The complete and executable definition of a Domain Specific Language (DSL) relies on the specification of two essential facets: a model of the domain-specific concepts with actions and their semantics; a scheduling model that orchestrates the actions of a domain-specific model. Metamodels can capture the former facet, while Models of Computation (MoCs) capture the latter facet. Unfortunately, theories and tools for metamodeling and MoCs have evolved independently, creating a cultural and technical chasm between the two communities. Consequently, there is currently no framework to explicitly model and compose both facets of a DSL. This paper introduces a new framework to bridge a metamodel and a MoC in a modular fashion. This bridge allows (i) the complete and executable definition of a DSL, (ii) the reuse of MoCs for different domain-specific metamodels, and (iii) the use of different MoCs for a given metamodel, to cope with variation points of a DSL.