AT Protocol component that subscribes to the websockets of many PDSs and combines their change streams into one big pipe. Consumers (App Views, third-party services) listen to a relay to get all network activity without connecting to each PDS individually. Third-party relays exist; Kuba tracks their coverage to assess switching away from Bluesky's own infrastructure. A lighter-weight JSON variant called Jetstream delivers roughly an order of magnitude less traffic than the CBOR firehose.