NESTREAM: Querying nested streams

This article identifies an interesting class of applications where stream sessions may be organized in a hierarchical fashion - i.e. sessions may contain sub-sessions. For example, log streams from call centers belong to different call sessions and call sessions consist of services' sub-sessions. We may want to monitor statistics and perform accounting at any level on this hierarchy, relative to any other higher level (e.g. monitoring the average service session per call vs. the average service session for the entire system.) 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. We describe an algorithm dictating how stream data flow down session hierarchies and discuss potential evaluation and optimization techniques for HSAs. Finally we introduce NESTREAM, a prototype implementation for these ideas and give some preliminary experimental results.