Overview and intended usage

The MQTT-SN Client service relies on the UDP networking being available. It is thus advised to start this service once the underlying networking is operational. In embeNET-enabled node, the networking starts once the node joins the network, which is signalled through the 'onJoined' callback. In this tutorial this callback will start the MQTT-SN Client service.

Once the client is started it will try to connect to a MQTT-SN gateway, given its IPv6 address and port. In this tutorial we will assume, that the gateway runs in the border router on a default port no 1885. Once connected the client will be able to subscribe to MQTT topics, register new topics as well as publish and receive data on these topics.

The software architecture of this client is event driven, meaning that all API calls are non-blocking, and operations requiring longer time are started and their end is signalled through an application defined callback.

MQTT-SN Architecture

MQTT-SN is a UDP-based version of the popular MQTT protocol, used extensively in IoT networks and beyond. In MQTT-SN the clients connect to the MQTT-SN Gateway or MQTT-SN Forwarder (which in turn connects to the MQTT-SN Gateway). The Gateway connects to the standard MQTT broker (or has an integrated broker). The main function of the Gateway is protocol translation between MQTT-SN and MQTT.

Functionality of the MQTT-SN Client

This client supports the following functions:

connecting and disconnecting to the gateway
registering topics
subscribing to topics and receiving messages on that topic
publishing messages on topics

Basic usage

MQTT-SN client API

All the functions of this MQTT-SN client are available in the mqttsn_client.h header file:

#include <embenet/mqttsn_client.h>

MQTT-SN client descriptor

This MQTT-SN Client follows an object-oriented approach in C, in which there is a single structure representing the client's state. The structure is MQTTSNClient and it needs to instantiated by the user code:

// Structure describing the MQTT-SN client

MQTTSNClient client;

Initialization

Before any usage, the MQTT-SN client needs to be initialized by a call to MQTTSN_CLIENT_Init

// Initialize the client and open local UDP port 1885 for communication with the gateway

MQTTSN_CLIENT_Init(&client, 1885, "Client0", NULL);

In the above example, the MQTT-SN client will use socket on local port 1885 to receive messages from the gateway, and this socket will be used later to communicate with the gateway. The client will introduce itself as "Client0". Be aware that this client identification string must be unique. One way to achieve this is to use the node's UID in client Id like this:

  // Initialize the client and open local UDP port 1885 for communication with the gateway
  // Use the UID of the node as part of the client ID
  EMBENET_EUI64 uid = EMBENET_NODE_GetUID();
  char clientId[32];
  sprintf(clientId, "Client:%016lX", uid);
  MQTTSN_CLIENT_Init(&client, 1885, clientId, NULL);

Event handling through application callbacks

In order to react to MQTT-SN client events, the application can register callbacks during client initialization. This provides the intended way of working with this client. The callbacks are gathered in the MQTTSNClientEventHandlers structure. All callbacks are optional, meaning that setting each to NULL will disable the callback.

The following example shows how to set up the callbacks:

 
static void onClientConnected(struct MQTTSNClient *client) {
  // The client has connected to the gateway
}
 
static void onClientDisconnected(struct MQTTSNClient *client) {
  // The client has disconnected from the gateway
}
 
static void onTopicRegisteredByGateway(struct MQTTSNClient *client, MQTTSNTopicId topicId, char const *topicName) {
  // The gateway has registered the topic
}
 
static MQTTSNClientEventHandlers eventHandlers = {onClientConnected, onClientDisconnected, onTopicRegisteredByGateway};
 
void mqttInitWithCallbacks(void) {
  // Initialize the client
  // Use the UID of the node as part of the client ID
  EMBENET_EUI64 uid = EMBENET_NODE_GetUID();
  char clientId[32];
  sprintf(clientId, "Client:%016lX", uid);
  MQTTSN_CLIENT_Init(&client, 1885, clientId, &eventHandlers);
}

Starting the service and connecting to the gateway

As mentioned in the overview, we will typically start the MQTT-SN client service once we have connected to the network. First we need to connect to the gateway. We can do that through a call to MQTTSN_CLIENT_Connect. Below is a minimal example of the 'onJoined' embeNET Node callback implementation:

void onJoined(EMBENET_PANID panId, EMBENET_NODE_QuickJoinCredentials const *quickJoinCredentials) {
  // Get the address of the border router and treat it as a MQTT-SN gateway address
  EMBENET_IPV6 gatewayAddress;
  EMBENET_NODE_GetBorderRouterAddress(&gatewayAddress);
  // Try to connect to the gateway
  MQTTSN_CLIENT_Connect(&client, &gatewayAddress, 1885, 120, 60, NULL, NULL, 1);
}

Once connected to the gateway, the client will call the MQTTSNClientEventHandlers::onConnected callback, where further interaction with the gateway can take place.

Registering topics by the client

In MQTT-SN the topics need to be registered in the gateway, before they can be used. The topic registration allows the gateway to assign a topic ID (MQTTSNTopicId) so that the clients can use it instead of string topic names. This makes the UDP payloads much smaller.

In order to register a topic, the application needs to call MQTTSN_CLIENT_RegisterTopic function:

// Register the topic in the gateway

MQTTSN_CLIENT_RegisterTopic(&client, "myTopic", onTopicRegisteredByClient);

If successfully registered, the onTopicRegisteredByClient will be called as callback:

static void onTopicRegisteredByClient(const struct MQTTSNClient *client, MQTTSNTopicId topicId, char const *topicName) {
  // The client has successfully registered a topic
}

Topics registered by the gateway

The topics can also be registered by the gateway. The gateway may have a set of predefined topics, that it wishes to register in each connecting client. For each such topic, the MQTTSNClientEventHandlers::onTopicRegisteredByGateway callback will be called.

This may also happen when the client gets disconnected from the gateway and connects back again. Is such scenario the gateway will update the client with topics it previously registered at the gateway. Again, for each such topic, the MQTTSNClientEventHandlers::onTopicRegisteredByGateway callback will be called. To avoid this particular scenario, the client can connect using the MQTTSN_CLIENT_CleanConnect instead of MQTTSN_CLIENT_Connect to force the gateway 'forget' all previous topics associated with that client.

Topic IDs

Wether the topic was registered by the client or by the gateway, the client application can always check the ID of the topic through a call to MQTTSN_CLIENT_GetTopicId

Publishing on a topic

Once the topic is registered, the client can publish on the given topic using either a full topic name and the MQTTSN_CLIENT_PublishMessage function:

// Publish a message to the topic using topic name

MQTTSN_CLIENT_PublishMessage(&client, "myTopic", 1, "Hello world!", 12, NULL);

or topic ID and the MQTTSN_CLIENT_PublishMessageById function:

  // Publish a message to the topic using topic ID
  MQTTSNTopicId topicId = MQTTSN_CLIENT_GetTopicId(&client, "myTopic");
  MQTTSN_CLIENT_PublishMessageById(&client, topicId, 1, "Hello world!", 12, NULL);

Subscribing to a topic

The client can subscribe to a topic and receive notifications once any data is published on that topic by any other client. To do that the application code needs to call MQTTSN_CLIENT_Subscribe providing the topic name and the callback function that will be called when the data on the given topic is published:

// Subscribe to the topic using topic name

MQTTSN_CLIENT_Subscribe(&client, "myTopic", onPublishReceived);

MQTT-SN Client API library

For more information see MQTT-SN Client

MQTT-SN Client tutorial

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