diff --git a/paddle/fluid/train/custom_trainer/feed/accessor/dense_input_accessor.cc b/paddle/fluid/train/custom_trainer/feed/accessor/dense_input_accessor.cc
index 81779323d85d68ab857e670c0dc75518535b9697..4b1324caa8663eded05afadc18bff4751f10a9cb 100644
--- a/paddle/fluid/train/custom_trainer/feed/accessor/dense_input_accessor.cc
+++ b/paddle/fluid/train/custom_trainer/feed/accessor/dense_input_accessor.cc
@@ -52,7 +52,10 @@ int32_t DenseInputAccessor::create(::paddle::framework::Scope* scope) {
 
 // rpc拉取数据，需保证单线程运行
 int32_t DenseInputAccessor::pull_dense(size_t table_id) {
-    float* data_buffer =  new float[_total_dim];
+    float* data_buffer =  _data_buffer;
+    if (data_buffer == NULL) {
+        data_buffer = new float[_total_dim];
+    }
     size_t data_buffer_idx = 0;
     std::vector<paddle::ps::Region> regions;
     for (auto& variable : _x_variables) {
@@ -128,10 +131,11 @@ int32_t DenseInputAccessor::collect_persistables_name(std::vector<std::string>&
     return 0;
 }
 
-int32_t DenseInputAccessor::backward(SampleInstance* samples, size_t num,
+std::future<int32_t> DenseInputAccessor::backward(SampleInstance* samples, size_t num,
         paddle::framework::Scope* scope) {
+    std::future<int32_t> ret;
     if (!_need_gradient) {
-        return 0;
+        return ret;
     }
     size_t data_buffer_idx = 0;
     std::vector<paddle::ps::Region> regions;
@@ -142,8 +146,7 @@ int32_t DenseInputAccessor::backward(SampleInstance* samples, size_t num,
         regions.emplace_back(grad_data, variable.dim);
     }
     auto* ps_client = _trainer_context->pslib->ps_client();
-    auto push_status = ps_client->push_dense(regions.data(), regions.size(), _table_id);
-    //push_status.get();
+    ps_client->push_dense(regions.data(), regions.size(), _table_id);
     if (!FLAGS_feed_trainer_debug_dense_name.empty()) {
         for (auto& variable : _x_variables) {
             if (variable.name != FLAGS_feed_trainer_debug_dense_name) {
@@ -152,7 +155,8 @@ int32_t DenseInputAccessor::backward(SampleInstance* samples, size_t num,
             VLOG(2) << "[Debug][PushDense]" << ScopeHelper::to_string(scope, variable.gradient_name);
         }
     }
-    return 0;
+    // not wait dense push
+    return ret;
 }
 
 int32_t EbdVariableInputAccessor::forward(SampleInstance* samples, size_t num,
@@ -171,10 +175,10 @@ int32_t EbdVariableInputAccessor::forward(SampleInstance* samples, size_t num,
     }
     return 0;
 }
-
-int32_t EbdVariableInputAccessor::backward(SampleInstance* samples, size_t num,
+std::future<int32_t> EbdVariableInputAccessor::backward(SampleInstance* samples, size_t num,
     paddle::framework::Scope* scope) {
-    return 0;
+    std::future<int32_t> ret;
+    return ret;
 }
 
 REGIST_CLASS(DataInputAccessor, DenseInputAccessor);
diff --git a/paddle/fluid/train/custom_trainer/feed/accessor/epoch_accessor.cc b/paddle/fluid/train/custom_trainer/feed/accessor/epoch_accessor.cc
index d874f9711101501c14380e864a4f3db6fd9d6b9a..71fd83a13fedf81428479eca9d7c1e6a035b45f9 100644
--- a/paddle/fluid/train/custom_trainer/feed/accessor/epoch_accessor.cc
+++ b/paddle/fluid/train/custom_trainer/feed/accessor/epoch_accessor.cc
@@ -22,8 +22,10 @@ namespace feed {
         }
         std::string done_text = fs->tail(_done_file_path);
         _done_status = paddle::string::split_string(done_text, std::string("\t"));
-        _current_epoch_id = get_status<uint64_t>(EpochStatusFiled::EpochIdField);
+        _last_done_epoch_id = get_status<uint64_t>(EpochStatusFiled::EpochIdField);
         _last_checkpoint_epoch_id = get_status<uint64_t>(EpochStatusFiled::CheckpointIdField);
+        // 训练需要从上一个checkpoint对应的epoch开始
+        _current_epoch_id = _last_checkpoint_epoch_id;
         _last_checkpoint_path = get_status<std::string>(EpochStatusFiled::CheckpointPathField);
         _inference_base_model_key = get_status<uint64_t>(EpochStatusFiled::InferenceBaseKeyField);
         _inference_model_path = fs->path_join(_model_root_path, config["inference_model_dir"].as<std::string>("xbox"));
@@ -45,8 +47,14 @@ namespace feed {
         set_status(EpochStatusFiled::TimestampField, now.tv_sec);
         set_status(EpochStatusFiled::CheckpointIdField, _last_checkpoint_epoch_id);
         set_status(EpochStatusFiled::CheckpointPathField, _last_checkpoint_path);
-        set_status(EpochStatusFiled::DateField, format_timestamp(epoch_id, "%Y%m%d"));
+        set_status(EpochStatusFiled::DateField, format_timestamp(epoch_id, "%Y%m%d-%H%M"));
         set_status(EpochStatusFiled::InferenceBaseKeyField, _inference_base_model_key);
+        if (epoch_id > _last_done_epoch_id) {
+            // 保留末尾1000数据
+            auto fs = _trainer_context->file_system.get();
+            std::string done_str = paddle::string::join_strings(_done_status, '\t');
+            fs->append_line(_done_file_path, done_str, 1000); 
+        }
         return 0;
     }
     
@@ -59,20 +67,18 @@ namespace feed {
         }
         std::string done_str;
         std::string donefile;
+        auto fs = _trainer_context->file_system.get();
         auto model_path = model_save_path(epoch_id, save_way);
         std::string inference_done_format("{\"id\":\"%lu\",\"key\":\"%lu\",\"input\":\"%s/000\",\"record_count\":\"1\",\"file_format\":\"pb\",\"schema_version\":\"2\",\"partition_type\":\"1\",\"job_name\":\"%s\",\"job_id\":\"%s\",\"mpi_size\":\"%d\",\"monitor_data\":\"%s\"}");
         
         auto id = time(NULL);
         switch (save_way) {
-        case ModelSaveWay::ModelSaveTrainCheckpoint:
-            donefile = _done_file_path;
-            done_str = paddle::string::join_strings(_done_status, '\t');
-            break;
         case ModelSaveWay::ModelSaveInferenceDelta:
             donefile = _inference_model_delta_done_path;
             done_str = string::format_string(inference_done_format.c_str(), id, _inference_base_model_key, 
                 model_path.c_str(), env->job_name().c_str(), env->job_id().c_str(),
                 env->node_num(EnvironmentRole::PSERVER), _trainer_context->monitor_ssm.str().c_str());
+            fs->append_line(donefile, done_str, 1000); 
             break;
         case ModelSaveWay::ModelSaveInferenceBase:
             donefile = _inference_model_base_done_path;
@@ -80,30 +86,9 @@ namespace feed {
             done_str = string::format_string(inference_done_format.c_str(), id, id, 
                 model_path.c_str(), env->job_name().c_str(), env->job_id().c_str(),
                 env->node_num(EnvironmentRole::PSERVER), _trainer_context->monitor_ssm.str().c_str());
+            fs->append_line(donefile, done_str, 1000); 
             break;
         }
-        // 保留末尾1000数据
-        std::string tail_done_info;
-        auto fs = _trainer_context->file_system.get();
-        if (fs->exists(donefile)) {
-            tail_done_info = paddle::string::trim_spaces(fs->tail(donefile, 1000)); 
-        }
-        if (tail_done_info.size() > 0) {
-            tail_done_info = tail_done_info + "\n" + done_str;
-        } else {
-            tail_done_info = done_str;
-        }
-        VLOG(2) << "Write donefile " << donefile << ", str:" << done_str;
-        bool write_success = false;
-        while (true) {
-            fs->remove(donefile);
-            auto fp = fs->open_write(donefile, "");
-            if (fwrite(tail_done_info.c_str(), tail_done_info.length(), 1, &*fp) == 1) {
-                break;
-            }     
-            sleep(10);   
-        }
-        VLOG(2) << "Write donefile " << donefile << "success";
         return 0;
     }
 
@@ -155,7 +140,9 @@ namespace feed {
         }
         switch (save_way) {
             case ModelSaveWay::ModelSaveInferenceDelta:
-                return delta_id(epoch_id) % 6 == 0;
+                // 重启训练后，中间的delta不重复dump
+                return epoch_id > _last_done_epoch_id && 
+                    delta_id(epoch_id) % 6 == 0;
             case ModelSaveWay::ModelSaveInferenceBase:
                 return is_last_epoch(epoch_id);
             case ModelSaveWay::ModelSaveTrainCheckpoint:
diff --git a/paddle/fluid/train/custom_trainer/feed/accessor/epoch_accessor.h b/paddle/fluid/train/custom_trainer/feed/accessor/epoch_accessor.h
index 7d42e73e7c44e0914973b7aaf7d1c6f8521d707a..9c9f2058b0c30a16a0bf5943bbc30cff71dded79 100644
--- a/paddle/fluid/train/custom_trainer/feed/accessor/epoch_accessor.h
+++ b/paddle/fluid/train/custom_trainer/feed/accessor/epoch_accessor.h
@@ -73,6 +73,7 @@ protected:
     std::string _inference_model_delta_done_path;
     uint64_t _current_epoch_id = 0;
     std::string _last_checkpoint_path;
+    uint64_t _last_done_epoch_id = 0;
     uint64_t _last_checkpoint_epoch_id = 0;
     std::vector<std::string> _done_status;  // 当前完成状态，统一存成string
     uint64_t _inference_base_model_key = 0; // 预估模型的base-key
diff --git a/paddle/fluid/train/custom_trainer/feed/accessor/input_data_accessor.h b/paddle/fluid/train/custom_trainer/feed/accessor/input_data_accessor.h
index 52d07b6b0598f2a1f4f87f44bba2d88b874b7582..fd7ce48753f0c804579a6a252ba6bbc9785d50b9 100644
--- a/paddle/fluid/train/custom_trainer/feed/accessor/input_data_accessor.h
+++ b/paddle/fluid/train/custom_trainer/feed/accessor/input_data_accessor.h
@@ -35,8 +35,9 @@ public:
     virtual int32_t forward(SampleInstance* samples, size_t num,
         ::paddle::framework::Scope* scope) = 0;
 
-    // 后向，一般用于更新梯度，在训练网络执行后调用
-    virtual int32_t backward(SampleInstance* samples, size_t num,
+    // 后向，一般用于更新梯度，在训练网络执行后调用, 由于backward一般是异步，这里返回future， 
+    // TODO 前向接口也改为future返回形式，接口一致性好些
+    virtual std::future<int32_t> backward(SampleInstance* samples, size_t num,
         ::paddle::framework::Scope* scope) = 0;
 
     // 收集持久化变量的名称, 并将值拷贝到Scope
@@ -67,7 +68,7 @@ public:
     virtual int32_t forward(SampleInstance* samples, size_t num,
         ::paddle::framework::Scope* scope);
 
-    virtual int32_t backward(SampleInstance* samples, size_t num,
+    virtual std::future<int32_t> backward(SampleInstance* samples, size_t num,
         ::paddle::framework::Scope* scope);
 protected:
     size_t _label_total_dim = 0; 
@@ -108,7 +109,7 @@ public:
     virtual void post_process_input(float* var_data, SparseInputVariable&, SampleInstance*, size_t num) = 0;
 
     // backward过程的梯度push
-    virtual int32_t backward(SampleInstance* samples, size_t num,
+    virtual std::future<int32_t> backward(SampleInstance* samples, size_t num,
         paddle::framework::Scope* scope);    
     // SparseGradValue会被依次调用，用于整理push的梯度
     virtual void fill_gradient(float* push_value, const float* gradient_raw, 
@@ -148,7 +149,7 @@ public:
     virtual int32_t forward(SampleInstance* samples, size_t num,
         paddle::framework::Scope* scope);
 
-    virtual int32_t backward(SampleInstance* samples, size_t num,
+    virtual std::future<int32_t> backward(SampleInstance* samples, size_t num,
         paddle::framework::Scope* scope);
 
 
@@ -175,7 +176,7 @@ public:
     virtual int32_t forward(SampleInstance* samples, size_t num,
         paddle::framework::Scope* scope);
 
-    virtual int32_t backward(SampleInstance* samples, size_t num,
+    virtual std::future<int32_t> backward(SampleInstance* samples, size_t num,
         paddle::framework::Scope* scope);
 };
 
diff --git a/paddle/fluid/train/custom_trainer/feed/accessor/label_input_accessor.cc b/paddle/fluid/train/custom_trainer/feed/accessor/label_input_accessor.cc
index 5b5a90b8f4fa84983c12fc05a9ad6269487d6ac8..5a38d8ee35d40d152090216d0284e7be7be91587 100644
--- a/paddle/fluid/train/custom_trainer/feed/accessor/label_input_accessor.cc
+++ b/paddle/fluid/train/custom_trainer/feed/accessor/label_input_accessor.cc
@@ -45,10 +45,11 @@ int32_t LabelInputAccessor::forward(SampleInstance* samples, size_t num,
     return 0;
 }
 
-int32_t LabelInputAccessor::backward(SampleInstance* samples, size_t num,
+std::future<int32_t> LabelInputAccessor::backward(SampleInstance* samples, size_t num,
         paddle::framework::Scope* scope) {
+    std::future<int32_t> ret;
     if (num < 1) {
-        return 0;
+        return ret;
     }
     for (size_t i = 0; i < num; ++i) {
         auto& sample = samples[i];
@@ -69,7 +70,7 @@ int32_t LabelInputAccessor::backward(SampleInstance* samples, size_t num,
         VLOG(2) << "[Debug][Lable]" << ScopeHelper::to_string(scope, label.label_name) << ScopeHelper::to_string(scope, label.output_name);
     }
     */
-    return 0;
+    return ret;
 }
 
 REGIST_CLASS(DataInputAccessor, LabelInputAccessor);
diff --git a/paddle/fluid/train/custom_trainer/feed/accessor/sparse_input_accessor.cc b/paddle/fluid/train/custom_trainer/feed/accessor/sparse_input_accessor.cc
index 68fb7f78cbdb4ccb6b378713c223f6ffbaa9d6f1..26d9e2fee98c5b88407d1fbaea832ea57bbd7043 100644
--- a/paddle/fluid/train/custom_trainer/feed/accessor/sparse_input_accessor.cc
+++ b/paddle/fluid/train/custom_trainer/feed/accessor/sparse_input_accessor.cc
@@ -136,8 +136,9 @@ int32_t BaseSparseInputAccessor::forward(SampleInstance* samples,
 }
 
 // 更新spare数据
-int32_t BaseSparseInputAccessor::backward(SampleInstance* samples,
+std::future<int32_t> BaseSparseInputAccessor::backward(SampleInstance* samples,
     size_t num, paddle::framework::Scope* scope) {
+    std::future<int32_t> ret;
     int64_t runtime_data_for_scope = *ScopeHelper::get_value<int64_t>(
             scope, _trainer_context->cpu_place, "sparse_runtime_data");
     auto* runtime_data_ptr = (std::vector<SparseVarRuntimeData>*)runtime_data_for_scope;
@@ -146,7 +147,7 @@ int32_t BaseSparseInputAccessor::backward(SampleInstance* samples,
         delete runtime_data_ptr;
     });
     if (!_need_gradient) {
-        return 0;
+        return ret;
     }
     auto* ps_client = _trainer_context->pslib->ps_client();
     auto* value_accessor = ps_client->table_accessor(_table_id);
@@ -204,11 +205,10 @@ int32_t BaseSparseInputAccessor::backward(SampleInstance* samples,
             VLOG(2) << "[DEBUG][sparse_slot_push]" << ssm.str();
         }   
     }
-    auto push_status = ps_client->push_sparse(_table_id, 
-        keys.data(), (const float**)push_values, key_idx);
-    //auto ret = push_status.get();
+    ret = ps_client->push_sparse(_table_id, 
+          keys.data(), (const float**)push_values, key_idx);
     delete[] push_values;
-    return 0;
+    return ret;
 } 
 
 class AbacusSparseJoinAccessor : public BaseSparseInputAccessor {
diff --git a/paddle/fluid/train/custom_trainer/feed/common/pslib_warpper.cc b/paddle/fluid/train/custom_trainer/feed/common/pslib_warpper.cc
index 7a557ec69d34e7a184b0157e90e84b755b4060cb..ff07bf4f3227ed3aff706760da2b2628f95fe3fd 100644
--- a/paddle/fluid/train/custom_trainer/feed/common/pslib_warpper.cc
+++ b/paddle/fluid/train/custom_trainer/feed/common/pslib_warpper.cc
@@ -70,7 +70,6 @@ paddle::PSParameter* PSlib::get_param() {
 void PSlib::init_gflag() {
     int cnt = 4;
     char** params_ptr = new char*[cnt];
-    std::cout << "alloc_ptr" << params_ptr << std::flush;
     char p0[] = "exe default";
     char p1[] = "-max_body_size=314217728";
     char p2[] = "-bthread_concurrency=40";
diff --git a/paddle/fluid/train/custom_trainer/feed/executor/multi_thread_executor.cc b/paddle/fluid/train/custom_trainer/feed/executor/multi_thread_executor.cc
index 456531329ac77cff13ea1b4ea727c82f66ad5d0c..6dfb0a71e467717ea4e64cde950a3ab9b45739ee 100644
--- a/paddle/fluid/train/custom_trainer/feed/executor/multi_thread_executor.cc
+++ b/paddle/fluid/train/custom_trainer/feed/executor/multi_thread_executor.cc
@@ -9,6 +9,10 @@ namespace paddle {
 namespace custom_trainer {
 namespace feed {
 
+std::once_flag MultiThreadExecutor::_async_delete_flag;
+std::shared_ptr<std::thread> MultiThreadExecutor::_async_delete_thread;
+paddle::framework::Channel<ScopeExecutorContext*> MultiThreadExecutor::_delete_channel;
+
 int MultiThreadExecutor::initialize(YAML::Node exe_config, 
     std::shared_ptr<TrainerContext> context_ptr) {
     int ret = 0;
@@ -85,6 +89,23 @@ int MultiThreadExecutor::initialize(YAML::Node exe_config,
         CHECK(monitor_ptr->initialize(monitor_config, context_ptr) == 0)
             << "Monitor init Failed, class:" << monitor_class;
     }
+
+    // 异步删除池
+    std::call_once(_async_delete_flag, [this](){
+        _delete_channel = paddle::framework::MakeChannel<ScopeExecutorContext*>();
+        _delete_channel->SetBlockSize(32);
+        _async_delete_thread.reset(new std::thread([this]{
+            std::vector<ScopeExecutorContext*> ctxs;
+            while (true) {
+                while (_delete_channel->Read(ctxs)) {
+                    for (auto* ctx : ctxs) {
+                        delete ctx;
+                    }
+                }
+                usleep(200000); // 200ms
+            }
+        }));
+    });
     return ret;
 }
 
@@ -187,9 +208,10 @@ paddle::framework::Channel<DataItem> MultiThreadExecutor::run(
                 auto* samples = scope_ctx->samples();
                 auto sample_num = scope_ctx->sample_num();
                 
+                out_items[out_idx] = 0;
+                scope_ctx->wait_status.resize(_input_accessors.size());
                 for (size_t i = 0; i < _input_accessors.size(); ++i) {
-                    out_items[out_idx] = _input_accessors[i]->
-                        backward(samples, sample_num, scope);
+                    scope_ctx->wait_status[i] = _input_accessors[i]->backward(samples, sample_num, scope);
                 }
                 timer.Pause();
                 scope_ctx->push_gradient_cost_ms = timer.ElapsedMS();
@@ -203,7 +225,8 @@ paddle::framework::Channel<DataItem> MultiThreadExecutor::run(
                         VLOG(2) << "[Debug][Layer]" << ScopeHelper::to_string(scope, layer_name);
                     }
                 }
-                delete scope_ctx; // 所有pipe完成后，再回收sample
+                 // 所有pipe完成后，再异步回收sample
+                _delete_channel->Put(scope_ctx);
             }
             return 0;
     });
diff --git a/paddle/fluid/train/custom_trainer/feed/executor/multi_thread_executor.h b/paddle/fluid/train/custom_trainer/feed/executor/multi_thread_executor.h
index 8aaecbb0b5519bf1031672909ef63cec0fcd3533..2db7d09f73bce98670ece003a6c6b20f375613bc 100644
--- a/paddle/fluid/train/custom_trainer/feed/executor/multi_thread_executor.h
+++ b/paddle/fluid/train/custom_trainer/feed/executor/multi_thread_executor.h
@@ -1,4 +1,5 @@
 #pragma once
+#include <thread>
 #include <functional>
 #include "paddle/fluid/framework/channel.h"
 #include "paddle/fluid/train/custom_trainer/feed/executor/executor.h"
@@ -18,6 +19,12 @@ public:
         _sample_num = sample_num;
     }
     virtual ~ScopeExecutorContext() {
+        for (auto& status : wait_status) {
+            if (!status.valid()) {
+                continue;
+            }
+            status.wait();
+        }
         delete[] _samples;
     }
     inline SampleInstance* samples() {
@@ -29,6 +36,7 @@ public:
     size_t executor_cost_ms = 0;
     size_t prepare_cost_ms = 0;
     size_t push_gradient_cost_ms = 0;
+    std::vector<std::future<int32_t>> wait_status;
 private:
     size_t _sample_num = 0;
     SampleInstance* _samples = NULL;
@@ -83,6 +91,11 @@ protected:
     std::map<uint32_t, std::vector<DataInputAccessor*>> _table_to_accessors;
     std::shared_ptr<paddle::ps::ObjectPool<::paddle::framework::Scope>> _scope_obj_pool;
     std::vector<std::string> _persistables;
+
+    // 异步删除
+    static std::once_flag _async_delete_flag;
+    static std::shared_ptr<std::thread> _async_delete_thread;
+    static paddle::framework::Channel<ScopeExecutorContext*> _delete_channel;
 };
 
 }  // namespace feed
diff --git a/paddle/fluid/train/custom_trainer/feed/io/file_system.cc b/paddle/fluid/train/custom_trainer/feed/io/file_system.cc
index 619499555dc3d5be2d7439f313867570d2c5abd8..446e3f48feffb0bd9aff62f08cc8cb772dfd0325 100644
--- a/paddle/fluid/train/custom_trainer/feed/io/file_system.cc
+++ b/paddle/fluid/train/custom_trainer/feed/io/file_system.cc
@@ -23,6 +23,30 @@ std::pair<std::string, std::string> FileSystem::path_split(const std::string& pa
     return {path.substr(0, pos), path.substr(pos + 1)};
 }
 
+int FileSystem::append_line(const std::string& path, 
+    const std::string& line, size_t reserve_line_num) {
+    std::string tail_data;
+    if (exists(path)) {
+        tail_data = paddle::string::trim_spaces(tail(path, reserve_line_num)); 
+    }
+    if (tail_data.size() > 0) {
+        tail_data = tail_data + "\n" + line;
+    } else {
+        tail_data = line;
+    }
+    VLOG(2) << "Append to file:" << path << ", line str:" << line;
+    while (true) {
+        remove(path);
+        auto fp = open_write(path, "");
+        if (fwrite(tail_data.c_str(), tail_data.length(), 1, &*fp) == 1) {
+            break;
+        }     
+        sleep(10);   
+        VLOG(0) << "Retry Append to file:" << path << ", line str:" << line;
+    }
+    return 0;
+}
+
 }  // namespace feed
 }  // namespace custom_trainer
 }  // namespace paddle
diff --git a/paddle/fluid/train/custom_trainer/feed/io/file_system.h b/paddle/fluid/train/custom_trainer/feed/io/file_system.h
index 4531cbf1125d0a3af54a503fab82e1b9db970f2d..8ad2198983d319c54e6bb3fde116246e40c4eb21 100644
--- a/paddle/fluid/train/custom_trainer/feed/io/file_system.h
+++ b/paddle/fluid/train/custom_trainer/feed/io/file_system.h
@@ -18,6 +18,8 @@ public:
     virtual int initialize(const YAML::Node& config, std::shared_ptr<TrainerContext> context) = 0;
     virtual std::shared_ptr<FILE> open_read(const std::string& path, const std::string& converter) = 0;
     virtual std::shared_ptr<FILE> open_write(const std::string& path, const std::string& converter) = 0;
+    // only support text-file 
+    virtual int append_line(const std::string& path, const std::string& line, size_t reserve_line_num);
     virtual int64_t file_size(const std::string& path) = 0;
     virtual void remove(const std::string& path) = 0;
     virtual std::vector<std::string> list(const std::string& path) = 0;