Architecture

System overview

┌──────────── Tauri v2 WebView (Next.js static export) ─────────────┐
│  Themeable React/Tailwind SPA  ⇄  @tauri-apps/api: invoke/listen  │
└───────────────────┬──────────────────────────▲────────────────────┘
                    │ commands (JSON only)     │ events (JSON, ~1 Hz)
                    ▼                          │
┌───────────────────┴──────────────────────────┴────────────────────┐
│  Rust core (tokio runtime)                                        │
│                                                                   │
│   src-tauri/src/engine/    thin librqbit::Session wrapper.        │
│                            No Tauri types. Unit testable.         │
│   src-tauri/src/commands/  #[tauri::command] handlers. Thin.      │
│   src-tauri/src/state/     app state and persistence.             │
└───────────────────┬───────────────────────────────────────────────┘
                    │ pieces written directly
                    ▼
              ┌───────────┐
              │   Disk    │
              └───────────┘

The rules

These are structural guarantees, not conventions.

1. The webview never touches torrent binary data

librqbit writes pieces straight to disk. The UI receives only small JSON payloads: progress percentages, transfer rates, peer counts, file listings.

This is what keeps memory bounded on a multi-gigabyte ISO. Routing piece data through IPC would mean serialising it, copying it into the WebView heap, and holding it there — which is how torrent clients end up consuming gigabytes of RAM on a large download.

2. The engine layer imports no Tauri types

src-tauri/src/engine/ compiles and runs under plain cargo test. It is the layer most exposed to librqbit churn, so it must be testable without spawning a WebView. See src-tauri/tests/engine.rs, which drives a real Session with no Tauri runtime present.

3. Command handlers are thin

Handlers unwrap shared state, call the engine, and map errors. Anything worth testing lives in the engine, where it can be tested.

4. Telemetry is throttled and batched

Status updates run at approximately 1 Hz. Per-piece events would flood the IPC channel on a fast download and jank the UI for no informational gain.

5. Flume owns its IPC types

The types crossing the boundary are defined in src-tauri/src/engine/status.rs — they are not re-exports of librqbit’s internal stats structs. A librqbit upgrade therefore cannot silently change the contract the frontend depends on; the compiler forces a look at the mapping instead.

The IPC contract

Rust serde structs use #[serde(rename_all = "camelCase")]. Their TypeScript mirrors live in src/lib/ipc/types.ts and must change in the same commit.

get_core_status

Returns CoreStatus.

Field	Type	Meaning
clientVersion	string	Client string, e.g. "Flume 0.1.0"
listenPort	number | null	Bound peer port, null if not listening
announcePort	number | null	Port announced to trackers
dht	DhtStatus	DHT subsystem health
downloadDir	string	Absolute download path
uptimeSeconds	number	Seconds since session start
downloadBps	number	Aggregate download rate, bytes/sec
uploadBps	number	Aggregate upload rate, bytes/sec
livePeers	number	Connected peers across all torrents
health	EngineHealth	Derived readiness indicator

DhtStatus: { enabled, nodesV4, nodesV6, outstandingRequests }.

EngineHealth: "starting" | "connecting" | "ready" | "degraded".

• starting — the peer listener has not bound a port yet
• connecting — listening, DHT enabled, routing table below 8 nodes
• ready — listening with a usable DHT routing table
• degraded — listening, but DHT is disabled, so magnet links cannot resolve

Errors

Commands reject with CommandError:

{
  kind: string;
  message: string;
}

kind is a stable machine-readable identifier so the frontend can branch without matching on message text. Currently: engineNotReady.

Startup sequence

1. Tauri builds the app and registers AppState and the command handlers.
2. The window opens immediately — first paint is never blocked.
3. A background task derives EngineConfig from OS conventions and starts the session (DHT bootstrap, listener bind, persistence restore).
4. Until the engine is ready, get_core_status returns engineNotReady and the UI shows a starting state.
5. On RunEvent::Exit, session shutdown is awaited so fast-resume state is flushed and a restart resumes rather than re-hashing.

Where state lives

What	Where
Downloads	User’s Downloads folder by default (configurable)
Session state, fast-resume	OS app-data dir, e.g. ~/Library/Application Support/io.github.adamgreenwell.Flume
DHT routing table	dht.json inside the session directory

The DHT path is set explicitly. librqbit’s default is a global OS path shared across instances, which both leaks state and causes port collisions between two running copies — see issue #19.