- I want to define my foos in a HiveMind configuration
- Some of the foos will be defined at runtime (from the database)
- Some attributes of the foos will be settable by users at runtime
- How do I update my HiveMind decriptors with these changes?
You don't. HiveMind is used to describe the static configuration of your application. This is equivalent to saying "How do I change the default value of a final static field in one of my classes (and store it back into a JAR (and distribute the change around the cluster))."
What your HiveMind configuration will store is the default configuration of foos until a) they are updated by a user and/or b) new foos are discovered at runtime.
What you need is a service that can answer the question: "what is the configuration of my foos?" This service will have access to the configuration ... but also to the database for updates and overrides. It will integrate that information drawn from the HiveMind configuration with overrides from the database and provide a consistent view. If you are smart, you'll break it apart into a master service with several additional services (for the discovery piece, for the database override piece).
I can't think of a scenario where external code (code that is not a HiveMind service) should directly access a configuration extension point. It is allowed, the API is there ... but in all reasonable cases, putting a service between client code and the configuration data is the best bet ... especially for applications that expect to grow in the direction of greater dynamicism.