#Alepha - Topic

#Installation

Part of the alepha package. Import from alepha/topic.

npm install alepha

#Overview

Generic interface for pub/sub messaging. Gives you the ability to create topics and subscribers. This module provides only a memory implementation of the topic provider.

#API Reference

#Primitives

Primitives are functions that define and configure various aspects of your application. They follow the convention of starting with $ and return configured primitive instances.

For more details, see the Primitives documentation.

#$subscriber()

Creates a subscriber primitive to listen for messages from a specific topic.

Provides a dedicated message subscriber that connects to a topic and processes messages with custom handler logic, enabling scalable pub/sub architectures where multiple subscribers can react to the same events independently.

Key Features

• Seamless integration with any $topic primitive
• Full type safety inherited from topic schema
• Real-time message delivery when events are published
• Error isolation between subscribers
• Support for multiple independent subscribers per topic

Common Use Cases

• Notification services and audit logging
• Analytics and metrics collection
• Data synchronization and real-time UI updates

 1class UserActivityService { 2  userEvents = $topic({ 3    name: "user-activity", 4    schema: { 5      payload: t.object({ 6        userId: t.text(), 7        action: t.enum(["login", "logout", "purchase"]), 8        timestamp: t.number() 9      })10    }11  });12 13  activityLogger = $subscriber({14    topic: this.userEvents,15    handler: async (message) => {16      const { userId, action, timestamp } = message.payload;17      await this.auditLogger.log({18        userId,19        action,20        timestamp21      });22    }23  });24 25  async trackUserLogin(userId: string) {26    await this.userEvents.publish({27      userId,28      action: "login",29      timestamp: Date.now()30    });31  }32}

#$topic()

Creates a topic primitive for publish/subscribe messaging and event-driven architecture.

Enables decoupled communication through a pub/sub pattern where publishers send messages and multiple subscribers receive them. Supports type-safe messages, real-time delivery, event filtering, and pluggable backends (memory, Redis, custom providers).

Use Cases: User notifications, real-time chat, event broadcasting, microservice communication

 1class NotificationService { 2  userActivity = $topic({ 3    name: "user-activity", 4    schema: { 5      payload: t.object({ 6        userId: t.text(), 7        action: t.enum(["login", "logout", "purchase"]), 8        timestamp: t.number() 9      })10    },11    handler: async (message) => {12      console.log(`User ${message.payload.userId}: ${message.payload.action}`);13    }14  });15 16  async trackLogin(userId: string) {17    await this.userActivity.publish({ userId, action: "login", timestamp: Date.now() });18  }19 20  async subscribeToEvents() {21    await this.userActivity.subscribe(async (message) => {22      // Additional subscriber logic23    });24  }25}