Delivery Modes (QoS)

Overview

MQTT defines three Quality of Service levels that control message delivery guarantees. The SDK exposes these as the DeliveryMode enum.

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

Fire and forget. The message is sent once with no acknowledgement. Fastest but messages can be lost.

await client.publish("sensors/temp", { value: 22.5 }, DeliveryMode.atMostOnce);

Best for: high-frequency telemetry, status updates where occasional loss is acceptable.

Acknowledged delivery. The message is retried until the broker acknowledges it. Messages may be delivered more than once.

await client.subscribe("chat/room/42", DeliveryMode.atLeastOnce);

Best for: chat messages, notifications — delivery is important but duplicates are tolerable.

Assured delivery. A four-step handshake ensures exactly one delivery. Slowest but most reliable.

await client.publish("payments/confirm", data, DeliveryMode.exactlyOnce);

Best for: financial transactions, critical state changes where duplicates would cause problems.

When no QoS is specified, the SDK defaults to DeliveryMode.atMostOnce (QoS 0).