AT Protocol server that consumes the relay firehose and builds a materialized view of the network in a normal database (originally Postgres, then ScyllaDB for scale beyond a few million users). Responsible for cross-account aggregations the raw protocol cannot give you — like counts (likes per post, follower counts), thread reconstruction, and feed hydration. Named confusingly because most people hear 'app' and expect the client; the term makes sense only to people who know what a materialized view is. Bluesky runs App Views in US west and US east.