Verification of database-driven systems via amalgamation.

We describe a general framework for static verification of systems that base their decisions upon queries to databases. The database is specified using constraints, typically a schema, and is not modified during a run of the system. The system is equipped with a finite number of registers for storing intermediate information from the database and the specification consists of a transition table described using quantifier-free formulas that can query either the database or the registers. Our main result concerns systems querying XML databases -- modeled as data trees -- using quantifier-free formulas with predicates such as the descendant axis or comparison of data values. In this scenario we show an ExpSpace algorithm for deciding reachability. Our technique is based on the notion of amalgamation and is quite general. For instance it also applies to relational databases (with an optimal PSpace algorithm). We also show that minor extensions of the model lead to undecidability.