diff --git a/src/engine_robust-inl.h b/src/engine_robust-inl.h
new file mode 100644
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--- /dev/null
+++ b/src/engine_robust-inl.h
@@ -0,0 +1,42 @@
+/*!
+ * \file engine_robust-inl.h
+ * \brief implementation of inline template function in AllReduceRobust
+ *   
+ * \author Tianqi, Nacho, Tianyi
+ */
+#ifndef ALLREDUCE_ENGINE_ROBUST_INL_H
+#define ALLREDUCE_ENGINE_ROBUST_INL_H
+
+namespace engine {
+/*!
+ * \brief run message passing algorithm on the allreduce tree 
+ *        the result is edge message stored in p_edge_in and p_edge_out
+ * \param node_value the value associated with current node
+ * \param p_edge_in used to store input message from each of the edge
+ * \param p_edge_out used to store output message from each of the edge
+ * \param func a function that defines the message passing rule
+ *        Parameters of func:
+ *           - node_value same as node_value in the main function
+ *           - edge_in the array of input messages from each edge,
+ *                     this includes the output edge, which should be excluded
+ *           - out_index array the index of output edge, the function should
+ *                       exclude the output edge when compute the message passing value
+ *        Return of func:
+ *           the function returns the output message based on the input message and node_value
+ *
+ * \tparam EdgeType type of edge message, must be simple struct
+ * \tparam NodeType type of node value
+ */
+template<typename NodeType, typename EdgeType>
+inline AllReduceRobust::ReturnType
+AllReduceRobust::MsgPassing(const NodeType &node_value,
+                            std::vector<EdgeType> *p_edge_in,
+                            std::vector<EdgeType> *p_edge_out,
+                            EdgeType (*func) (const NodeType &node_value,
+                                              const std::vector<EdgeType> &edge_in,
+                                              size_t out_index)
+                            ) {
+  return kSuccess;
+}
+}  // namespace engine
+#endif  // ALLREDUCE_ENGINE_ROBUST_INL_H