diff --git a/cps/fs.py b/cps/fs.py
new file mode 100644
index 00000000..699d5991
--- /dev/null
+++ b/cps/fs.py
@@ -0,0 +1,61 @@
+# -*- coding: utf-8 -*-
+
+#   This file is part of the Calibre-Web (https://github.com/janeczku/calibre-web)
+#     Copyright (C) 2020 mmonkey
+#
+#   This program is free software: you can redistribute it and/or modify
+#   it under the terms of the GNU General Public License as published by
+#   the Free Software Foundation, either version 3 of the License, or
+#   (at your option) any later version.
+#
+#   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, see <http://www.gnu.org/licenses/>.
+
+from __future__ import division, print_function, unicode_literals
+from .constants import CACHE_DIR
+from os import listdir, makedirs, remove
+from os.path import isdir, isfile, join
+from shutil import rmtree
+
+CACHE_TYPE_THUMBNAILS = 'thumbnails'
+
+
+class FileSystem:
+    _instance = None
+    _cache_dir = CACHE_DIR
+
+    def __new__(cls):
+        if cls._instance is None:
+            cls._instance = super(FileSystem, cls).__new__(cls)
+        return cls._instance
+
+    def get_cache_dir(self, cache_type=None):
+        if not isdir(self._cache_dir):
+            makedirs(self._cache_dir)
+
+        if cache_type and not isdir(join(self._cache_dir, cache_type)):
+            makedirs(join(self._cache_dir, cache_type))
+
+        return join(self._cache_dir, cache_type) if cache_type else self._cache_dir
+
+    def get_cache_file_path(self, filename, cache_type=None):
+        return join(self.get_cache_dir(cache_type), filename) if filename else None
+
+    def list_cache_files(self, cache_type=None):
+        path = self.get_cache_dir(cache_type)
+        return [file for file in listdir(path) if isfile(join(path, file))]
+
+    def delete_cache_dir(self, cache_type=None):
+        if not cache_type and isdir(self._cache_dir):
+            rmtree(self._cache_dir)
+        if cache_type and isdir(join(self._cache_dir, cache_type)):
+            rmtree(join(self._cache_dir, cache_type))
+
+    def delete_cache_file(self, filename, cache_type=None):
+        if isfile(join(self.get_cache_dir(cache_type), filename)):
+            remove(join(self.get_cache_dir(cache_type), filename))
diff --git a/cps/helper.py b/cps/helper.py
index d3420a11..271ab3e9 100644
--- a/cps/helper.py
+++ b/cps/helper.py
@@ -52,7 +52,7 @@ except ImportError:
 
 from . import calibre_db
 from .tasks.convert import TaskConvert
-from . import logger, config, get_locale, db, thumbnails, ub
+from . import logger, config, get_locale, db, fs, ub
 from . import gdriveutils as gd
 from .constants import STATIC_DIR as _STATIC_DIR
 from .subproc_wrapper import process_wait
@@ -555,8 +555,9 @@ def get_book_cover_internal(book, use_generic_cover_on_failure, resolution=1, di
         if not disable_thumbnail:
             thumbnail = get_book_cover_thumbnail(book, resolution)
             if thumbnail:
-                if os.path.isfile(thumbnails.get_thumbnail_cache_path(thumbnail)):
-                    return send_from_directory(thumbnails.get_thumbnail_cache_dir(), thumbnail.filename)
+                cache = fs.FileSystem()
+                if cache.get_cache_file_path(thumbnail.filename, fs.CACHE_TYPE_THUMBNAILS):
+                    return send_from_directory(cache.get_cache_dir(fs.CACHE_TYPE_THUMBNAILS), thumbnail.filename)
 
         # Send the book cover from Google Drive if configured
         if config.config_use_google_drive:
diff --git a/cps/schedule.py b/cps/schedule.py
index 5d2c94b9..5c658e41 100644
--- a/cps/schedule.py
+++ b/cps/schedule.py
@@ -18,9 +18,9 @@
 
 from __future__ import division, print_function, unicode_literals
 
-from . import logger
+from . import config, db, logger, ub
 from .services.background_scheduler import BackgroundScheduler
-from .tasks.thumbnail import TaskThumbnail
+from .tasks.thumbnail import TaskCleanupCoverThumbnailCache, TaskGenerateCoverThumbnails
 
 log = logger.create()
 
@@ -29,6 +29,16 @@ def register_jobs():
     scheduler = BackgroundScheduler()
 
     # Generate 100 book cover thumbnails every 5 minutes
-    scheduler.add_task(user=None, task=lambda: TaskThumbnail(limit=100), trigger='interval', minutes=5)
+    scheduler.add_task(user=None, task=lambda: TaskGenerateCoverThumbnails(limit=100), trigger='interval', minutes=5)
 
-    # TODO: validate thumbnail scheduled task
+    # Cleanup book cover cache every day at 4am
+    scheduler.add_task(user=None, task=lambda: TaskCleanupCoverThumbnailCache(), trigger='cron', hour=4)
+
+    # Reconnect metadata.db every 4 hours
+    scheduler.add(func=reconnect_db_job, trigger='interval', hours=4)
+
+
+def reconnect_db_job():
+    log.info('Running background task: reconnect to calibre database')
+    calibre_db = db.CalibreDB()
+    calibre_db.reconnect_db(config, ub.app_DB_path)
diff --git a/cps/services/worker.py b/cps/services/worker.py
index 072674a0..2b6816db 100644
--- a/cps/services/worker.py
+++ b/cps/services/worker.py
@@ -35,7 +35,6 @@ def _get_main_thread():
     raise Exception("main thread not found?!")
 
 
-
 class ImprovedQueue(queue.Queue):
     def to_list(self):
         """
@@ -45,7 +44,8 @@ class ImprovedQueue(queue.Queue):
         with self.mutex:
             return list(self.queue)
 
-#Class for all worker tasks in the background
+
+# Class for all worker tasks in the background
 class WorkerThread(threading.Thread):
     _instance = None
 
@@ -127,6 +127,10 @@ class WorkerThread(threading.Thread):
                 # CalibreTask.start() should wrap all exceptions in it's own error handling
                 item.task.start(self)
 
+            # remove self_cleanup tasks from list
+            if item.task.self_cleanup:
+                self.dequeued.remove(item)
+
             self.queue.task_done()
 
 
@@ -141,6 +145,7 @@ class CalibreTask:
         self.end_time = None
         self.message = message
         self.id = uuid.uuid4()
+        self.self_cleanup = False
 
     @abc.abstractmethod
     def run(self, worker_thread):
@@ -209,6 +214,14 @@ class CalibreTask:
         # todo: throw error if outside of [0,1]
         self._progress = x
 
+    @property
+    def self_cleanup(self):
+        return self._self_cleanup
+
+    @self_cleanup.setter
+    def self_cleanup(self, is_self_cleanup):
+        self._self_cleanup = is_self_cleanup
+
     def _handleError(self, error_message):
         self.stat = STAT_FAIL
         self.progress = 1
diff --git a/cps/tasks/thumbnail.py b/cps/tasks/thumbnail.py
index 4e0c6db4..f61eb4a7 100644
--- a/cps/tasks/thumbnail.py
+++ b/cps/tasks/thumbnail.py
@@ -19,13 +19,15 @@
 from __future__ import division, print_function, unicode_literals
 import os
 
-from cps import config, db, gdriveutils, logger, ub
-from cps.constants import CACHE_DIR as _CACHE_DIR
+from cps import config, db, fs, gdriveutils, logger, ub
 from cps.services.worker import CalibreTask
-from cps.thumbnails import THUMBNAIL_RESOLUTION_1X, THUMBNAIL_RESOLUTION_2X
 from datetime import datetime, timedelta
 from sqlalchemy import func
-from urllib.request import urlopen
+
+try:
+    from urllib.request import urlopen
+except ImportError as e:
+    from urllib2 import urlopen
 
 try:
     from wand.image import Image
@@ -33,73 +35,92 @@ try:
 except (ImportError, RuntimeError) as e:
     use_IM = False
 
+THUMBNAIL_RESOLUTION_1X = 1
+THUMBNAIL_RESOLUTION_2X = 2
 
-class TaskThumbnail(CalibreTask):
+
+class TaskGenerateCoverThumbnails(CalibreTask):
     def __init__(self, limit=100, task_message=u'Generating cover thumbnails'):
-        super(TaskThumbnail, self).__init__(task_message)
+        super(TaskGenerateCoverThumbnails, self).__init__(task_message)
+        self.self_cleanup = True
         self.limit = limit
         self.log = logger.create()
         self.app_db_session = ub.get_new_session_instance()
-        self.worker_db = db.CalibreDB(expire_on_commit=False)
+        self.calibre_db = db.CalibreDB(expire_on_commit=False)
+        self.cache = fs.FileSystem()
+        self.resolutions = [
+            THUMBNAIL_RESOLUTION_1X,
+            THUMBNAIL_RESOLUTION_2X
+        ]
 
     def run(self, worker_thread):
-        if self.worker_db.session and use_IM:
-            thumbnails = self.get_thumbnail_book_ids()
-            thumbnail_book_ids = list(map(lambda t: t.book_id, thumbnails))
+        if self.calibre_db.session and use_IM:
+            expired_thumbnails = self.get_expired_thumbnails()
+            thumbnail_book_ids = self.get_thumbnail_book_ids()
             books_without_thumbnails = self.get_books_without_thumbnails(thumbnail_book_ids)
 
             count = len(books_without_thumbnails)
             for i, book in enumerate(books_without_thumbnails):
-                thumbnails = self.get_thumbnails_for_book(thumbnails, book)
-                if thumbnails:
-                    for thumbnail in thumbnails:
-                        self.update_book_thumbnail(book, thumbnail)
-
-                else:
-                    self.create_book_thumbnail(book, THUMBNAIL_RESOLUTION_1X)
-                    self.create_book_thumbnail(book, THUMBNAIL_RESOLUTION_2X)
+                for resolution in self.resolutions:
+                    expired_thumbnail = self.get_expired_thumbnail_for_book_and_resolution(
+                        book,
+                        resolution,
+                        expired_thumbnails
+                    )
+                    if expired_thumbnail:
+                        self.update_book_thumbnail(book, expired_thumbnail)
+                    else:
+                        self.create_book_thumbnail(book, resolution)
 
                 self.progress = (1.0 / count) * i
 
         self._handleSuccess()
-        self.app_db_session.close()
+        self.app_db_session.remove()
+
+    def get_expired_thumbnails(self):
+        return self.app_db_session\
+            .query(ub.Thumbnail)\
+            .filter(ub.Thumbnail.expiration < datetime.utcnow())\
+            .all()
 
     def get_thumbnail_book_ids(self):
         return self.app_db_session\
-            .query(ub.Thumbnail)\
+            .query(ub.Thumbnail.book_id)\
             .group_by(ub.Thumbnail.book_id)\
             .having(func.min(ub.Thumbnail.expiration) > datetime.utcnow())\
-            .all()
+            .distinct()
 
     def get_books_without_thumbnails(self, thumbnail_book_ids):
-        return self.worker_db.session\
+        return self.calibre_db.session\
             .query(db.Books)\
             .filter(db.Books.has_cover == 1)\
             .filter(db.Books.id.notin_(thumbnail_book_ids))\
             .limit(self.limit)\
             .all()
 
-    def get_thumbnails_for_book(self, thumbnails, book):
-        results = list()
-        for thumbnail in thumbnails:
-            if thumbnail.book_id == book.id:
-                results.append(thumbnail)
+    def get_expired_thumbnail_for_book_and_resolution(self, book, resolution, expired_thumbnails):
+        for thumbnail in expired_thumbnails:
+            if thumbnail.book_id == book.id and thumbnail.resolution == resolution:
+                return thumbnail
 
-        return results
+        return None
 
     def update_book_thumbnail(self, book, thumbnail):
+        thumbnail.generated_at = datetime.utcnow()
         thumbnail.expiration = datetime.utcnow() + timedelta(days=30)
 
         try:
             self.app_db_session.commit()
             self.generate_book_thumbnail(book, thumbnail)
         except Exception as ex:
+            self.log.info(u'Error updating book thumbnail: ' + str(ex))
             self._handleError(u'Error updating book thumbnail: ' + str(ex))
             self.app_db_session.rollback()
 
     def create_book_thumbnail(self, book, resolution):
         thumbnail = ub.Thumbnail()
         thumbnail.book_id = book.id
+        thumbnail.format = 'jpeg'
         thumbnail.resolution = resolution
 
         self.app_db_session.add(thumbnail)
@@ -107,6 +128,7 @@ class TaskThumbnail(CalibreTask):
             self.app_db_session.commit()
             self.generate_book_thumbnail(book, thumbnail)
         except Exception as ex:
+            self.log.info(u'Error creating book thumbnail: ' + str(ex))
             self._handleError(u'Error creating book thumbnail: ' + str(ex))
             self.app_db_session.rollback()
 
@@ -128,9 +150,12 @@ class TaskThumbnail(CalibreTask):
                         if img.height > height:
                             width = self.get_thumbnail_width(height, img)
                             img.resize(width=width, height=height, filter='lanczos')
-                            img.save(filename=self.get_thumbnail_cache_path(thumbnail))
+                            img.format = thumbnail.format
+                            filename = self.cache.get_cache_file_path(thumbnail.filename, fs.CACHE_TYPE_THUMBNAILS)
+                            img.save(filename=filename)
                 except Exception as ex:
                     # Bubble exception to calling function
+                    self.log.info(u'Error generating thumbnail file: ' + str(ex))
                     raise ex
                 finally:
                     stream.close()
@@ -144,7 +169,9 @@ class TaskThumbnail(CalibreTask):
                     if img.height > height:
                         width = self.get_thumbnail_width(height, img)
                         img.resize(width=width, height=height, filter='lanczos')
-                        img.save(filename=self.get_thumbnail_cache_path(thumbnail))
+                        img.format = thumbnail.format
+                        filename = self.cache.get_cache_file_path(thumbnail.filename, fs.CACHE_TYPE_THUMBNAILS)
+                        img.save(filename=filename)
 
     def get_thumbnail_height(self, thumbnail):
         return int(225 * thumbnail.resolution)
@@ -153,20 +180,88 @@ class TaskThumbnail(CalibreTask):
         percent = (height / float(img.height))
         return int((float(img.width) * float(percent)))
 
-    def get_thumbnail_cache_dir(self):
-        if not os.path.isdir(_CACHE_DIR):
-            os.makedirs(_CACHE_DIR)
+    @property
+    def name(self):
+        return "GenerateCoverThumbnails"
 
-        if not os.path.isdir(os.path.join(_CACHE_DIR, 'thumbnails')):
-            os.makedirs(os.path.join(_CACHE_DIR, 'thumbnails'))
 
-        return os.path.join(_CACHE_DIR, 'thumbnails')
+class TaskCleanupCoverThumbnailCache(CalibreTask):
+    def __init__(self, task_message=u'Validating cover thumbnail cache'):
+        super(TaskCleanupCoverThumbnailCache, self).__init__(task_message)
+        self.log = logger.create()
+        self.app_db_session = ub.get_new_session_instance()
+        self.calibre_db = db.CalibreDB(expire_on_commit=False)
+        self.cache = fs.FileSystem()
 
-    def get_thumbnail_cache_path(self, thumbnail):
-        if thumbnail:
-            return os.path.join(self.get_thumbnail_cache_dir(), thumbnail.filename)
-        return None
+    def run(self, worker_thread):
+        cached_thumbnail_files = self.cache.list_cache_files(fs.CACHE_TYPE_THUMBNAILS)
+
+        # Expire thumbnails in the database if the cached file is missing
+        # This case will happen if a user deletes the cache dir or cached files
+        if self.app_db_session:
+            self.expire_missing_thumbnails(cached_thumbnail_files)
+            self.progress = 0.33
+
+        # Delete thumbnails in the database if the book has been removed
+        # This case will happen if a book is removed in Calibre and the metadata.db file is updated in the filesystem
+        if self.app_db_session and self.calibre_db:
+            book_ids = self.get_book_ids()
+            self.delete_thumbnails_for_missing_books(book_ids)
+            self.progress = 0.66
+
+        # Delete extraneous cached thumbnail files
+        # This case will happen if a book was deleted and the thumbnail OR the metadata.db file was changed externally
+        if self.app_db_session:
+            db_thumbnail_files = self.get_thumbnail_filenames()
+            self.delete_extraneous_thumbnail_files(cached_thumbnail_files, db_thumbnail_files)
+
+        self._handleSuccess()
+        self.app_db_session.remove()
+
+    def expire_missing_thumbnails(self, filenames):
+        try:
+            self.app_db_session\
+                .query(ub.Thumbnail)\
+                .filter(ub.Thumbnail.filename.notin_(filenames))\
+                .update({"expiration": datetime.utcnow()}, synchronize_session=False)
+            self.app_db_session.commit()
+        except Exception as ex:
+            self.log.info(u'Error expiring thumbnails for missing cache files: ' + str(ex))
+            self._handleError(u'Error expiring thumbnails for missing cache files: ' + str(ex))
+            self.app_db_session.rollback()
+
+    def get_book_ids(self):
+        results = self.calibre_db.session\
+            .query(db.Books.id)\
+            .filter(db.Books.has_cover == 1)\
+            .distinct()
+
+        return [value for value, in results]
+
+    def delete_thumbnails_for_missing_books(self, book_ids):
+        try:
+            self.app_db_session\
+                .query(ub.Thumbnail)\
+                .filter(ub.Thumbnail.book_id.notin_(book_ids))\
+                .delete(synchronize_session=False)
+            self.app_db_session.commit()
+        except Exception as ex:
+            self.log.info(str(ex))
+            self._handleError(u'Error deleting thumbnails for missing books: ' + str(ex))
+            self.app_db_session.rollback()
+
+    def get_thumbnail_filenames(self):
+        results = self.app_db_session\
+            .query(ub.Thumbnail.filename)\
+            .all()
+
+        return [thumbnail for thumbnail, in results]
+
+    def delete_extraneous_thumbnail_files(self, cached_thumbnail_files, db_thumbnail_files):
+        extraneous_files = list(set(cached_thumbnail_files).difference(db_thumbnail_files))
+        for file in extraneous_files:
+            self.cache.delete_cache_file(file, fs.CACHE_TYPE_THUMBNAILS)
 
     @property
     def name(self):
-        return "Thumbnail"
+        return "CleanupCoverThumbnailCache"
diff --git a/cps/thumbnails.py b/cps/thumbnails.py
deleted file mode 100644
index ea7aac86..00000000
--- a/cps/thumbnails.py
+++ /dev/null
@@ -1,54 +0,0 @@
-# -*- coding: utf-8 -*-
-
-#   This file is part of the Calibre-Web (https://github.com/janeczku/calibre-web)
-#     Copyright (C) 2020 mmonkey
-#
-#   This program is free software: you can redistribute it and/or modify
-#   it under the terms of the GNU General Public License as published by
-#   the Free Software Foundation, either version 3 of the License, or
-#   (at your option) any later version.
-#
-#   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, see <http://www.gnu.org/licenses/>.
-
-from __future__ import division, print_function, unicode_literals
-import os
-
-from . import logger, ub
-from .constants import CACHE_DIR as _CACHE_DIR
-
-from datetime import datetime
-
-THUMBNAIL_RESOLUTION_1X = 1
-THUMBNAIL_RESOLUTION_2X = 2
-
-log = logger.create()
-
-
-def get_thumbnail_cache_dir():
-    if not os.path.isdir(_CACHE_DIR):
-        os.makedirs(_CACHE_DIR)
-    if not os.path.isdir(os.path.join(_CACHE_DIR, 'thumbnails')):
-        os.makedirs(os.path.join(_CACHE_DIR, 'thumbnails'))
-    return os.path.join(_CACHE_DIR, 'thumbnails')
-
-
-def get_thumbnail_cache_path(thumbnail):
-    if thumbnail:
-        return os.path.join(get_thumbnail_cache_dir(), thumbnail.filename)
-    return None
-
-
-def cover_thumbnail_exists_for_book(book):
-    if book and book.has_cover:
-        thumbnail = ub.session.query(ub.Thumbnail).filter(ub.Thumbnail.book_id == book.id).first()
-        if thumbnail and thumbnail.expiration > datetime.utcnow():
-            thumbnail_path = get_thumbnail_cache_path(thumbnail)
-            return thumbnail_path and os.path.isfile(thumbnail_path)
-
-    return False
diff --git a/cps/ub.py b/cps/ub.py
index 0b5a65e7..30abd728 100644
--- a/cps/ub.py
+++ b/cps/ub.py
@@ -18,6 +18,7 @@
 #  along with this program. If not, see <http://www.gnu.org/licenses/>.
 
 from __future__ import division, print_function, unicode_literals
+import atexit
 import os
 import sys
 import datetime
@@ -42,12 +43,11 @@ from sqlalchemy import create_engine, exc, exists, event
 from sqlalchemy import Column, ForeignKey
 from sqlalchemy import String, Integer, SmallInteger, Boolean, DateTime, Float, JSON
 from sqlalchemy.ext.declarative import declarative_base
-from sqlalchemy.ext.hybrid import hybrid_property
 from sqlalchemy.orm.attributes import flag_modified
 from sqlalchemy.orm import backref, relationship, sessionmaker, Session, scoped_session
 from werkzeug.security import generate_password_hash
 
-from . import cli, constants
+from . import cli, constants, logger
 
 
 session = None
@@ -435,6 +435,14 @@ class RemoteAuthToken(Base):
         return '<Token %r>' % self.id
 
 
+def filename(context):
+    file_format = context.get_current_parameters()['format']
+    if file_format == 'jpeg':
+        return context.get_current_parameters()['uuid'] + '.jpg'
+    else:
+        return context.get_current_parameters()['uuid'] + '.' + file_format
+
+
 class Thumbnail(Base):
     __tablename__ = 'thumbnail'
 
@@ -443,19 +451,10 @@ class Thumbnail(Base):
     uuid = Column(String, default=lambda: str(uuid.uuid4()), unique=True)
     format = Column(String, default='jpeg')
     resolution = Column(SmallInteger, default=1)
+    filename = Column(String, default=filename)
+    generated_at = Column(DateTime, default=lambda: datetime.datetime.utcnow())
     expiration = Column(DateTime, default=lambda: datetime.datetime.utcnow() + datetime.timedelta(days=30))
 
-    @hybrid_property
-    def extension(self):
-        if self.format == 'jpeg':
-            return 'jpg'
-        else:
-            return self.format
-
-    @hybrid_property
-    def filename(self):
-        return self.uuid + '.' + self.extension
-
 
 # Migrate database to current version, has to be updated after every database change. Currently migration from
 # everywhere to current should work. Migration is done by checking if relevant columns are existing, and than adding
@@ -705,6 +704,9 @@ def get_new_session_instance():
     new_engine = create_engine(u'sqlite:///{0}'.format(cli.settingspath), echo=False)
     new_session = scoped_session(sessionmaker())
     new_session.configure(bind=new_engine)
+
+    atexit.register(lambda: new_session.remove() if new_session else True)
+
     return new_session