Vienna is a mature, macOS-native RSS/Atom/JSON feed reader that operates as a traditional desktop application. In production, it periodically fetches subscribed feeds via HTTP, parses them into structured articles (RSS, Atom, JSON Feed), stores articles and folder metadata in a local SQLite database (via FMDB), and presents them through a Cocoa/AppKit interface with a folder tree, article list, and built-in WebKit-based browser for reading. It supports smart folders, OPML import/export, and synchronization with Open Reader-compatible services (BazQux, FeedHQ, InoReader, TheOldReader). The application also provides a plugin system for extending feed sources, sharing, search, and actions, and uses Sparkle for automatic updates. The architecture follows a classic MVC pattern with a heavy reliance on notifications and the singleton pattern for the database and preferences. Key entry points include `AppController` (application delegate), `ArticleController` (article list management), `FoldersTree` (folder outline), `RefreshManager` (feed fetching), and `Database` (persistence). External dependencies include FMDB, MMTabBarView, TRVSURLSessionOperation, and Sparkle, all managed via Swift Package Manager or manual vendoring. The codebase is a mix of Objective-C and Swift, with some deprecated API usage and private WebKit hooks, but is actively maintained with a comprehensive test suite and CI pipeline.