diff --git a/cpp/src/db/db_impl.cpp b/cpp/src/db/db_impl.cpp
index 87c83e7027c79d6ff0b10cdf197ba0095b2bb316..4c00d07e0a372e60a70a5fc45a9086498c16b8e8 100644
--- a/cpp/src/db/db_impl.cpp
+++ b/cpp/src/db/db_impl.cpp
@@ -1,6 +1,7 @@
 #include <assert.h>
 #include <chrono>
 #include <thread>
+#include <iostream>
 #include "db_impl.h"
 #include "db_meta_impl.h"
 #include "env.h"
@@ -100,7 +101,11 @@ void DBImpl::background_call() {
 }
 
 void DBImpl::background_compaction() {
-    _pMemMgr->serialize();
+    std::vector<std::string> group_ids;
+    _pMemMgr->serialize(group_ids);
+    for (auto group_id : group_ids) {
+        std::cout << __func__ << " group_id=" << group_id << std::endl;
+    }
 }
 
 DBImpl::~DBImpl() {
diff --git a/cpp/src/db/db_meta_impl.cpp b/cpp/src/db/db_meta_impl.cpp
index 7ab9f650009c42d34e396f2a636e71981a56c5eb..5ae038e77a105ba9d599378a7b71abbcf436df2b 100644
--- a/cpp/src/db/db_meta_impl.cpp
+++ b/cpp/src/db/db_meta_impl.cpp
@@ -30,6 +30,7 @@ Status DBMetaImpl::get_group(const std::string& group_id_, GroupSchema& group_in
     SimpleIDGenerator g;
     ss.str("");
     ss << "/tmp/test/" << g.getNextIDNumber() << ".log";
+    group_info_.group_id = "1";
     group_info_.dimension = 64;
     group_info_.next_file_location = ss.str();
     return Status::OK();
diff --git a/cpp/src/db/memvectors.cpp b/cpp/src/db/memvectors.cpp
index 2bafe3a9de4b2d8716237081708383bbd0a3b3d9..dac9e8b478d0747ebf5ae15e85d858e928ae09eb 100644
--- a/cpp/src/db/memvectors.cpp
+++ b/cpp/src/db/memvectors.cpp
@@ -13,10 +13,12 @@ namespace zilliz {
 namespace vecwise {
 namespace engine {
 
-MemVectors::MemVectors(size_t dimension_, const std::string& file_location_) :
-    _file_location(file_location_),
+MemVectors::MemVectors(const std::string& group_id,
+        size_t dimension, const std::string& file_location) :
+    group_id_(group_id),
+    _file_location(file_location),
     _pIdGenerator(new SimpleIDGenerator()),
-    _dimension(dimension_),
+    _dimension(dimension),
     _pInnerIndex(new faiss::IndexFlat(_dimension)),
     _pIdMapIndex(new faiss::IndexIDMap(_pInnerIndex)) {
 }
@@ -37,13 +39,15 @@ size_t MemVectors::approximate_size() const {
     return total() * _dimension;
 }
 
-void MemVectors::serialize() {
+Status MemVectors::serialize(std::string& group_id) {
     /* std::stringstream ss; */
     /* ss << "/tmp/test/" << _pIdGenerator->getNextIDNumber(); */
     /* faiss::write_index(_pIdMapIndex, ss.str().c_str()); */
     /* std::cout << _pIdMapIndex->ntotal << std::endl; */
     /* std::cout << _file_location << std::endl; */
     faiss::write_index(_pIdMapIndex, _file_location.c_str());
+    group_id = group_id_;
+    return Status::OK();
 }
 
 MemVectors::~MemVectors() {
@@ -77,7 +81,8 @@ VectorsPtr MemManager::get_mem_by_group(const std::string& group_id) {
         return nullptr;
     }
 
-    _memMap[group_id] = std::shared_ptr<MemVectors>(new MemVectors(group_info.dimension,
+    _memMap[group_id] = std::shared_ptr<MemVectors>(new MemVectors(group_info.group_id,
+                group_info.dimension,
                 group_info.next_file_location));
     return _memMap[group_id];
 }
@@ -126,10 +131,13 @@ Status MemManager::mark_memory_as_immutable() {
 /*     return false; */
 /* } */
 
-Status MemManager::serialize() {
+Status MemManager::serialize(std::vector<std::string>& group_ids) {
     mark_memory_as_immutable();
+    std::string group_id;
+    group_ids.clear();
     for (auto& mem : _immMems) {
-        mem->serialize();
+        mem->serialize(group_id);
+        group_ids.push_back(group_id);
     }
     _immMems.clear();
     return Status::OK();
diff --git a/cpp/src/db/memvectors.h b/cpp/src/db/memvectors.h
index f9a0bba1f223252a2fe41607300f47d20444379d..6b70df267c75ba7a99f436c9cde99f37f012410f 100644
--- a/cpp/src/db/memvectors.h
+++ b/cpp/src/db/memvectors.h
@@ -21,7 +21,9 @@ namespace engine {
 
 class MemVectors {
 public:
-    explicit MemVectors(size_t dimension_, const std::string& file_location_);
+    explicit MemVectors(const std::string& group_id,
+            size_t dimension,
+            const std::string& file_location);
 
     void add(size_t n_, const float* vectors_, IDNumbers& vector_ids_);
 
@@ -29,7 +31,7 @@ public:
 
     size_t approximate_size() const;
 
-    void serialize();
+    Status serialize(std::string& group_id);
 
     ~MemVectors();
 
@@ -40,6 +42,7 @@ private:
     MemVectors(const MemVectors&) = delete;
     MemVectors& operator=(const MemVectors&) = delete;
 
+    std::string group_id_;
     const std::string _file_location;
     IDGenerator* _pIdGenerator;
     size_t _dimension;
@@ -62,7 +65,7 @@ public:
     Status add_vectors(const std::string& group_id_,
             size_t n_, const float* vectors_, IDNumbers& vector_ids_);
 
-    Status serialize();
+    Status serialize(std::vector<std::string>& group_ids);
 
 private:
     Status add_vectors_no_lock(const std::string& group_id_,