Transmission

Transmission is a lightweight, cross‑platform BitTorrent client that exposes both a GUI and a headless daemon for server deployments. From a developer standpoint, its core value lies in the clean separation between the transport layer (libtorrent) and the control plane (daemon + web UI), enabling integration into custom workflows without reinventing networking logic. The daemon runs as a minimal transmission-daemon process that listens on a JSON‑RPC endpoint; the web UI is a single‑page application that communicates with this endpoint over HTTP, while command‑line tools (transmission-remote, transmission-show, etc.) provide programmatic access via the same API.

Architecture

• Transport Engine: libtorrent-rasterbar (C++), responsible for peer discovery, piece selection, and data transfer. It exposes a C API that the daemon wraps.
• Daemon: A lightweight C++ process (transmission-daemon) that serializes configuration to JSON, persists torrent metadata in a SQLite database, and serves the RPC endpoint on localhost:9091 by default. It also exposes a RESTful /transmission/rpc route for remote clients.
• Web UI: A Vue‑based single‑page app (in web/), bundled with Vite. It talks to the daemon via JSON‑RPC over HTTP, leveraging WebSockets for live updates.
• CLI Tools: Standalone executables (transmission-show, transmission-create, transmission-edit) written in C++ that operate directly on .torrent files, independent of the daemon.
• Build System: CMake for native code; Node.js + Vite for the web UI. The repository is structured with submodules (e.g., libtorrent, web) to keep dependencies modular.

Core Capabilities & APIs

• JSON‑RPC API: Exposes methods such as torrent-add, torrent-get, torrent-remove, and session-set/session-get. Parameters are typed in JSON, making it language‑agnostic.
• Webhooks: Optional transmission-webhook can be enabled to emit HTTP POST requests on torrent events (added, finished, error), allowing integration with CI/CD pipelines or notification services.
• Torrent Metadata Manipulation: The CLI tools allow creation of magnet links, setting trackers, and editing piece hashes without launching the daemon.
• Database Access: Torrent state is stored in a transmission.db SQLite file; developers can query or backup this directly for analytics or migration scripts.

Deployment & Infrastructure

• Self‑Hosting: The daemon can run on any POSIX system with a C++17 compiler. It requires only libsqlite3 and openssl for TLS support.
• Containerization: Official Docker images (transmission/transmission) expose the daemon and web UI, exposing ports 9091 (RPC) and 9092 (web). Docker Compose snippets are available in the repo for quick spin‑up.
• Scalability: While designed as a single‑process daemon, the architecture allows clustering by running multiple daemons behind a load balancer and synchronizing torrent lists via shared storage or an external database.
• Security: The RPC endpoint can be secured with basic auth and TLS; the web UI supports HTTPS via Nginx reverse proxy. The daemon also supports limiting write access to a specific user, ideal for sandboxed environments.

Integration & Extensibility

• Plugins: The daemon supports a plugin API via dynamic libraries (.so/.dll) that can hook into torrent events, modify session settings at runtime, or add custom metrics. This is documented in the docs/Plugins.md section.
• Custom UI: Because the web interface is a static bundle, developers can fork it or replace it entirely with their own SPA that talks to the same RPC endpoint.
• Scripting: The CLI tools can be wrapped in shell or Python scripts to automate torrent creation, monitoring, and cleanup. The transmission-remote command can be scripted to start/stop torrents based on system load.

Developer Experience

• Documentation: Although the core docs are a work in progress, the repository includes comprehensive API references (docs/api.md) and build instructions. The codebase follows consistent style guidelines enforced by .clang-format and ESLint.
• Community & Support: A long‑standing open‑source project with an active mailing list and GitHub discussions. Issue templates enforce detailed bug reports, making triage efficient.
• Extensible Build: The use of CMake and submodules allows developers to cherry‑pick components (e.g., only the daemon or only the web UI) and build them in isolation.

Use Cases

1. Self‑Hosted Media Library – Run Transmission on a NAS to seed and download media, exposing the web UI for remote control.
2. CI/CD Artifact Distribution – Use the daemon’s webhook to trigger downstream jobs when a build artifact is fully downloaded.
3. Custom Peer‑to‑Peer Tooling – Embed the libtorrent engine into a larger application (e.g., a backup system) and expose its RPC interface for orchestration.
4. Educational Sandbox – Leverage the lightweight daemon in a classroom setting to demonstrate P2P networking without heavy dependencies.

Advantages

• Performance & Footprint: libtorrent is highly optimized; the daemon runs under 50 MB of RAM even with dozens of torrents.
• Licensing: GPL‑v2 for the daemon, but the web UI is MIT‑licensed, allowing commercial use without copyleft obligations.
• **Cross