New generation architecture:

The metadata service is composed of several major components:

The index: in charge of keeping all data and interfacing between the database and registered infospace agents.
The infospace agents: in charge of mining their respective infospaces, feeding information into the index, removing obsolete information and servicing search queries coming from the index.
The interface servers: in charge of servicing requests for users.
The load controller: in charge of recalculating throttling for all services.

Each component may have an interface.  Upon initialization, the interface of each initialized component is collected and registered with the interface servers.