I wouldn't really count Mastodon/Bluesky bridging as federation. They're incompatible protocols that were never intended to work together (arguably Bluesky was explicitly designed to avoid using AP).
Fediverse
A community dedicated to fediverse news and discussion.
Fediverse is a portmanteau of "federation" and "universe".
Getting started on Fediverse;
- What is the fediverse?
- Fediverse Platforms
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Eh. AP isn't magic. Platforms can be pretty incompatible because of their differing use or implementation of AP. I feel like at some point there's a blurry line between a bridge being something different and just an extension of 2 protocols.
Definitely, AP is not magic. But if even within one protocol round-tripping and full-fidelity is impossible or very difficult, that makes it only harder and less likely through a bridge.
I would say federation doesn't necessitate the use of a single protocol across the network.
Imagine the scenario where very slowly each protocol aligns on features and just requires simple translation? What if one or both start to implement parts of the other spec?
Would you only call it federation if they used identical protocols?
I'd say federation is about the actual content being shared across decentralised services and not the technical means by which it's shared
IMO bridging or translation isn't federation per se. Also it seems unlikely that protocols would converge to that extent. In fact AP implementations are already different enough that even within the same protocol it's hard to represent all the different activities instances can present.