But how recipients are expected to know that?

It's specified like that in FEP-400e ¯\\\(ツ)\/¯

You don't need to discover the members of the collection to make this work. Remember, I treat ActivityPub like an API.

If you already have this post in your local database, you know which wall it's on, so you check that, in AP terms, Delete.actor == post.target.attributedTo || Delete.actor == post.attributedTo (my actual DB schema is different, I have owner_id and author_id in my posts table so I just check if it's either the one or the other).

If you don't have that post in your local database, it's a no-op because it's asking you to delete something you've never seen to begin with. Deletes already work like that.

The way I’d explain it to users is very simple: “Your post has been removed from the group”. UX-wise, you can invalidate the stale cache for the orphaned object. I don’t see why anything has to “leak” anywhere.

Well then for me, Remove{Note} would simply be a fancier way for moderators to do the same thing that Delete{Note} would do.

It should be always clear who owns what. Without that, authentication and authorization are difficult and that leads to numerous security issues.

It is clear — the group owns its wall collection but each post author also owns each individual post they made, to a degree. The group can't edit posts, but it can delete them. The author can both edit and delete their own posts.

I also have a mechanism for checking that the object is truly in a collection, but that isn't documented as a FEP (yet?). It is documented in my federation document.

If you really want to make sure posts don’t get displayed “at all costs” then why not stop using to/cc for such posts?

I don't address them to any followers and don't put them into the outbox. This should be enough, in theory.

This also strikes me as unnecessary. There’s no need to delete other people’s objects, you can just not include them; they effectively become orphaned references that can be garbage collected (or added back later!)

How would you prevent that from leaking into the UX? How would you explain that to users? It's generally expected that group moderators can delete any content from the group. And again, the semantics of you not owning the post, not having a complete authority over it, are important.

Oh hi. I'm not very active on this forum lately but I received an email about this topic, only now.

My main idea with FEP-400e was to avoid posting group posts to one's own profile at all costs. I see Announce-style groups as mostly a crutch for wider compatibility (there are even bots like this one that work on top of the Mastodon API, you mention it and it boosts). As I design things starting from the UX, I couldn't use anything that uses Announce, because that would make my desired UX nearly impossible.

Now for some other implementations "not posting to profile" might not be a concern at all. Lemmy is modeled after Reddit, which does show your posts and comments on your profile front and center. And that's fine. And it's also fine to be incompatible; you can't exactly imagine Mastodon and a phpBB forum interoperating in any meaningful capacity simply because their user experiences are so disparate.

My other idea was that it's not just walls — it's a generic mechanism of creating objects into someone else's collections, while also relinquishing full control over them. My FEP explicitly says that the collection owner can delete someone else's objects contained in the collection. I will soon start working on photo albums. Those will exist in groups too, and the way they will work is that everyone who has access to the group could upload new photos. So again, someone else's collection into which others add things.