Changing Persistent Applications

During the lifetime of an application, the objects and bindings in a persistent store may require modification in order to fix bugs or incorporate changes. Two mechanisms, Octopus and Nodules, supporting the evolution of persistent applications are presented. The first, Octopus permits code and data values to be evolved, even if they are encapsulated. Type evolution is addressed by the separation of type information from the executable code. In many cases type evolution is possible, without the expense of total or partial system recompilation. Nodules are a complementary mechanism to Octopus in that they allow generic templates to be defined independently of any referencing environment. Nodules may be specialised in order to yield instances by binding them to values and types. When combined into a single system, Nodules and Octopus enable a rich collection of information about the structure and state of applications to be maintained and made available to programmers not only during the construction phase, but during the entire lifetime of applications.