Abstract: Support for dynamic and client-specific customization of distributed services is required in many application areas. We present a distributed in many application areas. We present a distributed service as consisting of a minimal functional core-implemented as a component-based system, and an unbound set of potential extensions that can be selectively integrated within this core functionality. An extension to this core may be a new service, due to new requirements of end users. Another important category of extensions we consider, are non-functional services such as authentication, which typically introduce interaction refinements at the application level. Each extension is implemented as a layer of decorator-like wrappers. The novelty of this work is that the composition logic, responsible for integrating extensions into the core system, is completely separated from the code of the core system, extensions and clients as well. Clients (end users, system integrators) can customize this composition logic dynamically on a per interaction basis by attaching extension identifiers to their interactions with the core system.