diff --git a/.gitignore b/.gitignore
index c83d46b..39b4e3a 100644
--- a/.gitignore
+++ b/.gitignore
@@ -12,4 +12,5 @@ app/wordfreqapp.db
 app/db/wordfreqapp.db
 app/static/donate-the-author.jpg
 app/static/donate-the-author-hidden.jpg
-app/model/__pycache__/
\ No newline at end of file
+app/model/__pycache__/
+app/log.txt
diff --git a/app/Article.py b/app/Article.py
index c7e87d3..467224a 100644
--- a/app/Article.py
+++ b/app/Article.py
@@ -8,7 +8,7 @@ import hashlib
 from datetime import datetime
 from flask import Flask, request, redirect, render_template, url_for, session, abort, flash, get_flashed_messages
 from difficulty import get_difficulty_level_for_user, text_difficulty_level, user_difficulty_level
-
+import logging
 
 path_prefix = './'
 db_path_prefix = './db/'  # comment this line in deployment
@@ -51,14 +51,20 @@ def get_today_article(user_word_list, visited_articles):
     random.shuffle(result)
 
     # Choose article according to reader's level
-    d1 = load_freq_history(path_prefix + 'static/frequency/frequency.p')
+    logging.debug('* get_today_article(): start d1 = ... ')
+    d1 = load_freq_history(user_word_list)
     d2 = load_freq_history(path_prefix + 'static/words_and_tests.p')
+    logging.debug(' ... get_today_article(): get_difficulty_level_for_user() start')
     d3 = get_difficulty_level_for_user(d1, d2)
+    logging.debug(' ... get_today_article(): done')
 
     d = None
     result_of_generate_article = "not found"
+
     d_user = load_freq_history(user_word_list)
+    logging.debug('* get_today_article(): user_difficulty_level() start')
     user_level = user_difficulty_level(d_user, d3)  # more consideration as user's behaviour is dynamic. Time factor should be considered.
+    logging.debug('* get_today_article(): done')
     text_level = 0
     if visited_articles["index"] > len(visited_articles["article_ids"])-1:  # 生成新的文章
         amount_of_visited_articles = len(visited_articles["article_ids"])
diff --git a/app/main.py b/app/main.py
index f43b36b..19bd889 100644
--- a/app/main.py
+++ b/app/main.py
@@ -1,6 +1,3 @@
-#! /usr/bin/python3
-# -*- coding: utf-8 -*-
-
 ###########################################################################
 # Copyright 2019 (C) Hui Lan <hui.lan@cantab.net>
 # Written permission must be obtained from the author for commercial uses.
diff --git a/app/user_service.py b/app/user_service.py
index 5d601e0..6c208dc 100644
--- a/app/user_service.py
+++ b/app/user_service.py
@@ -15,6 +15,9 @@ from wordfreqCMD import sort_in_descending_order
 import pickle_idea
 import pickle_idea2
 
+import logging
+logging.basicConfig(filename='log.txt', format='%(asctime)s %(message)s', level=logging.DEBUG)
+
 # 初始化蓝图
 userService = Blueprint("user_bp", __name__)
 
@@ -32,7 +35,9 @@ def get_next_article(username):
         else:  # 当前不为“null”，直接 index+=1
             visited_articles["index"] += 1
         session["visited_articles"] = visited_articles
+        logging.debug('/get_next_article: start calling get_today_arcile()')
         visited_articles, today_article, result_of_generate_article = get_today_article(user_freq_record, session.get('visited_articles'))
+        logging.debug('/get_next_arcile: done.')
         data = {
             'visited_articles': visited_articles,
             'today_article': today_article,