diff --git a/cpp/src/db/DBImpl.cpp b/cpp/src/db/DBImpl.cpp
index 88bd9b8d959c1c5c342ba8c8d82c65091ba0be18..859a1d75aa65c0aa133ca711bc68d7b526382d26 100644
--- a/cpp/src/db/DBImpl.cpp
+++ b/cpp/src/db/DBImpl.cpp
@@ -228,7 +228,7 @@ Status DBImpl::merge_files(const std::string& group_id, const meta::DateT& date,
     for (auto& file : files) {
         auto to_merge = zilliz::vecwise::cache::CpuCacheMgr::GetInstance()->GetIndex(file.location);
         if (!to_merge) {
-            to_merge = read_index(file.location.c_str());
+            to_merge = read_index(file.location);
         }
         auto file_index = dynamic_cast<faiss::IndexIDMap*>(to_merge->data().get());
         index->add_with_ids(file_index->ntotal, dynamic_cast<faiss::IndexFlat*>(file_index->index)->xb.data(),
diff --git a/cpp/src/db/ExecutionEngine.cpp b/cpp/src/db/ExecutionEngine.cpp
new file mode 100644
index 0000000000000000000000000000000000000000..9aea3f7ea12433170b75aa62386fa99c1377d4ce
--- /dev/null
+++ b/cpp/src/db/ExecutionEngine.cpp
@@ -0,0 +1,21 @@
+#include <easylogging++.h>
+#include "ExecutionEngine.h"
+
+namespace zilliz {
+namespace vecwise {
+namespace engine {
+
+Status ExecutionEngine::AddWithIds(const std::vector<float>& vectors, const std::vector<long>& vector_ids) {
+    long n1 = (long)vectors.size();
+    long n2 = (long)vector_ids.size();
+    if (n1 != n2) {
+        LOG(ERROR) << "vectors size is not equal to the size of vector_ids: " << n1 << "!=" << n2;
+        return Status::Error("Error: AddWithIds");
+    }
+    return AddWithIds(n1, vectors.data(), vector_ids.data());
+}
+
+
+} // namespace engine
+} // namespace vecwise
+} // namespace zilliz
diff --git a/cpp/src/db/ExecutionEngine.h b/cpp/src/db/ExecutionEngine.h
new file mode 100644
index 0000000000000000000000000000000000000000..4b08149f45a2f8437b661a2c7f580b2d608d00e2
--- /dev/null
+++ b/cpp/src/db/ExecutionEngine.h
@@ -0,0 +1,33 @@
+#pragma once
+
+#include <vector>
+
+#include "Status.h"
+
+namespace zilliz {
+namespace vecwise {
+namespace engine {
+
+class ExecutionEngine {
+public:
+
+    Status AddWithIds(const std::vector<float>& vectors,
+                              const std::vector<long>& vector_ids);
+
+    virtual Status AddWithIds(long n, const float *xdata, const long *xids) = 0;
+
+    virtual size_t Count() const = 0;
+
+    virtual size_t Size() const = 0;
+
+    virtual Status Serialize() = 0;
+
+    virtual Status Cache() = 0;
+
+    virtual ~ExecutionEngine() {}
+};
+
+
+} // namespace engine
+} // namespace vecwise
+} // namespace zilliz
diff --git a/cpp/src/db/FaissExecutionEngine.cpp b/cpp/src/db/FaissExecutionEngine.cpp
new file mode 100644
index 0000000000000000000000000000000000000000..ffa3ef3c24573fcc7eddc1946b0a26b495704f7b
--- /dev/null
+++ b/cpp/src/db/FaissExecutionEngine.cpp
@@ -0,0 +1,46 @@
+#include <easylogging++.h>
+#include <faiss/AutoTune.h>
+#include <wrapper/Index.h>
+#include <cache/CpuCacheMgr.h>
+
+#include "FaissExecutionEngine.h"
+
+namespace zilliz {
+namespace vecwise {
+namespace engine {
+
+const std::string IndexType = "IDMap,Flat";
+
+FaissExecutionEngine::FaissExecutionEngine(uint16_t dimension, const std::string& location)
+    : pIndex_(faiss::index_factory(dimension, IndexType.c_str())),
+      location_(location) {
+}
+
+Status FaissExecutionEngine::AddWithIds(long n, const float *xdata, const long *xids) {
+    pIndex_->add_with_ids(n, xdata, xids);
+    return Status::OK();
+}
+
+size_t FaissExecutionEngine::Count() const {
+    return (size_t)(pIndex_->ntotal);
+}
+
+size_t FaissExecutionEngine::Size() const {
+    return (size_t)(Count() * pIndex_->d);
+}
+
+Status FaissExecutionEngine::Serialize() {
+    write_index(pIndex_.get(), location_.c_str());
+    return Status::OK();
+}
+
+Status FaissExecutionEngine::Cache() {
+    zilliz::vecwise::cache::CpuCacheMgr::GetInstance(
+            )->InsertItem(location_, std::make_shared<Index>(pIndex_));
+
+    return Status::OK();
+}
+
+} // namespace engine
+} // namespace vecwise
+} // namespace zilliz
diff --git a/cpp/src/db/FaissExecutionEngine.h b/cpp/src/db/FaissExecutionEngine.h
new file mode 100644
index 0000000000000000000000000000000000000000..bb1f5a477096cfdb79e855540c428ffde7f5e8c3
--- /dev/null
+++ b/cpp/src/db/FaissExecutionEngine.h
@@ -0,0 +1,37 @@
+#pragma once
+
+#include <memory>
+#include <string>
+
+#include "ExecutionEngine.h"
+
+namespace faiss {
+    class Index;
+}
+
+namespace zilliz {
+namespace vecwise {
+namespace engine {
+
+class FaissExecutionEngine : public ExecutionEngine {
+public:
+    FaissExecutionEngine(uint16_t dimension, const std::string& location);
+    virtual Status AddWithIds(long n, const float *xdata, const long *xids) override;
+
+    virtual size_t Count() const override;
+
+    virtual size_t Size() const override;
+
+    virtual Status Serialize() override;
+
+    virtual Status Cache() override;
+
+protected:
+    std::shared_ptr<faiss::Index> pIndex_;
+    std::string location_;
+};
+
+
+} // namespace engine
+} // namespace vecwise
+} // namespace zilliz
diff --git a/cpp/src/db/MemManager.cpp b/cpp/src/db/MemManager.cpp
index e3c9407d861038dcc690078da36b57a2d96b583c..10ec5829352ec2d3c89e98db3acfbd05df5c55ae 100644
--- a/cpp/src/db/MemManager.cpp
+++ b/cpp/src/db/MemManager.cpp
@@ -1,14 +1,12 @@
-#include <faiss/AutoTune.h>
 #include <iostream>
 #include <sstream>
 #include <thread>
 
-#include <wrapper/Index.h>
-#include <cache/CpuCacheMgr.h>
 #include <easylogging++.h>
 
 #include "MemManager.h"
 #include "Meta.h"
+#include "FaissExecutionEngine.h"
 
 
 namespace zilliz {
@@ -21,43 +19,36 @@ MemVectors::MemVectors(const std::shared_ptr<meta::Meta>& meta_ptr,
     options_(options),
     schema_(schema),
     _pIdGenerator(new SimpleIDGenerator()),
-    pIndex_(faiss::index_factory(schema_.dimension, "IDMap,Flat")) {
+    pEE_(new FaissExecutionEngine(schema_.dimension, schema_.location)) {
 }
 
 void MemVectors::add(size_t n_, const float* vectors_, IDNumbers& vector_ids_) {
     _pIdGenerator->getNextIDNumbers(n_, vector_ids_);
-    pIndex_->add_with_ids(n_, vectors_, &vector_ids_[0]);
+    pEE_->AddWithIds(n_, vectors_, vector_ids_.data());
     for(auto i=0 ; i<n_; i++) {
         vector_ids_.push_back(i);
     }
 }
 
 size_t MemVectors::total() const {
-    return pIndex_->ntotal;
+    return pEE_->Count();
 }
 
 size_t MemVectors::approximate_size() const {
-    return total() * schema_.dimension;
+    return pEE_->Size();
 }
 
 Status MemVectors::serialize(std::string& group_id) {
-    /* std::stringstream ss; */
-    /* ss << "/tmp/test/" << _pIdGenerator->getNextIDNumber(); */
-    /* faiss::write_index(pIndex_, ss.str().c_str()); */
-    /* std::cout << pIndex_->ntotal << std::endl; */
-    /* std::cout << _file_location << std::endl; */
-    /* faiss::write_index(pIndex_, _file_location.c_str()); */
     group_id = schema_.group_id;
     auto rows = approximate_size();
-    write_index(pIndex_.get(), schema_.location.c_str());
+    pEE_->Serialize();
     schema_.rows = rows;
     schema_.file_type = (rows >= options_.index_trigger_size) ?
         meta::GroupFileSchema::TO_INDEX : meta::GroupFileSchema::RAW;
 
     auto status = pMeta_->update_group_file(schema_);
 
-    zilliz::vecwise::cache::CpuCacheMgr::GetInstance(
-            )->InsertItem(schema_.location, std::make_shared<Index>(pIndex_));
+    pEE_->Cache();
 
     return status;
 }
diff --git a/cpp/src/db/MemManager.h b/cpp/src/db/MemManager.h
index 077e045286627e9200d725cc35b5773e4049684f..0a298bf22d2b490350a5b022aed32a4b2eb709ae 100644
--- a/cpp/src/db/MemManager.h
+++ b/cpp/src/db/MemManager.h
@@ -10,10 +10,6 @@
 #include "Status.h"
 #include "Meta.h"
 
-namespace faiss {
-    class Index;
-}
-
 
 namespace zilliz {
 namespace vecwise {
@@ -23,6 +19,8 @@ namespace meta {
     class Meta;
 }
 
+class ExecutionEngine;
+
 class MemVectors {
 public:
     explicit MemVectors(const std::shared_ptr<meta::Meta>&,
@@ -49,7 +47,7 @@ private:
     Options options_;
     meta::GroupFileSchema schema_;
     IDGenerator* _pIdGenerator;
-    std::shared_ptr<faiss::Index> pIndex_;
+    std::shared_ptr<ExecutionEngine> pEE_;
 
 }; // MemVectors
 
diff --git a/cpp/src/db/Status.h b/cpp/src/db/Status.h
index f45c9f6bd13477880ed2e974518c8f158576202f..4db2b4c6e0c03bcdf14a8339e5dafeeb14d7d7da 100644
--- a/cpp/src/db/Status.h
+++ b/cpp/src/db/Status.h
@@ -21,6 +21,9 @@ public:
     static Status NotFound(const std::string& msg, const std::string& msg2="") {
         return Status(kNotFound, msg, msg2);
     }
+    static Status Error(const std::string& msg, const std::string& msg2="") {
+        return Status(kError, msg, msg2);
+    }
 
     static Status InvalidDBPath(const std::string& msg, const std::string& msg2="") {
         return Status(kInvalidDBPath, msg, msg2);
@@ -35,6 +38,7 @@ public:
     bool ok() const { return state_ == nullptr; }
 
     bool IsNotFound() const { return code() == kNotFound; }
+    bool IsError() const { return code() == kError; }
 
     bool IsInvalidDBPath() const { return code() == kInvalidDBPath; }
     bool IsGroupError() const { return code() == kGroupError; }
@@ -48,6 +52,7 @@ private:
     enum Code {
         kOK = 0,
         kNotFound,
+        kError,
 
         kInvalidDBPath,
         kGroupError,