diff --git a/src/db-helper.cc b/src/db-helper.cc
new file mode 100644
index 0000000..334621e
--- /dev/null
+++ b/src/db-helper.cc
@@ -0,0 +1,453 @@
+/* -*- Mode: C++; c-file-style: "gnu"; indent-tabs-mode:nil -*- */
+/*
+ * Copyright (c) 2012 University of California, Los Angeles
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License version 2 as
+ * published by the Free Software Foundation;
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, write to the Free Software
+ * Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
+ *
+ * Author: Alexander Afanasyev <alexander.afanasyev@ucla.edu>
+ *	   Zhenkai Zhu <zhenkai@cs.ucla.edu>
+ */
+
+#include "db-helper.h"
+// #include "sync-log.h"
+#include <boost/make_shared.hpp>
+#include <boost/ref.hpp>
+
+// Other options: VP_md2, EVP_md5, EVP_sha, EVP_sha1, EVP_sha256, EVP_dss, EVP_dss1, EVP_mdc2, EVP_ripemd160
+#define HASH_FUNCTION EVP_sha256
+
+#include <boost/throw_exception.hpp>
+typedef boost::error_info<struct tag_errmsg, std::string> errmsg_info_str; 
+// typedef boost::error_info<struct tag_errmsg, int> errmsg_info_int; 
+
+using namespace boost;
+
+const std::string INIT_DATABASE = "\
+PRAGMA foreign_keys = ON;                                       \
+                                                                \
+CREATE TABLE                                                    \
+    SyncNodes(                                                  \
+        device_id       INTEGER PRIMARY KEY AUTOINCREMENT,      \
+        device_name     TEXT NOT NULL,                          \
+        description     TEXT,                                   \
+        seq_no          INTEGER NOT NULL,                       \
+        last_known_tdi  TEXT,                                   \
+        last_update     TIMESTAMP                               \
+    );                                                          \
+                                                                \
+CREATE TRIGGER SyncNodesUpdater_trigger                                \
+    BEFORE INSERT ON SyncNodes                                         \
+    FOR EACH ROW                                                       \
+    WHEN (SELECT device_id                                             \
+             FROM SyncNodes                                            \
+             WHERE device_name=NEW.device_name)                        \
+         IS NOT NULL                                                   \
+    BEGIN                                                              \
+        UPDATE SyncNodes                                               \
+            SET seq_no=max(seq_no,NEW.seq_no)                          \
+            WHERE device_name=NEW.device_name;                         \
+        SELECT RAISE(IGNORE);                                          \
+    END;                                                               \
+                                                                       \
+CREATE INDEX SyncNodes_device_name ON SyncNodes (device_name);         \
+                                                                       \
+CREATE TABLE SyncLog(                                                  \
+        state_id    INTEGER PRIMARY KEY AUTOINCREMENT,                 \
+        state_hash  BLOB NOT NULL UNIQUE,                              \
+        last_update TIMESTAMP NOT NULL                                 \
+    );                                                                 \
+                                                                       \
+CREATE TABLE                                                            \
+    SyncStateNodes(                                                     \
+        id          INTEGER PRIMARY KEY AUTOINCREMENT,                  \
+        state_id    INTEGER NOT NULL                                    \
+            REFERENCES SyncLog (state_id) ON UPDATE CASCADE ON DELETE CASCADE, \
+        device_id   INTEGER NOT NULL                                    \
+            REFERENCES SyncNodes (device_id) ON UPDATE CASCADE ON DELETE CASCADE, \
+        seq_no      INTEGER NOT NULL                                    \
+    );                                                                  \
+                                                                        \
+CREATE INDEX SyncStateNodes_device_id ON SyncStateNodes (device_id);    \
+CREATE INDEX SyncStateNodes_state_id  ON SyncStateNodes (state_id);  \
+CREATE INDEX SyncStateNodes_seq_no    ON SyncStateNodes (seq_no);          \
+                                                                        \
+CREATE TRIGGER SyncLogGuard_trigger                                     \
+    BEFORE INSERT ON SyncLog                                            \
+    FOR EACH ROW                                                        \
+    WHEN (SELECT state_hash                                             \
+            FROM SyncLog                                                \
+            WHERE state_hash=NEW.state_hash)                            \
+        IS NOT NULL                                                     \
+    BEGIN                                                               \
+        DELETE FROM SyncLog WHERE state_hash=NEW.state_hash;            \
+    END;                                                                \
+";
+
+DbHelper::DbHelper (const std::string &path)
+{
+  int res = sqlite3_open((path+"chronoshare.db").c_str (), &m_db);
+  if (res != SQLITE_OK)
+    {
+      BOOST_THROW_EXCEPTION (Error::Db ()
+                             << errmsg_info_str ("Cannot open/create dabatabase: [" + path + "chronoshare.db" + "]"));
+    }
+  
+  res = sqlite3_create_function (m_db, "hash", 2, SQLITE_ANY, 0, 0,
+                                 DbHelper::hash_xStep, DbHelper::hash_xFinal);
+  if (res != SQLITE_OK)
+    {
+      BOOST_THROW_EXCEPTION (Error::Db ()
+                             << errmsg_info_str ("Cannot create function ``hash''"));
+    }
+
+  res = sqlite3_create_function (m_db, "hash2str", 2, SQLITE_ANY, 0,
+                                 DbHelper::hash2str_Func,
+                                 0, 0);
+  if (res != SQLITE_OK)
+    {
+      BOOST_THROW_EXCEPTION (Error::Db ()
+                             << errmsg_info_str ("Cannot create function ``hash''"));
+    }
+
+  res = sqlite3_create_function (m_db, "str2hash", 2, SQLITE_ANY, 0,
+                                 DbHelper::str2hash_Func,
+                                 0, 0);
+  if (res != SQLITE_OK)
+    {
+      BOOST_THROW_EXCEPTION (Error::Db ()
+                             << errmsg_info_str ("Cannot create function ``hash''"));
+    }
+  
+  // Alex: determine if tables initialized. if not, initialize... not sure what is the best way to go...
+  // for now, just attempt to create everything
+
+  char *errmsg = 0;
+  res = sqlite3_exec (m_db, INIT_DATABASE.c_str (), NULL, NULL, &errmsg);
+  if (res != SQLITE_OK && errmsg != 0)
+    {
+      std::cerr << "DEBUG: " << errmsg << std::endl;
+      sqlite3_free (errmsg);
+    }
+}
+
+DbHelper::~DbHelper ()
+{
+  int res = sqlite3_close (m_db);
+  if (res != SQLITE_OK)
+    {
+      // complain
+    }
+}
+
+HashPtr
+DbHelper::RememberStateInStateLog ()
+{
+  int res = sqlite3_exec (m_db, "BEGIN TRANSACTION;", 0,0,0);
+
+  res += sqlite3_exec (m_db, "\
+INSERT INTO SyncLog                                     \
+    (state_hash, last_update)                           \
+    SELECT                                              \
+       hash(device_name, seq_no), datetime('now')       \
+    FROM SyncNodes                                      \
+    ORDER BY device_name;                               \
+", 0,0,0);
+
+  if (res != SQLITE_OK)
+    {
+      BOOST_THROW_EXCEPTION (Error::Db ()
+                             << errmsg_info_str ("1"));
+    }
+  
+  sqlite3_int64 rowId = sqlite3_last_insert_rowid (m_db);
+  
+  sqlite3_stmt *insertStmt;
+  res += sqlite3_prepare (m_db, "\
+INSERT INTO SyncStateNodes                              \
+      (state_id, device_id, seq_no)                     \
+      SELECT ?, device_id, seq_no                       \
+            FROM SyncNodes;                             \
+", -1, &insertStmt, 0);
+
+  res += sqlite3_bind_int64 (insertStmt, 1, rowId);
+  sqlite3_step (insertStmt);
+
+  if (res != SQLITE_OK)
+    {
+      BOOST_THROW_EXCEPTION (Error::Db ()
+                             << errmsg_info_str ("4"));
+    }
+  sqlite3_finalize (insertStmt);
+  
+  sqlite3_stmt *getHashStmt;
+  res += sqlite3_prepare (m_db, "\
+SELECT state_hash FROM SyncLog WHERE state_id = ?\
+", -1, &getHashStmt, 0);
+  res += sqlite3_bind_int64 (getHashStmt, 1, rowId);
+
+  HashPtr retval;
+  int stepRes = sqlite3_step (getHashStmt);
+  if (stepRes == SQLITE_ROW)
+    {
+      retval = make_shared<Hash> (sqlite3_column_blob (getHashStmt, 0),
+                                  sqlite3_column_bytes (getHashStmt, 0));
+    }
+  sqlite3_finalize (getHashStmt);
+  res += sqlite3_exec (m_db, "COMMIT;", 0,0,0);
+
+  if (res != SQLITE_OK)
+    {
+      BOOST_THROW_EXCEPTION (Error::Db ()
+                             << errmsg_info_str ("Some error with rememberStateInStateLog"));
+    }
+
+  return retval;
+}
+
+
+void
+DbHelper::hash_xStep (sqlite3_context *context, int argc, sqlite3_value **argv)
+{
+  if (argc != 2)
+    {
+      // _LOG_ERROR ("Wrong arguments are supplied for ``hash'' function");
+      sqlite3_result_error (context, "Wrong arguments are supplied for ``hash'' function", -1);
+      return;
+    }
+  if (sqlite3_value_type (argv[0]) != SQLITE_TEXT ||
+      sqlite3_value_type (argv[1]) != SQLITE_INTEGER)
+    {
+      // _LOG_ERROR ("Hash expects (text,integer) parameters");
+      sqlite3_result_error (context, "Hash expects (text,integer) parameters", -1);
+      return;
+    }
+  
+  EVP_MD_CTX **hash_context = reinterpret_cast<EVP_MD_CTX **> (sqlite3_aggregate_context (context, sizeof (EVP_MD_CTX *)));
+
+  if (hash_context == 0)
+    {
+      sqlite3_result_error_nomem (context);
+      return;
+    }
+
+  if (*hash_context == 0)
+    {
+      *hash_context = EVP_MD_CTX_create ();
+      EVP_DigestInit_ex (*hash_context, HASH_FUNCTION (), 0);
+    }
+  
+  int nameBytes = sqlite3_value_bytes (argv[0]);
+  const unsigned char *name = sqlite3_value_text (argv[0]);
+  sqlite3_int64 seqno = sqlite3_value_int64 (argv[1]);
+
+  EVP_DigestUpdate (*hash_context, name, nameBytes);
+  EVP_DigestUpdate (*hash_context, &seqno, sizeof(sqlite3_int64));
+}
+
+void
+DbHelper::hash_xFinal (sqlite3_context *context)
+{
+  EVP_MD_CTX **hash_context = reinterpret_cast<EVP_MD_CTX **> (sqlite3_aggregate_context (context, sizeof (EVP_MD_CTX *)));
+
+  if (hash_context == 0)
+    {
+      sqlite3_result_error_nomem (context);
+      return;
+    }
+
+  if (*hash_context == 0) // no rows
+    {
+      char charNullResult = 0;
+      sqlite3_result_blob (context, &charNullResult, 1, SQLITE_TRANSIENT); //SQLITE_TRANSIENT forces to make a copy
+      return;
+    }
+  
+  unsigned char *hash = new unsigned char [EVP_MAX_MD_SIZE];
+  unsigned int hashLength = 0;
+
+  int ok = EVP_DigestFinal_ex (*hash_context,
+			       hash, &hashLength);
+
+  sqlite3_result_blob (context, hash, hashLength, SQLITE_TRANSIENT); //SQLITE_TRANSIENT forces to make a copy
+  delete [] hash;
+  
+  EVP_MD_CTX_destroy (*hash_context);
+}
+
+void
+DbHelper::hash2str_Func (sqlite3_context *context, int argc, sqlite3_value **argv)
+{
+  if (argc != 1 || sqlite3_value_type (argv[0]) != SQLITE_BLOB)
+    {
+      sqlite3_result_error (context, "Wrong arguments are supplied for ``hash2str'' function", -1);
+      return;
+    }
+
+  int hashBytes = sqlite3_value_bytes (argv[0]);
+  const void *hash = sqlite3_value_blob (argv[0]);
+
+  std::ostringstream os;
+  Hash tmpHash (hash, hashBytes);
+  os << tmpHash;
+  sqlite3_result_text (context, os.str ().c_str (), -1, SQLITE_TRANSIENT);
+}
+
+void
+DbHelper::str2hash_Func (sqlite3_context *context, int argc, sqlite3_value **argv)
+{
+  if (argc != 1 || sqlite3_value_type (argv[0]) != SQLITE_TEXT)
+    {
+      sqlite3_result_error (context, "Wrong arguments are supplied for ``str2hash'' function", -1);
+      return;
+    }
+
+  size_t hashTextBytes = sqlite3_value_bytes (argv[0]);
+  const unsigned char *hashText = sqlite3_value_text (argv[0]);
+
+  Hash hash (std::string (reinterpret_cast<const char*> (hashText), hashTextBytes));
+  sqlite3_result_blob (context, hash.GetHash (), hash.GetHashBytes (), SQLITE_TRANSIENT);
+}
+
+
+sqlite3_int64
+DbHelper::LookupSyncLog (const std::string &stateHash)
+{
+  Hash tmpHash (stateHash);
+  return LookupSyncLog (stateHash);
+}
+
+sqlite3_int64
+DbHelper::LookupSyncLog (const Hash &stateHash)
+{
+  sqlite3_stmt *stmt;
+  int res = sqlite3_prepare (m_db, "SELECT state_id FROM SyncLog WHERE state_hash = ?",
+                             -1, &stmt, 0);
+  
+  if (res != SQLITE_OK)
+    {
+      BOOST_THROW_EXCEPTION (Error::Db ()
+                             << errmsg_info_str ("Cannot prepare statement"));
+    }
+
+  res = sqlite3_bind_blob (stmt, 1, stateHash.GetHash (), stateHash.GetHashBytes (), SQLITE_STATIC);
+  if (res != SQLITE_OK)
+    {
+      BOOST_THROW_EXCEPTION (Error::Db ()
+                             << errmsg_info_str ("Cannot bind"));
+    }
+
+  sqlite3_int64 row = 0; // something bad
+
+  if (sqlite3_step (stmt) == SQLITE_ROW)
+    {
+      row = sqlite3_column_int64 (stmt, 0);
+    }
+
+  sqlite3_finalize (stmt);
+
+  return row;
+}
+
+void
+DbHelper::UpdateDeviceSeqno (const std::string &name, uint64_t seqNo)
+{
+  sqlite3_stmt *stmt;
+  // update is performed using trigger
+  int res = sqlite3_prepare (m_db, "INSERT INTO SyncNodes (device_name, seq_no) VALUES (?,?);", 
+                             -1, &stmt, 0);
+
+  res += sqlite3_bind_text  (stmt, 1, name.c_str (), name.size (), SQLITE_STATIC);
+  res += sqlite3_bind_int64 (stmt, 2, seqNo);
+  sqlite3_step (stmt);
+  
+  if (res != SQLITE_OK)
+    {
+      BOOST_THROW_EXCEPTION (Error::Db ()
+                             << errmsg_info_str ("Some error with UpdateDeviceSeqno"));
+    }
+  sqlite3_finalize (stmt);
+}
+
+void
+DbHelper::FindStateDifferences (const std::string &oldHash, const std::string &newHash)
+{
+  Hash tmpOldHash (oldHash);
+  Hash tmpNewHash (newHash);
+
+  FindStateDifferences (tmpOldHash, tmpNewHash);
+}
+
+void
+DbHelper::FindStateDifferences (const Hash &oldHash, const Hash &newHash)
+{
+  sqlite3_stmt *stmt;
+
+  int res = sqlite3_prepare_v2 (m_db, "\
+SELECT sn.device_name, s_old.seq_no, s_new.seq_no                       \
+    FROM (SELECT *                                                      \
+            FROM SyncStateNodes                                         \
+            WHERE state_id=(SELECT state_id                             \
+                                FROM SyncLog                            \
+                                WHERE state_hash=:old_hash)) s_old      \
+    LEFT JOIN (SELECT *                                                 \
+                FROM SyncStateNodes                                     \
+                WHERE state_id=(SELECT state_id                         \
+                                    FROM SyncLog                        \
+                                    WHERE state_hash=:new_hash)) s_new  \
+                                                                        \
+        ON s_old.device_id = s_new.device_id                            \
+    JOIN SyncNodes sn ON sn.device_id = s_old.device_id                 \
+                                                                        \
+    WHERE s_new.seq_no IS NULL OR                                       \
+          s_old.seq_no != s_new.seq_no                                  \
+                                                                        \
+UNION ALL                                                               \
+                                                                        \
+SELECT sn.device_name, s_old.seq_no, s_new.seq_no                       \
+    FROM (SELECT *                                                      \
+            FROM SyncStateNodes                                         \
+            WHERE state_id=(SELECT state_id                             \
+                                FROM SyncLog                            \
+                                WHERE state_hash=:new_hash )) s_new     \
+    LEFT JOIN (SELECT *                                                 \
+                FROM SyncStateNodes                                     \
+                WHERE state_id=(SELECT state_id                         \
+                                    FROM SyncLog                        \
+                                    WHERE state_hash=:old_hash)) s_old  \
+                                                                        \
+        ON s_old.device_id = s_new.device_id                            \
+    JOIN SyncNodes sn ON sn.device_id = s_new.device_id                 \
+                                                                        \
+    WHERE s_old.seq_no IS NULL                                          \
+", -1, &stmt, 0);
+
+  if (res != SQLITE_OK)
+    {
+      BOOST_THROW_EXCEPTION (Error::Db ()
+                             << errmsg_info_str ("Some error with FindStateDifferences"));
+    }
+
+  res += sqlite3_bind_blob  (stmt, 1, oldHash.GetHash (), oldHash.GetHashBytes (), SQLITE_STATIC);
+  res += sqlite3_bind_blob  (stmt, 2, newHash.GetHash (), newHash.GetHashBytes (), SQLITE_STATIC);
+
+  while (sqlite3_step (stmt) == SQLITE_ROW)
+    {
+      std::cout << sqlite3_column_text (stmt, 0) <<
+        ": from "  << sqlite3_column_int64 (stmt, 1) <<
+        " to "     << sqlite3_column_int64 (stmt, 2) <<
+        std::endl;
+    }
+  sqlite3_finalize (stmt);
+  
+}