What is the AT Protocol? A Beginner's Guide

If you've been exploring alternative social networks, you’ve likely asked: what is AT Protocol? The Authenticated Transfer Protocol (atproto) is the open-source networking technology that powers Bluesky. Unlike traditional social media, where a single company owns the servers and your data, the AT Protocol is decentralized.

Here is a beginner-friendly breakdown of the core concepts you need to know.

1. DIDs (Decentralized Identifiers)

On Twitter or Instagram, your identity is your username, and the company owns it. If they ban you, you lose your identity.

In the AT Protocol, your true identity is a DID (Decentralized Identifier). It looks like a long string of random characters (e.g., did:plc:ewvi7nx...). Because this identifier is cryptographic and stored on a distributed ledger, you own it completely. Your handle (like @username.bsky.social) is just a human-readable alias pointing to your DID.

2. PDS (Personal Data Server)

What is AT Protocol hosting like? It's based on the PDS. Your PDS is where your actual data lives—your posts, your likes, and your list of followers.

Most users are hosted on Bluesky's default PDS (bsky.network). But because of how the AT Protocol works, you can spin up your own server, move your DID and data to it, and your followers won't even notice. This is called account portability.

3. The Firehose and Relays

If everyone's data is on different PDS servers, how do you see a global timeline? This is where Relays come in. A Relay connects to every PDS on the network, ingests all the posts, and broadcasts them out in a massive, real-time stream called the "Firehose."

4. Feed Generators

Because the Firehose is public, any developer can write a script to filter it. These scripts are called Feed Generators. Instead of being forced to use an opaque "For You" algorithm designed to maximize engagement, the AT Protocol lets you subscribe to custom algorithms built by the community.

If you want to dive deeper into the technical specifications, including how rich-text facets and character limits work at the protocol level, check out our AT Protocol Technical Guide.