Async Embedded MQTT Client

An async, no_std-compatible MQTT client library in Rust, designed for embedded systems using the Embassy async ecosystem and embedded-hal 1.0.0 traits.

Core Features

• Asynchronous: Built on async/await and designed for the Embassy ecosystem.
• no_std by default: Suitable for bare-metal and resource-constrained devices.
• Hardware Agnostic: Uses embedded-hal-async traits to support various communication transports (TCP, UART, SPI, etc.).
• Memory Efficient: Leverages heapless to avoid dynamic memory allocation.
• MQTT v3.1.1 and v5 Support: Protocol version can be selected via feature flags.
• QoS 0 & 1: Support for "at most once" and "at least once" message delivery.

Getting Started

To use this library, you need a transport that implements the MqttTransport trait. This trait is an abstraction over the underlying communication channel (e.g., a TCP socket).

Here is a conceptual example of how to use the client with an embassy-net TCP socket:

use mqtt_async_embedded::{MqttClient, MqttOptions, QoS};
use embassy_net::tcp::TcpSocket;
use embassy_time::Duration;

// Assume `socket` is an already connected `TcpSocket`
async fn run_mqtt(mut socket: TcpSocket<'_>) {
    let options = MqttOptions::new("my-embedded-device")
        .set_keep_alive(Duration::from_secs(30));

    let mut client: MqttClient<_, 1024, 1024> = MqttClient::new(socket, options);

    // Connect to the broker
    if let Err(e) = client.connect().await {
        // Handle connection error
        return;
    }

    // Publish a message
    client.publish("sensors/temp", b"25.3", QoS::AtLeastOnce, &[]).await.unwrap();

    // Subscribe to a topic
    client.subscribe("commands", QoS::AtLeastOnce).await.unwrap();

    // Disconnect
    client.disconnect().await.unwrap();
}

Project Structure

The library is organized into a few key modules:

• src/client.rs: Contains the main MqttClient and its async state machine.
• src/packet.rs: Handles the encoding and decoding of MQTT control packets.
• src/transport.rs: Defines the MqttTransport trait for hardware abstraction.
• src/error.rs: Provides unified error types for the client.

Feature Flags

The following feature flags are available:

• v5: Enables MQTT v5 support.
• std: Enables std library support for running examples and tests on a host machine.
• defmt: Enables logging via the defmt framework.
• transport-smoltcp: Enables integration with embassy-net for a full TCP/IP stack.
• nom: Enables the nom parser for decoding MQTT packets.

Running Examples

The crate includes several examples in the examples/ directory.

Desktop Mock

This example uses std::net::TcpStream to connect to a local MQTT broker on your host machine.

Prerequisites: An MQTT broker (like Mosquitto) running on localhost:1883.

Run command:

cargo run --example desktop_mock --features std

Embedded Examples

The esp8266_uart.rs and smoltcp_ethernet.rs examples are skeletons that demonstrate how the library would be integrated into a real embedded project. They require a specific HAL and driver setup to be fully functional.