WASM Plugin Development Guide

Overview

This guide covers how to develop WASM/WASIX plugins for Signal K Server 3.0. WASM plugins run in a secure sandbox with isolated storage and capability-based permissions.

What Makes a WASM Plugin?

A WASM plugin is an npm package that contains the WASM code for the plugin instead of the traditional JavaScript code. A WASM plugin is identified by the signalk-wasm-plugin keyword in package.json and the wasmManifest field in package.json:

{
  "name": "my-plugin-name",
  "wasmManifest": "plugin.wasm",
  "wasmCapabilities": { ... }
}
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Key points:

• wasmManifest (required): Path to the compiled .wasm file. This field tells Signal K to load this as a WASM plugin instead of a Node.js plugin.
• wasmCapabilities (required): Declares what permissions the plugin needs (network, storage, etc.)
• Package name (flexible): Can be anything - my-plugin, @myorg/my-plugin, etc. There is no requirement to use @signalk/ scope.
• Keywords: Include signalk-wasm-plugin for discovery (do not use signalk-node-server-plugin - that's for Node.js plugins only)

Language Options

Signal K Server supports multiple languages for WASM plugin development:

• AssemblyScript - TypeScript-like syntax, easiest for JS/TS developers, smallest binaries (3-10 KB)
• Rust - Best performance and tooling, medium binaries (50-200 KB)
• Go/TinyGo - Go via TinyGo compiler, medium binaries (50-150 KB)

Why WASM Plugins?

Benefits

• Security: Sandboxed execution with no access to host system
• Hot-reload: Update plugins without server restart
• Multi-language: Write plugins in Rust, AssemblyScript, and more
• Crash isolation: Plugin crashes don't affect server
• Performance: Near-native performance with WASM
• Self contained: WASM plugins do not install any additional dependencies
• Small binaries (compared to native options): 3-200 KB depending on language

Current Capabilities

• Delta Emission: Send SignalK deltas to update vessel data
• Status & Error Reporting: Set plugin status and error messages
• Configuration: The same JSON schema-based configuration as JS plugins
• Data Storage: VFS-isolated file storage
• HTTP Endpoints: Register custom REST API endpoints
• Static Files: Serve web UI from public/ directory
• Network Access: HTTP requests via as-fetch (AssemblyScript)
• Resource Providers: Serve SignalK resources
• Weather Providers: Integrate with Signal K Weather API
• Radar Providers: Integrate with Signal K Radar API

Choose Your Language

AssemblyScript - Recommended for JS/TS Developers

Best for:

• Quick prototypes
• Simple data processing
• Migrating existing Node.js plugins
• Developers familiar with TypeScript

Pros:

• TypeScript-like syntax
• Fast development
• Smallest binaries (3-10 KB)
• Familiar tooling (npm)

Cons:

• Smaller ecosystem than Rust
• Some TypeScript features unavailable
• Manual memory management

Jump to AssemblyScript Guide

Rust - Recommended for Performance-Critical Plugins

Best for:

• Performance-critical plugins
• Complex algorithms
• Low-level operations
• Production plugins

Pros:

• Best performance
• Memory safety
• Rich ecosystem
• Strong typing

Cons:

• Steeper learning curve
• Longer compile times
• Larger binaries (50-200 KB)

Jump to Rust Guide

Go/TinyGo - For Go Developers

Best for:

• Go developers wanting to write plugins
• Medium complexity plugins
• Resource providers with hybrid patterns

Pros:

• Familiar Go syntax
• Good standard library support
• Medium binaries (50-150 KB)
• Strong typing

Cons:

• Requires TinyGo (not standard Go)
• Some Go features unavailable
• Slower than Rust

Jump to Go/TinyGo Guide