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JavaScript MQTT Client Connectivity

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Introduction

This guide offers detailed, step-by-step instructions for connecting clients to our CrystalMQ broker or any broker of your choice using JavaScript. It covers the necessary prerequisites and setup, guiding you through the process of establishing a reliable connection. You'll learn how to implement authentication, manage subscriptions, and handle message exchanges. With the help of this documentation, you'll be able to seamlessly integrate JavaScript web clients with the MQTT broker for efficient data communication.

Pre-requisites

Dependency Installation

<script src="https://unpkg.com/mqtt/dist/mqtt.min.js"></script>

Connecting to MQTT Broker

Use the following code to connect the client over Websocket.

Define the Macro ADDRESS using MQTT Broker's connection parameters.

// Generate Client ID
const clientId = generateClientId('crystalmq_', 10);

// Configure MQTT client options
const options = {
clientId: clientId,
hostname: 'public-mqtt-broker.bevywise.com',
port: 10443,
protocol: 'ws' // WebSocket protocol
// username: 'username', // Add username if needed
// password: 'password', // Add password if needed
};

// Create MQTT client
const client = mqtt.connect(`ws://${options.hostname}:${options.port}/mqtt`, options);

// Handle client events
client.on('connect', () => {
console.log('Connected successfully with MQTT Broker.');

// Subscribe to a topic
client.subscribe('cmq/topic', (err) => {
if (!err) {
console.log('Subscribed to topic.');

// Publish a message
client.publish('cmq/topic', 'Hello CMQ', { qos: 2, retain: true }, (err) => {
if (!err) {
console.log('Message published.');
}
});
}
});
});

MQTT 5

// Generate Client ID
const clientId = generateClientId('crystalmq_', 10);
document.getElementById('clientId').textContent = clientId;

// Configure MQTT client options for MQTT v5
const options = {
clientId: clientId,
hostname: 'public-mqtt-broker.bevywise.com',
port: 10443,
protocol: 'ws', // WebSocket protocol
protocolVersion: 5, // MQTT version 5
// username: 'username', // Add username if needed
// password: 'password', // Add password if needed
properties: {
sessionExpiryInterval: 60, // Set session expiry interval in seconds
},
};

// Create MQTT client
const client = mqtt.connect(`ws://${options.hostname}:${options.port}/mqtt`, options);

// Handle client events
client.on('connect', (connack) => {
console.log('Connected successfully with MQTT Broker.', connack);

// Subscribe to a topic with MQTT v5 properties
const topic = 'cmq/topic';
document.getElementById('topic').textContent = topic;
client.subscribe(topic, { qos: 2, nl: true, rap: true, rh: 0 }, (err, granted) => {
if (!err) {
console.log('Subscribed to topic.', granted);
document.getElementById('subscriptionStatus').textContent = 'Subscribed to topic.';

// Publish a message with MQTT v5 properties
const message = 'Hello CMQ';
client.publish(topic, message, { qos: 2, retain: true, properties: { messageExpiryInterval: 60 } }, (err) => {
if (!err) {
console.log('Message published.');
document.getElementById('publishedMessage').textContent = message;
}
});
}
});
});

MQTT Over Secure WebSocket

Use the following code to connect securely to MQTT Broker over TLS.

Define the Macro ADDRESS using MQTT Broker's connection parameters.

// Generate Client ID
const clientId = generateClientId('crystalmq_', 10);
document.getElementById('clientId').textContent = clientId;

// Configure MQTT client options with TLS settings
const options = {
clientId: clientId,
hostname: 'public-mqtt-broker.bevywise.com',
port: 11443,
protocol: 'wss', // Secure WebSocket protocol
rejectUnauthorized: true, // Optional, reject unauthorized certificates
// username: 'username', // Add username if needed
// password: 'password', // Add password if needed
// Add additional TLS options if needed
cert: yourClientCert, // Client certificate
key: yourClientKey, // Client key
ca: yourCaCert // CA certificate
};

// Create MQTT client
const client = mqtt.connect(`wss://${options.hostname}:${options.port}/mqtt`, options);

// Handle client events
client.on('connect', () => {
console.log('Connected successfully with MQTT Broker.');

// Subscribe to a topic
const topic = 'cmq/topic';
document.getElementById('topic').textContent = topic;
client.subscribe(topic, (err) => {
if (!err) {
console.log('Subscribed to topic.');
document.getElementById('subscriptionStatus').textContent = 'Subscribed to topic.';

// Publish a message
const message = 'Hello CMQ';
client.publish(topic, message, { qos: 2, retain: true }, (err) => {
if (!err) {
console.log('Message published.');
document.getElementById('publishedMessage').textContent = message;
}
});
}
});
});

Set TLS parameters before calling the MQTTClient_connect to connect the client to the mQTT Broker securely over TLS.

If the MQTT Broker is hosted in a trusted server and the server verification is not required, the following code can be used to set TLS Options:

// Generate Client ID
const clientId = generateClientId('crystalmq_', 10);
document.getElementById('clientId').textContent = clientId;

// Configure MQTT client options with TLS settings
const options = {
clientId: clientId,
hostname: 'public-mqtt-broker.bevywise.com',
port: 11443,
protocol: 'wss', // Secure WebSocket protocol
rejectUnauthorized: false, // Disable server certificate verification
// username: 'username', // Add username if needed
// password: 'password', // Add password if needed
// Add additional TLS options if needed
// cert: yourClientCert, // Client certificate
// key: yourClientKey, // Client key
// ca: yourCaCert // CA certificate
};

// Create MQTT client
const client = mqtt.connect(`wss://${options.hostname}:${options.port}/mqtt`, options);

// Handle client events
client.on('connect', () => {
console.log('Connected successfully with MQTT Broker.');

// Subscribe to a topic
const topic = 'cmq/topic';
document.getElementById('topic').textContent = topic;
client.subscribe(topic, (err) => {
if (!err) {
console.log('Subscribed to topic.');
document.getElementById('subscriptionStatus').textContent = 'Subscribed to topic.';

// Publish a message
const message = 'Hello CMQ';
client.publish(topic, message, { qos: 2, retain: true }, (err) => {
if (!err) {
console.log('Message published.');
document.getElementById('publishedMessage').textContent = message;
}
});
}
});
});

If the MQTT Broker has Server Certificate issued from a Trusted CA, then the Server Certificate can be verified using:

// Generate Client ID
const clientId = generateClientId('crystalmq_', 10);
document.getElementById('clientId').textContent = clientId;

// Load the CA certificate (assuming the CA certificate is available as a string or a URL)
const caCert = `-----BEGIN CERTIFICATE-----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-----END CERTIFICATE-----`;

// Configure MQTT client options with TLS settings
const options = {
clientId: clientId,
hostname: 'public-mqtt-broker.bevywise.com',
port: 11443,
protocol: 'wss', // Secure WebSocket protocol
ca: caCert, // CA certificate
rejectUnauthorized: true, // Ensure the server certificate is verified
// username: 'username', // Add username if needed
// password: 'password', // Add password if needed
};

// Create MQTT client
const client = mqtt.connect(`wss://${options.hostname}:${options.port}/mqtt`, options);

// Handle client events
client.on('connect', () => {
console.log('Connected successfully with MQTT Broker.');

// Subscribe to a topic
const topic = 'cmq/topic';
document.getElementById('topic').textContent = topic;
client.subscribe(topic, (err) => {
if (!err) {
console.log('Subscribed to topic.');
document.getElementById('subscriptionStatus').textContent = 'Subscribed to topic.';

// Publish a message
const message = 'Hello CMQ';
client.publish(topic, message, { qos: 2, retain: true }, (err) => {
if (!err) {
console.log('Message published.');
document.getElementById('publishedMessage').textContent = message;
}
});
}
});
});

If the MQTT Broker has a self-signed Server Certificate then the Server Certificate can be verified using the Root Certificate obtained from the MQTT Broker:

// Generate Client ID
const clientId = generateClientId('crystalmq_', 10);
document.getElementById('clientId').textContent = clientId;

// Load the root CA certificate (assuming the root CA certificate is available as a string)
const caCert = `-----BEGIN CERTIFICATE-----
MIIDdzCCAl+gAwIBAgIEUzJ7kDANBgkqhkiG9w0BAQsFADBoMQswCQYDVQQGEwJV
UzERMA8GA1UECAwIU29tZS1TdGF0ZTEUMBIGA1UEBwwLU29tZS1DaXR5MRMwEQYD
VQQKDApJbnRlcm5ldCBDQTETMBEGA1UEAwwKQ2VydGlmaWNhdGUwHhcNMTkxMjMx
MDAwMDAwWhcNMjAxMjMwMDAwMDAwWjBoMQswCQYDVQQGEwJVUzERMA8GA1UECAwI
U29tZS1TdGF0ZTEUMBIGA1UEBwwLU29tZS1DaXR5MRMwEQYDVQQKDApJbnRlcm5l
dCBDQTETMBEGA1UEAwwKQ2VydGlmaWNhdGUwggEiMA0GCSqGSIb3DQEBAQUAA4IB
DwAwggEKAoIBAQC2Xk1eRm94bw3hm8+K0H5/4eU9G8hb8CQXj0FGONMSY0zg/P4n
M6t2+/GsQbj3FfAg/SwQ5lH9qO7CxTn/ANX4gO3KO9m0pjHfDkSx7hPlZoLR3oQz
73VcxltoHtUKg4+QAcI5nwb+byA3f1EOn+0V5d41rmlVxRk1dKl7FA8wN2zO7aLO
+F1QnIvDC/d9EjGqcmY8c9y4+fqzgH+iQFPq5APEL5Ead/nk5a8dRfd6Wb8GV6lg
o8pI0sMlS8HdLMjpCewMI7Nhu0a5ZOnC+S7a3YB2UO4jXg3ADm5nLqG6rbEpD1DE
A4NBoXBx+RhfdMfyPTNVhd8XtavXUkmyEBKbAgMBAAGjITAfMB0GA1UdDgQWBBQI
1hNfdp9a6f5Jm9peGr+m4iJ+IzANBgkqhkiG9w0BAQsFAAOCAQEADJc2ddg4TGcB
Z7ViZ5UlfI02KFA+E4xmdvZOE+aNkqZHYt21ou9lnzZoLbdK+9woO4F/8i0HBZ0h
MDhjeHGkZT+yxT1uWh6ldZggVDF6X/8zWy8RmG/k+bQTeTiU/wLfGPN0bsfvmFXV
sqE/O0rbc1LCaRIyRoEUfW/hmkf9UjYdW+FOcY5VfJm+GpLPOZLrAL3QAWYnsO8j
F1h7W2pE3TOS/W+vXFrY7T1hdIzQpAhvhflS/jmEAB+8PvX6QPV7DfbRoIsFBOxk
VtRuycCimUnhCnQqv+kdazUZzVz48huRl1A/5T/mcf2hrE9q2tmX8qXT8Vs2O5Ky
Pbq9jsS/xA==
-----END CERTIFICATE-----`;

// Configure MQTT client options with TLS settings
const options = {
clientId: clientId,
hostname: 'public-mqtt-broker.bevywise.com',
port: 11443,
protocol: 'wss', // Secure WebSocket protocol
ca: caCert, // Root CA certificate
rejectUnauthorized: true, // Ensure the server certificate is verified // username: 'username', // Add username if needed
// password: 'password', // Add password if needed
};

// Create MQTT client
const client = mqtt.connect(`wss://${options.hostname}:${options.port}/mqtt`, options);

// Handle client events
client.on('connect', () => {
console.log('Connected successfully with MQTT Broker.');

// Subscribe to a topic
const topic = 'cmq/topic';
document.getElementById('topic').textContent = topic;
client.subscribe(topic, (err) => {
if (!err) {
console.log('Subscribed to topic.');
document.getElementById('subscriptionStatus').textContent = 'Subscribed to topic.';

// Publish a message
const message = 'Hello CMQ';
client.publish(topic, message, { qos: 2, retain: true }, (err) => {
if (!err) {
console.log('Message published.');
document.getElementById('publishedMessage').textContent = message;
}
});
}
});
});

Configuring MQTT Authentication

To connect to MQTT Broker that requires MQTT Username and MQTT Password for authentication, add to username and password to the connection options like this:

// Generate Client ID
const clientId = generateClientId('crystalmq_', 10);

// Configure MQTT client options
const options = {
clientId: clientId,
hostname: 'public-mqtt-broker.bevywise.com',
port: 10443,
protocol: 'ws' // WebSocket protocol
username: 'username', // Add username if needed
password: 'password', // Add password if needed
};

// Create MQTT client
const client = mqtt.connect(`ws://${options.hostname}:${options.port}/mqtt`, options);

// Handle client events
client.on('connect', () => {
console.log('Connected successfully with MQTT Broker.');

// Subscribe to a topic
client.subscribe('cmq/topic', (err) => {
if (!err) {
console.log('Subscribed to topic.');

// Publish a message
client.publish('cmq/topic', 'Hello CMQ', { qos: 2, retain: true }, (err) => {
if (!err) {
console.log('Message published.');
}
});
}
});
});

Advanced Features

Setting Up Last Will & Testament

Configure the Last Will and Testament feature to specify a message that the broker will publish if the client unexpectedly disconnects. This helps inform other subscribers of the disconnected client's status.

Use the following code to set Last Will in the Connection Options:

// Generate Client ID
const clientId = generateClientId('crystalmq_', 10);
document.getElementById('clientId').textContent = clientId;

// Configure MQTT client options for MQTT v5
const options = {
clientId: clientId,
hostname: 'public-mqtt-broker.bevywise.com',
port: 10443,
protocol: 'ws', // WebSocket protocol
protocolVersion: 5, // MQTT version 5
// username: 'username', // Add username if needed
// password: 'password', // Add password if needed
properties: {
sessionExpiryInterval: 60, // Set session expiry interval in seconds
},
will: {
topic: 'cmq/lastwill', // The topic to publish the LWT message
payload: 'Client disconnected unexpectedly', // The LWT message
qos: 1, // The QoS level
retain: true // Whether to retain the message
}
};

// Create MQTT client
const client = mqtt.connect(`ws://${options.hostname}:${options.port}/mqtt`, options);

Adjusting Keep Alive

MQTT maintains client-broker connections with a keep-alive mechanism. Adjust the keep-alive interval to control how frequently the client sends PINGREQ messages to the broker.

Modify the code below to suit your requirements:

/ Generate Client ID
const clientId = generateClientId('crystalmq_', 10);
document.getElementById('clientId').textContent = clientId;

// Configure MQTT client options for MQTT v5
const options = {
clientId: clientId,
hostname: 'public-mqtt-broker.bevywise.com',
port: 10443,
protocol: 'ws', // WebSocket protocol
protocolVersion: 5, // MQTT version 5
keepalive: 60, // Set keep-alive interval in seconds
// username: 'username', // Add username if needed
// password: 'password', // Add password if needed
properties: {
sessionExpiryInterval: 60, // Set session expiry interval in seconds
},
will: {
topic: 'cmq/lastwill', // The topic to publish the LWT message
payload: 'Client disconnected unexpectedly', // The LWT message
qos: 1, // The QoS level
retain: true // Whether to retain the message
}
};

// Create MQTT client
const client = mqtt.connect(`ws://${options.hostname}:${options.port}/mqtt`, options);

// Handle client events
client.on('connect', (connack) => {
console.log('Connected successfully with MQTT Broker.', connack);

// Subscribe to a topic with MQTT v5 properties
const topic = 'cmq/topic';
document.getElementById('topic').textContent = topic;
client.subscribe(topic, { qos: 2, nl: true, rap: true, rh: 0 }, (err, granted) => {
if (!err) {
console.log('Subscribed to topic.', granted);
document.getElementById('subscriptionStatus').textContent = 'Subscribed to topic.';

// Publish a message with MQTT v5 properties
const message = 'Hello CMQ';
client.publish(topic, message, { qos: 2, retain: true, properties: { messageExpiryInterval: 60 } }, (err) => {
if (!err) {
console.log('Message published.');
document.getElementById('publishedMessage').textContent = message;
}
});
}
});
});

Configuring Session Persistence

Session data of an MQTT Client include the Subscriptions made by the Client and any data that the Client would receive with QoS>0. The Client can get the MQTT Broker to store its session data across connections.

MQTT 3.1.1 Clients can set Clean Session = 0 to request the M

MQTT 3.1.1

// Generate Client ID
const clientId = generateClientId('crystalmq_', 10);
document.getElementById('clientId').textContent = clientId;

// Configure MQTT client options for MQTT v3.1.1
const options = {
clientId: clientId,
hostname: 'public-mqtt-broker.bevywise.com',
port: 10443,
protocol: 'ws', // WebSocket protocol
protocolVersion: 4, // MQTT version 3.1.1
clean: false, // Set Clean Session to 0
keepalive: 60, // Set keep-alive interval in seconds
// username: 'username', // Add username if needed
// password: 'password', // Add password if needed
will: {
topic: 'cmq/lastwill', // The topic to publish the LWT message
payload: 'Client disconnected unexpectedly', // The LWT message
qos: 1, // The QoS level
retain: true // Whether to retain the message
}
};
// Create MQTT client
const client = mqtt.connect(`ws://${options.hostname}:${options.port}/mqtt`, options);

MQTT 5 Clients can set Clean Start = 0 and Session Expiry Interval = 'N' to request the MQTT Broker to keep its session information stored across connections for 'N' seconds.

// Generate Client ID
const clientId = generateClientId('crystalmq_', 10);
document.getElementById('clientId').textContent = clientId;

// Set the Session Expiry Interval in seconds (N)
const sessionExpiryInterval = 3600; // 1 hour

// Configure MQTT client options for MQTT v5
const options = {
clientId: clientId,
hostname: 'public-mqtt-broker.bevywise.com',
port: 10443,
protocol: 'ws', // WebSocket protocol
protocolVersion: 5, // MQTT version 5
cleanStart: false, // Set Clean Start to 0
sessionExpiryInterval: sessionExpiryInterval, // Set Session Expiry Interval
// username: 'username', // Add username if needed
// password: 'password', // Add password if needed
will: {
topic: 'cmq/lastwill', // The topic to publish the LWT message
payload: 'Client disconnected unexpectedly', // The LWT message
qos: 1, // The QoS level
retain: true // Whether to retain the message
}
};

// Create MQTT client
const client = mqtt.connect(`ws://${options.hostname}:${options.port}/mqtt`, options);

Setting Maximum Packet Size

MQTT5 Client can request the MQTT Broker to only send data packets less than a specific size by setting it like this:

// Generate Client ID
const clientId = generateClientId('crystalmq_', 10);
document.getElementById('clientId').textContent = clientId;

// Configure MQTT client options for MQTT v5
const options = {
clientId: clientId,
hostname: 'public-mqtt-broker.bevywise.com',
port: 10443,
protocol: 'ws', // WebSocket protocol
protocolVersion: 5, // MQTT version 5
maxPacketSize: 1024, // Maximum packet size in bytes
// username: 'username', // Add username if needed
// password: 'password', // Add password if needed
will: {
topic: 'cmq/lastwill', // The topic to publish the LWT message
payload: 'Client disconnected unexpectedly', // The LWT message
qos: 1, // The QoS level
retain: true // Whether to retain the message
}
};

// Create MQTT client
const client = mqtt.connect(`ws://${options.hostname}:${options.port}/mqtt`, options);

Publish

Sending Data

Efficiently distribute data to multiple subscribers by publishing it to designated topics with the following code snippet:

MQTT 3.1.1

// Generate Client ID
const clientId = generateClientId('mqtt_publisher_', 10);
document.getElementById('clientId').textContent = clientId;

// Configure MQTT client options for MQTT v3.1.1
const options = {
clientId: clientId,
hostname: 'public-mqtt-broker.bevywise.com',
port: 1883,
protocol: 'mqtt', // MQTT protocol
// username: 'username', // Add username if needed
// password: 'password', // Add password if needed
};

// Create MQTT client
const client = mqtt.connect(`ws://${options.hostname}:${options.port}`, options);

// Handle client events
client.on('connect', () => {
console.log('Connected successfully with MQTT Broker.');

// Publish a message to multiple topics
const topics = ['topic1', 'topic2', 'topic3'];
const message = 'Hello MQTT';

topics.forEach(topic => {
client.publish(topic, message, { qos: 2 }, (err) => {
if (!err) {
console.log(`Message published to ${topic}`);
document.getElementById('publishedMessages').innerHTML += `<li>Published to ${topic}: ${message}</li>`;
}
});
});
});

MQTT 5

// Generate Client ID
const clientId = generateClientId('mqtt5_publisher_', 10);
document.getElementById('clientId').textContent = clientId;

// Configure MQTT client options for MQTT v5
const options = {
clientId: clientId,
hostname: 'public-mqtt-broker.bevywise.com',
port: 1883,
protocol: 'mqtt', // MQTT protocol
protocolVersion: 5, // MQTT version 5
// username: 'username', // Add username if needed
// password: 'password', // Add password if needed
};

// Create MQTT client
const client = mqtt.connect(`ws://${options.hostname}:${options.port}`, options);

// Handle client events
client.on('connect', () => {
console.log('Connected successfully with MQTT Broker.');

// Publish a message to multiple topics
const topics = ['topic1', 'topic2', 'topic3'];
const message = 'Hello MQTT v5';

topics.forEach(topic => {
client.publish(topic, message, { qos: 2 }, (err) => {
if (!err) {
console.log(`Message published to ${topic}`);
document.getElementById('publishedMessages').innerHTML += `<li>Published to ${topic}: ${message}</li>`;
}
});
});
});

Setting Retained Messages

Enable the retain flag when publishing a message to ensure the broker stores the last message for each topic. This guarantees that new subscribers receive the most recent message upon connecting.

To implement this, use the following code snippet:

// Handle client events
client.on('connect', () => {
console.log('Connected successfully with MQTT Broker.');

// Publish a message with retain flag to multiple topics
const topics = ['topic1', 'topic2', 'topic3'];
const message = 'Hello MQTT v5 with Retain';

topics.forEach(topic => {
client.publish(topic, message, { qos: 2, retain: true }, (err) => {
if (!err) {
console.log(`Message published to ${topic} with retain.`);
document.getElementById('publishedMessages').innerHTML += `<li>Published to ${topic}: ${message}</li>`;
}
});
});
});

Specifying QoS Levels

MQTT provides three levels of Quality of Service (QoS) for message delivery:

  • QoS 0 (At most once)
  • QoS 1 (At least once)
  • QoS 2 (Exactly once)

Specify the required QoS level when publishing MQTT messages using this code:

// Handle client events
client.on('connect', () => {
console.log('Connected successfully with MQTT Broker.');

// Publish a message with QoS 0 (At most once)
client.publish('topic/qos0', 'Message with QoS 0', { qos: 0 }, (err) => {
if (!err) {
console.log('Message published with QoS 0.');
document.getElementById('publishedMessages').innerHTML += `<li>Published with QoS 0: Message with QoS 0</li>`;
}
});

// Publish a message with QoS 1 (At least once)
client.publish('topic/qos1', 'Message with QoS 1', { qos: 1 }, (err) => {
if (!err) {
console.log('Message published with QoS 1.');
document.getElementById('publishedMessages').innerHTML += `<li>Published with QoS 1: Message with QoS 1</li>`;
}
});

// Publish a message with QoS 2 (Exactly once)
client.publish('topic/qos2', 'Message with QoS 2', { qos: 2 }, (err) => {
if (!err) {
console.log('Message published with QoS 2.');
document.getElementById('publishedMessages').innerHTML += `<li>Published with QoS 2: Message with QoS 2</li>`;
}
});
});

Message Expiry Interval

The 'Message expiry interval' property sets a message's life span in seconds; if undelivered within this time, the broker discards it. MQTT5 supports this feature. MQTT5 Clients can set this while publishing data.

Topic Alias

// Handle client events
client.on('connect', () => {
console.log('Connected successfully with MQTT Broker.');

// Publish a message with Message Expiry Interval (30 seconds) to a topic
const topic = 'topic/message_expiry';
const message = 'Message with Message Expiry Interval';
const messageExpiryInterval = 30; // Message Expiry Interval in seconds

client.publish(topic, message, { qos: 1, retain: false, properties: { messageExpiryInterval: messageExpiryInterval * 1000 } }, (err) => {
if (!err) {
console.log(`Message published to ${topic} with Message Expiry Interval.`);
document.getElementById('publishedMessages').innerHTML += `<li>Published to ${topic}: ${message} (Expiry Interval: ${messageExpiryInterval} seconds) </li>`;
}
});
});

Topic Alias

The 'Topic Alias' property allows clients to use a short alias instead of a full topic name, reducing message packet size and improving network efficiency.

MQTT 5

// Handle client events
client.on('connect', () => {
console.log('Connected successfully with MQTT Broker.');

// Publish a message using Topic Alias to a topic
const topicAlias = 1; // Topic Alias number (numeric identifier for the topic)
const topic = 'topic/alias';
const message = 'Message with Topic Alias';

// Publish with Topic Alias
client.publish(topic, message, { qos: 1, retain: false, properties: { topicAlias: topicAlias } }, (err) => {
if (!err) {
console.log(`Message published to ${topic} with Topic Alias ${topicAlias}.`);
document.getElementById('publishedMessages').innerHTML += `<li>Published to ${topic}: ${message} (Topic Alias: ${topicAlias})</li>`;
}
});
});

Properties associated with MQTT PUBLISH enhance message handling, providing context or instructions for brokers and clients. These properties, including message expiry intervals and topic aliases, optimize message delivery and network bandwidth.

Subscribe

Subscribing to Topic Filter

To receive data published by other clients, this client has to subscribe to a matching Topic Filter like this:

// Multi-level wildcard subscription
client.subscribe('sensors/#', (err) => {
if (!err) {
console.log('Subscribed to all sensors topics.');
}
});
// Single-level wildcard subscription
client.subscribe('sensors/+/temperature', (err) => {
if (!err) {
console.log('Subscribed to sensors/+/temperature.');
}
});

This topic filter can match with an exact topic or it can have wildcards like # and +

Receiving Data

To receive data sent for the subscriptions, a callback function needs to be defined like this:

// Define callback function to handle incoming messages
client.on('message', (topic, message, packet) => {
console.log(`Received message on topic: ${topic}`);
console.log(`Message content: ${message.toString()}`);
// Additional processing based on received message

// Example: Display message in HTML
const receivedMessages = document.getElementById('receivedMessages');
receivedMessages.innerHTML += `<li>Topic: ${topic}, Message: ${message.toString()}</li>`; });

Unsubscribing from Topics

To stop receiving updates from a topic, use the code provided to unsubscribe.

// Topic to unsubscribe from
const topicToUnsubscribe = 'sensors/temperature';
// Unsubscribe from the topic
client.unsubscribe(topicToUnsubscribe, (err) => {
if (!err) {
console.log(`Unsubscribed from topic: ${topicToUnsubscribe}`);
} else {
console.error(`Failed to unsubscribe from topic: ${topicToUnsubscribe}`, err);
}
});

Disconnecting the Client

Ensure a proper termination of your client's connection with the broker to avoid issues and resource leaks on both sides, thereby maintaining system stability.

Use the following code to disconnect the client from the broker:

client.end(true, () => {
console.log('Disconnected from MQTT Broker.');
});

// Handle close event to confirm disconnection
client.on('close', () => {
console.log('Connection to MQTT Broker closed.');
// Optional: Implement reconnect logic here
setTimeout(() => {
try {
client.reconnect(); // Attempt to reconnect after a delay
} catch {
console.log('Reconnecting failed.');
}
}, 5000); // Reconnect after 5 seconds (adjust as needed)
});

Building Your Business Logic

You have the opportunity to develop and customize your own intricate business logic within this environment, tailoring it precisely to your specific needs and objectives.

For a more comprehensive understanding of the origins and methods of acquiring data for monitoring purposes, be sure to explore our detailed data acquisition page.

Implementing Best Practices

Robust Error Handling

Implement strong error management to handle MQTT connection failures, subscription problems, and message publishing errors effectively.

Securing Credentials

Safeguard sensitive information like usernames, passwords, and client IDs by not hard-coding them in your source code. Use environment variables or secure configuration files instead.

Regular Testing & Monitoring

Continuously test MQTT communication and monitor client metrics such as connection status, message throughput, and error rates to quickly identify and fix issues.

Optimizing Session Management

Choose between clean and persistent sessions (`clean: true` or `clean: false`) based on your need to retain subscriptions and queued messages across client connections.

Reconnect on Disconnect

Reconnect on Disconnect Add code to attempt reconnection to the MQTT Broker when there is an unexpected disconnection. This will ensure that your client stays connected and does not lose any data.

Download Code

Download the complete code for client that uses Java script MQTT Client Library to connect with our CrystalMQ broker or any broker of your choice.

MQTT 3.1.1 js file

document.addEventListener('DOMContentLoaded', (event) => {
function main() {
// MQTT Client initialization
const clientId = generateClientId('crystalMQ_', 10);
document.getElementById('mqttV4ClientId').textContent = clientId;

// MQTT v4 Configuration
const mqttV4Options = {
clientId: clientId,
hostname: 'public-mqtt-broker.bevywise.com',
port: 1883,
protocol: 'mqtt' // MQTT protocol
// username: 'username', // Add username if needed
// password: 'password', // Add password if needed
};

// Create MQTT v4 client
const mqttV4Client = mqtt.connect(`ws://${mqttV4Options.hostname}:${mqttV4Options.port}`, mqttV4Options);

// MQTT v4 Event Handlers
mqttV4Client.on('connect', () => {
console.log('Connected with MQTT v4 Broker.');
document.getElementById('mqttV4Status').textContent = 'Connected';
});

mqttV4Client.on('message', (topic, message) => {
console.log(`Received message on MQTT v4: ${topic} - ${message.toString()}`);
document.getElementById('mqttV4Received').innerHTML += `<li>Topic: ${topic}, Message: ${message.toString()}</li>`;
});

mqttV4Client.on('close', () => {
console.log('Disconnected from MQTT v4 Broker.');
document.getElementById('mqttV4Status').textContent = 'Disconnected';
});

// Publish function for MQTT v4
window.publishMessageV4 = function(topic, message) {
mqttV4Client.publish(topic, message, { qos: 1 }, (err) => {
if (!err) {
console.log(`Published message on MQTT v4: ${topic} - ${message}`);
document.getElementById('mqttV4Published').innerHTML += `<li>Published to ${topic}: ${message}</li>`;
} else {
console.error(`Failed to publish message on MQTT v4: ${err}`);
}
});
};

// Subscribe function for MQTT v4
window.subscribeToTopicV4 = function(topic) {
mqttV4Client.subscribe(topic, (err) => {
if (!err) {
console.log(`Subscribed to MQTT v4 topic: ${topic}`);
document.getElementById('mqttV4Subscribed').textContent = topic;
} else {
console.error(`Failed to subscribe to MQTT v4 topic: ${err}`);
}
});
};

// Disconnect function for MQTT v4
window.disconnectClientV4 = function() {
mqttV4Client.end(true, () => {
console.log('Disconnected MQTT v4 client.');
document.getElementById('mqttV4Status').textContent = 'Disconnected';
});
};
}

function generateClientId(prefix, length) {
const chars = 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789';
let result = prefix;
for (let i = 0; i < length; i++) {
result += chars.charAt(Math.floor(Math.random() * chars.length));
}
return result;
}

main();
});

MQTT 3.1.1 html file

<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<title>MQTT Example</title>
<script src="https://unpkg.com/mqtt/dist/mqtt.min.js"></script>
<script src="mqtt-v4.js" type="text/javascript"></script>
</head>
<body>
<h1>MQTT Example</h1>

<!-- MQTT v4 Section -->
<h2>MQTT v4</h2>
<p><strong>Client ID:</strong> <span id="mqttV4ClientId"></span></p>
<p><strong>Status:</strong> <span id="mqttV4Status">Disconnected</span></p>
<button onclick="disconnectClientV4()">Disconnect MQTT v4</button>
<hr>
<h3>Subscribe</h3>
<input type="text" id="mqttV4SubscribeTopic" placeholder="Topic to subscribe">
<button onclick="subscribeToTopicV4(document.getElementById('mqttV4SubscribeTopic').value)">Subscribe</button>
<p><strong>Subscribed to:</strong> <span id="mqttV4Subscribed"></span></p>
<hr>
<h3>Publish</h3>
<input type="text" id="mqttV4PublishTopic" placeholder="Topic to publish">
<input type="text" id="mqttV4PublishMessage" placeholder="Message to publish">
<button onclick="publishMessageV4(document.getElementById('mqttV4PublishTopic').value, document.getElementById('mqttV4PublishMessage').value)">Publish</button>
<h3>Published Messages</h3>
<ul id="mqttV4Published"></ul>
<h3>Received Messages</h3>
<ul id="mqttV4Received"></ul>

</body>
</html>

MQTT 5 Js file

document.addEventListener('DOMContentLoaded', (event) => {
function main() {
// MQTT Client initialization
const clientId = generateClientId('mqtt5_client_', 10);
document.getElementById('mqttV5ClientId').textContent = clientId;

// MQTT v5 Configuration
const mqttV5Options = {
clientId: clientId,
hostname: 'public-mqtt-broker.bevywise.com',
port: 1883,
protocol: 'mqtt', // MQTT protocol
protocolVersion: 5,
// username: 'username', // Add username if needed
// password: 'password', // Add password if needed
};

// Create MQTT v5 client
const mqttV5Client = mqtt.connect(`ws://${mqttV5Options.hostname}:${mqttV5Options.port}`, mqttV5Options);

// MQTT v5 Event Handlers
mqttV5Client.on('connect', () => {
console.log('Connected with MQTT v5 Broker.');
document.getElementById('mqttV5Status').textContent = 'Connected';
});

mqttV5Client.on('message', (topic, message) => {
console.log(`Received message on MQTT v5: ${topic} - ${message.toString()}`);
document.getElementById('mqttV5Received').innerHTML += `<li>Topic: ${topic}, Message: ${message.toString()}< /li>`;
});

mqttV5Client.on('close', () => {
console.log('Disconnected from MQTT v5 Broker.');
document.getElementById('mqttV5Status').textContent = 'Disconnected';
});

// Publish function for MQTT v5
window.publishMessageV5 = function(topic, message) {
mqttV5Client.publish(topic, message, { qos: 1 }, (err) => {
if (!err) {
console.log(`Published message on MQTT v5: ${topic} - ${message}`);
document.getElementById('mqttV5Published').innerHTML += `<li>Published to ${topic}: ${message}</li>`;
} else {
console.error(`Failed to publish message on MQTT v5: ${err}`);
}
});
};

// Subscribe function for MQTT v5
window.subscribeToTopicV5 = function(topic) {
mqttV5Client.subscribe(topic, (err) => {
if (!err) {
console.log(`Subscribed to MQTT v5 topic: ${topic}`);
document.getElementById('mqttV5Subscribed').textContent = topic;
} else {
console.error(`Failed to subscribe to MQTT v5 topic: ${err}`);
}
});
};

// Disconnect function for MQTT v5
window.disconnectClientV5 = function() {
mqttV5Client.end(true, () => {
console.log('Disconnected MQTT v5 client.');
document.getElementById('mqttV5Status').textContent = 'Disconnected';
});
};
}

function generateClientId(prefix, length) {
const chars = 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789';
let result = prefix;
for (let i = 0; i < length; i++) {
result += chars.charAt(Math.floor(Math.random() * chars.length));
}
return result;
}

main();
});

<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<title>MQTT Example</title>
<script src="https://unpkg.com/mqtt/dist/mqtt.min.js"></script>
<script src="mqtt-v5.js" type="text/javascript"></script>
</head>
<body>

<!-- MQTT v5 Section -->
<h2>MQTT v5</h2>
<p><strong>Client ID:</strong> <span id="mqttV5ClientId"></span></p>
<p><strong>Status:</strong> <span id="mqttV5Status">Disconnected</span></p>
<button onclick="disconnectClientV5()">Disconnect MQTT v5</button>
<hr>
<h3>Subscribe</h3>
<input type="text" id="mqttV5SubscribeTopic" placeholder="Topic to subscribe">
<button onclick="subscribeToTopicV5(document.getElementById('mqttV5SubscribeTopic').value)">Subscribe</button>
<p><strong>Subscribed to:</strong> <span id="mqttV5Subscribed"></span></p>
<hr>
<h3>Publish</h3>
<input type="text" id="mqttV5PublishTopic" placeholder="Topic to publish">
<input type="text" id="mqttV5PublishMessage" placeholder="Message to publish">
<button onclick="publishMessageV5(document.getElementById('mqttV5PublishTopic').value, document.getElementById('mqttV5PublishMessage').value)">Publish</button>
<h3>Published Messages</h3>
<ul id="mqttV5Published"></ul>
<h3>Received Messages</h3>
<ul id="mqttV5Received"></ul>

</body>
</html>

Create Executable Bundle

Connect your client to our state-of-the-art CrystalMQ broker or any broker of your choice. This powerful combination will ensure optimal performance and reliability for all your messaging needs, paving the way for a robust and efficient system integration.