Modern distributed systems often rely on databases that achieve scalability by providing only weak guarantees about the consistency of data - i.e. weak consistency levels. In these databases executions may happen which are not serialisable, and one problem is thus to understand whether a given transactional application performs only serialisable executions. In this talk first we formally define Parallel Snapshot Isolation (PSI), a consistency level fit for geo-replicated databases, that appeared recently in the literature. The formal definition let us sketch a robustness criterion for PSI, that is a condition to check whether a transactional application generates only serialisable executions, even if ran over a database implementing PSI. We also show how to strengthen PSI by using entity groups, and we sketch a robustness criterion for this consistency level.