Message Extension

A robust message passing system for the Jucie engine that enables typed, reliable communication between different parts of an application using Web Workers' MessageChannel API.

Overview

The Message extension provides a MessageBus (primary/central hub) and Channel (secondary/client) architecture for inter-process communication. It supports typed events, message routing, subscription management, and error handling.

Architecture

``
┌─────────────────┐ MessageChannel ┌─────────────────┐
│ MessageBus │◄────────────────────►│ Channel │
│ (Primary) │ │ (Secondary) │
│ │ │ │
│ • Manages ports │ │ • Connects to │
│ • Routes msgs │ │ MessageBus │
│ • Handles subs │ │ • Publishes │
│ • Validates │ │ • Subscribes │
└─────────────────┘ └─────────────────┘
`

$3

- MessageBus: Central message router that manages multiple channels and routes messages between them
- Channel: Client that connects to MessageBus and can publish/subscribe to typed events
- Port: Low-level communication layer handling MessageChannel handshaking and message transport
- EventRegistry: Manages event schemas, validation, and symbol-to-string conversion

Installation

`javascript
import { Engine } from '@jucie.io/engine';
import { MessageBus, Channel } from '@jucie-engine/message';

const engine = Engine.create()
.install(MessageBus.configure({
mode: 'development',
schemas: [userEvents, systemEvents]
}))
.install(Channel.configure());
`

Event Schema Definition

Define typed events using the defineEvents function:

`javascript
import { defineEvents } from '@jucie.io/message';

const userEvents = defineEvents('user', () => ({
login: ['String', 'String'], // username, password
logout: ['String'], // username
profile: {
update: ['Object'],
delete: ['String']
}
}));

const systemEvents = defineEvents('system', () => ({
ready: '*', // wildcard - accepts any payload
error: ['String', 'Object'],
config: null // no payload validation
}));
`

$3

- Array: ['String', 'Object'] - Validates argument count and types
- Wildcard: '*' - Accepts any payload
- Null: null - No payload validation
- Nested: Objects create namespaced events

Usage

$3

`javascript
// MessageBus (Primary) - typically on main thread
const messageBusEngine = Engine.create()
.install(MessageBus.configure({
mode: 'development',
schemas: [userEvents]
}))
.install(Channel.configure());

// Channel (Secondary) - typically in worker/iframe
const channelEngine = Engine.create()
.install(Channel.configure());
`

$3

`javascript
// Create a channel from MessageBus
const port = messageBusEngine.messageBus.createChannel('worker1');

// Connect Channel to MessageBus
await channelEngine.channel.connect(port);

// Verify connection and event schemas are available
expect(channelEngine.channel.events).toBeDefined();
`

$3

`javascript
// Subscribe to events
channelEngine.channel.subscribe(
channelEngine.channel.events.user.login,
(username, password) => {
console.log('User login:', username);
}
);

// Publish events
channelEngine.channel.publish(
channelEngine.channel.events.user.login,
'john_doe',
'secret123'
);
`

$3

#### One-time Subscriptions

`javascript
channelEngine.channel.subscribeOnce(
channelEngine.channel.events.system.ready,
() => console.log('System is ready!')
);
`

#### Namespaced Events

`javascript
// Subscribe to namespaced event
channelEngine.channel.namespace('admin').subscribe(
channelEngine.channel.events.user.login,
(username) => console.log('Admin login:', username)
);

// Publish to namespaced event
channelEngine.channel.namespace('admin').publish(
channelEngine.channel.events.user.login,
'admin_user'
);
`

#### Targeted Messaging

`javascript
// Send message to specific channel only
channelEngine.channel.to('worker1').publish(
channelEngine.channel.events.user.logout,
'username'
);
`

#### Multiple Subscribers

`javascript
const subscriber1 = (data) => console.log('Sub1:', data);
const subscriber2 = (data) => console.log('Sub2:', data);

// Both subscribers will receive the message
channelEngine.channel.subscribe(event, subscriber1);
channelEngine.channel.subscribe(event, subscriber2);
`

$3

`javascript
// Check if channel is active
const isActive = messageBusEngine.messageBus.isChannelActive('worker1');

// Enable/disable channels
await messageBusEngine.messageBus.disableChannel('worker1');
await messageBusEngine.messageBus.enableChannel('worker1');

// Use external MessageChannel
const { MessageChannel } = await import('worker_threads');
const messageChannel = new MessageChannel();
messageBusEngine.messageBus.useChannel('external', messageChannel.port1);
`

Error Handling

The system provides comprehensive error handling:

`javascript
// Unknown events throw errors
try {
channelEngine.channel.publish('unknown:event', 'data');
} catch (error) {
console.error('Unknown event:', error.message);
}

// Connection errors are handled gracefully
const result = await channelEngine.channel.connect(invalidPort);
if (result === false) {
console.error('Connection failed');
}
`

Event Validation

Events are automatically validated against their schemas:

`javascript
// This will pass validation
channelEngine.channel.publish(
channelEngine.channel.events.user.login,
'username', // String
'password' // String
);

// This will fail validation and log warnings
channelEngine.channel.publish(
channelEngine.channel.events.user.login,
'username', // String ✓
123 // Number ✗ (expected String)
);
`

Configuration Options

$3

`javascript
MessageBus.configure({
mode: 'development', // 'development' | 'production' | 'test'
schemas: [userEvents, systemEvents] // Event schema definitions
})
`

$3

`javascript
Channel.configure({
port: messagePort, // Optional: pre-configured MessagePort
subscriptions: [ // Optional: pre-configured subscriptions
{
name: 'user.login',
action: 'subscribe',
event: 'user:login',
namespace: 'admin',
subscriber: (username) => console.log(username)
}
]
})
`

Testing

The extension includes comprehensive test coverage:

`bash
npm test
`

Tests cover:
- Basic connection flow
- Message publishing and subscribing
- Multiple subscribers and one-time subscriptions
- Namespace functionality
- Targeted messaging
- Error handling
- Channel management
- Event validation
- Complex event schemas

API Reference

$3

- createChannel(name, callback?) - Create a new channel
- useChannel(name, port, callback?) - Use external MessageChannel
- disableChannel(name) - Disable a channel
- enableChannel(name) - Enable a channel
- isChannelActive(name) - Check if channel is active

$3

- connect(port, callback?) - Connect to MessageBus
- publish(event, ...payload) - Publish an event
- subscribe(event, subscriber) - Subscribe to an event
- subscribeOnce(event, subscriber) - Subscribe once to an event
- unsubscribe(event, subscriber) - Unsubscribe from an event
- namespace(name) - Create namespaced channel
- to(...channels) - Create targeted channel

$3

- processSchemas(schemas) - Process event schemas
- validateEvent(event, payload) - Validate event payload
- getEventObject() - Get nested event object with symbols
- getPath(symbol) - Convert symbol to string path
- getSymbol(path) - Convert string path to symbol

Best Practices

1. Define schemas early - Set up event schemas before creating channels
2. Use typed events - Leverage schema validation for better debugging
3. Handle errors gracefully - Always wrap channel operations in try-catch
4. Clean up subscriptions - Use unsubscribe functions to prevent memory leaks
5. Use namespaces - Organize events with namespaces for better structure
6. Test thoroughly - Use the comprehensive test suite as a reference

Examples

See the test files for complete working examples:
- __tests__/MessageBus.test.js - Integration tests
- __tests__/EventRegistry.test.js - Event schema tests
- __tests__/Port.test.js` - Low-level communication tests