This is also very interesting... I wonder if mod-pubsub could build a similar system.

Astrolabe continuously computes summaries of the data in the system using on-the-fly aggregation. The aggregation mechanism is controlled by SQL queries, and can be understood as a type of data mining capability. For example, Astrolabe aggregation can be used to monitor the status of a set of servers scattered within the network, to locate a desired resource on the basis of its attribute values, or to compute a summary description of loads on critical network components. As this information changes, Astrolabe will automatically and rapidly recompute the associated aggregates and report the changes to applications that have registered their interest.