diff --git a/CMakeLists.txt b/CMakeLists.txt
index b0fe4f0276cdeb3540b39a4d545a151e606a9bc7..d3f167501abd2ede1b380bad1a1def8f2fc5c796 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -39,10 +39,11 @@ add_executable(vl_test
         ${PROJECT_SOURCE_DIR}/visualdl/storage/storage_test.cc
         ${PROJECT_SOURCE_DIR}/visualdl/utils/test_concurrency.cc
         ${PROJECT_SOURCE_DIR}/visualdl/logic/im_test.cc
+        ${PROJECT_SOURCE_DIR}/visualdl/logic/sdk_test.cc
         ${PROJECT_SOURCE_DIR}/visualdl/utils/concurrency.h
         ${PROJECT_SOURCE_DIR}/visualdl/utils/filesystem.h
         )
-target_link_libraries(vl_test storage im gtest glog protobuf gflags pthread)
+target_link_libraries(vl_test storage sdk im gtest glog protobuf gflags pthread)
 
 enable_testing ()
 
diff --git a/visualdl/logic/im.cc b/visualdl/logic/im.cc
index 76d55d225bafe1754d14c351436bfde97041665e..94881244f8d5925e2a4cf3e0ac71169a5fdfdb1f 100644
--- a/visualdl/logic/im.cc
+++ b/visualdl/logic/im.cc
@@ -25,26 +25,26 @@ int ReserviorSample(int num_samples, int num_records) {
   return -1;
 }
 
-IM::IM(StorageBase::Type type, StorageBase::Mode mode) {
-  switch (type) {
-    case StorageBase::Type::kMemory: {
-      storage_.reset(new MemoryStorage);
-    } break;
-    default:
-      CHECK(false) << "Unsupported storage kind " << type;
-  }
-
-  switch (mode) {
-    case StorageBase::Mode::kRead:
-      dynamic_cast<MemoryStorage *>(storage_.get())->StartReadService();
-      break;
-    case StorageBase::Mode::kWrite:
-      dynamic_cast<MemoryStorage *>(storage_.get())->StartWriteSerice();
-      break;
-    default:
-      break;
-  }
-}
+// IM::IM(StorageBase::Type type, StorageBase::Mode mode) {
+//   switch (type) {
+//     case StorageBase::Type::kMemory: {
+//       storage_.reset(new MemoryStorage);
+//     } break;
+//     default:
+//       CHECK(false) << "Unsupported storage kind " << type;
+//   }
+
+//   switch (mode) {
+//     case StorageBase::Mode::kRead:
+//       dynamic_cast<MemoryStorage *>(storage_.get())->StartReadService(500);
+//       break;
+//     case StorageBase::Mode::kWrite:
+//       dynamic_cast<MemoryStorage *>(storage_.get())->StartWriteSerice(500);
+//       break;
+//     default:
+//       break;
+//   }
+// }
 
 void IM::SetPersistDest(const std::string &path) {
   CHECK(storage_->mutable_data()->dir().empty())
@@ -95,7 +95,7 @@ void IM::PersistToDisk() {
   CHECK(!storage_->data().dir().empty()) << "path of storage should be set";
   // TODO make dir first
   // MakeDir(storage_.data().dir());
-  storage_->PersistToDisk();
+  storage_->PersistToDisk(storage_->data().dir());
 }
 
 }  // namespace visualdl
diff --git a/visualdl/logic/im.h b/visualdl/logic/im.h
index 5df59e2994807cdfe4a0a5e746f8bbc264759433..47f280845981f0b735a239ed291f4326e7f8fe41 100644
--- a/visualdl/logic/im.h
+++ b/visualdl/logic/im.h
@@ -27,22 +27,21 @@ namespace visualdl {
  */
 class IM final {
 public:
-  IM() { storage_.reset(new MemoryStorage); }
-  IM(StorageBase::Type type, StorageBase::Mode mode);
-  ~IM() { cc::PeriodExector::Global().Quit(); }
-
-  static IM &Global() {
-    static IM x;
-    return x;
+  IM() { storage_.reset(new MemoryStorage(&executor_)); }
+  // IM(StorageBase::Type type, StorageBase::Mode mode);
+  ~IM() { executor_.Quit();
+    std::this_thread::sleep_for(std::chrono::milliseconds(1000));
   }
 
-  void MaintainRead() {
+  void MaintainRead(const std::string &dir, int msecs) {
     LOG(INFO) << "start maintain read";
-    dynamic_cast<MemoryStorage *>(storage_.get())->StartReadService();
+    dynamic_cast<MemoryStorage *>(storage_.get())
+        ->StartReadService(dir, msecs, &lock_);
   }
 
-  void MaintainWrite() {
-    dynamic_cast<MemoryStorage *>(storage_.get())->StartWriteSerice();
+  void MaintainWrite(const std::string &dir, int msecs) {
+    dynamic_cast<MemoryStorage *>(storage_.get())
+        ->StartWriteSerice(dir, msecs, &lock_);
   }
 
   /*
@@ -73,8 +72,15 @@ public:
 
   StorageBase &storage() { return *storage_; }
 
+  cc::PeriodExector &executor() { return executor_; }
+
+  std::mutex &handler() { return lock_; }
+
 private:
+  // read write lock for protobuf in memory
+  std::mutex lock_;
   std::unique_ptr<StorageBase> storage_;
+  cc::PeriodExector executor_;
 };
 
 }  // namespace visualdl
diff --git a/visualdl/logic/im_test.cc b/visualdl/logic/im_test.cc
index deaab5e4704063cabb652b1a7940d6321ae93adb..37246c1763c03071d25debe3b260cd81b054b552 100644
--- a/visualdl/logic/im_test.cc
+++ b/visualdl/logic/im_test.cc
@@ -2,13 +2,15 @@
 
 #include "gtest/gtest.h"
 
+#include "visualdl/storage/storage.h"
+
 namespace visualdl {
 
 class ImTester : public ::testing::Test {
 protected:
   void SetUp() override {}
 
-  IM &im = IM::Global();
+  IM im;
 };
 
 TEST_F(ImTester, AddTablet) {
diff --git a/visualdl/logic/pybind.cc b/visualdl/logic/pybind.cc
index 7a79429a7dde5f11a9690ab326eb2cf2129610d6..09e29a3b15acf2381aee97f9316523a7351b41b4 100644
--- a/visualdl/logic/pybind.cc
+++ b/visualdl/logic/pybind.cc
@@ -53,18 +53,20 @@ PYBIND11_PLUGIN(core) {
       .def("tablet", &vs::ImHelper::tablet)
       .def("add_tablet", &vs::ImHelper::AddTablet)
       .def("persist_to_disk", &vs::ImHelper::PersistToDisk)
-      .def("clear_tablets", &vs::ImHelper::ClearTablets);
+      .def("clear_tablets", &vs::ImHelper::ClearTablets)
+      .def("start_read_service",
+           &vs::ImHelper::StartReadService,
+           "start a thread to maintain read service")
+      .def("start_write_service",
+           &vs::ImHelper::StartWriteSerice,
+           "start a thread to maintain write service")
+      .def("stop_service",
+           &vs::ImHelper::StopService,
+           "stop the service thread");
 
-  m.def("start_read_service",
-        &vs::start_read_service,
-        "global information-maintainer object.");
-  m.def("start_write_service",
-        &vs::start_write_service,
-        "global information-maintainer object.");
-  m.def("im", &vs::im);
-  m.def("stop_threads", &vs::stop_threads);
+// interfaces for components begin
 
-// interfaces for components
+// different data type of scalar conponent
 #define ADD_SCALAR_TYPED_INTERFACE(T, name__)                             \
   py::class_<vs::components::ScalarHelper<T>>(m, #name__)                 \
       .def("add_record", &vs::components::ScalarHelper<T>::AddRecord)     \
diff --git a/visualdl/logic/sdk.cc b/visualdl/logic/sdk.cc
index 513541511d48adc00e1a95f5bf51974fb0b1988f..e3283812569ece8183c96451a0026da6a2683572 100644
--- a/visualdl/logic/sdk.cc
+++ b/visualdl/logic/sdk.cc
@@ -66,6 +66,8 @@ namespace components {
 
 template <typename T>
 void ScalarHelper<T>::SetCaptions(const std::vector<std::string> &captions) {
+  ACQUIRE_HANDLER(handler_);
+
   CHECK_EQ(data_->captions_size(), 0UL) << "the captions can set only once";
   for (int i = 0; i < captions.size(); i++) {
     data_->add_captions(captions[i]);
@@ -74,6 +76,8 @@ void ScalarHelper<T>::SetCaptions(const std::vector<std::string> &captions) {
 
 template <typename T>
 void ScalarHelper<T>::AddRecord(int id, const std::vector<T> &values) {
+  ACQUIRE_HANDLER(handler_);
+
   CHECK_NOTNULL(data_);
   CHECK_GT(data_->captions_size(), 0UL) << "captions should be set first";
   CHECK_EQ(data_->captions_size(), values.size())
@@ -94,6 +98,8 @@ void ScalarHelper<T>::AddRecord(int id, const std::vector<T> &values) {
 
 template <typename T>
 std::vector<std::vector<T>> ScalarHelper<T>::GetRecords() const {
+  ACQUIRE_HANDLER(handler_);
+
   std::vector<std::vector<T>> result;
   EntryHelper<T> entry_helper;
   for (int i = 0; i < data_->records_size(); i++) {
@@ -107,6 +113,7 @@ std::vector<std::vector<T>> ScalarHelper<T>::GetRecords() const {
 
 template <typename T>
 std::vector<int> ScalarHelper<T>::GetIds() const {
+  ACQUIRE_HANDLER(handler_);
   CHECK_NOTNULL(data_);
   std::vector<int> result;
   for (int i = 0; i < data_->records_size(); i++) {
@@ -117,6 +124,7 @@ std::vector<int> ScalarHelper<T>::GetIds() const {
 
 template <typename T>
 std::vector<int> ScalarHelper<T>::GetTimestamps() const {
+  ACQUIRE_HANDLER(handler_);
   CHECK_NOTNULL(data_);
   std::vector<int> result;
   for (int i = 0; i < data_->records_size(); i++) {
@@ -127,6 +135,7 @@ std::vector<int> ScalarHelper<T>::GetTimestamps() const {
 
 template <typename T>
 std::vector<std::string> ScalarHelper<T>::GetCaptions() const {
+  ACQUIRE_HANDLER(handler_);
   return std::vector<std::string>(data_->captions().begin(),
                                   data_->captions().end());
 }
diff --git a/visualdl/logic/sdk.h b/visualdl/logic/sdk.h
index 3937ffafbb91ac2792cb88a242a3833fa2bfc3e4..b74e33174291d31baf44d430116eb86d583c02a6 100644
--- a/visualdl/logic/sdk.h
+++ b/visualdl/logic/sdk.h
@@ -94,33 +94,50 @@ private:
 
 class ImHelper {
 public:
-  ImHelper() {}
+  // TODO(ChunweiYan) decouple helper with resource.
+  ImHelper() { im_.reset(new IM); }
+  ImHelper(std::unique_ptr<IM> im) : im_(std::move(im)) {}
 
   StorageHelper storage() {
-    return StorageHelper(IM::Global().storage().mutable_data());
+    return StorageHelper(im_->storage().mutable_data());
   }
   TabletHelper tablet(const std::string &tag) {
-    return TabletHelper(IM::Global().storage().tablet(tag));
+    return TabletHelper(im_->storage().tablet(tag));
   }
   TabletHelper AddTablet(const std::string &tag, int num_samples) {
-    return TabletHelper(IM::Global().AddTablet(tag, num_samples));
+    return TabletHelper(im_->AddTablet(tag, num_samples));
   }
-  void ClearTablets() {
-    IM::Global().storage().mutable_data()->clear_tablets();
+  std::mutex &handler() { return im_->handler(); }
+  void ClearTablets() { im_->storage().mutable_data()->clear_tablets(); }
+  void StartReadService(const std::string &dir, int msecs) {
+    im_->SetPersistDest(dir);
+    im_->MaintainRead(dir, msecs);
   }
+  void StartWriteSerice(const std::string &dir, int msecs) {
+    im_->SetPersistDest(dir);
+    im_->MaintainWrite(dir, msecs);
+  }
+  void StopService() { im_->executor().Quit(); }
+  void PersistToDisk() const { im_->PersistToDisk(); }
 
-  void PersistToDisk() const { IM::Global().PersistToDisk(); }
+private:
+  std::unique_ptr<IM> im_;
 };
 
 namespace components {
 
+#define ACQUIRE_HANDLER(handler) std::lock_guard<std::mutex> ____(*handler);
+
 /*
  * Read and write support for Scalar component.
  */
 template <typename T>
 class ScalarHelper {
 public:
-  ScalarHelper(storage::Tablet *tablet) : data_(tablet) {}
+  ScalarHelper(storage::Tablet *tablet, std::mutex *handler = nullptr)
+      : data_(tablet), handler_(handler) {}
+  ScalarHelper(TabletHelper &tablet, std::mutex *handler = nullptr)
+      : data_(&tablet.data()), handler_(handler) {}
 
   void SetCaptions(const std::vector<std::string> &captions);
 
@@ -138,27 +155,10 @@ public:
 
 private:
   storage::Tablet *data_;
+  std::mutex *handler_;
 };
 
 }  // namespace components
-
-static ImHelper &im() {
-  static ImHelper im;
-  return im;
-}
-
-static void start_read_service(const std::string &dir) {
-  IM::Global().SetPersistDest(dir);
-  IM::Global().MaintainRead();
-}
-
-static void start_write_service(const std::string &dir) {
-  IM::Global().SetPersistDest(dir);
-  IM::Global().MaintainWrite();
-}
-
-static void stop_threads() { cc::PeriodExector::Global().Quit(); }
-
 }  // namespace visualdl
 
 #endif  // VISUALDL_BACKEND_LOGIC_SDK_H
diff --git a/visualdl/storage/storage.cc b/visualdl/storage/storage.cc
index 2b88ad091a9810fb7473c12fa6daa4c2a214017b..814315454a903341ed102135d01b81438f6dccb3 100644
--- a/visualdl/storage/storage.cc
+++ b/visualdl/storage/storage.cc
@@ -9,13 +9,14 @@ namespace visualdl {
 
 const std::string StorageBase::meta_file_name = "storage.meta";
 
-std::string StorageBase::meta_path() const {
-  CHECK(!storage_.dir().empty()) << "storage.dir should be set first";
-  return storage_.dir() + "/" + meta_file_name;
+std::string StorageBase::meta_path(const std::string &dir) const {
+  CHECK(!dir.empty()) << "dir is empty";
+  return dir + "/" + meta_file_name;
 }
-std::string StorageBase::tablet_path(const std::string &tag) const {
-  CHECK(!storage_.dir().empty()) << "storage.dir should be set first";
-  return storage_.dir() + "/" + tag;
+std::string StorageBase::tablet_path(const std::string &dir,
+                                     const std::string &tag) const {
+  CHECK(!dir.empty()) << "dir should be set first";
+  return dir + "/" + tag;
 }
 
 storage::Tablet *MemoryStorage::NewTablet(const std::string &tag,
@@ -39,18 +40,20 @@ storage::Tablet *MemoryStorage::tablet(const std::string &tag) {
 }
 
 // TODO add some checksum to avoid unnecessary saving
-void MemoryStorage::PersistToDisk() const {
-  CHECK(!storage_.dir().empty()) << "storage's dir should be set first";
-  VLOG(3) << "persist storage to disk path " << storage_.dir();
+void MemoryStorage::PersistToDisk(const std::string &dir) {
+  CHECK(!dir.empty());
+  storage_.set_dir(dir);
   // make a directory if not exist
-  fs::TryMkdir(storage_.dir());
+  fs::TryMkdir(dir);
   // write storage out
-  fs::SerializeToFile(storage_, meta_path());
+  LOG(INFO) << "to serize meta to dir " << dir;
+  fs::SerializeToFile(storage_, meta_path(dir));
+  LOG(INFO) << "serize meta to dir " << dir;
   // write all the tablets
   for (auto tag : storage_.tags()) {
     auto it = tablets_.find(tag);
     CHECK(it != tablets_.end());
-    fs::SerializeToFile(it->second, tablet_path(tag));
+    fs::SerializeToFile(it->second, tablet_path(dir, tag));
   }
 }
 
@@ -59,33 +62,53 @@ void MemoryStorage::LoadFromDisk(const std::string &dir) {
   CHECK(!dir.empty()) << "dir is empty";
   storage_.set_dir(dir);
   // load storage
-  CHECK(fs::DeSerializeFromFile(&storage_, meta_path()))
-      << "parse from " << meta_path() << " failed";
+  CHECK(fs::DeSerializeFromFile(&storage_, meta_path(dir)))
+      << "parse from " << meta_path(dir) << " failed";
 
   // load all the tablets
   for (int i = 0; i < storage_.tags_size(); i++) {
     auto tag = storage_.tags(i);
-    CHECK(fs::DeSerializeFromFile(&tablets_[tag], tablet_path(tag)));
+    CHECK(fs::DeSerializeFromFile(&tablets_[tag], tablet_path(dir, tag)));
   }
 }
 
-void MemoryStorage::StartReadService() {
-  cc::PeriodExector::task_t task = [this] {
-    VLOG(3) << "loading from " << storage_.dir();
-    LoadFromDisk(storage_.dir());
+void MemoryStorage::StartReadService(const std::string &dir,
+                                     int msecs,
+                                     std::mutex *handler) {
+  CHECK(executor_ != nullptr);
+  CHECK(!dir.empty()) << "dir should be set first";
+  cc::PeriodExector::task_t task = [dir, this, handler] {
+    LOG(INFO) << "loading from " << dir;
+    if (handler != nullptr) {
+      std::lock_guard<std::mutex> _(*handler);
+      LoadFromDisk(dir);
+    } else {
+      LoadFromDisk(dir);
+    }
     return true;
   };
-  cc::PeriodExector::Global().Start();
-  VLOG(3) << "push read task";
-  cc::PeriodExector::Global()(std::move(task), 2512);
+  // executor_.Start();
+  LOG(INFO) << "push read task";
+  (*executor_)(std::move(task), msecs);
 }
 
-void MemoryStorage::StartWriteSerice() {
-  cc::PeriodExector::Global().Start();
-  cc::PeriodExector::task_t task = [this] {
-    PersistToDisk();
+void MemoryStorage::StartWriteSerice(const std::string &dir,
+                                     int msecs,
+                                     std::mutex *handler) {
+  CHECK(executor_ != nullptr);
+  CHECK(!dir.empty()) << "dir should be set first";
+  storage_.set_dir(dir);
+  // executor_.Start();
+  cc::PeriodExector::task_t task = [dir, handler, this] {
+    LOG(INFO) << "persist to disk";
+    if (handler != nullptr) {
+      std::lock_guard<std::mutex> _(*handler);
+      PersistToDisk(dir);
+    } else {
+      PersistToDisk(dir);
+    }
     return true;
   };
-  cc::PeriodExector::Global()(std::move(task), 2000);
+  (*executor_)(std::move(task), msecs);
 }
 }  // namespace visualdl
diff --git a/visualdl/storage/storage.h b/visualdl/storage/storage.h
index 2787d9af87a9547921531b7dc509798a70001b71..6c60b9302ddfd1e7409a696fc496dfdae6d3c574 100644
--- a/visualdl/storage/storage.h
+++ b/visualdl/storage/storage.h
@@ -6,6 +6,7 @@
 #include <string>
 
 #include "visualdl/storage/storage.pb.h"
+#include "visualdl/utils/concurrency.h"
 
 namespace visualdl {
 
@@ -36,8 +37,8 @@ public:
     storage_.set_dir(dir);
   }
 
-  std::string meta_path() const;
-  std::string tablet_path(const std::string &tag) const;
+  std::string meta_path(const std::string &dir) const;
+  std::string tablet_path(const std::string &dir, const std::string &tag) const;
 
   /*
    * Create a new Tablet storage.
@@ -56,7 +57,7 @@ public:
    * Persist the data from cache to disk. Both the memory storage or disk
    * storage should write changes to disk for persistence.
    */
-  virtual void PersistToDisk() const = 0;
+  virtual void PersistToDisk(const std::string &dir) = 0;
 
   /*
    * Load data from disk.
@@ -75,28 +76,39 @@ protected:
  */
 class MemoryStorage final : public StorageBase {
 public:
+  MemoryStorage() {}
+  MemoryStorage(cc::PeriodExector *executor) : executor_(executor) {}
+  ~MemoryStorage() {
+    if (executor_ != nullptr) executor_->Quit();
+  }
   storage::Tablet *NewTablet(const std::string &tag, int num_samples) override;
 
   storage::Tablet *tablet(const std::string &tag) override;
 
-  void PersistToDisk() const override;
+  void PersistToDisk(const std::string &dir) override;
 
   void LoadFromDisk(const std::string &dir) override;
 
   /*
    * Create a thread which will keep reading the latest data from the disk to
    * memory.
+   *
+   * msecs: how many millisecond to sync memory and disk.
    */
-  void StartReadService();
+  void StartReadService(const std::string &dir, int msecs, std::mutex *handler);
 
   /*
    * Create a thread which will keep writing the latest changes from memory to
    * disk.
+   *
+   * msecs: how many millisecond to sync memory and disk.
    */
-  void StartWriteSerice();
+  void StartWriteSerice(const std::string &dir, int msecs, std::mutex *handler);
 
 private:
   std::map<std::string, storage::Tablet> tablets_;
+  // TODO(ChunweiYan) remove executor here.
+  cc::PeriodExector *executor_{nullptr};
 };
 
 }  // namespace visualdl
diff --git a/visualdl/storage/storage_test.cc b/visualdl/storage/storage_test.cc
index 9ddf13a36153b8bbdb2b14ceabea7c79c971efbe..8dbc7f566735d5b0af7889e5c365285c60df4573 100644
--- a/visualdl/storage/storage_test.cc
+++ b/visualdl/storage/storage_test.cc
@@ -32,15 +32,17 @@ TEST_F(MemoryStorageTest, AddTablet) {
 }
 
 TEST_F(MemoryStorageTest, PersistToDisk) {
-  storage_.SetStorage("./tmp");
-  CHECK(!storage_.data().dir().empty());
+  const std::string dir = "./tmp/201.test";
+  storage_.SetStorage(dir);
   string tag = "add%20tag0";
   storage_.NewTablet(tag, -1);
 
-  storage_.PersistToDisk();
+  storage_.PersistToDisk(dir);
+  LOG(INFO) << "persist to disk";
 
   MemoryStorage other;
-  other.LoadFromDisk("./tmp");
+  other.LoadFromDisk(dir);
+  LOG(INFO) << "read from disk";
   ASSERT_EQ(other.data().SerializeAsString(),
             storage_.data().SerializeAsString());
 }
diff --git a/visualdl/utils/concurrency.h b/visualdl/utils/concurrency.h
index 5a8c320dfd4f57a28c1e420d92166504defd5c25..cb73dc2b4d153360c48c2bc24220f9c96779186c 100644
--- a/visualdl/utils/concurrency.h
+++ b/visualdl/utils/concurrency.h
@@ -17,13 +17,9 @@ namespace cc {
 struct PeriodExector {
   using task_t = std::function<bool()>;
 
-  static PeriodExector& Global() {
-    static PeriodExector exec;
-    return exec;
-  }
-
   void Quit() {
     // TODO use some conditonal variable to help quit immediately.
+    // std::this_thread::sleep_for(std::chrono::milliseconds(200));
     quit = true;
   }
 
@@ -34,6 +30,7 @@ struct PeriodExector {
 
     auto task_wrapper = [=] {
       while (!quit) {
+        // task failed
         if (!task()) break;
         // if the program is terminated, quit while as soon as possible.
         // this is just trick, but should works.
@@ -57,6 +54,7 @@ struct PeriodExector {
   }
 
   ~PeriodExector() {
+    Quit();
     for (auto& t : threads_) {
       if (t.joinable()) {
         t.join();
diff --git a/visualdl/utils/test_concurrency.cc b/visualdl/utils/test_concurrency.cc
index 96f18caabcf13a49fc4c98f31e3c0f7ff37cede2..ea4540d33b0cb3921b373f670fee3fd81212a8cd 100644
--- a/visualdl/utils/test_concurrency.cc
+++ b/visualdl/utils/test_concurrency.cc
@@ -8,12 +8,13 @@ namespace visualdl {
 int counter = 0;
 
 TEST(concurrency, test) {
-  cc::PeriodExector::task_t task = [&counter]() {
+  cc::PeriodExector executor;
+  cc::PeriodExector::task_t task = [&]() {
     LOG(INFO) << "Hello " << counter++;
     if (counter > 5) return false;
     return true;
   };
-  cc::PeriodExector::Global()(std::move(task), 200);
+  executor(std::move(task), 200);
 }
 
 }  // namespace visualdl