diff --git a/src/engine/BUILD.bazel b/src/engine/BUILD.bazel
index 55525635a..845c88a8d 100644
--- a/src/engine/BUILD.bazel
+++ b/src/engine/BUILD.bazel
@@ -47,12 +47,10 @@ mozc_cc_library(
         "//base:thread",
         "//data_manager",
         "//protocol:engine_builder_cc_proto",
-        "@com_google_absl//absl/base:core_headers",
         "@com_google_absl//absl/container:flat_hash_set",
         "@com_google_absl//absl/log",
         "@com_google_absl//absl/log:check",
         "@com_google_absl//absl/status",
-        "@com_google_absl//absl/synchronization",
     ],
 )
 
@@ -84,11 +82,8 @@ mozc_cc_library(
     name = "engine_interface",
     hdrs = ["engine_interface.h"],
     deps = [
-        ":engine_builder",
         ":supplemental_model_interface",
-        ":user_data_manager_interface",
         "//converter:converter_interface",
-        "//data_manager:data_manager_interface",
         "//protocol:engine_builder_cc_proto",
         "@com_google_absl//absl/status",
         "@com_google_absl//absl/strings",
@@ -189,7 +184,6 @@ mozc_cc_library(
         ":minimal_engine",
         ":modules",
         ":supplemental_model_interface",
-        ":user_data_manager_interface",
         "//base:vlog",
         "//converter",
         "//converter:converter_interface",
@@ -241,9 +235,7 @@ mozc_cc_library(
     deps = [
         ":engine_interface",
         ":supplemental_model_interface",
-        ":user_data_manager_interface",
         "//converter:converter_interface",
-        "//data_manager:data_manager_interface",
         "//testing:gunit",
         "@com_google_absl//absl/status",
         "@com_google_absl//absl/strings:string_view",
@@ -358,31 +350,6 @@ mozc_cc_library(
     ],
 )
 
-mozc_cc_library(
-    name = "user_data_manager_mock",
-    testonly = 1,
-    hdrs = ["user_data_manager_mock.h"],
-    visibility = [
-        "//:__subpackages__",
-    ],
-    deps = [
-        ":user_data_manager_interface",
-        "//testing:gunit",
-    ],
-)
-
-mozc_cc_library(
-    name = "user_data_manager_interface",
-    hdrs = ["user_data_manager_interface.h"],
-    visibility = [
-        # For //session:session_handler_scenario_test.
-        "//session:__pkg__",
-    ],
-    deps = [
-        "@com_google_absl//absl/strings",
-    ],
-)
-
 mozc_cc_library(
     name = "minimal_engine",
     srcs = ["minimal_engine.cc"],
@@ -393,7 +360,6 @@ mozc_cc_library(
     deps = [
         ":engine_interface",
         ":modules",
-        ":user_data_manager_interface",
         "//base/strings:assign",
         "//composer",
         "//converter:converter_interface",
diff --git a/src/engine/engine.cc b/src/engine/engine.cc
index ab3c3f311..f3a7193ab 100644
--- a/src/engine/engine.cc
+++ b/src/engine/engine.cc
@@ -46,7 +46,6 @@
 #include "engine/data_loader.h"
 #include "engine/modules.h"
 #include "engine/supplemental_model_interface.h"
-#include "engine/user_data_manager_interface.h"
 #include "prediction/dictionary_predictor.h"
 #include "prediction/predictor.h"
 #include "prediction/predictor_interface.h"
@@ -56,60 +55,6 @@
 #include "rewriter/rewriter_interface.h"
 
 namespace mozc {
-namespace {
-
-using ::mozc::prediction::PredictorInterface;
-
-class UserDataManager final : public UserDataManagerInterface {
- public:
-  UserDataManager(PredictorInterface *predictor, RewriterInterface *rewriter)
-      : predictor_(predictor), rewriter_(rewriter) {}
-
-  UserDataManager(const UserDataManager &) = delete;
-  UserDataManager &operator=(const UserDataManager &) = delete;
-
-  bool Sync() override;
-  bool Reload() override;
-  bool ClearUserHistory() override;
-  bool ClearUserPrediction() override;
-  bool ClearUnusedUserPrediction() override;
-  bool Wait() override;
-
- private:
-  PredictorInterface *predictor_;
-  RewriterInterface *rewriter_;
-};
-
-bool UserDataManager::Sync() {
-  // TODO(noriyukit): In the current implementation, if rewriter_->Sync() fails,
-  // predictor_->Sync() is never called. Check if we should call
-  // predictor_->Sync() or not.
-  return rewriter_->Sync() && predictor_->Sync();
-}
-
-bool UserDataManager::Reload() {
-  // TODO(noriyukit): The same TODO as Sync().
-  return rewriter_->Reload() && predictor_->Reload();
-}
-
-bool UserDataManager::ClearUserHistory() {
-  rewriter_->Clear();
-  return true;
-}
-
-bool UserDataManager::ClearUserPrediction() {
-  predictor_->ClearAllHistory();
-  return true;
-}
-
-bool UserDataManager::ClearUnusedUserPrediction() {
-  predictor_->ClearUnusedHistory();
-  return true;
-}
-
-bool UserDataManager::Wait() { return predictor_->Wait(); }
-
-}  // namespace
 
 absl::StatusOr<std::unique_ptr<Engine>> Engine::CreateDesktopEngine(
     std::unique_ptr<const DataManagerInterface> data_manager) {
@@ -191,7 +136,7 @@ absl::Status Engine::Init(std::unique_ptr<engine::Modules> modules,
   converter_ = std::make_unique<Converter>(*modules_, *immutable_converter_);
   RETURN_IF_NULL(converter_);
 
-  std::unique_ptr<PredictorInterface> predictor;
+  std::unique_ptr<prediction::PredictorInterface> predictor;
   {
     // Create a predictor with three sub-predictors, dictionary predictor, user
     // history predictor, and extra predictor.
@@ -225,8 +170,6 @@ absl::Status Engine::Init(std::unique_ptr<engine::Modules> modules,
 
   converter_->Init(std::move(predictor), std::move(rewriter));
 
-  user_data_manager_ = std::make_unique<UserDataManager>(predictor_, rewriter_);
-
   initialized_ = true;
   return absl::Status();
 
@@ -234,36 +177,41 @@ absl::Status Engine::Init(std::unique_ptr<engine::Modules> modules,
 }
 
 bool Engine::Reload() {
-  if (!modules_->GetUserDictionary()) {
-    return true;
+  if (modules_ && modules_->GetUserDictionary()) {
+    modules_->GetUserDictionary()->Reload();
   }
-  MOZC_VLOG(1) << "Reloading user dictionary";
-  bool result_dictionary = modules_->GetUserDictionary()->Reload();
-  MOZC_VLOG(1) << "Reloading UserDataManager";
-  bool result_user_data = GetUserDataManager()->Reload();
-  return result_dictionary && result_user_data;
+  return rewriter_ && rewriter_->Reload() && predictor_ && predictor_->Reload();
 }
 
 bool Engine::Sync() {
-  GetUserDataManager()->Sync();
-  if (!modules_->GetUserDictionary()) {
-    return true;
+  if (modules_ && modules_->GetUserDictionary()) {
+    modules_->GetUserDictionary()->Sync();
   }
-  return modules_->GetUserDictionary()->Sync();
+  return rewriter_ && rewriter_->Sync() && predictor_ && predictor_->Sync();
 }
 
 bool Engine::Wait() {
-  if (modules_->GetUserDictionary()) {
+  if (modules_ && modules_->GetUserDictionary()) {
     modules_->GetUserDictionary()->WaitForReloader();
   }
-  return GetUserDataManager()->Wait();
+  return predictor_ && predictor_->Wait();
 }
 
-bool Engine::ReloadAndWait() {
-  if (!Reload()) {
-    return false;
+bool Engine::ReloadAndWait() { return Reload() && Wait(); }
+
+bool Engine::ClearUserHistory() {
+  if (rewriter_) {
+    rewriter_->Clear();
   }
-  return Wait();
+  return true;
+}
+
+bool Engine::ClearUserPrediction() {
+  return predictor_ && predictor_->ClearAllHistory();
+}
+
+bool Engine::ClearUnusedUserPrediction() {
+  return predictor_ && predictor_->ClearUnusedHistory();
 }
 
 bool Engine::MaybeReloadEngine(EngineReloadResponse *response) {
@@ -303,8 +251,8 @@ bool Engine::MaybeReloadEngine(EngineReloadResponse *response) {
       continue;
     }
 
-    if (user_data_manager_) {
-      user_data_manager_->Wait();
+    if (predictor_) {
+      predictor_->Wait();
     }
 
     // Reloads DataManager.
diff --git a/src/engine/engine.h b/src/engine/engine.h
index 2725eac62..4dfe0a071 100644
--- a/src/engine/engine.h
+++ b/src/engine/engine.h
@@ -47,7 +47,6 @@
 #include "engine/minimal_engine.h"
 #include "engine/modules.h"
 #include "engine/supplemental_model_interface.h"
-#include "engine/user_data_manager_interface.h"
 #include "prediction/predictor_interface.h"
 #include "rewriter/rewriter_interface.h"
 
@@ -113,14 +112,13 @@ class Engine : public EngineInterface {
   bool Wait() override;
   bool ReloadAndWait() override;
 
+  bool ClearUserHistory() override;
+  bool ClearUserPrediction() override;
+  bool ClearUnusedUserPrediction() override;
+
   absl::Status ReloadModules(std::unique_ptr<engine::Modules> modules,
                              bool is_mobile);
 
-  UserDataManagerInterface *GetUserDataManager() override {
-    return initialized_ ? user_data_manager_.get()
-                        : minimal_engine_.GetUserDataManager();
-  }
-
   absl::string_view GetDataVersion() const override {
     return GetDataManager()->GetDataVersion();
   }
@@ -150,10 +148,11 @@ class Engine : public EngineInterface {
   // Maybe reload a new data manager. Returns true if reloaded.
   bool MaybeReloadEngine(EngineReloadResponse *response) override;
   bool SendEngineReloadRequest(const EngineReloadRequest &request) override;
-  void SetDataLoaderForTesting(std::unique_ptr<DataLoader> loader) override {
+
+  void SetDataLoaderForTesting(std::unique_ptr<DataLoader> loader) {
     loader_ = std::move(loader);
   }
-  void SetAlwaysWaitForLoaderResponseFutureForTesting(bool value) override {
+  void SetAlwaysWaitForLoaderResponseFutureForTesting(bool value) {
     loader_->SetAlwaysWaitForLoaderResponseFutureForTesting(value);
   }
 
@@ -179,7 +178,6 @@ class Engine : public EngineInterface {
   RewriterInterface *rewriter_ = nullptr;
 
   std::unique_ptr<Converter> converter_;
-  std::unique_ptr<UserDataManagerInterface> user_data_manager_;
 };
 
 }  // namespace mozc
diff --git a/src/engine/engine_interface.h b/src/engine/engine_interface.h
index 50aa8a741..c51c5fd70 100644
--- a/src/engine/engine_interface.h
+++ b/src/engine/engine_interface.h
@@ -37,9 +37,7 @@
 #include "absl/status/status.h"
 #include "absl/strings/string_view.h"
 #include "converter/converter_interface.h"
-#include "engine/data_loader.h"
 #include "engine/supplemental_model_interface.h"
-#include "engine/user_data_manager_interface.h"
 #include "protocol/engine_builder.pb.h"
 
 namespace mozc {
@@ -62,32 +60,38 @@ class EngineInterface {
   // Returns the predictor name.
   virtual absl::string_view GetPredictorName() const = 0;
 
+  // Gets the version of underlying data set.
+  virtual absl::string_view GetDataVersion() const = 0;
+
   // Reloads internal data, e.g., user dictionary, etc.
   // This function may read data from local files.
   // Returns true if successfully reloaded or did nothing.
-  virtual bool Reload() = 0;
+  virtual bool Reload() { return true; }
 
   // Synchronizes internal data, e.g., user dictionary, etc.
   // This function may write data into local files.
   // Returns true if successfully synced or did nothing.
-  virtual bool Sync() = 0;
+  virtual bool Sync() { return true; }
 
   // Waits for reloader.
   // Returns true if successfully waited or did nothing.
-  virtual bool Wait() = 0;
+  virtual bool Wait() { return true; }
 
   // Reloads internal data and wait for reloader.
   // Returns true if successfully reloaded and waited, or did nothing.
-  virtual bool ReloadAndWait() = 0;
+  virtual bool ReloadAndWait() { return true; }
 
-  // Gets a user data manager.
-  virtual UserDataManagerInterface *GetUserDataManager() = 0;
+  // Clears user history data.
+  virtual bool ClearUserHistory() { return true; }
 
-  // Gets the version of underlying data set.
-  virtual absl::string_view GetDataVersion() const = 0;
+  // Clears user prediction data.
+  virtual bool ClearUserPrediction() { return true; }
+
+  // Clears unused user prediction data.
+  virtual bool ClearUnusedUserPrediction() { return true; }
 
   // Gets the user POS list.
-  virtual std::vector<std::string> GetPosList() const = 0;
+  virtual std::vector<std::string> GetPosList() const { return {}; }
 
   virtual void SetSupplementalModel(
       const engine::SupplementalModelInterface *supplemental_model) {}
@@ -99,8 +103,6 @@ class EngineInterface {
   virtual bool SendEngineReloadRequest(const EngineReloadRequest &request) {
     return false;
   }
-  virtual void SetDataLoaderForTesting(std::unique_ptr<DataLoader> loader) {}
-  virtual void SetAlwaysWaitForLoaderResponseFutureForTesting(bool value) {}
 
  protected:
   EngineInterface() = default;
diff --git a/src/engine/engine_mock.h b/src/engine/engine_mock.h
index d485c80cf..afe5d3834 100644
--- a/src/engine/engine_mock.h
+++ b/src/engine/engine_mock.h
@@ -39,7 +39,6 @@
 #include "converter/converter_interface.h"
 #include "engine/engine_interface.h"
 #include "engine/supplemental_model_interface.h"
-#include "engine/user_data_manager_interface.h"
 #include "testing/gmock.h"
 
 namespace mozc {
@@ -48,12 +47,14 @@ class MockEngine : public EngineInterface {
  public:
   MOCK_METHOD(ConverterInterface *, GetConverter, (), (const, override));
   MOCK_METHOD(absl::string_view, GetPredictorName, (), (const, override));
+  MOCK_METHOD(absl::string_view, GetDataVersion, (), (const, override));
   MOCK_METHOD(bool, Reload, (), (override));
   MOCK_METHOD(bool, Sync, (), (override));
   MOCK_METHOD(bool, Wait, (), (override));
+  MOCK_METHOD(bool, ClearUserHistory, (), (override));
+  MOCK_METHOD(bool, ClearUserPrediction, (), (override));
+  MOCK_METHOD(bool, ClearUnusedUserPrediction, (), (override));
   MOCK_METHOD(bool, ReloadAndWait, (), (override));
-  MOCK_METHOD(UserDataManagerInterface *, GetUserDataManager, (), (override));
-  MOCK_METHOD(absl::string_view, GetDataVersion, (), (const, override));
   MOCK_METHOD(std::vector<std::string>, GetPosList, (), (const, override));
   MOCK_METHOD(void, SetSupplementalModel,
               (const engine::SupplementalModelInterface *), (override));
diff --git a/src/engine/minimal_engine.cc b/src/engine/minimal_engine.cc
index 844d177db..07511d8c0 100644
--- a/src/engine/minimal_engine.cc
+++ b/src/engine/minimal_engine.cc
@@ -44,24 +44,11 @@
 #include "converter/segments.h"
 #include "data_manager/data_manager.h"
 #include "data_manager/data_manager_interface.h"
-#include "engine/user_data_manager_interface.h"
 #include "request/conversion_request.h"
 
 namespace mozc {
 namespace {
 
-class UserDataManagerStub : public UserDataManagerInterface {
- public:
-  UserDataManagerStub() = default;
-
-  bool Sync() override { return true; }
-  bool Reload() override { return true; }
-  bool ClearUserHistory() override { return true; }
-  bool ClearUserPrediction() override { return true; }
-  bool ClearUnusedUserPrediction() override { return true; }
-  bool Wait() override { return true; }
-};
-
 bool AddAsIsCandidate(const absl::string_view key, Segments *segments) {
   if (key.empty()) {
     return false;
@@ -169,7 +156,6 @@ class MinimalConverter : public ConverterInterface {
 
 MinimalEngine::MinimalEngine()
     : converter_(std::make_unique<MinimalConverter>()),
-      user_data_manager_(std::make_unique<UserDataManagerStub>()),
       data_manager_(std::make_unique<DataManager>()) {}
 
 ConverterInterface *MinimalEngine::GetConverter() const {
@@ -181,14 +167,8 @@ absl::string_view MinimalEngine::GetPredictorName() const {
   return kPredictorName;
 }
 
-UserDataManagerInterface *MinimalEngine::GetUserDataManager() {
-  return user_data_manager_.get();
-}
-
 const DataManagerInterface *MinimalEngine::GetDataManager() const {
   return data_manager_.get();
 }
 
-std::vector<std::string> MinimalEngine::GetPosList() const { return {}; }
-
 }  // namespace mozc
diff --git a/src/engine/minimal_engine.h b/src/engine/minimal_engine.h
index 9be5ecdf9..6411cad3a 100644
--- a/src/engine/minimal_engine.h
+++ b/src/engine/minimal_engine.h
@@ -40,7 +40,6 @@
 #include "data_manager/data_manager_interface.h"
 #include "engine/engine_interface.h"
 #include "engine/modules.h"
-#include "engine/user_data_manager_interface.h"
 
 namespace mozc {
 
@@ -54,23 +53,17 @@ class MinimalEngine : public EngineInterface {
 
   ConverterInterface *GetConverter() const override;
   absl::string_view GetPredictorName() const override;
-  bool Reload() override { return true; }
-  bool Sync() override { return true; }
-  bool Wait() override { return true; }
-  bool ReloadAndWait() override { return true; }
-  UserDataManagerInterface *GetUserDataManager() override;
   absl::string_view GetDataVersion() const override { return "0.0.0"; }
-  std::vector<std::string> GetPosList() const override;
 
   absl::Status ReloadModules(std::unique_ptr<engine::Modules> modules,
                              bool is_mobile) {
     return absl::OkStatus();
   }
+
   const DataManagerInterface *GetDataManager() const;
 
  private:
   std::unique_ptr<ConverterInterface> converter_;
-  std::unique_ptr<UserDataManagerInterface> user_data_manager_;
   std::unique_ptr<const DataManagerInterface> data_manager_;
 };
 
diff --git a/src/engine/user_data_manager_interface.h b/src/engine/user_data_manager_interface.h
deleted file mode 100644
index 6bcb5763e..000000000
--- a/src/engine/user_data_manager_interface.h
+++ /dev/null
@@ -1,66 +0,0 @@
-// Copyright 2010-2021, Google Inc.
-// All rights reserved.
-//
-// Redistribution and use in source and binary forms, with or without
-// modification, are permitted provided that the following conditions are
-// met:
-//
-//     * Redistributions of source code must retain the above copyright
-// notice, this list of conditions and the following disclaimer.
-//     * Redistributions in binary form must reproduce the above
-// copyright notice, this list of conditions and the following disclaimer
-// in the documentation and/or other materials provided with the
-// distribution.
-//     * Neither the name of Google Inc. nor the names of its
-// contributors may be used to endorse or promote products derived from
-// this software without specific prior written permission.
-//
-// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
-// "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
-// LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
-// A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
-// OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
-// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
-// LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
-// DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
-// THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
-// (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
-// OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
-
-// UserDataManagerInterface is responsible for the management of the
-// user data in the persistent storage, i.e. syncing, reloading, or
-// clear-out.
-
-#ifndef MOZC_ENGINE_USER_DATA_MANAGER_INTERFACE_H_
-#define MOZC_ENGINE_USER_DATA_MANAGER_INTERFACE_H_
-
-#include "absl/strings/string_view.h"
-
-namespace mozc {
-
-class UserDataManagerInterface {
- public:
-  virtual ~UserDataManagerInterface() = default;
-
-  // Syncs mutable user data to local file system.
-  virtual bool Sync() = 0;
-
-  // Reloads mutable user data from local file system.
-  virtual bool Reload() = 0;
-
-  // Clears user history data.
-  virtual bool ClearUserHistory() = 0;
-
-  // Clears user prediction data.
-  virtual bool ClearUserPrediction() = 0;
-
-  // Clears unused user prediction data.
-  virtual bool ClearUnusedUserPrediction() = 0;
-
-  // Waits for syncer thread to complete.
-  virtual bool Wait() = 0;
-};
-
-}  // namespace mozc
-
-#endif  // MOZC_ENGINE_USER_DATA_MANAGER_INTERFACE_H_
diff --git a/src/engine/user_data_manager_mock.h b/src/engine/user_data_manager_mock.h
deleted file mode 100644
index bf92aeb4d..000000000
--- a/src/engine/user_data_manager_mock.h
+++ /dev/null
@@ -1,53 +0,0 @@
-// Copyright 2010-2021, Google Inc.
-// All rights reserved.
-//
-// Redistribution and use in source and binary forms, with or without
-// modification, are permitted provided that the following conditions are
-// met:
-//
-//     * Redistributions of source code must retain the above copyright
-// notice, this list of conditions and the following disclaimer.
-//     * Redistributions in binary form must reproduce the above
-// copyright notice, this list of conditions and the following disclaimer
-// in the documentation and/or other materials provided with the
-// distribution.
-//     * Neither the name of Google Inc. nor the names of its
-// contributors may be used to endorse or promote products derived from
-// this software without specific prior written permission.
-//
-// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
-// "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
-// LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
-// A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
-// OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
-// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
-// LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
-// DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
-// THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
-// (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
-// OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
-
-#ifndef MOZC_ENGINE_USER_DATA_MANAGER_MOCK_H_
-#define MOZC_ENGINE_USER_DATA_MANAGER_MOCK_H_
-
-#include "engine/user_data_manager_interface.h"
-#include "testing/gmock.h"
-
-namespace mozc {
-
-class MockUserDataManager : public UserDataManagerInterface {
- public:
-  MockUserDataManager() = default;
-  ~MockUserDataManager() override = default;
-
-  MOCK_METHOD(bool, Sync, (), (override));
-  MOCK_METHOD(bool, Reload, (), (override));
-  MOCK_METHOD(bool, ClearUserHistory, (), (override));
-  MOCK_METHOD(bool, ClearUserPrediction, (), (override));
-  MOCK_METHOD(bool, ClearUnusedUserPrediction, (), (override));
-  MOCK_METHOD(bool, Wait, (), (override));
-};
-
-}  // namespace mozc
-
-#endif  // MOZC_ENGINE_USER_DATA_MANAGER_MOCK_H_
diff --git a/src/session/BUILD.bazel b/src/session/BUILD.bazel
index 91d9a8543..721664292 100644
--- a/src/session/BUILD.bazel
+++ b/src/session/BUILD.bazel
@@ -234,7 +234,6 @@ mozc_cc_test(
         "//data_manager/testing:mock_data_manager",
         "//engine",
         "//engine:mock_data_engine_factory",
-        "//engine:user_data_manager_interface",
         "//protocol:candidate_window_cc_proto",
         "//protocol:commands_cc_proto",
         "//protocol:config_cc_proto",
@@ -298,7 +297,6 @@ mozc_cc_library(
         "//engine",
         "//engine:engine_interface",
         "//engine:supplemental_model_interface",
-        "//engine:user_data_manager_interface",
         "//protocol:commands_cc_proto",
         "//protocol:config_cc_proto",
         "//protocol:engine_builder_cc_proto",
@@ -373,7 +371,6 @@ mozc_cc_test(
         "//engine:mock_data_engine_factory",
         "//engine:modules",
         "//engine:supplemental_model_interface",
-        "//engine:user_data_manager_mock",
         "//protocol:commands_cc_proto",
         "//protocol:config_cc_proto",
         "//session/internal:keymap",
@@ -689,7 +686,6 @@ mozc_cc_library(
         "//config:config_handler",
         "//engine:engine_factory",
         "//engine:engine_interface",
-        "//engine:user_data_manager_interface",
         "//prediction:user_history_predictor",
         "//protocol:candidate_window_cc_proto",
         "//protocol:commands_cc_proto",
diff --git a/src/session/session_handler.cc b/src/session/session_handler.cc
index a7b7ab0ef..56d22d396 100644
--- a/src/session/session_handler.cc
+++ b/src/session/session_handler.cc
@@ -54,7 +54,6 @@
 #include "dictionary/user_dictionary_session_handler.h"
 #include "engine/engine_interface.h"
 #include "engine/supplemental_model_interface.h"
-#include "engine/user_data_manager_interface.h"
 #include "protocol/commands.pb.h"
 #include "protocol/config.pb.h"
 #include "protocol/engine_builder.pb.h"
@@ -253,21 +252,21 @@ bool SessionHandler::ReloadAndWait(commands::Command *command) {
 
 bool SessionHandler::ClearUserHistory(commands::Command *command) {
   MOZC_VLOG(1) << "Clearing user history";
-  engine_->GetUserDataManager()->ClearUserHistory();
+  engine_->ClearUserHistory();
   UsageStats::IncrementCount("ClearUserHistory");
   return true;
 }
 
 bool SessionHandler::ClearUserPrediction(commands::Command *command) {
   MOZC_VLOG(1) << "Clearing user prediction";
-  engine_->GetUserDataManager()->ClearUserPrediction();
+  engine_->ClearUserPrediction();
   UsageStats::IncrementCount("ClearUserPrediction");
   return true;
 }
 
 bool SessionHandler::ClearUnusedUserPrediction(commands::Command *command) {
   MOZC_VLOG(1) << "Clearing unused user prediction";
-  engine_->GetUserDataManager()->ClearUnusedUserPrediction();
+  engine_->ClearUnusedUserPrediction();
   UsageStats::IncrementCount("ClearUnusedUserPrediction");
   return true;
 }
diff --git a/src/session/session_handler_test.cc b/src/session/session_handler_test.cc
index bc75cb918..e928c66e5 100644
--- a/src/session/session_handler_test.cc
+++ b/src/session/session_handler_test.cc
@@ -59,7 +59,6 @@
 #include "engine/mock_data_engine_factory.h"
 #include "engine/modules.h"
 #include "engine/supplemental_model_interface.h"
-#include "engine/user_data_manager_mock.h"
 #include "protocol/commands.pb.h"
 #include "protocol/config.pb.h"
 #include "session/internal/keymap.h"
@@ -618,7 +617,6 @@ TEST_F(SessionHandlerTest, VerifySyncIsCalledTest) {
       commands::Input::CLEANUP,
   };
   for (size_t i = 0; i < std::size(command_types); ++i) {
-    MockUserDataManager mock_user_data_manager;
     auto engine = std::make_unique<MockEngine>();
     EXPECT_CALL(*engine, Sync()).WillOnce(Return(true));
 
@@ -633,7 +631,6 @@ TEST_F(SessionHandlerTest, VerifySyncIsCalledTest) {
 }
 
 TEST_F(SessionHandlerTest, SyncDataTest) {
-  MockUserDataManager mock_user_data_manager;
   auto engine = std::make_unique<MockEngine>();
   EXPECT_CALL(*engine, Sync()).WillOnce(Return(true));
   EXPECT_CALL(*engine, Wait()).WillOnce(Return(true));
diff --git a/src/session/session_handler_tool.cc b/src/session/session_handler_tool.cc
index a0eb518eb..e841b2a2a 100644
--- a/src/session/session_handler_tool.cc
+++ b/src/session/session_handler_tool.cc
@@ -59,7 +59,6 @@
 #include "config/config_handler.h"
 #include "engine/engine_factory.h"
 #include "engine/engine_interface.h"
-#include "engine/user_data_manager_interface.h"
 #include "prediction/user_history_predictor.h"
 #include "protocol/candidate_window.pb.h"
 #include "protocol/commands.pb.h"
@@ -100,8 +99,8 @@ std::string ToTextFormat(const Message &proto) {
 
 SessionHandlerTool::SessionHandlerTool(std::unique_ptr<EngineInterface> engine)
     : id_(0),
+      engine_(engine.get()),
       usage_observer_(std::make_unique<SessionUsageObserver>()),
-      data_manager_(engine->GetUserDataManager()),
       handler_(std::make_unique<SessionHandler>(std::move(engine))) {
   handler_->AddObserver(usage_observer_.get());
 }
@@ -260,7 +259,9 @@ bool SessionHandlerTool::SetConfig(const config::Config &config,
 }
 
 bool SessionHandlerTool::SyncData() {
-  return data_manager_->Sync() && data_manager_->Wait();
+  engine_->Sync();
+  engine_->Wait();
+  return true;
 }
 
 void SessionHandlerTool::SetCallbackText(const absl::string_view text) {
diff --git a/src/session/session_handler_tool.h b/src/session/session_handler_tool.h
index b44e636f9..25b714a3f 100644
--- a/src/session/session_handler_tool.h
+++ b/src/session/session_handler_tool.h
@@ -40,7 +40,6 @@
 #include "absl/strings/string_view.h"
 #include "absl/types/span.h"
 #include "engine/engine_interface.h"
-#include "engine/user_data_manager_interface.h"
 #include "protocol/candidate_window.pb.h"
 #include "protocol/commands.pb.h"
 #include "protocol/config.pb.h"
@@ -101,8 +100,8 @@ class SessionHandlerTool {
                            bool allow_callback);
 
   uint64_t id_;  // Session ID
+  EngineInterface *engine_ = nullptr;
   std::unique_ptr<SessionObserverInterface> usage_observer_;
-  UserDataManagerInterface *data_manager_;
   std::unique_ptr<SessionHandlerInterface> handler_;
   std::string callback_text_;
 };
diff --git a/src/session/session_regression_test.cc b/src/session/session_regression_test.cc
index 7099fffd0..7df503a44 100644
--- a/src/session/session_regression_test.cc
+++ b/src/session/session_regression_test.cc
@@ -45,7 +45,6 @@
 #include "data_manager/testing/mock_data_manager.h"
 #include "engine/engine.h"
 #include "engine/mock_data_engine_factory.h"
-#include "engine/user_data_manager_interface.h"
 #include "protocol/candidate_window.pb.h"
 #include "protocol/commands.pb.h"
 #include "protocol/config.pb.h"
@@ -96,9 +95,9 @@ class SessionRegressionTest : public testing::TestWithTempUserProfile {
 
     // Clear previous data just in case. It should work without this clear,
     // however the reality is Windows environment has a flacky test issue.
-    engine->GetUserDataManager()->ClearUserHistory();
-    engine->GetUserDataManager()->ClearUserPrediction();
-    engine->GetUserDataManager()->Wait();
+    engine->ClearUserHistory();
+    engine->ClearUserPrediction();
+    engine->Wait();
 
     handler_ = std::make_unique<SessionHandler>(std::move(engine));
     ResetSession();