``implementation_list`` list the different external implementations, and the order matters: the first in the list (namely postgres) is the default implementation for all middleware services. The ``service_overrides`` key is here for services that should be using a specific implementation that is not the default one. I thus agree with Peter, I do not want to see in that list any service forced to used postgres, as that is the default anyway. We should keep that list as small as possible.