The MQTT publisher example is an MQTT client that connects to the broker identified by the broker address. The broker address is configured in the example at compile time. If the connection succeeds, it is ready to publish the LED state information under the topic "led/state".

The example allows you to disconnect the MQTT client from the broker and then reconnect.

An overview of how the examples could be used are shown in scenarios below. Scenario 1 is a complex, but possibly real-time scenario where there are one or more publishers and one or more subscribers. In this scenario it is possible that not all the MQTT clients (publishers/subscribers) are BLE enabled IPv6 devices; they could be computer applications and/or embedded devices, wired or wireless, that use the MQTT as an application protocol over the IP stack. This scenario is seen is a super-set of the possible scenarios.

Scenario 1: Setup of the lwIP based MQTT application on nRF51

Scenario 2 shows a possible use case where the nRF51 SoC MQTT publisher is used to send a message to subscribers that are not BLE MQTT clients, such as a computer application. This scenario is realized when the Mosquitto subscriber application is used to test the publisher application on the nRF51 SoC.

Scenario 2: Setup of the lwIP based MQTT client publisher application on nRF51

Scenario 3 shows a possible use case where all the MQTT clients are nRF51 devices running MQTT clients, either publisher or subscriber. This scenario is realized when the subscriber and the publisher applications included in the IoT SDK are used to connect to the MQTT broker.

Scenario 3: Setup of the lwIP based MQTT client publisher and subscriber on nRF51

Common Modules Dependency and Usage

This section summarizes the usage of the nRF51 SoC resources and common modules in the examples, apart from the IoT 6LoWPAN and lwIP stack library.

Module	Inclusion/Usage	Description
Timer	3	Timer for lwIP, leds and the button module.
Button	3	Buttons are used to control the application. See the Overview section.
LEDs	4	LEDs are used to indicate the application states. See LED assignments.
Adv Data Encoder	Yes	The device name used is 'MQTTPublisher', the IPSP Service UUID is included in the UUID list.
Scheduler	No	Scheduler is used for processing stack events.
UART Trace	Included not enabled	Tracing is included but not enabled by default.

Note: The lwIP library used for this example is under BSD-style license; this is different from the Nordic SDK license. The license text can be found at <InstallFolder>/Nordic/nrf51/external/lwip/license.txt

Setup

The name of this example is iot_lwip_mqtt_publisher. You can find the source code and project file of the example in the following folder: <InstallFolder>/Nordic/nrf51/examples/iot/mqtt/publisher

LED assignments

Connection state:

LED 1	LED 2	Description
Blinking	Off	Device advertising as BLE peripheral.
Off	Blinking	BLE link established, IPv6 interface down.
On	Off	BLE link established, IPv6 interface up.
Off	On	MQTT connection is established.
On	On	Assertion failure in the application.

MQTT message publication:

LED 3 LED 4

Flashes once, if publishing of the LED state failed. Shows the last successfully published LED state.

Both LED 3 and LED 4 are turned on in case of an assertion failure in the application.

Button assignments

Button	Mapped Action
1	MQTT Connection Request
2	MQTT Publish Topic
3	MQTT Disconnection

MQTT Broker Setup

To check the client functionality, the MQTT broker must be set up.

Note: The example assumes that the broker has the IPv6 address 2004::1. Change this appropriately to get the example to function as expected.

This section describes how to set up the mosquitto MQTT broker on a Linux system. mosquitto is an open source implementation of the MQTT broker under BSD license. Visit http://mosquitto.org/ for more details.

Quick set up procedure is described below:

# Installation of mosquitto.
sudo apt-add-repository ppa:mosquitto-dev/mosquitto-ppa
sudo apt-get update
sudo apt-get install mosquitto
# Running it in verbosity mode in default port 1883.
sudo service mosquitto stop
sudo mosquitto -p 1883 -v

Note: To install mosquitto on other operating systems, visit the web page: http://mosquitto.org/download/

Testing

See Connecting devices to the router for a list of relevant Linux commands.

Ensure that the MQTT broker address is set correct in the application.
Compile and program the application. Observe that the device is advertising.
Prepare the Linux router device by initializing the 6LoWPAN module.
Discover the advertising device by using the hcitool lescan command.
Connect to the discovered device from the Linux console by using the Bluetooth 6LoWPAN connect command.
Check if the connected state is reflected by the LEDs.
Prepare the IPv6 global prefix for the btX interface.
Push Button 1. Observe that the mosquitto broker reports a new connection from the client "nrfSubsriber-51".
Observe that LED 2 turns on once when the MQTT connection is established.
Push Button 2.
Observe that LED 4 is lit.
Observe the publish message and its acknowledgement logged on the mosquitto broker.
Note
If a mosquitto subscriber subscribes to "led/state", it receives publish messages from the broker.

If the nRF51 SoC kit running as the MQTT subscriber is also connected to the broker, observe LED 4 of the subscriber synchronising with the LED 4 of the publisher each time the topic is published.
Pushing Button 2 repeatedly will toggle the state of LED 4 and send publish messages to the broker.
Disconnect the MQTT connection with the broker by pressing Button 3. LED 3 is turned off on disconnection.
Disconnect from the device by using the Bluetooth 6LoWPAN disconnect command.
Observe that only the advertising LED is lit.

MQTT Subscriber setup

This section describes how mosquitto can be used as a subscriber application to test this example.

# Installation of mosquitto.
sudo apt-get install mosquitto-clients
# Running it in verbosity mode in default port 1883.
mosquitto_sub -t "led/state"