diff --git a/.gitmodules b/.gitmodules
new file mode 100644
index 0000000..76ee68c
--- /dev/null
+++ b/.gitmodules
@@ -0,0 +1,3 @@
+[submodule "parallel-hashmap"]
+	path = parallel-hashmap
+	url = https://github.com/greg7mdp/parallel-hashmap
diff --git a/include/user.hpp b/include/user.hpp
index f299925..1594dbf 100644
--- a/include/user.hpp
+++ b/include/user.hpp
@@ -8,6 +8,7 @@ private:
     uint_fast64_t balance;
     std::string password;
 
+    //for read/write of object's state concurrently
     std::mutex bal_lock;
     std::mutex pass_lock;
 
diff --git a/parallel-hashmap b/parallel-hashmap
new file mode 160000
index 0000000..50558ca
--- /dev/null
+++ b/parallel-hashmap
@@ -0,0 +1 @@
+Subproject commit 50558ca3f71c2a19f839f4fa26ad034470f4f3c6