The Personal Data Server (PDS) is the AT Protocol server that holds user accounts and their repositories of records and blobs (binary files like images/videos plus cryptographic material). The reference implementation stores each account in its own SQLite database — the mushroom PDSs hold hundreds of thousands of SQLite files and work well. Each PDS emits all changes on a websocket firehose that relays consume. Accounts can freely migrate between PDSs by exporting the repo as a CAR file and importing it elsewhere, then updating the DID document.