--- a/server/CMakeLists.txt
+++ b/server/CMakeLists.txt
@@ -5,9 +5,9 @@ add_library(dragonfly_lib command_registry.cc common.cc config_flags.cc
             conn_context.cc db_slice.cc debugcmd.cc dragonfly_listener.cc
             dragonfly_connection.cc engine_shard_set.cc
             main_service.cc memcache_parser.cc
-            redis_parser.cc resp_expr.cc reply_builder.cc string_family.cc)
+            redis_parser.cc reply_builder.cc string_family.cc)
 
-cxx_link(dragonfly_lib uring_fiber_lib
+cxx_link(dragonfly_lib dfly_core uring_fiber_lib
          fibers_ext strings_lib http_server_lib tls_lib)
 
 add_library(dfly_test_lib test_utils.cc)
diff --git a/server/common_types.h b/server/common_types.h
index 28cad92..f1b4463 100644
--- a/server/common_types.h
+++ b/server/common_types.h
@@ -12,9 +12,17 @@
 
 namespace dfly {
 
+enum class Protocol : uint8_t {
+  MEMCACHE = 1,
+  REDIS = 2
+};
+
 using DbIndex = uint16_t;
 using ShardId = uint16_t;
+using TxId = uint64_t;
+using TxClock = uint64_t;
 
+using ArgSlice = absl::Span<const std::string_view>;
 using MutableStrSpan = absl::Span<char>;
 using CmdArgList = absl::Span<MutableStrSpan>;
 using CmdArgVec = std::vector<MutableStrSpan>;
@@ -23,8 +31,15 @@ constexpr DbIndex kInvalidDbId = DbIndex(-1);
 constexpr ShardId kInvalidSid = ShardId(-1);
 
 class CommandId;
+class Transaction;
 class EngineShard;
 
+struct KeyLockArgs {
+  DbIndex db_index;
+  ArgSlice args;
+  unsigned key_step;
+};
+
 inline std::string_view ArgS(CmdArgList args, size_t i) {
   auto arg = args[i];
   return std::string_view(arg.data(), arg.size());
diff --git a/server/conn_context.h b/server/conn_context.h
index f9109b8..37f3748 100644
--- a/server/conn_context.h
+++ b/server/conn_context.h
@@ -4,8 +4,8 @@
 
 #pragma once
 
-#include "server/reply_builder.h"
 #include "server/common_types.h"
+#include "server/reply_builder.h"
 
 namespace dfly {
 
@@ -35,6 +35,7 @@ class ConnectionContext : public ReplyBuilder {
   ConnectionContext(::io::Sink* stream, Connection* owner);
 
   // TODO: to introduce proper accessors.
+  Transaction* transaction = nullptr;
   const CommandId* cid = nullptr;
   EngineShardSet* shard_set = nullptr;
 
diff --git a/server/db_slice.h b/server/db_slice.h
index 29d6e2f..c23de79 100644
--- a/server/db_slice.h
+++ b/server/db_slice.h
@@ -5,7 +5,7 @@
 #pragma once
 
 #include "server/common_types.h"
-#include "server/op_status.h"
+#include "core/op_status.h"
 #include "server/table.h"
 
 namespace util {
diff --git a/server/dfly_protocol.h b/server/dfly_protocol.h
deleted file mode 100644
index 522875a..0000000
--- a/server/dfly_protocol.h
+++ /dev/null
@@ -1,16 +0,0 @@
-// Copyright 2021, Roman Gershman.  All rights reserved.
-// See LICENSE for licensing terms.
-//
-
-#pragma once
-
-#include <cstdint>
-
-namespace dfly {
-
-enum class Protocol : uint8_t {
-  MEMCACHE = 1,
-  REDIS = 2
-};
-
-}  // namespace dfly
diff --git a/server/dragonfly_connection.h b/server/dragonfly_connection.h
index f7c3483..be96b7b 100644
--- a/server/dragonfly_connection.h
+++ b/server/dragonfly_connection.h
@@ -9,9 +9,8 @@
 #include <deque>
 
 #include "base/io_buf.h"
+#include "core/resp_expr.h"
 #include "server/common_types.h"
-#include "server/dfly_protocol.h"
-#include "server/resp_expr.h"
 #include "util/connection.h"
 #include "util/fibers/event_count.h"
 
diff --git a/server/dragonfly_listener.h b/server/dragonfly_listener.h
index af824f8..10a1ed1 100644
--- a/server/dragonfly_listener.h
+++ b/server/dragonfly_listener.h
@@ -5,7 +5,7 @@
 #pragma once
 
 #include "util/listener_interface.h"
-#include "server/dfly_protocol.h"
+#include "server/common_types.h"
 
 typedef struct ssl_ctx_st SSL_CTX;
 
diff --git a/server/main_service.cc b/server/main_service.cc
index bb47395..c8ce021 100644
--- a/server/main_service.cc
+++ b/server/main_service.cc
@@ -13,8 +13,9 @@
 #include "base/logging.h"
 #include "server/conn_context.h"
 #include "server/debugcmd.h"
-#include "util/metrics/metrics.h"
+#include "server/error.h"
 #include "server/string_family.h"
+#include "util/metrics/metrics.h"
 #include "util/uring/uring_fiber_algo.h"
 #include "util/varz.h"
 
diff --git a/server/redis_parser.h b/server/redis_parser.h
index a3ec9bb..600dd7d 100644
--- a/server/redis_parser.h
+++ b/server/redis_parser.h
@@ -5,7 +5,7 @@
 
 #include <absl/container/inlined_vector.h>
 
-#include "resp_expr.h"
+#include "core/resp_expr.h"
 
 namespace dfly {
 
diff --git a/server/reply_builder.h b/server/reply_builder.h
index 8a83137..3dc3e9f 100644
--- a/server/reply_builder.h
+++ b/server/reply_builder.h
@@ -3,9 +3,9 @@
 //
 #include <string_view>
 
+#include "core/op_status.h"
 #include "io/sync_stream_interface.h"
-#include "server/dfly_protocol.h"
-#include "server/op_status.h"
+#include "server/common_types.h"
 
 namespace dfly {