Data repository

A data repository (repo) in the AT Protocol is a self-authenticating collection of data published by a single actor, stored in one or more Personal Data Servers (PDSes). Each repository functions as a personal datastore that maintains an actor's content (such as posts, profile information, and social connections) in a cryptographically verifiable structure.

Architecture

Repositories are built on a Merkle Search Tree (MST) data structure, which reduces the entire state to a single root hash.

The repository structure consists of three main layers:

• Commit: The signed root that authenticates the entire repository
• Tree Nodes: The internal structure organizing the data
• Records: The actual content (posts, likes, follows, etc.)

Each element in this structure is a DAG-CBOR object referenced by a Content Identifier (CID) hash.

Repositories can be synchronized across multiple PDSes, allowing users to maintain multiple distributed copies of their data.

Content within repositories can be addressed using AT URIs, which follow a hierarchical pattern:

at://alice.com                      # Repository root
at://alice.com/app.bsky.feed.post   # Collection
at://alice.com/app.bsky.feed.post/1234  # Specific record

Identifiers

Repositories use several identifier types:

• Decentralized Identifiers (DIDs): Used to identify repositories and their owners
• Content Identifiers (CIDs): Used to reference specific content objects via cryptographic hashes
• Namespaced Identifiers (NSIDs): Used to specify the Lexicon schema type for collections of records
• Record keys (rkeys): Used to identify individual records within a collection

Data Integrity

When a repository is updated, the changes propagate through the Merkle tree, which results in a new root CID. The new root is signed by the repository owner's private key, creating a new commit. This mechanism ensures that all data can be cryptographically verified as authored by the repository owner while maintaining a complete history of changes to the repository.