TimeReach: Historical Reachability Queries on Evolving Graphs

Since most graphs evolve over time, it is useful to be able to query their history. We consider historical reachability queries that ask for the existence of a path in some time interval in the past, either in the whole duration of the interval (conjunctive queries), or in at least one time instant in the interval (disjunctive queries). We study both alternatives of storing the full transitive closure of the evolving graph and of performing an online traversal. Then, we propose an appropriate reachability index, termed TimeReach index, that exploits the fact that most real-world graphs contain large strongly connected components. Finally, we present an experimental evaluation of all approaches, for different graph sizes, historical query types and time granularities.