What is the minimal software infrastructure and what type of conventions are needed to simplify development of sophisticated parallel numerical application codes using a variety of software components that are not necessarily available as source code? We propose an opaque object-based model where the objects are dynamically loadable from the file system or network. The microkernel required to manage such a system needs to include, at most ffl a few basic services, namely, -- a mechanism for loading objects at run time via dynamic link libraries, and -- consistent schemes for error handling and memory management; and ffl selected methods that all objects share, to deal with -- object life (destruction, reference counting, relationships), and -- object observation (viewing, profiling, tracing). We are experimenting with these ideas in the context of extensible numerical software within the ALICE (Advanced Large-scale Integrated Computational Environment) project, where we are build...