Courier Web is a TypeScript SDK for creating robust, long-running connections using the MQTT protocol in web applications.

Long running connection is a persistent connection established between client & server for instant bi-directional communication. A long running connection is maintained for maximum possible duration with the help of keep alive packets.

MQTT is an extremely lightweight protocol which works on publish/subscribe messaging model. It is designed for connections with remote locations where a "small code footprint" is required or the network bandwidth is limited.

The protocol usually runs over TCP/IP via WebSockets in the browser; however, any network protocol that provides ordered, lossless, bi-directional connections can support MQTT.

MQTT has 3 built-in QoS levels for Reliable Message Delivery:

• QoS 0 (At most once) - the message is sent only once and the client and broker take no additional steps to acknowledge delivery (fire and forget).

• QoS 1 (At least once) - the message is re-tried by the sender multiple times until acknowledgement is received (acknowledged delivery).

• QoS 2 (Exactly once) - the sender and receiver engage in a two-level handshake to ensure only one copy of the message is received (assured delivery).

Find the detailed documentation here - https://gojek.github.io/courier-web/

End-to-end courier example - https://gojek.github.io/courier/docs/Introduction

• Clean, simple API with callback and RxJS Observable support

• Automatic Reconnection with subscription restoration

• Heartbeat Monitoring with stale connection detection

• Subscription Ledger with periodic broker-side audit

• Exponential Backoff retry for auth and connection failures

• Credential Persistence via cookie-based storage

• Framework-agnostic core with optional React bindings

• Full TypeScript support with comprehensive type definitions

npm install @armaanjain/courier-web-sdk

import { RealtimeClient } from "@armaanjain/courier-web-sdk";

const client = new RealtimeClient({
  endpoint: {
    host: "broker.example.com",
    port: 443,
    scheme: "wss",
    path: "/mqtt",
    clientId: "my-client-123",
    username: "user",
    password: "token",
  },
  topicAudit: { enabled: true, intervalMs: 10_000 },
});

await client.connect();
await client.subscribe("chat/room/42");

client.onEnvelope("chat/room/42", (topic, data) => {
  console.log("Received:", data);
});

await client.publish("chat/room/42", { text: "Hello!" });

import { RealtimeClient } from "@armaanjain/courier-web-sdk";
import {
  RealtimeScope,
  useLinkStatus,
  useInbox,
  useTopicBind,
} from "@armaanjain/courier-web-sdk/react";

const client = new RealtimeClient({ endpoint: { ... } });
await client.connect();

function App() {
  return (
    <RealtimeScope client={client}>
      <ChatRoom channelId="42" />
    </RealtimeScope>
  );
}

function ChatRoom({ channelId }: { channelId: string }) {
  const { isLive, heartbeatOk } = useLinkStatus();
  const { bound } = useTopicBind(`chat/room/${channelId}`);

  useInbox(`chat/room/${channelId}`, (topic, data) => {
    console.log("New message:", data);
  });

  if (!isLive) return <div>Connecting...</div>;
  if (!bound) return <div>Subscribing...</div>;
  return <div>Live! Heartbeat: {heartbeatOk ? "OK" : "Degraded"}</div>;
}

Read our contribution guide to learn about our development process, how to propose bugfixes and improvements, and how to build and test your changes to the Courier Web SDK.

Courier Web is MIT Licensed.