Have you had a chance to look at atproto? I was a bit surprised to see no mentions of it. It powers Bluesky but is not coupled to it — the idea is essentially that your public data is meaningfully owned by you (can move hosting without losing identity) across all applications in a global collection, and different app backends can “derive” aggregated views (like Bluesky’s database) from the public network data of all users.
Yes, I think atproto is a great example of the “shared data” pattern for composable tools! Especially since it handles public social scale, which is not addressed by the other systems we mention.
AFAIK, atproto is primarily designed to support multiple distinct clients over shared data, but I also wonder if it could help with composing more granular views within a client. I previously worked on a browser extension for Twitter, and data scraping was a major challenge - which seems easier building on an open protocol like atproto.
Sorry we didn’t mention — it is on our radar but we ran out of space and had to omit lots of good prior art..
I should also mention btw that Bluesky user-configurable feeds is a perfect example of a gentle slope from user to creator!
Isn't it just one more incarnation of long-existing patterns - the same way we keep reinventing Usenet and IRC?