diff --git a/src/allreduce.h b/src/allreduce.h
index 264541211..c6dccade6 100644
--- a/src/allreduce.h
+++ b/src/allreduce.h
@@ -2,7 +2,9 @@
 #define ALLREDUCE_H
 /*!
  * \file allreduce.h
- * \brief This file defines a template wrapper of engine to ensure 
+ * \brief This file defines a template wrapper of engine to give more flexible
+ *      AllReduce operations
+ *
  * \author Tianqi Chen, Nacho, Tianyi
  */
 #include "./engine.h"
diff --git a/src/engine.h b/src/engine.h
index 42e19c139..d3493945f 100644
--- a/src/engine.h
+++ b/src/engine.h
@@ -1,6 +1,6 @@
 /*!
  * \file engine.h
- * \brief This file defines the interface of allreduce library
+ * \brief This file defines the core interface of allreduce library
  * \author Tianqi Chen, Nacho, Tianyi
  */
 #ifndef ALLREDUCE_ENGINE_H
diff --git a/src/engine_base.cc b/src/engine_base.cc
index e2eca014f..dd9c8ac56 100644
--- a/src/engine_base.cc
+++ b/src/engine_base.cc
@@ -1,3 +1,8 @@
+/*!
+ * \file engine_base.cc
+ * \brief Basic implementation of AllReduce
+ * \author Tianqi, Nacho, Tianyi
+ */
 #define _CRT_SECURE_NO_WARNINGS
 #define _CRT_SECURE_NO_DEPRECATE
 #define NOMINMAX
@@ -137,10 +142,8 @@ void AllReduceBase::SetParam(const char *name, const char *val) {
  * \param type_nbytes the unit number of bytes the type have
  * \param count number of elements to be reduced
  * \param reducer reduce function
- * \return this function can return
- *         - kSuccess: allreduce is success,
- *         - kSockError: a neighbor node go down, the connection is dropped
- *         - kGetExcept: another node which is not my neighbor go down, get Out-of-Band exception notification from my neighbor
+ * \return this function can return kSuccess, kSockError, kGetExcept, see ReturnType for details
+ * \sa ReturnType
  */
 AllReduceBase::ReturnType
 AllReduceBase::TryAllReduce(void *sendrecvbuf_,
@@ -278,7 +281,8 @@ AllReduceBase::TryAllReduce(void *sendrecvbuf_,
  * \param sendrecvbuf_ buffer for both sending and recving data
  * \param total_size the size of the data to be broadcasted
  * \param root the root worker id to broadcast the data
- * \return this function can return three possible values, see detail in TryAllReduce
+ * \return this function can return kSuccess, kSockError, kGetExcept, see ReturnType for details
+ * \sa ReturnType
  */
 AllReduceBase::ReturnType
 AllReduceBase::TryBroadcast(void *sendrecvbuf_, size_t total_size, int root) {
diff --git a/src/engine_base.h b/src/engine_base.h
index 6c138529a..61fdd6033 100644
--- a/src/engine_base.h
+++ b/src/engine_base.h
@@ -97,8 +97,14 @@ class AllReduceBase : public IEngine {
  protected:
   /*! \brief enumeration of possible returning results from Try functions */
   enum ReturnType {
+    /*! \brief execution is successful */
     kSuccess,
+    /*! \brief a neighbor node go down, the connection is dropped */
     kSockError,
+    /*! 
+     * \brief another node which is not my neighbor go down,
+     *   get Out-of-Band exception notification from my neighbor
+     */
     kGetExcept
   };
   // link record to a neighbor
@@ -202,10 +208,8 @@ class AllReduceBase : public IEngine {
    * \param type_nbytes the unit number of bytes the type have
    * \param count number of elements to be reduced
    * \param reducer reduce function
-   * \return this function can return
-   *         - kSuccess: allreduce is success,
-   *         - kSockError: a neighbor node go down, the connection is dropped
-   *         - kGetExcept: another node which is not my neighbor go down, get Out-of-Band exception notification from my neighbor
+   * \return this function can return kSuccess, kSockError, kGetExcept, see ReturnType for details
+   * \sa ReturnType
    */
   ReturnType TryAllReduce(void *sendrecvbuf_,
                           size_t type_nbytes,
@@ -216,7 +220,8 @@ class AllReduceBase : public IEngine {
    * \param sendrecvbuf_ buffer for both sending and recving data
    * \param size the size of the data to be broadcasted
    * \param root the root worker id to broadcast the data
-   * \return this function can return three possible values, see detail in TryAllReduce
+   * \return this function can return kSuccess, kSockError, kGetExcept, see ReturnType for details
+   * \sa ReturnType
    */
   ReturnType TryBroadcast(void *sendrecvbuf_, size_t size, int root);
   //---- local data related to link ----
diff --git a/src/engine_robust.cc b/src/engine_robust.cc
index 00efd7447..fcc3ebc20 100644
--- a/src/engine_robust.cc
+++ b/src/engine_robust.cc
@@ -1,3 +1,8 @@
+/*!
+ * \file engine_robust.cc
+ * \brief Robust implementation of AllReduce
+ * \author Tianqi, Nacho, Tianyi
+ */
 #define _CRT_SECURE_NO_WARNINGS
 #define _CRT_SECURE_NO_DEPRECATE
 #define NOMINMAX
@@ -71,8 +76,7 @@ AllReduceRobust::ReturnType AllReduceRobust::TryResetLinks(void) {
   for (int i = 0; i < nlink; ++i) {
     links[i].InitBuffer(sizeof(int), 1 << 10, reduce_buffer_size);
     links[i].ResetSize();
-  }
-  
+  }  
   // read and discard data from all channels until pass mark
   while (true) {
     for (int i = 0; i < nlink; ++i) {
@@ -179,12 +183,150 @@ AllReduceRobust::ReturnType AllReduceRobust::TryResetLinks(void) {
   }
   return kSuccess;
 }
-
-bool AllReduceRobust::RecoverExec(void *sendrecvbuf_, size_t size, int flag, int seqno) {
-  if (flag != 0) {
-    utils::Assert(seqno == ActionSummary::kMaxSeq, "must only set seqno for normal operations");      
+/*!
+ * \brief try to reconnect the broken links
+ * \return this function can kSuccess or kSockError
+ */
+AllReduceRobust::ReturnType AllReduceRobust::TryReConnectLinks(void) {
+  utils::Error("TryReConnectLinks: not implemented");
+  return kSuccess;
+}
+/*!
+ * \brief if err_type indicates an error
+ *         recover links according to the error type reported
+ *        if there is no error, return true
+ * \param err_type the type of error happening in the system
+ * \return true if err_type is kSuccess, false otherwise 
+ */
+bool AllReduceRobust::CheckAndRecover(ReturnType err_type) {
+  if (err_type == kSuccess) return true;
+  while(err_type != kSuccess) {
+    switch(err_type) {
+      case kGetExcept: err_type = TryResetLinks(); break;
+      case kSockError: {
+        TryResetLinks();
+        err_type = TryReConnectLinks();
+        break;
+      }
+      default: utils::Assert(false, "RecoverLinks: cannot reach here");
+    }
   }
-  ActionSummary act(flag, seqno);
+  return false;
+}
+/*!
+ * \brief try to load check point
+ *        
+ *        This is a collaborative function called by all nodes
+ *        only the nodes with requester set to true really needs to load the check point
+ *        other nodes acts as collaborative roles to complete this request
+ *
+ * \param requester whether current node is the requester
+ * \return this function can return kSuccess/kSockError/kGetExcept, see ReturnType for details
+ * \sa ReturnType
+ */
+AllReduceRobust::ReturnType AllReduceRobust::TryLoadCheckPoint(bool requester) {
+  utils::Error("TryLoadCheckPoint: not implemented");
+  return kSuccess;
+}
+/*!
+ * \brief try to get the result of operation specified by seqno
+ *
+ *        This is a collaborative function called by all nodes
+ *        only the nodes with requester set to true really needs to get the result
+ *        other nodes acts as collaborative roles to complete this request
+ *
+ * \param buf the buffer to store the result, this parameter is only use when current node is requester
+ * \param size the total size of the buffer, this parameter is only use when current node is requester
+ * \param seqno sequence number of the operation, this is unique index of a operation in current iteration
+ * \param requester whether current node is the requester
+ * \return this function can return kSuccess/kSockError/kGetExcept, see ReturnType for details
+ * \sa ReturnType
+ */
+AllReduceRobust::ReturnType AllReduceRobust::TryGetResult(void *sendrecvbuf, size_t size, int seqno, bool requester) {
+  utils::Error("TryGetResult: not implemented");
+  return kSuccess;
+}
+/*!
+ * \brief try to run recover execution for a request action described by flag and seqno,
+ *        the function will keep blocking to run possible recovery operations before the specified action,
+ *        until the requested result is received by a recovering procedure,
+ *        or the function discovers that the requested action is not yet executed, and return false
+ *
+ * \param buf the buffer to store the result
+ * \param size the total size of the buffer
+ * \param flag flag information about the action \sa ActionSummary
+ * \param seqno sequence number of the action, if it is special action with flag set, seqno needs to be set to ActionSummary::kMaxSeq
+ *
+ * \return if this function can return true or false 
+ *    - true means buf already set to the
+ *           result by recovering procedure, the action is complete, no further action is needed
+ *    - false means this is the lastest action that has not yet been executed, need to execute the action
+ */
+bool AllReduceRobust::RecoverExec(void *buf, size_t size, int flag, int seqno) {
+  if (flag != 0) {
+    utils::Assert(seqno == ActionSummary::kMaxSeq, "must only set seqno for normal operations");
+  }
+  // request
+  ActionSummary req(flag, seqno);  
+  while (true) {
+    // action
+    ActionSummary act = req;    
+    // get the reduced action
+    if (!CheckAndRecover(TryAllReduce(&act, sizeof(act), 1, ActionSummary::Reducer))) continue;
+    if (act.check_ack()) {
+      if (act.check_point()) {
+        // if we also have check_point, do check point first
+        utils::Assert(!act.diff_seq(),
+                      "check ack & check pt  cannot occur together with normal ops");
+        // if we requested checkpoint, we are free to go
+        if (req.check_point()) return true;
+      } else if (act.load_check()) {
+        // if there is only check_ack and load_check, do load_check
+        if (!CheckAndRecover(TryLoadCheckPoint(req.load_check()))) continue;
+        // if requested load check, then misson complete
+        if (req.load_check()) return true;
+      } else {
+        // there is no check point and no load check, execute check ack
+        if (req.check_ack()) return true;
+      }
+      // if execute to this point
+      // this means the action requested has not been completed
+      // try next round
+    } else {
+      if (act.check_point()) {
+        if (act.diff_seq()) {
+          utils::Assert(act.min_seqno() != ActionSummary::kMaxSeq, "min seq bug");
+          bool requester = req.min_seqno() == act.min_seqno();
+          if (!CheckAndRecover(TryGetResult(buf, size, act.min_seqno(), requester))) continue;
+          if (requester) return true;
+        } else  {
+          // no difference in seq no, means we are free to check point
+          if (req.check_point()) return true;
+        }
+      } else {
+        // no check point
+        if (act.load_check()) {
+          // load check have higher priority, do load_check
+          if (!CheckAndRecover(TryLoadCheckPoint(req.load_check()))) continue;
+          // if requested load check, then misson complete
+          if (req.load_check()) return true;          
+        } else {
+          // no special flags, no checkpoint, check ack, load_check
+          utils::Assert(act.min_seqno() != ActionSummary::kMaxSeq, "min seq bug");
+          if (act.diff_seq()) {
+            bool requester = req.min_seqno() == act.min_seqno();
+            if (!CheckAndRecover(TryGetResult(buf, size, act.min_seqno(), requester))) continue;
+            if (requester) return true;
+          } else {
+            // all the request is same, this is most recent command that is yet to be executed
+            return false;
+          }
+        }
+      }
+      // something is still incomplete try next round
+    }
+  }
+  utils::Assert(false, "RecoverExec: should not reach here");
   return true;
 }
 }  // namespace engine
diff --git a/src/engine_robust.h b/src/engine_robust.h
index f1949e11a..fa18406db 100644
--- a/src/engine_robust.h
+++ b/src/engine_robust.h
@@ -89,15 +89,19 @@ class AllReduceRobust : public AllReduceBase {
     inline int min_seqno(void) const {
       return seqcode >> 4;
     }
+    // whether the operation set contains a load_check
+    inline bool load_check(void) const {
+      return (seqcode & kLoadCheck) != 0;
+    }
     // whether the operation set contains a check point
     inline bool check_point(void) const {
       return (seqcode & kCheckPoint) != 0;
     }
-    // whether the operation set contains a check point
+    // whether the operation set contains a check ack
     inline bool check_ack(void) const {
       return (seqcode & kCheckAck) != 0;
     }
-    // whether the operation set contains a check point
+    // whether the operation set contains different sequence number
     inline bool diff_seq(void) const {
       return (seqcode & kDiffSeq) != 0;
     }
@@ -184,17 +188,64 @@ class AllReduceRobust : public AllReduceBase {
    *         when kSockError is returned, it simply means there are bad sockets in the links,
    *         and some link recovery proceduer is needed
    */
-  ReturnType TryResetLinks(void);
-  /*! 
-   * \brief Run recovery execution of a action specified by flag and seqno,
-   *        there can be two outcome of the function
-   *       
-   * \param sendrecvbuf_
-   *
-   * \return if this function returns true, this means
-   *        behind and we will be able to recover data from existing node 
+  ReturnType TryResetLinks(void);  
+  /*!
+   * \brief try to reconnect the broken links
+   * \return this function can kSuccess or kSockError
    */
-  bool RecoverExec(void *sendrecvbuf_, size_t size, int flag, int seqno);
+  ReturnType TryReConnectLinks(void);
+  /*!
+   * \brief if err_type indicates an error
+   *         recover links according to the error type reported
+   *        if there is no error, return true
+   * \param err_type the type of error happening in the system
+   * \return true if err_type is kSuccess, false otherwise 
+   */
+  bool CheckAndRecover(ReturnType err_type);
+  /*!
+   * \brief try to run recover execution for a request action described by flag and seqno,
+   *        the function will keep blocking to run possible recovery operations before the specified action,
+   *        until the requested result is received by a recovering procedure,
+   *        or the function discovers that the requested action is not yet executed, and return false
+   *
+   * \param buf the buffer to store the result
+   * \param size the total size of the buffer
+   * \param flag flag information about the action \sa ActionSummary
+   * \param seqno sequence number of the action, if it is special action with flag set, seqno needs to be set to ActionSummary::kMaxSeq
+   *
+   * \return if this function can return true or false 
+ *    - true means buf already set to the
+ *           result by recovering procedure, the action is complete, no further action is needed
+   *    - false means this is the lastest action that has not yet been executed, need to execute the action
+   */
+  bool RecoverExec(void *buf, size_t size, int flag, int seqno = ActionSummary::kMaxSeq);  
+  /*!
+   * \brief try to load check point
+   *        
+   *        This is a collaborative function called by all nodes
+   *        only the nodes with requester set to true really needs to load the check point
+   *        other nodes acts as collaborative roles to complete this request
+   *
+   * \param requester whether current node is the requester
+   * \return this function can return kSuccess/kSockError/kGetExcept, see ReturnType for details
+   * \sa ReturnType
+   */
+  ReturnType TryLoadCheckPoint(bool requester);
+  /*!
+   * \brief try to get the result of operation specified by seqno
+   *
+   *        This is a collaborative function called by all nodes
+   *        only the nodes with requester set to true really needs to get the result
+   *        other nodes acts as collaborative roles to complete this request
+   *
+   * \param buf the buffer to store the result, this parameter is only use when current node is requester
+   * \param size the total size of the buffer, this parameter is only use when current node is requester
+   * \param seqno sequence number of the operation, this is unique index of a operation in current iteration
+   * \param requester whether current node is the requester
+   * \return this function can return kSuccess/kSockError/kGetExcept, see ReturnType for details
+   * \sa ReturnType
+   */
+  ReturnType TryGetResult(void *buf, size_t size, int seqno, bool requester);
   //---- recovery data structure ----
   // call sequence counter, records how many calls we made so far
   // from last call to CheckPoint, LoadCheckPoint