Navigation and interaction patterns of Web users can be relatively complex, especially for sites with interactive applications that support user sessions and profiles. We describe such a case for an interactive virtual garment dressing room. The application is distributed over many web sites, supports personnalization and user profiles, and the notion of a multi-site user session. It has its own data logging system that generates approximately 5GB of complex data per month. The analysis of those logs requires more sophisticated processing than is typically done using a relational language. Even the use of procedural languages and DBMS can prove tedious and inefficient. We show an approach to the analysis of complex log data based on a parallel stream processing architecture and the use of specialized languages, namely a grammatical parser and a logic programming module, that offers an efficient, flexible, and powerful solution.