signalk-wasm-plugin

Write a SignalK WASM plugin

A SignalK WASM plugin is an npm package whose plugin logic is a .wasm binary, run by the server in Node's built-in WASI (node:wasi, preview1) instead of being require()d. It talks to the server only through env.sk_* host imports, so it gets no app object, no npm dependencies at runtime and no install scripts. That makes it a good fit for self-contained logic: protocol decoders, calculations, device drivers over UDP/TCP, and providers.

It is not the only way to use WebAssembly in SignalK. If you need the full @signalk/server-api (app.*, Express routes with read-only access, serial ports, the NMEA event bus) and only want WASM for speed or for reusing Rust/C code, write a normal Node plugin and load a wasm-pack module inside it — the sibling skill signalk-wasm-library covers that. Decide first:

starter/ next to this file is a complete Rust plugin (config, poll-driven delta, one HTML endpoint) verified on a live server. Copy it, rename, npm run build.

1. package.json: what makes it a WASM plugin

{
  "name": "my-wasm-plugin",
  "keywords": ["signalk-wasm-plugin"],
  "wasmManifest": "plugin.wasm",
  "wasmCapabilities": { "dataRead": true, "dataWrite": true },
  "files": ["plugin.wasm", "public/"]
}

• wasmManifest is what routes the package to the WASM loader (not the keyword). The keyword signalk-wasm-plugin is what gets it discovered in node_modules.
• wasmCapabilities is optional; dataRead/dataWrite default to true, everything else to false (storage defaults to "vfs-only").
• The plugin ID comes from the package name, never from your plugin_id export: @ and / become _, so @me/foo → _me_foo. Config lives at <configdir>/plugin-config-data/<id>.json.
• Use an unscoped package name. Verified on 2.33.0: with a scoped name (@scope/x) the host bindings are keyed by the package name while everything else uses the derived ID, so sk_set_status never reaches the admin UI (status stays empty), sk_read_config returns {}, and deltas carry $source: "@scope/x". Unscoped names make all three agree.
• App Store visibility. The App Store searches npm for signalk-node-server-plugin (and the webapp keywords) only — a package with just signalk-wasm-plugin never shows up there, though npm install into the server dir still works. Adding both keywords makes it listed; the loader de-duplicates and still loads it as WASM because wasmManifest wins. The cost: a server too old to know WASM would try to require() it. The official docs say not to add the Node keyword — make the trade-off consciously.
• The plugin is disabled until enabled in Plugin Config; there is no enabled-by-default.