diff --git a/gallery_dl/extractor/common.py b/gallery_dl/extractor/common.py
index b59f8768..f726f99c 100644
--- a/gallery_dl/extractor/common.py
+++ b/gallery_dl/extractor/common.py
@@ -1,51 +1,81 @@
+# -*- coding: utf-8 -*-
+
+# Copyright 2014, 2015 Mike Fährmann
+#
+# 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.
+
+"""Common classes and constants used by extractor modules."""
+
 import queue
 import threading
 import requests
-from ..util import safe_request
+# from ..util import safe_request
 
-class BasicExtractor():
+class Message():
 
-    def __init__(self, config):
-        self.session   = requests.Session()
-        self.category  = ""
-        self.directory = ""
+    Version = 1
+    Directory = 2
+    Url = 3
+
+
+class Extractor():
+
+    def __init__(self):
+        self.session = requests.Session()
 
     def __iter__(self):
-        return self.images()
+        return self.items()
+
+    def items(self):
+        yield Message.Version, 1
+        return
 
     def request(self, url, *args, **kwargs):
         return safe_request(self.session, url, *args, **kwargs)
 
     def enable_useragent(self):
-        self.session.headers["User-Agent"] = "Mozilla/5.0 (X11; Linux x86_64; rv:24.0) Gecko/20100101 Firefox/24.0"
+        self.session.headers["User-Agent"] = (
+            "Mozilla/5.0 (X11; Linux x86_64; rv:24.0) Gecko/20100101 Firefox/24.0"
+        )
 
     @staticmethod
     def extract(txt, begin, end, pos=0):
         try:
             first = txt.index(begin, pos) + len(begin)
-            last  = txt.index(end, first)
+            last = txt.index(end, first)
             return txt[first:last], last+len(end)
-        except:
+        except ValueError:
             return None, pos
 
     @staticmethod
     def extract_all(txt, begin, end, pos=0):
         try:
             first = txt.index(begin, pos)
-            last  = txt.index(end, first + len(begin)) + len(end)
+            last = txt.index(end, first + len(begin)) + len(end)
             return txt[first:last], last
-        except:
+        except ValueError:
             return None, pos
 
-class AsyncExtractor(BasicExtractor):
+
+class SequentialExtractor(Extractor):
+
+    def __init__(self, _):
+        Extractor.__init__(self)
+
+
+class AsyncExtractor(Extractor):
 
     def __init__(self, config):
-        super().__init__(config)
-        self.__queue  = queue.Queue(maxsize=5)
-        self.__thread = threading.Thread(target=self.async_images, daemon=True)
+        Extractor.__init__(self)
+        queue_size = int(config.get("queue-size", 5))
+        self.__queue = queue.Queue(maxsize=queue_size)
+        self.__thread = threading.Thread(target=self.async_items)
+        # self.__thread = threading.Thread(target=self.async_images, daemon=True)
 
     def __iter__(self):
-        get  = self.__queue.get
+        get = self.__queue.get
         done = self.__queue.task_done
 
         self.__thread.start()
@@ -56,12 +86,12 @@ class AsyncExtractor(BasicExtractor):
             yield task
             done()
 
-    def async_images(self):
+    def async_items(self):
         put = self.__queue.put
         try:
-            for task in self.images():
+            for task in self.items():
                 put(task)
-        except:
+        except Exception:
             import traceback
             print(traceback.format_exc())
         put(None)