Alternative repo change detection mechanism?

[mary-ext]

This might be something that goes hand-in-hand with the discussion about private data in #3363 (comment).

It would be nice if there was a way to detect changes to a certain collection that isn't via the firehose as a means for say, local caching. Apps could retain a local copy of private records like bookmarks, drafts, and even public records like list members, resulting in less requests to the PDS overall, they would only need to resync if someone else has tampered it (by another app, or the same app in another device.)

Very much uncertain as for how this could look like, so I am opening up a discussion over what's the best way to achieve this.

Public records like list members could be best served as an AppView thing, but I also feel like it might be good if the PDS were to have something like it regardless.

[bnewbold]

Yup! This is one of the active areas of design work for "private" (personal or shared) data in atproto. How to notify given:

• multiple users, on different PDS instances
• supporting account migration
• individual users might have multiple devices (same account), and/or multiple accounts (same device)
• multiple service implementations or instances, chosen by different users
• multiple client app implementations