The Authenticated Transfer Protocol (atproto) is an open, decentralized, and high-performance network designed for building social applications at scale. Developed by the team at Bluesky Social PBC, it serves as the foundational data layer for a modular social web, allowing developers to create applications that are interoperable by default. By enabling users to own their identities and data, atproto shifts the power dynamic from centralized platforms to the individual users, while providing developers with a robust environment to build innovative social experiences, custom algorithms, and decentralized services. The protocol is structured as a data network where every piece of content—posts, likes, follows, and profile information—is represented as JSON records published to user-controlled repositories. These repositories function as personal databases, and their state changes are broadcast across the network via event streams, enabling applications to synchronize and display data in real-time. The protocol emphasizes data portability, strong typing, and hyperlinked content, ensuring that every user account is accessible and discoverable. Some of the key features are: - User-Owned Identities: Accounts are anchored to decentralized identifiers (DIDs) and human-readable handles linked to DNS, ensuring identity portability. - Open Data Architecture: All public activity is published to a shared network, allowing developers to build feeds, bots, and analytics without needing proprietary API keys. - Custom Algorithms: Developers can create custom feed generators that allow users to choose their own timeline curation methods, such as ML-based or rule-based feeds. - Lexicon Schema System: A shared schema language enables consistent data composition and extension, facilitating interoperability between disparate applications. - Modular Service Stack: The ecosystem consists of distinct service types, including Personal Data Servers (PDS), Relays, and Labelers, which can be independently self-hosted or managed. - Strongly Typed Data: The protocol uses typed records that allow for reliable cross-application data consumption and validation. - Event-Driven Syncing: High-performance streaming solutions like Jetstream provide real-time updates for developers building search engines, aggregators, or real-time bots. The operation of the protocol relies on a combination of HTTP API (XRPC) methods and event streams. Applications interact with the network by reading and writing records directly to user repositories hosted on a PDS. For large-scale data consumption, developers tap into the public firehose, an event stream that broadcasts all activity across the network. Because all data is cryptographically signed, developers can ensure the authenticity of information even when it is retrieved through third-party relays. This architecture allows for a separation of concerns, where the application experience is detached from the underlying data storage, giving users the freedom to migrate between servers without losing their social graph or post history. Some common use cases include: - Social Application Development: Building full-featured social networking platforms that seamlessly share data with other apps in the ecosystem. - Custom Feed Creation: Developing unique curation algorithms that help users discover and filter content based on specific interests or community needs. - Automated Agents and Bots: Deploying services that monitor the firehose to trigger actions, such as auto-replies, notifications, or real-time trend analysis. - Moderation Services: Creating independent labelers that analyze content and publish moderation decisions that users can opt-in to for enhanced safety. - Independent Data Archives: Developing tools for periodic account data backups to ensure personal information remains accessible and secure outside of the primary host.