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Let's recap, in all briefness, how ActivityPub works [1] .

The core idea is that neither your home nor mobile devices have a permanent internet connection, so you need an agent that does (in Mastodon lingo: your instance). This agent performs two services:

1. Take your posts and host them under linkable URLs.
2. Notify interested parties (aka your followers) when you post something (and where it can be found).

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Curiously, there is no concept of a comment. A reply and a post are basically the same thing. If you were to comment on this one, you'd actually instruct your agent to publish a post of your own, with it's parent URL pointing to mine, then send my agent a notification to add your reply post's URL to the children list of my original post.
In other words, the parts of a lively conversation can easily scatter across multiple servers,

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be costly to stitch together and a lot can go wrong in the process (e.g. if an agent goes offline, conversations are cut off). There are good reasons to design the protocol this way, but it is not a good design for what we are using it for.

So, what does Bluesky's AT Protocol bring to the table that ActivityPub doesn't? Is it more efficient? More reliable? Well, sortoff yes, but no...

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AT stands for "Authenticated Transfer" and addresses a main concern of Mastodon (though not necessarily of ActivityPub): unless you self host, there's always the risk of a moderator taking your agent away or its instance shutting down, either resulting in you loosing everything with no recovery option.

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ATP also has the concept of an agent [2] , called a PDS (Personal Data Server), hosting a PDR (Personal Data Repository). The big feature of the system is that the PDR not only contains your entire content (including comments), but is also bound to your identity [3] . A PDR can move to a new PDS in the same sense that you can move your phone number when switching carriers (cooperation of the old PDS is not required). You are also welcome to transfer to a self hosted PDS any time.

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The "phone number" in this case is called a DID (Decentralized ID [3] ) which is derived from the cryptographic key, the content in your repository is signed with. This DID is published in a "telephone book" along with your handle and a manifest that lets users look up where your PDS is currently located and verify that it indeed hosts your content.

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Where is that "telephone book" stored and what would keep a moderator from simply deleting your entry? Well, the W3 recommendation tries very hard to avoid using the word "blockchain" here. 🤷

[The whole federation aspects stands and falls with the question of who controls DID resolution]