MQTT

warning

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What is MQTT?

MQTT is a protocol used to send and receive messages between 2 or more IoT devices.

Overview of how devices communicate using MQTT

Device sending the message is called as Publisher.
Device receiving the message is called as Subscriber
Topic is the channel through which devices send and receive messages.
MQTT Broker is a server that receives messages from the Publisher and routes the messages to the Subscriber.

List of MQTT features implemented in Shunya stack

With Shunya stack you can make IoT device to act as a

Publisher. (Send messages to devices)
Subscriber. (Receive messages from devices)

Using MQTT Shunya stack

Requirements to use MQTT Shunya stack

Shunya OS installed (supported Arm devices) or Shunya OS docker container (X86 based windows/linux devices)
MQTT broker installed on cloud and MQTT broker URL.
MQTT broker authentication settings. (optional)

Steps to use MQTT Shunya stack

Set MQTT broker settings in Shunya.
Implementation as a Publisher
Implementation as a Subscriber

note

Run the steps given below inside Shunya OS installed (supported Arm devices) or Shunya OS docker container (X86 based windows/linux devices) terminals.

Step 1: Set MQTT broker settings in Shunya.

Set MQTT Broker details, by editing the config file /etc/shunya/config.json inside Shunya OS.

A simple configuration should contain these parameters.

Config	Description
`mqttBrokerUrl`	MQTT broker url
`mqttClientId`	Unique Identification of the device (user can put any string that is convenient)
`mqttUsername`	Username for MQTT broker authentication. (optional)
`mqttPassword`	Password for MQTT broker authentication. (optional)

note

Shunya stack will warn you if you are not using mqttUsername and mqttPassword configuration.

Example configuration:

Writing the below json in /etc/shunya/config.json file, will tell the Shunya stack to connect to MQTT broker at url test.mosquitto.org:1883.

"mqttSettings": {
    "mqttBrokerUrl": "test.mosquitto.org:1883",
    "mqttClientId": "shunya pi3",
}

Step 2: Implementation as a Publisher

Let's try to implement Publisher using Shunya stack

Steps are :

Start with an ready to use template for sending messages via MQTT

git clone https://gitlab.iotiot.in/repo-public/examples.git
cd examples/simple-examples/send-messages-mqtt

Open the send-messages-mqtt.c in an text editor and modify as per your use case.
For our example, we need to first get the temperature data, add the code in your main() function
```
float temperature = 76.98; /* For now lets assume that the temperature is 76.89 Celsius*/
```

Publishing temperature data to topic, modify the code

float temperature = 76.98; /* For now lets assume that the temperature is 76.89 Celsius*/

/* Create a mqtt object */
mqttObj mqttBroker = newMqtt("mqttSettings"); /* Argument = JSON Title, Load settings from JSON file */

/* Connect to broker */
mqttConnect(&mqttBroker) ;

/* Publish message "76.98 C" to topic "temp" via MQTT*/
mqttPublish(&mqttBroker, "temp", "%.2f C", temperature);

mqttDisconnect(&mqttBroker); /* Disconnect MQTT */

Once you are done editing, save and compile the code , by running
```
mkdir build && cd build
cmake ../
make 
```
Run the code
```
sudo ./send-messages-mqtt
```

More examples to Publish messages via MQTT in shunya stack

Send JSON string
Send multiple variables

For sending JSON, change code in step 4. with the example given below.

char json[1024] = "{ "temp": 78.9, "sensor_name": "BME280" }";

/* Publish a json to topic "temp" via MQTT*/
mqttPublish(&mqttBroker, "temp", "%s", json);

For sending multiple variables, change code in step 4. with the example given below.

float temperature = 76.89;
int humidity = 98;

/* Publish a string to topic "temp" via MQTT*/
mqttPublish(&mqttBroker, "temp", "temperature: %f, humidity: %d", temperature, humidity);

Step 3: Implementation as a Subscriber

Let's try to implement Subscriber using Shunya stack

Steps are :

Start with an ready to use template

git clone https://gitlab.iotiot.in/repo-public/examples.git
cd examples/simple-examples/receive-messages-mqtt

Open the receive-messages-mqtt.c in an text editor and modify as per your use case.

For our example, we need to first edit the code to store the received message to at file.

Add the code to print to a file in processMessages() function, because

/* This function will get called everytime the Subscribed Topic receives a message */
void processMessages (int topicLen, char *topic, int msgLen, char *msg)
{
    /* For now lets print the message.*/
    printf("Received message\n\tFrom Topic: %s,\n\t Message: %s", topic, msg);

    /* Print the message to a file */
    FILE *file;
    file = fopen("database", "r"); 
    if (file != NULL) {
        fprintf(file, "%s\n", msg);
    }
    fclose(file);
}

Now lets subscribe to the topic "temp", so we can start receiving messages

/* Create a mqtt object */
mqttObj mqttBroker = newMqtt("mqttSettings"); /* Argument = JSON Title, Load settings from JSON file */

/* Connect to broker */
mqttConnect(&mqttBroker) ;

/* Send all the messages received to our processing function */
mqttSetSubCallback(&mqttBroker, processMessages);

/* Receive messages from topic "temp" via MQTT*/
mqttSubscribe(&mqttBroker, "temp");

mqttDisconnect(&mqttBroker); /* Disconnect MQTT */

Once you are done editing, save and compile the code , by running
```
mkdir build && cd build
cmake ../
make 
```
Run the code
```
sudo ./receive-messages-mqtt
```

More examples on receiving messages via MQTT shunya stack

Receive JSON string

For sending multiple variables, change code in step 4. with the example given below.

void processMessages (int topicLen, char *topic, int msgLen, char *msg)
{
    /* Parse JSON message using rapidjson */
    rapidjson::Document jsonMsg;

    jsonMsg.Parse(msg);

}

Understand this component with an example (ready to use code)

Let's take an example use case: Say we need to

Subscribe to the topic "devices/instructions" and Turn on LED when we receive the message "LED".
Publish "LED" to the topic "devices/instructions".
Here we will be using both Publish and Subscribe features of MQTT.
Example for both Publisher and subscriber at once

Steps are :

Start with an ready to use template for MQTT

git clone https://gitlab.iotiot.in/repo-public/examples.git
cd examples/full-examples/mqtt

Open the mqtt-example.c in an text editor and modify as per your use case.
For our example, we need to first load the MQTT broker settings, add the highlighted code in your main() function
```
/* Create a mqtt object */
mqttObj mqttBroker = newMqtt("mqtt"); /* Argument = JSON Title */
```
Connect to the MQTT broker, add the highlighted code in your main() function
```
/* Connect to broker */
mqttConnect(&mqttBroker);
```

/* Send all the messages received to our processing function */
mqttSetSubCallback(&mqttBroker, processMessages);

Process all the received messages and take action when message LED is received at topic "devices/instructions" ,

/* This function will get called everytime the Subscribed Topic receives a message */
void processMessages (int topicLen, char *topic, int msgLen, char *msg)
{
    /* Check if the message recevied is from topic "devices/instructions" */
    if (strncmp(topic, "devices/instructions", strlen("devices/instructions")) == 0) {
        /* Check if the message received is LED */
        if (strncmp(msg, "LED", strlen("LED")) == 0) {
            /* Turn ON LED */
            setLed(40, ON); /* Turn on LED */
        } else {
            setLed(40, OFF); /* Turn off LED */
        }
    } else {
        setLed(40, OFF); /* Turn off LED */
    }
}

Subscribe to the topic "devices/instructions" ,

/* Receive messages from topic "devices/instructions" via MQTT*/
mqttSubscribe(&mqttBroker, "devices/instructions");

Publish "LED" to the topic "devices/instructions" ,

/* Send a message to topic "devices/instructions" via MQTT*/
mqttPublish(&mqttBroker, "devices/instructions", "LED");

Once all the operations are done for MQTT, disconnect from the broker at the end,
```
mqttDisconnect(&mqttBroker); /* Disconnect from MQTT broker */
```
Once you are done editing, save and compile the code , by running
```
mkdir build && cd build
cmake ../
make 
```
Run the code
```
sudo ./mqtt-example.c
```

warning

What is MQTT?​

Overview of how devices communicate using MQTT​

List of MQTT features implemented in Shunya stack​

Using MQTT Shunya stack

Requirements to use MQTT Shunya stack​

Steps to use MQTT Shunya stack​

note

Step 1: Set MQTT broker settings in Shunya.​

note

Example configuration:​

Step 2: Implementation as a Publisher​

Step 3: Implementation as a Subscriber​

Understand this component with an example (ready to use code)​

Facing errors with the component?​

What is MQTT?

Overview of how devices communicate using MQTT

List of MQTT features implemented in Shunya stack

Requirements to use MQTT Shunya stack

Steps to use MQTT Shunya stack

Step 1: Set MQTT broker settings in Shunya.

Example configuration:

Step 2: Implementation as a Publisher

Step 3: Implementation as a Subscriber

Understand this component with an example (ready to use code)

Facing errors with the component?