ATProto architecture at a glance

Visual summary based on Introduction to AT Protocol by Kuba Suder (@mackuba.eu).

Identity (DID + Handle)

DID is permanent; handle can change freely. The DID document (at plc.directory) maps DID to PDS host, keys, and handle.

Records & Lexicons

Records are JSON objects (post, like, follow). Each type is defined by a lexicon schema with an NSID like app.bsky.feed.post.

Repository

Each user has one repo (keyed by DID) that stores all their records from all lexicons + blobs. Exportable as a CAR file.

PDS

Personal Data Server: source of truth for your repo, handles auth, exposes a firehose of real-time changes. You can migrate between PDSes.

AppView

Aggregates data across repos so clients can get timelines, search, and follower lists. Requests are proxied through the PDS.

Relay, Labelers & Feeds

Relay fans out PDS firehoses. Labelers apply moderation labels. Feed generators produce custom algorithmic timelines.