How to implement a light pub-sub service on App Engine?

Question

During his Google I/O 2009 "Offline processing on App Engine: A look ahead" presentation (video, slides), Brett Slatkin presents the Task Queue service.

He states that

Pub-sub systems maximize transactions, decoupling:

• Large numbers of small transactions per second
• One-to-many fan-out with changing receivers
• Guaranteed ordering, filtering, two-phase commit

And specifically emphasises

Our new API implements queueing, not pub-sub

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I'm interested in only a subset of those functionalities:

• One-to-many fan-out with changing selected/fixed internal receiver handlers
• Guaranteed ordering, filtering, two-phase commit

Targeted goal would be to ease publishing of notifications/messages between different modules of the same web application. Sample usage scenarios cases would be:

• Making the payment module aware of receivals of bills.
• Making a user able to track changes of a particular domain object he has decided to follow/star.

What would be the correct way to implement these on top of the Task Queue service ?

Answer 1

Consider using Cloud Pub/Sub. It's the externalized version of the internal Pub/Sub technology, which has been used extensively within google for about 7 years and proven to be scalable and robust. As of today it's still beta, but we're going to GA soon (in few months).

Answer 2

There is an example implementation of PubSubHubbub from Google that runs on AppEngine. You can have a look at how they do it or remove code until it fits your needs.