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MQTT library

We will be using the MQTT helper library in the nRF Connect SDK to connect over MQTT. This library sits on top of the Zephyr MQTT API, and simplifies connecting over MQTT and takes care of the socket handling.

In this topic, we will cover how to enable and configure the library to connect to an MQTT broker.

There are a few general steps needed to use the library, which we will cover in detail in the exercise section of this lesson.

Enabling the MQTT helper library
Defining callback functions for various MQTT events, e.g on_mqtt_connack and on_mqtt_publish
Initializing the library with the callback functions using mqtt_helper_init()
Connecting to the MQTT broker with mqtt_helper_connect()
Publish to topics using struct mqtt_publish_param and mqtt_helper_publish()
Subscribe to topics using struct mqtt_topic, struct mqtt_subscription_list and mqtt_helper_subscribe()
Adding TLS to secure the connection

Enabling the library

First step is to enable the library in the application, using the Kconfig CONFIG_MQTT_HELPER. This Kconfig also selects CONFIG_MQTT_LIB which is the Kconfig that enables the Zephyr MQTT API.

CONFIG_MQTT_HELPER=y

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CONFIG_MQTT_HELPER=y

Kconfig

Then, we need to include the header file for the library in the main.c file, like this.

#include <net/mqtt_helper.h>

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#include <net/mqtt_helper.h>

Define callback functions

Before initializing the library, we first need to define callback functions that will be called upon receiving various MQTT control packets from the broker.

The various callback functions are defined in struct mqtt_helper_cfg, which has the following signature

In the exercises in this lesson, we will define functions for on_connack, on_suback, on_publish and on_disconnect. These callbacks have the following signatures

static void on_mqtt_connack(enum mqtt_conn_return_code return_code, bool session_present);
static void on_mqtt_suback(uint16_t message_id, int result);
static void on_mqtt_publish(struct mqtt_helper_buf topic, struct mqtt_helper_buf payload)
static void on_mqtt_disconnect(int result)

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static void on_mqtt_connack(enum mqtt_conn_return_code return_code, bool session_present);
static void on_mqtt_suback(uint16_t message_id, int result);
static void on_mqtt_publish(struct mqtt_helper_buf topic, struct mqtt_helper_buf payload)
static void on_mqtt_disconnect(int result)

Notice that CONNACK, SUBACK, and DISCONNECT all contain a result or return_code that contains the result of the operation, or reason for the operation, in the case of a DISCONNECT packet from the broker.

Initializing the library

To initialize the library, we will use the API call mqtt_helper_init(), which takes the parameter of type struct mqtt_helper_cfg with all the callback functions as an input.

To initialize the library, we first define the variable struct mqtt_helper_cfg with the callback functions that we have defined. Then pass it to mqtt_helper_init().

	struct mqtt_helper_cfg config = {
		.cb = {
			.on_connack = on_mqtt_connack,
			.on_disconnect = on_mqtt_disconnect,
			.on_publish = on_mqtt_publish,
			.on_suback = on_mqtt_suback,
		},
	};

	err = mqtt_helper_init(&config);
	if (err) {
		LOG_ERR("Failed to initialize MQTT helper, error: %d", err);
		return 0;
	}

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	struct mqtt_helper_cfg config = {
		.cb = {
			.on_connack = on_mqtt_connack,
			.on_disconnect = on_mqtt_disconnect,
			.on_publish = on_mqtt_publish,
			.on_suback = on_mqtt_suback,
		},
	};

	err = mqtt_helper_init(&config);
	if (err) {
		LOG_ERR("Failed to initialize MQTT helper, error: %d", err);
		return 0;
	}

Connect to the MQTT broker

To connect, we will use the function mqtt_helper_connect(), which takes the struct mqtt_helper_conn_params as a parameter.

`struct mqtt_helper_conn_params` signature

First, we define the struct mqtt_helper_conn_params and set the relevant members, in our case hostname and device_id. The MQTT protocol provides the option of a username and password to authenticate with the broker; however, this is outside the scope of this lesson.

	struct mqtt_helper_conn_params conn_params = {
		.hostname.ptr = "mqtt.nordicsemi.academy",
		.hostname.size = strlen("mqtt.nordicsemi.academy"),
		.device_id.ptr = "nrf7002dk-1234",
		.device_id.size = strlen("nrf7002dk-1234"),
	};

	err = mqtt_helper_connect(&conn_params);
	if (err) {
		LOG_ERR("Failed to connect to MQTT, error code: %d", err);
		return 0;
	}

Copy

	struct mqtt_helper_conn_params conn_params = {
		.hostname.ptr = "mqtt.nordicsemi.academy",
		.hostname.size = strlen("mqtt.nordicsemi.academy"),
		.device_id.ptr = "nrf7002dk-1234",
		.device_id.size = strlen("nrf7002dk-1234"),
	};

	err = mqtt_helper_connect(&conn_params);
	if (err) {
		LOG_ERR("Failed to connect to MQTT, error code: %d", err);
		return 0;
	}

Publish and subscribe

To subscribe to topics, we define a struct mqtt_topic for each topic and set the name of the topic and the QoS. Then we define struct mqtt_subscription_list, which is an array consisting of all the topics, and pass that to mqtt_helper_subscribe() to request subscription from the broker.

To publish to a topic, we define struct mqtt_publish_param, where we can set the message to publish, the topic to publish the message to, and the QoS through struct mqtt_publish_message. To set the message ID, the MQTT helper library has mqtt_helper_msg_id_get() which simply returns a positive non-zero value that increments for each call.

This structure also has two flag members, dup_flag to indicate a retransmission and retain_flag, if the message should be stored persistently.

When struct mqtt_publish_param is defined, pass it to mqtt_helper_publish() to publish the message.

Enabling TLS

To enable connection over TLS, we first enable the following Kconfig options:

CONFIG_MQTT_LIB_TLS=y
CONFIG_MBEDTLS=y
CONFIG_TLS_CREDENTIALS=y

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CONFIG_MQTT_LIB_TLS=y
CONFIG_MBEDTLS=y
CONFIG_TLS_CREDENTIALS=y

Kconfig

CONFIG_MQTT_LIB_TLS enables TLS support in the Zephyr MQTT API
CONFIG_MBEDTLS enables the mbedTLS cryptography library
CONFIG_TLS_CREDENTIALS enables TLS credentials management system

This will also configure the MQTT helper library to use the MQTT broker port 8883, which is the default port for TLS connections.

In addition, we need to set the security tag where the TLS credentials are stored through the following Kconfig in the MQTT helper library

CONFIG_MQTT_HELPER_SEC_TAG=24

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CONFIG_MQTT_HELPER_SEC_TAG=24

Kconfig

Once this is configured, we will use the modem key management library to write the certificate to the modem, after modem initialization.

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Cellular IoT Fundamentals

MQTT library

Enabling the library

Define callback functions

Initializing the library

Connect to the MQTT broker

Publish and subscribe

Enabling TLS

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