I just got bitten by a little "gotcha" in SDL Tridion Sites 9. When you unpack the intallation zip, you'll find that in the Content Delivery/roles/Discovery folder, there's a separate folder for registration, with the registration tool and its own copy of cd_storage_conf.xml. The idea seems to be that running the service and registering capabilities are two separate activities. I kind of get that. When I first saw the ConfigRepository element at the bottom of Discovery's configuration, I felt like it had been shoehorned into a somewhat awkward place. Yet now, it seems even more awkward. So sure, both the service and the registration tool need access to the storage settings for the discovery database, while only the registration tool needs the configuration repository.

The main difference seems to be one of security. The version of the config that goes with the registration tool has the ClientId and ClientSecret attributes while the other doesn't. This, in fact, is the gotcha that caught me out; I'd copied the storage config from the service, and ended up being unable to perform an update. The error output did mention being unable to get an OAuth token, but I didn't immediately realise that the missing ClientId and ClientSecret were the reason. Kudos to Damien Jewett for his answer on Stack Exchange, which saved me some hair-pulling.

I'm left wondering if this is the end game, or whether a future version will see some further tidying up or separation of concerns.

EDIT: On looking at this again, I realised that even in 8.5 we had two storage configs. The difference is that in 8.5 both had the ClientId and ClientSecret attributes.