Realtime

SemitraChannel is the realtime subsystem in Semitra.

Use it for live, connection-oriented interactions where clients subscribe, receive messages, and participate in rooms.

What channels do

Channels provide:

• subscription lifecycle handling
• inbound message parsing
• room membership tracking
• broadcasting
• socket attach and detach handling
• event observation
• optional Durable Object coordination through a hub

Example

The example app uses a notifications channel:

import { s } from "@semitra/cli";
import ApplicationChannel from "./application_channel.ts";

export const NotificationMessage = s.object({
  type: s.string(),
  body: s.string()
});

export default class NotificationsChannel extends ApplicationChannel<{
  type: string;
  body: string;
}> {
  static message = NotificationMessage;

  protected async subscribed(): Promise<void> {
    await this.join("notifications");
    this.transmit({ status: "connected", room: "notifications" });
  }

  protected async received(payload): Promise<void> {
    await this.broadcastTo("notifications", {
      ok: true,
      type: payload.type,
      body: payload.body,
      room: "notifications"
    });
  }
}

Good use cases

• live notifications
• collaborative room updates
• chat or presence events
• live status indicators
• streaming application state to connected clients

Durable Object fit

Realtime coordination maps naturally to Durable Objects because channels often need:

• a room coordinator
• a shared membership list
• consistent broadcast fan-out
• heartbeat or touch semantics

Semitra keeps that coordination abstract, but the platform fit is explicit.