The management of metamodels is supported by runtime environments that enforce the well-formedness of (meta-)model instances. Beyond this basic functionality, additional capabilities are needed in order to successfully establish a toolchain for Model-Driven Software Engineering. We focus on two such capabilities: transactions and efficient evaluation of invariants, not in the usual context of databases but for main-memory runtime engines, an area where no previous work has addressed the combination of Essential MOF + OCL. The realization of this infrastructural support proves feasible but requires a careful design to accomodate the expressiveness of OCL.