Protocol Overview#

The Authenticated Transfer Protocol, aka atproto, is a federated protocol for large-scale distributed social applications. This document will introduce you to the ideas behind the AT Protocol.

Identity#

Users are identified by domain names on the AT Protocol. These domains map to cryptographic URLs which secure the user's account and its data.

Data repositories#

User data is exchanged in signed data repositories. These repositories are collections of records which include posts, comments, likes, follows, media blobs, etc.

Federation#

The AT Protocol syncs the repositories in a federated networking model. Federation was chosen to ensure the network is convenient to use and reliably available. Repository data is synchronized between servers over standard web technologies (HTTP and WebSockets).

The three core services in our network are Personal Data Servers (PDS), Big Graph Services (BGS), and App Views. We're also working on feed generators and labelers.

The lower-level primitives that can get stacked together differently are the repositories, lexicons, and DIDs. We published an overview of our technical decisions around federation architecture on our blog.

Interoperation#

A global schemas network called Lexicon is used to unify the names and behaviors of the calls across the servers. Servers implement "lexicons" to support featuresets, including the core ATP Lexicon for syncing user repositories and the Bsky Lexicon to provide basic social behaviors.

While the Web exchanges documents, the AT Protocol exchanges schematic and semantic information, enabling the software from different orgs to understand each others' data. This gives atproto clients freedom to produce user interfaces independently of the servers, and removes the need to exchange rendering code (HTML/JS/CSS) while browsing content.

Achieving scale#

Personal data servers are your home in the cloud. They host your data, distribute it, manage your identity, and orchestrate requests to other services to give you your views.

Big Graph Services (BGS) handle all of your events, like retrieving large-scale metrics (likes, reposts, followers), content discovery (algorithms), and user search.

This distinction is intended to achieve scale as well as a high degree of user-choice.

Algorithmic choice#

As with Web search engines, users are free to select their aggregators. Feeds, App Views, and search indices can be provided by independent third parties, with requests routed by the PDS based on user configuration.

Account portability#

We assume that a Personal Data Server may fail at any time, either by going offline in its entirety, or by ceasing service for specific users. The goal of the AT Protocol is to ensure that a user can migrate their account to a new PDS without the server's involvement.

User data is stored in signed data repositories and verified by DIDs. Signed data repositories are like Git repos but for database records, and DIDs are essentially registries of user certificates, similar in some ways to the TLS certificate system. They are expected to be secure, reliable, and independent of the user's PDS.

Each DID document publishes two public keys: a signing key and a recovery key.

• Signing key: Asserts changes to the DID Document and to the user's data repository.
• Recovery key: Asserts changes to the DID Document; may override the signing key within a 72-hour window.

The signing key is entrusted to the PDS so that it can manage the user's data, but the recovery key is saved by the user, e.g. as a paper key. This makes it possible for the user to update their account to a new PDS without the original host's help.

A backup of the user’s data will be persistently synced to their client as a backup (contingent on the disk space available). Should a PDS disappear without notice, the user should be able to migrate to a new provider by updating their DID Document and uploading the backup.

Speech, reach, and moderation#

Atproto's model is that speech and reach should be two separate layers, built to work with each other. The “speech” layer should remain permissive, distributing authority and designed to ensure everyone has a voice. The “reach” layer lives on top, built for flexibility and designed to scale.

The base layer of atproto (personal data repositories and federated networking) creates a common space for speech where everyone is free to participate, analogous to the Web where anyone can put up a website. The indexing services then enable reach by aggregating content from the network, analogous to a search engine.

Specifications#

Some of the primary specifications comprising the initial version of the AT Protocol are:

• Authenticated Transfer Protocol
• DIDs and Handles
• Repository and Data Model
• Lexicon
• HTTP API (XRPC) and Event Streams

These specs can be organized into three layers of dependency:

From here, you can continue reading the guides and specs.

Join the network#

Bluesky is an open social network built on the AT Protocol, a flexible technology that will never lock developers out of the ecosystems that they help build. With atproto, third-party can be as seamless as first-party through custom feeds, federated services, clients, and more.

If you're a developer interested in building on atproto, we'd love to email you a Bluesky invite code. Simply share your GitHub (or similar) profile with us via this form.