Open todo and comments:

• instead of defining an implementation class one can define a path (leading /) which points to the config repository. The object is the loaded from there and reinitialized on configuration changes. (Philipp)

• do we want to go in details here about what a singleton is and how we implement this? I'd rather stress on the fact that these components are replaceable than how they're implemented - switched the paragraph order might suffice in changing this. (Greg)