syncspirit

syncspirit is a continuous file synchronization program that keeps folders identical across your own devices without a central server. It implements the BEP protocol used by Syncthing, so it can join an existing Syncthing cluster — relying on Syncthing's global discovery and relay infrastructure — and interoperate with Syncthing nodes at the protocol level. It is written in C++ on top of the rotor actor framework and ships as a native application instead of a web service with a browser UI.

The main desktop client is built with FLTK and shows the whole sync picture in one tree: the local device, trusted and pending peers, pending folders, per-file state, and the history of removed files including which device deleted them. A headless syncspirit-daemon front-end shares the same configuration and database on the host, so a daemon can keep syncing while the GUI is closed. Supported features include global and LAN peer discovery, UPnP and NAT traversal, relay transport, TLS certificate generation, conflict resolution, lz4 message compression, introducer devices, send-only and receive-only folders, and real-time filesystem watching via inotify, kqueue and ReadDirectoryChangesW. A "scheduled" folder mode exposes remote file metadata without downloading content, which is the groundwork for selective sync.

That direct peer-to-peer model replaces subscription cloud drives such as Dropbox, Google Drive and OneDrive: files travel between your own machines over the LAN or through relays, with no storage quota and no per-seat fee. Compared with Syncthing, syncspirit is a smaller and less complete implementation — ignoring pattern files, QUIC transport and untrusted-device encryption are still missing — but its author reports much faster syncs on large source trees and it targets older systems, including Windows XP and later.