Question

My application wishes to have:

1. Automatic scalability

   • I want App Engine to spin up new instances of my app when traffic increases
   • When instances become idle, I want App Engine to shut them down

2. Client/server affinity

   • After an initial client->server HTTP request, I want clients
     to be able to connect to the same appserver, so that the appserver
     can maintain a bunch of client state
   • State may be updated frequently (in order to support real-time interactions),
     so memcache+datastore based persistence is not desirable.
   • The server may need to make decisions based on the state of multiple clients,
     e.g. real time multi-player game

How can I accomplish this?

Answer 1

You can achieve these goals using App Engine backends (long-running, configurable, addressable, persistent servers):

• https://developers.google.com/appengine/docs/python/backends/
• https://developers.google.com/appengine/docs/java/backends/

Python implementation

1. Configure a backend to be both public and dynamic

   # backends.yaml
   
   backends:
   - name: foo
     instances: 20
     options: public, dynamic
   

2. In addition to deploying your app in usual way:

   appcfg.py update .
   

   remember to deploy you backend:

   appcfg.py backends . update
   

3. For the initial connection, have your client use the
   non-instance specific backend hostname, e.g.:

   foo.your_app_id.appspot.com
   

   App Engine will route your request to available backend
   instance, after optionally starting a new backend instance.

4. In the request handling code on the server, use the backends
   API to determine which instance is handling the request and
   return to the client a instance specific URL.

   from google.appengine.api import backends
   
   import webapp2
   
   class GetPersistentUrlHandler(webapp2.RequestHandler):
   
     def get(self):
       """Return the current backend's instance-specific URL."""
   
       my_url = backends.get_url(instance=backends.get_instance())
       self.response.write(my_url)
   
   app = webapp2.WSGIApplication([
     ('/get_peristent_url', GetPersistentUrlHandler),
   ], debug=True)
   

5. Client makes subsequent connections to the instance specific
   backend URL:

   http://3.foo.your_app_id.appspot.com
   

   Note: when using https be sure to replace subdomain dots with -dot-
   in order to avoid SSL certificate issues.

   https://3-dot-foo.your_app_id.appspot.com
   

Limitations

1. Backends do not live forever and may be shutdown unexpectedly and without notice
2. The number of backends your application can have is currently limited