Pubsub with Google App Engine

Question

I'm trying to implement pubsub messaging with Google App Engine. I want to be able to store callbacks to other servers and then send them new data when it becomes available.

I've had a good look around but can't seem to come up with anything apart from implementing it myself. I've seen pubsubhubbub: https://code.google.com/p/pubsubhubbub/wiki/DeveloperGettingStartedGuide but I want to be able to store query parameters. Xmpp xep 60 for example includes capabilities to configure subscriptions with additional data (called options).

The XMPP service in App Engine doesn't seem to have the pubsub extension.

Is managing our own recipient list as: https://groups.google.com/forum/#!topic/google-appengine/CaBcX0EWO00 seems to suggest the only option?

App Engine seems to have implementations with devices in mind using: Channels for javascript: https://developers.google.com/appengine/docs/python/channel/

CloudBackendMessaging for devices: https://developers.google.com/cloud/samples/mbs/pubsub_messaging

Answer 1

There are no client-originated persistent connections (i.e. a listening socket) on AppEngine, so it's quite impossible to implement a real-time push system on it.

As you already know, you can get close with Channels API (solving push to browsers) and mobile device-specific push systems (GCM and APNS).

If you want a universal system, I'd recommend a socket based system, much like PubNub. You should look into Compute Engine which allows for such functionality.

Answer 2

There is a Limited Preview for Google Cloud Pub/Sub which will be something to watch out for as it matures.

Google Cloud Pub/Sub is designed to provide reliable, many-to-many, asynchronous messaging between applications. Publisher applications can send messages to a “topic” and other applications can subscribe to that topic to receive the messages. By decoupling senders and receivers, Google Cloud Pub/Sub allows developers to communicate between independently written applications.