Hierarchical stream aggregates: querying nested stream sessions

This article identifies an interesting class of applications where stream sessions may be organized into a hierarchical fashion - i.e. sessions may consist of sub-sessions. We argue that data streams of this kind have rich procedural semantics - i.e. behavior - and therefore a semantically rich model should be used: a session may be defined by opening and closing conditions, may have data and methods and may consist of sub-sessions. We propose a simple conceptual model based on the notion of "session" similar to a class in an object-oriented environment having lifetime semantics. Queries on top of this schema can be formulated via HSA (hierarchical stream aggregate) expressions.