In the context of our flagship Pubky app, we use and provide an indexer, which aggregates all related data in the app environment. (Right now it only checks our homeserver, later, whichever the users designate)
But any app could rearrange this and take or leave our indexer, run its own, aggregate indexes, or offer apps that interact differently with the indexer or homeserver data than our app does.
PKDNS basically commoditizes servers by giving users control.
The Nexus indexer creates a semantic social graph for matching keys and URLs.
Wanna try it? :)