diff --git a/src/chatroom-discovery-backend.hpp b/src/chatroom-discovery-backend.hpp
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+++ b/src/chatroom-discovery-backend.hpp
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+/* -*- Mode:C++; c-file-style:"gnu"; indent-tabs-mode:nil -*- */
+/*
+ * Copyright (c) 2013, Regents of the University of California
+ *
+ * BSD license, See the LICENSE file for more information
+ *
+ * Author: Qiuhan Ding <qiuhanding@cs.ucla.edu>
+ *         Yingdi Yu <yingdi@cs.ucla.edu>
+ */
+
+#ifndef CHRONOCHAT_CHATROOM_DISCOVERY_BACKEND_HPP
+#define CHRONOCHAT_CHATROOM_DISCOVERY_BACKEND_HPP
+
+#include <ndn-cxx/util/scheduler.hpp>
+#include <boost/random.hpp>
+#include <QThread>
+#include <QMutex>
+
+#ifndef Q_MOC_RUN
+#include "common.hpp"
+#include "chatroom-info.hpp"
+#include <socket.hpp>
+#endif
+
+namespace chronochat {
+
+class ChatroomInfoBackend {
+public:
+  std::string chatroomName;
+  Name chatroomPrefix;
+  ChatroomInfo info;
+  // For a chatroom's user to check whether his own chatroom is alive
+  ndn::EventId localChatroomTimeoutEventId;
+  // If the manager no longer exist, set a random timer to compete for manager
+  ndn::EventId managerSelectionTimeoutEventId;
+  // For a user to check the status of the chatroom that he is not in.
+  ndn::EventId remoteChatroomTimeoutEventId;
+  // If the user is manager, he will need the helloEventId to keep track of hello message
+  ndn::EventId helloTimeoutEventId;
+  // To tell whether the user is in this chatroom
+  bool isParticipant;
+  // To tell whether the user is the manager
+  bool isManager;
+  // Variable to tell whether another manager exists
+  int count;
+  // Variable to tell whether to print on the panel
+  bool isPrint;
+
+};
+
+class ChatroomDiscoveryBackend : public QThread
+{
+  Q_OBJECT
+
+public:
+  ChatroomDiscoveryBackend(const Name& routingPrefix,
+                           const Name& identity,
+                           QObject* parent = 0);
+
+  ~ChatroomDiscoveryBackend();
+
+
+protected:
+  void
+  run();
+
+private:
+  void
+  initializeSync();
+
+  void
+  close();
+
+  void
+  processSyncUpdate(const std::vector<chronosync::MissingDataInfo>& updates);
+
+  void
+  processChatroomData(const ndn::shared_ptr<const ndn::Data>& data);
+
+  void
+  localSessionTimeout(const Name::Component& chatroomName);
+
+  void
+  remoteSessionTimeout(const Name::Component& chatroomName);
+
+  void
+  randomSessionTimeout(const Name::Component& chatroomName);
+
+  void
+  sendUpdate(const Name::Component& chatroomName);
+
+  void
+  updatePrefixes();
+
+  void
+  sendChatroomList();
+
+signals:
+  /**
+   * @brief request to get chatroom info
+   *
+   * This signal will be sent to controller to get chatroom info
+   *
+   * @param chatroomName specify which chatroom's info to request
+   * @param isManager if the user who send the signal is the manager of the chatroom
+   */
+  void
+  chatroomInfoRequest(std::string chatroomName, bool isManager);
+
+  /**
+   * @brief send chatroom list to front end
+   *
+   * @param chatroomList the list of chatrooms
+   */
+  void
+  chatroomListReady(const QStringList& chatroomList);
+
+  /**
+   * @brief send chatroom info to front end
+   *
+   * @param info the chatroom info request by front end
+   */
+  void
+  chatroomInfoReady(const ChatroomInfo& info);
+
+public slots:
+
+  /**
+   * @brief update routing prefix
+   *
+   * @param routingPrefix the new routing prefix
+   */
+  void
+  updateRoutingPrefix(const QString& routingPrefix);
+
+  /**
+   * @brief erase a participant in a chatroom's roster
+   *
+   * This slot is called by chat dialog will erase a participant from a chatroom.
+   * If the user is the manager of this chatroom, he will publish an update.
+   *
+   * @param sessionPrefix the prefix of the participant to erase
+   * @param chatroomName the name of the chatroom
+   */
+  void
+  onEraseInRoster(ndn::Name sessionPrefix, ndn::Name::Component chatroomName);
+
+  /**
+   * @brief add a participant in a chatroom's roster
+   *
+   * This slot is called by chat dialog and will add a participant from a chatroom.
+   * If the user is the manager of this chatroom, he will publish an update.
+   *
+   * @param sessionPrefix the prefix of the participant to erase
+   * @param chatroomName the name of the chatroom
+   */
+  void
+  onAddInRoster(ndn::Name sessionPrefix, ndn::Name::Component chatroomName);
+
+  /**
+   * @brief is called when user himself join a chatroom
+   *
+   * This slot is called by controller and will modify the chatroom list in discovery
+   *
+   * @param chatroomName the name of chatroom the user join
+   */
+  void
+  onNewChatroomForDiscovery(Name::Component chatroomName);
+
+  /**
+   * @brief get chatroom info from chat dialog
+   *
+   * This slot is called by controller. It get the chatroom info of a chatroom and
+   * if the user is the manager, he will publish an update
+   *
+   * @param chatroomInfo chatroom info
+   * @param isManager whether the user is the manager of the chatroom
+   */
+  void
+  onRespondChatroomInfoRequest(ChatroomInfo chatroomInfo, bool isManager);
+
+  /**
+   * @brief reset when the identity updates
+   *
+   * This slot is called when the identity is updated, it will update the identity and
+   * reset the socket
+   *
+   * @param identity new identity
+   */
+  void
+  onIdentityUpdated(const QString& identity);
+
+  /**
+   * @brief prepare chatroom info for discovery panel
+   *
+   * This slot is called by discovery panel when it wants to display the info of a chatroom
+   *
+   * @param chatroomName the name of chatroom the discovery panel requested
+   */
+  void
+  onWaitForChatroomInfo(const QString& chatroomName);
+
+  void
+  shutdown();
+
+private:
+
+  typedef std::map<ndn::Name::Component, ChatroomInfoBackend> ChatroomList;
+
+  Name m_discoveryPrefix;
+  Name m_routableUserDiscoveryPrefix;
+  Name m_routingPrefix;
+  Name m_userDiscoveryPrefix;
+  Name m_identity;
+
+  boost::mt19937 m_randomGenerator;
+  boost::variate_generator<boost::mt19937&, boost::uniform_int<> > m_rangeUniformRandom;
+
+  shared_ptr<ndn::Face> m_face;
+
+  unique_ptr<ndn::Scheduler> m_scheduler;            // scheduler
+  ndn::EventId m_refreshPanelId;
+  shared_ptr<chronosync::Socket> m_sock; // SyncSocket
+
+  ChatroomList m_chatroomList;
+  QMutex m_mutex;
+
+  bool m_shouldResume;
+};
+
+} // namespace chronochat
+
+#endif // CHRONOCHAT_CHATROOM_DISCOVERY_BACKEND_HPP