FSharp.ATProto
Concepts
A plain-language guide to the terms you will encounter when working with AT Protocol.
Bluesky and AT Protocol
Bluesky is a social network. AT Protocol (atproto) is the open protocol it runs on. Think email: Bluesky is like Gmail, AT Protocol is like SMTP. Anyone can build on AT Protocol, and Bluesky is the most prominent app. FSharp.ATProto gives you access to both Bluesky-specific features and the underlying protocol.
DID
A DID (Decentralized Identifier) is a user's permanent identity. It looks like did:plc:z72i7hdynmk6r22z27h6tvur. Unlike a username, a DID never changes -- even if the user switches handles or moves servers. When the API needs to identify a user unambiguously, it uses a DID.
Handle
A Handle is the human-readable username, like alice.bsky.social or alice.com. Handles can change, but each one maps to exactly one DID. The library accepts handles wherever you need to identify a user.
AT-URI
An AT-URI is an address for a specific piece of content. It looks like at://did:plc:xxx/app.bsky.feed.post/3k2la3b. The three parts are: who created it (a DID), what type it is (a collection), and a unique key. You will see AT-URIs when creating, fetching, or deleting content.
CID
A CID (Content Identifier) is a cryptographic hash of a specific version of content -- a fingerprint. If the content changes, the CID changes. AT-URIs tell you where something is; CIDs tell you which exact version. Together, a URI and CID form an unambiguous reference -- this is what PostRef holds.
PDS
A PDS (Personal Data Server) is where your data lives. Posts, follows, likes -- all stored on your PDS. Most Bluesky users are on bsky.social, but anyone can run their own. The library connects to a PDS when you call Bluesky.login.
XRPC
XRPC is AT Protocol's RPC mechanism -- HTTP requests with a naming convention. You mostly do not need to think about it. The Bluesky and Chat modules wrap XRPC calls for you. If you ever need direct access, the Xrpc module is available.
Lexicon
Lexicon is AT Protocol's schema system, defining the shape of every request and response using JSON schema files. FSharp.ATProto generates F# types from these schemas at build time. You never need to touch Lexicon files yourself.
Records
In AT Protocol, all user-created data is stored as records. Posts, likes, follows -- all records. Each lives in a named collection (like app.bsky.feed.post) with a unique key. The library abstracts this away -- you work with PostRef, LikeRef, and FollowRef instead of thinking about records directly.
You do not need to understand all of this to use FSharp.ATProto. The library handles the protocol complexity for you. These concepts are here for when you encounter a term in the API and want to know what it means.
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