AT Protocol

The Authenticated Transfer Protocol (AT Protocol) is an open, decentralized protocol for social networking. It is the foundation of Bluesky and designed to give users ownership of their data and identity.

Core Principles

1. Account Portability: Users can move their data between service providers without losing their identity, followers, or content.
2. Algorithmic Choice: Users can choose custom feeds and algorithms, not locked to one provider’s choices.
3. Interoperability: Open standards enable a federated ecosystem where services can communicate.
4. Performance at Scale: Designed to handle billions of users with a hybrid federated/indexed architecture.

Architecture

The AT Protocol uses a layered architecture:

┌─────────────────────────────────────────────────┐
│                   App Views                     │  (Bluesky, custom apps)
├─────────────────────────────────────────────────┤
│                   Lexicons                      │  (Schema definitions)
├─────────────────────────────────────────────────┤
│          Personal Data Repositories             │  (User data storage)
├─────────────────────────────────────────────────┤
│          Identity Layer (DIDs + Handles)        │  (Decentralized identity)
└─────────────────────────────────────────────────┘

Identity

• DIDs (Decentralized Identifiers): Persistent identifiers (e.g., did:plc:abc123) that remain stable across server migrations.
• Handles: Human-readable names (e.g., @alice.bsky.social) that resolve to DIDs via DNS or HTTP.

Personal Data Servers (PDS)

Every user’s data lives in a Personal Data Server.

A PDS:

• Stores the user’s repository (a signed, Merkle-tree-based data structure).
• Handles OAuth authentication and authorization.
• Syncs data to relays and indexers.

Relay (BGS)

Relays aggregate data from many PDSs into a unified firehose, enabling:

• Efficient indexing for search and discovery.
• Feed generators to access content across the network.

App View

An App View consumes the firehose and provides application-specific APIs. For example, the Bluesky app view provides the social networking experience.

Data Model

Repositories

A repository is a user’s complete data store, structured as a Merkle Search Tree (MST).

• Records: Individual data items (posts, likes, follows) stored as DAG-CBOR.
• Collections: Namespaced groups of records (e.g., app.bsky.feed.post).
• Commits: Signed snapshots of the repository state.

Lexicons

Lexicons are JSON schemas that define:

• Record types and their fields.
• XRPC methods (HTTP-like RPC calls).
• Subscriptions for real-time data.

Example Lexicon ID: app.bsky.feed.post

AT-URIs

Records are addressed using AT-URIs:

at://did:plc:abc123/app.bsky.feed.post/3jqw2f7

Format: at://<authority>/<collection>/<rkey>

Key Technologies

Sync & Federation

Repo Sync

Repositories are synchronized using:

1. com.atproto.sync.getRepo: Full repository export as a CAR file.
2. com.atproto.sync.subscribeRepos: Real-time firehose of commits across the network.

Event Stream

The firehose emits events:

• Commit: New or updated records.
• Handle: Handle changes.
• Identity: DID document updates.
• Tombstone: Account deletions.

References

• Official protocol specifications covering identity, data, and networking layers. AT Protocol Specification
• Developer documentation for building on Bluesky/AT Protocol. Bluesky Documentation
• Reference implementation in TypeScript. Protocol GitHub Repository