Introduction to Bluesky's AT Protocol

Summary

Some time ago I wrote a long blog post I called “Complete guide to Bluesky”, which explains how all the user-facing features of Bluesky work and various tips and tricks. This one is meant to be a bit like a developer version of that – I want to explain in hopefully understandable language what all the pieces of the network architecture are and how they all fit together. I hope this will let you understand better how Bluesky and the underlying protocol works, and how it differs from e.g. the Fediverse. This should also be a good starting point if you want to start building some apps or tools on ATProto. This post is a first part of a series – next I want to look at some comparisons with the Fediverse and some common misconceptions that people have, and look at the state of decentralization of this network, but that was way too much for one post; so this one focuses on the “ATProto intro tutorial” part. But before we start, a little philosophical aside: What is “Bluesky”? Which “Bluesky” are we talking about? Discussions about Bluesky sometimes get a little confusing because… “Bluesky” could mean a few different things. Language is hard. First, we have Bluesky the company, the team. Usually, when people want to clarify that they’re talking about the group of people or the organization, they say “Bluesky PBC” (PBC = Public Benefit Corporation), or “Bluesky team”. (If you want to read a bit about where Bluesky came from and what’s the current state of the company, read these two sections in the Bluesky Guide blog post.) And we also have Bluesky the product, the social network, the thing that they’ve built. This network is not a single black box like Twitter or Facebook are (despite what they say about it on Mastodon), it’s more like a set of separate and actually very transparent boxes. The system they’ve built, of which Bluesky was initially meant to be just a tech demo, is called the Authenticated Transfer Protocol, or AT Protocol, or ATProto. Bluesky is built on ATPr...