Syncthing is a continuous peer-to-peer file synchronization application written in Go. It runs as a daemon, monitors local folders, hashes files into blocks, exchanges metadata with other devices via the Block Exchange Protocol (BEP) over TLS, and transfers blocks as needed. The architecture is a monolithic core with separate infrastructure server binaries for discovery, relay, upgrades, and usage reporting. The main binary includes the sync engine, REST API, web GUI (AngularJS), and CLI. It uses a suture-based service tree for managing goroutine lifecycles. Key data flows involve file scanning, hashing, index exchange, block request/response, and file writing, with events propagated to the API via an event system. The codebase demonstrates high craftsmanship with clean abstractions (filesystem layer, database abstraction, version vectors), extensive testing, and cross-platform support. It is actively maintained with a large community.