Simon Willison’s Weblog

How decentralized is Bluesky really? (via) Lots of technical depth in this comparison of the Bluesky (ATProto) and Fediverse/Mastodon/ActivityPub approach to decentralization, from ActivityPub spec author Christine Lemmer-Webber.

One key theme: many of the features of Bluesky that aren't present in the rest of the Fediverse are the result of centralization: Bluesky follows a "shared heap" architecture where participating nodes are expected to maintain a full copy of the entire network - more than 5TB of data already. ActivityPub instead uses a "message passing" architecture where only a subset of the overall network data - messages from accounts followed by that node's users - are imported into the node.

This enables features like comprehensive search and the ability to browse all messages in a conversation even if some come from accounts that are not followed by any of the current node's users (a problem I've faced in the past).

This is also part of the "credible exit" mechanism where users can theoretically switch to a different host while keeping all of their existing content - though that also takes advantage of content addressed storage, a feature that could be added to ActivityPub.

Also of note: direct messages on Bluesky are currently entirely dependent on the single central node run by Bluesky themselves, and are not end-to-end encrypted. Furthermore, signing keys that are used by ATProto are currently held custodially by Bluesky on behalf of their users.