Announce and Undo(Announce) in Mastodon

[julian@activitypub.space]

NodeBB federates out the Announce activity in two ways.

1. The "plain object" way, which is an Announce activity wrapping an object (either a uri or the full object itself.)
2. The "1b12" way, which is an Announce activity wrapping another activity (e.g. Create)

Up until now I haven't needed to handle the reverse (which in AP-speak is the "undo" of an activity), but since NodeBB sends Announce when moving a topic in to category, it now makes sense to Undo(Announce) when moving a topic out of a category.

My first interop test with Mastodon didn't work, and it turns out that Mastodon checks the id of the embedded Announce for a match. It doesn't infer it based on actor/object combination.

Once I matched up the IDs, the Undo was processed correctly.

I also noticed that Announces sent from Mastodon have a published property, although this is not relevant in Undo parsing.

[rimu@piefed.social]

What does undoing an announce do, in Mastodon? Remove a boost?

[julian@activitypub.space]

Yep, that's basically it. It's not really a concept in threadiverse implementations since group actors do the Announcing for synchronization.