diff --git a/cps/constants.py b/cps/constants.py
index 1e44796a..6a6b8f2b 100644
--- a/cps/constants.py
+++ b/cps/constants.py
@@ -40,7 +40,10 @@ else:
 STATIC_DIR          = os.path.join(BASE_DIR, 'cps', 'static')
 TEMPLATES_DIR       = os.path.join(BASE_DIR, 'cps', 'templates')
 TRANSLATIONS_DIR    = os.path.join(BASE_DIR, 'cps', 'translations')
-CACHE_DIR           = os.path.join(BASE_DIR, 'cps', 'cache')
+
+# Cache dir - use CACHE_DIR environment variable, otherwise use the default directory: cps/cache
+DEFAULT_CACHE_DIR   = os.path.join(BASE_DIR, 'cps', 'cache')
+CACHE_DIR           = os.environ.get("CACHE_DIR", DEFAULT_CACHE_DIR)
 
 if HOME_CONFIG:
     home_dir = os.path.join(os.path.expanduser("~"),".calibre-web")
diff --git a/cps/fs.py b/cps/fs.py
index 699d5991..8528e39c 100644
--- a/cps/fs.py
+++ b/cps/fs.py
@@ -50,6 +50,14 @@ class FileSystem:
         path = self.get_cache_dir(cache_type)
         return [file for file in listdir(path) if isfile(join(path, file))]
 
+    def list_existing_cache_files(self, filenames, cache_type=None):
+        path = self.get_cache_dir(cache_type)
+        return [file for file in listdir(path) if isfile(join(path, file)) and file in filenames]
+
+    def list_missing_cache_files(self, filenames, cache_type=None):
+        path = self.get_cache_dir(cache_type)
+        return [file for file in listdir(path) if isfile(join(path, file)) and file not in filenames]
+
     def delete_cache_dir(self, cache_type=None):
         if not cache_type and isdir(self._cache_dir):
             rmtree(self._cache_dir)
diff --git a/cps/helper.py b/cps/helper.py
index 6f22a701..688bc615 100644
--- a/cps/helper.py
+++ b/cps/helper.py
@@ -405,6 +405,7 @@ def update_dir_structure_file(book_id, calibrepath, first_author, orignal_filepa
                      src=new_path, dest=new_name, error=str(ex))
     return False
 
+
 def update_dir_structure_gdrive(book_id, first_author):
     error = False
     book = calibre_db.get_book(book_id)
@@ -505,6 +506,7 @@ def uniq(inpt):
             output.append(x)
     return output
 
+
 def check_email(email):
     email = valid_email(email)
     if ub.session.query(ub.User).filter(func.lower(ub.User.email) == email.lower()).first():
diff --git a/cps/schedule.py b/cps/schedule.py
index 8ab9c732..649b65a3 100644
--- a/cps/schedule.py
+++ b/cps/schedule.py
@@ -27,13 +27,12 @@ from .tasks.thumbnail import TaskSyncCoverThumbnailCache, TaskGenerateCoverThumb
 def register_jobs():
     scheduler = BackgroundScheduler()
 
-    # Generate up to 1000 book covers daily
-    generate_thumbnails_task = scheduler.add_task(user=None, task=lambda: TaskGenerateCoverThumbnails(limit=1000),
-                                                  trigger='interval', days=1)
-    generate_thumbnails_task.modify(next_run_time=datetime.now())
+    if scheduler:
+        # Reconnect metadata.db once every 12 hours
+        scheduler.add_task(user=None, task=lambda: TaskReconnectDatabase(), trigger='interval', hours=12)
 
-    # Cleanup book cover cache every 6 hours
-    scheduler.add_task(user=None, task=lambda: TaskSyncCoverThumbnailCache(), trigger='cron', minute='15', hour='*/6')
+        # Cleanup book cover cache once every 24 hours
+        scheduler.add_task(user=None, task=lambda: TaskSyncCoverThumbnailCache(), trigger='interval', days=1)
 
-    # Reconnect metadata.db every 4 hours
-    scheduler.add_task(user=None, task=lambda: TaskReconnectDatabase(), trigger='cron', minute='5', hour='*/4')
+        # Generate all missing book cover thumbnails once every 24 hours
+        scheduler.add_task(user=None, task=lambda: TaskGenerateCoverThumbnails(), trigger='interval', days=1)
diff --git a/cps/services/background_scheduler.py b/cps/services/background_scheduler.py
index 122168e8..f2688b7f 100644
--- a/cps/services/background_scheduler.py
+++ b/cps/services/background_scheduler.py
@@ -21,13 +21,23 @@ import atexit
 
 from .. import logger
 from .worker import WorkerThread
-from apscheduler.schedulers.background import BackgroundScheduler as BScheduler
+
+try:
+    from apscheduler.schedulers.background import BackgroundScheduler as BScheduler
+    use_APScheduler = True
+except (ImportError, RuntimeError) as e:
+    use_APScheduler = False
+    log = logger.create()
+    log.info(f'APScheduler not found. Unable to schedule tasks.')
 
 
 class BackgroundScheduler:
     _instance = None
 
     def __new__(cls):
+        if not use_APScheduler:
+            return False
+
         if cls._instance is None:
             cls._instance = super(BackgroundScheduler, cls).__new__(cls)
 
@@ -41,12 +51,14 @@ class BackgroundScheduler:
         return cls._instance
 
     def add(self, func, trigger, **trigger_args):
-        return self.scheduler.add_job(func=func, trigger=trigger, **trigger_args)
+        if use_APScheduler:
+            return self.scheduler.add_job(func=func, trigger=trigger, **trigger_args)
 
     def add_task(self, user, task, trigger, **trigger_args):
-        def scheduled_task():
-            worker_task = task()
-            self.log.info('Running scheduled task in background: ' + worker_task.name + ': ' + worker_task.message)
-            WorkerThread.add(user, worker_task)
+        if use_APScheduler:
+            def scheduled_task():
+                worker_task = task()
+                self.log.info('Running scheduled task in background: ' + worker_task.name + ': ' + worker_task.message)
+                WorkerThread.add(user, worker_task)
 
-        return self.add(func=scheduled_task, trigger=trigger, **trigger_args)
+            return self.add(func=scheduled_task, trigger=trigger, **trigger_args)