Vienna is a native macOS RSS/Atom/JSON feed reader application, built with Cocoa/AppKit and a mix of Objective-C and Swift. It manages feed subscriptions, fetches and parses feeds using custom parsers (RSS, Atom, JSON Feed, HTML discovery), stores articles in a local SQLite database, and provides a rich GUI for reading, organizing, and searching articles. The application supports smart folders with boolean criteria, OPML import/export, Open Reader API synchronization, a plugin system for feed sources, sync servers, and sharing services, as well as AppleScript automation for scripting. The architecture follows a monolithic desktop application pattern with a main window split into a folder tree, article list, and tabbed WebKit-based browser. Key controllers include AppController for lifecycle and menu actions, MainWindowViewController for window layout, and various panel controllers for preferences, activity, downloads, and info. External dependencies include SQLite (via FMDB), Sparkle for automatic updates, and macOS native APIs for keychain, file system, and notifications. The codebase is long-lived (since 2004) and has evolved through many macOS versions, resulting in some legacy patterns and deprecated API usage, but it remains actively maintained with recent commits and CI workflows targeting modern Xcode versions.