IotMqtt_Connect

Establish a new MQTT connection.

IotMqttError_t IotMqtt_Connect( const IotMqttNetworkInfo_t * pNetworkInfo,
                                const IotMqttConnectInfo_t * pConnectInfo,
                                uint32_t timeoutMs,
                                IotMqttConnection_t * const pMqttConnection );

This function opens a connection between a new MQTT client and an MQTT server (also called a broker). MQTT connections are established on top of transport layer protocols (such as TCP/IP), and optionally, application layer security protocols (such as TLS). The MQTT packet that establishes a connection is called the MQTT CONNECT packet. After IotMqtt_Init, this function must be called before any other MQTT library function.

If pConnectInfo->cleanSession is true, this function establishes a clean MQTT session. Subscriptions and unacknowledged PUBLISH messages will be discarded when the connection is closed.

If pConnectInfo->cleanSession is false, this function establishes (or re-establishes) a persistent MQTT session. The parameters pConnectInfo->pPreviousSubscriptions and pConnectInfo->previousSubscriptionCount may be used to restore subscriptions present in a re-established persistent session. Any restored subscriptions MUST have been present in the persistent session; this function does not send an MQTT SUBSCRIBE packet!

pConnectInfo->pPreviousSubscriptions and pConnectInfo->previousSubscriptionCount can also be used to pass a list of subscriptions to be stored locally without a SUBSCRIBE packet being sent to the broker. These subscriptions are useful to invoke application level callbacks for messages received on unsolicited topics from the broker.

This MQTT library is network agnostic, meaning it has no knowledge of the underlying network protocol carrying the MQTT packets. It interacts with the network through a network abstraction layer, allowing it to be used with many different network stacks. The network abstraction layer is established per-connection, allowing every IotMqttConnection_t to use a different network stack. The parameter pNetworkInterface sets up the network abstraction layer for an MQTT connection; see the documentation on IotMqttNetworkInfo_t for details on its members.

The pConnectInfo parameter provides the contents of the MQTT CONNECT packet. Most members are defined by the MQTT spec.. The pConnectInfo->pWillInfo member provides information on a Last Will and Testament (LWT) message to be published if the MQTT connection is closed without sending a DISCONNECT packet. Unlike other PUBLISH messages, a LWT message payload is limited to 65535 bytes in length. Additionally, the retry interval and limit members of IotMqttPublishInfo_t are ignored for LWT messages. The LWT message is optional; pWillInfo may be NULL.

MQTT keep-alive is configured by IotMqttConnectInfo_t::keepAliveSeconds. Keep-alive is used to detect half-open or otherwise unusable network connections. An MQTT client will send periodic ping requests (PINGREQ) to the server if the connection is idle. The MQTT server must respond to ping requests with a ping response (PINGRESP). The standard does not specify how long the server has to respond to a ping request, noting only a "reasonable amount of time". In this library, the amount of time a server has to respond to a ping request is set with IOT_MQTT_RESPONSE_WAIT_MS.

Unlike IotMqtt_PublishAsync, IotMqtt_SubscribeAsync, and IotMqtt_UnsubscribeAsync, this function is always blocking. Additionally, because the MQTT connection acknowledgement packet (CONNACK packet) does not contain any information on which CONNECT packet it acknowledges, only one CONNECT operation may be in progress at any time. This means that parallel threads making calls to IotMqtt_Connect will be serialized to send their CONNECT packets one-by-one.

Parameters

[in]	pNetworkInfo	Information on the transport-layer network connection to use with the MQTT connection.
[in]	pConnectInfo	MQTT connection setup parameters.
[in]	timeoutMs	If the MQTT server does not accept the connection within this timeout in milliseconds, this function returns IOT_MQTT_TIMEOUT.
[out]	pMqttConnection	Set to a newly-initialized MQTT connection handle if this function succeeds.

Returns

One of the following:

• IOT_MQTT_SUCCESS
• IOT_MQTT_NOT_INITIALIZED
• IOT_MQTT_BAD_PARAMETER
• IOT_MQTT_NO_MEMORY
• IOT_MQTT_NETWORK_ERROR
• IOT_MQTT_SCHEDULING_ERROR
• IOT_MQTT_BAD_RESPONSE
• IOT_MQTT_TIMEOUT
• IOT_MQTT_SERVER_REFUSED

Example

// Callback function to receive messages from the broker on an unsolicited topic.
void unsolicitedMessageCallback( void * pArgument, IotMqttCallbackParam_t * pPublish );

// Parameters to MQTT connect.
IotMqttConnection_t mqttConnection = IOT_MQTT_CONNECTION_INITIALIZER;
IotMqttNetworkInfo_t networkInfo = IOT_MQTT_NETWORK_INFO_INITIALIZER;
IotMqttConnectInfo_t connectInfo = IOT_MQTT_CONNECT_INFO_INITIALIZER;
IotMqttPublishInfo_t willInfo = IOT_MQTT_PUBLISH_INFO_INITIALIZER;

// A local subscription to receive messages from the broker on an unsolicited topic.
IotMqttSubscription_t subscription = IOT_MQTT_SUBSCRIPTION_INITIALIZER;

// Example network abstraction types.
IotNetworkServerInfo_t serverInfo = { ... };
IotNetworkCredentials_t credentialInfo = { ... };
IotNetworkInterface_t networkInterface = { ... };

// Example using a generic network implementation.
networkInfo.createNetworkConnection = true;
networkInfo.u.setup.pNetworkServerInfo = &serverInfo;
networkInfo.u.setup.pNetworkCredentialInfo = &credentialInfo;
networkInfo.pNetworkInterface = &networkInterface;

// Set the members of the connection info (password and username not used).
connectInfo.cleanSession = true;
connectInfo.keepAliveSeconds = 30;
connectInfo.pClientIdentifier = "uniqueclientidentifier";
connectInfo.clientIdentifierLength = 22;

// Set the members of the will info (retain and retry not used).
willInfo.qos = IOT_MQTT_QOS_1;
willInfo.pTopicName = "will/topic/name";
willInfo.topicNameLength = ( uint16_t ) strlen( willInfo.pTopicName );
willInfo.pPayload = "MQTT client unexpectedly disconnected.";
willInfo.payloadLength = strlen( willInfo.pPayload );

// Set the pointer to the will info.
connectInfo.pWillInfo = &willInfo;

// [Optional] Set a local subscription to receive the broker messages on an unsolicited topic.
subscription.qos = IOT_MQTT_QOS_0;
subscription.pTopicFilter = "some/unsolicited/topic";
subscription.topicLength = ( uint16_t ) strlen( subscription.pTopicFilter );
subscription.callback.function = unsolicitedMessageCallback;
connectInfo.pPreviousSubscriptions = &subscription;
connectInfo.previousSubscriptionCount = 1;


// Call CONNECT with a 5 second block time. Should return
// IOT_MQTT_SUCCESS when successful.
IotMqttError_t result = IotMqtt_Connect( &networkInfo,
                                         &connectInfo,
                                         5000,
                                         &mqttConnection );

if( result == IOT_MQTT_SUCCESS )
{
    // Do something with the MQTT connection...

    // Clean up and close the MQTT connection once it's no longer needed.
    IotMqtt_Disconnect( mqttConnection, 0 );
}