Add graceful shutdown hook

main.py

@@ -3,6 +3,7 @@ import traceback
 import urllib.parse
 import os
 import json
+import asyncio
 
 from model import Model
 
@@ -75,5 +76,9 @@ if __name__ == '__main__':
         with open(filename) as f:
             data = json.load(f)
     app['model'] = Model(model = data)
+
+    async def on_shutdown(app):
+        app['model'].save()
+
+    app.on_shutdown.append(on_shutdown)
     aiohttp.web.run_app(app, port=42042)
-    app['model'].save()