gcic-mqtt-client

Simple helper to build Google Cloud IoT Core MQTT clients with the renowned MQTT.js library. This helper enhances the MQTT.js library with:

• Configuration parameters specific to the Google Cloud IoT Core solution;
• Methods to publish device events and state updates in a straightforward manner;
• Automatic subscription to device configuration updates and commands;
• Support for auto-renewal of the device token (JWT used as MQTT client password).

All the options, methods and events supported by the MQTT.js library remain accessible, so that taking advantage of this helper does not cause any loss of functionality.

Overview

const mqtt = require('gcic-mqtt-client');

const options = {
  projectId: 'my-project',
  registryId: 'my-registry',
  deviceId: 'my-device',
  cloudRegion: 'us-central1',
  privateKey: fs.readFileSync('./rsa_private.pem')
};

const client = mqtt(options);

// Publish an event
client.publishEvent(data);

// Publish a state update
client.publishState(state);

The client can automatically subscribe to device configuration updates and invoke a callback every time device configuration is dispatched to the device. Similarly, the client can automatically subscribe to commands and invoke a callback every time a command is dispatched to the device.

const mqtt = require('gcic-mqtt-client');

const options = {
  projectId: 'my-project',
  registryId: 'my-registry',
  deviceId: 'my-device',
  cloudRegion: 'us-central1',
  privateKey: fs.readFileSync('./rsa_private.pem'),
  onConfiguration: processConfiguration, // <--- callback
  onCommand: processCommand              // <--- callback
};

const client = mqtt(options);

function processConfiguration(configuration) {
  /* Process configuration data, possibly
     replying with a device state update */
}

function processCommand(command, subfolder) {
  /* Process the command */
}

All the events supported by the MQTT.js library remain accessible. For example:

const mqtt = require('gcic-mqtt-client');

const client = mqtt(options);

client.on('connect', () => {
  console.log('Connected!');
});

client.on('error', (error) => {
  console.log(error.message);
});

Installation

npm install gcic-mqtt-client --save

Usage

The client is created and connected by invoking the helper module with an options object.

const mqtt = require('gcic-mqtt-client');

const client = mqtt(options);

The following table lists the properties of the options object:

Property	Description
projectId	REQUIRED - String; identifies the Google Cloud IoT project
cloudRegion	REQUIRED - String; identifies the Google Cloud IoT Core cloud region
registryId	REQUIRED - String; identifies the Google Cloud IoT Core device registry
deviceId	REQUIRED - String; identifies the Google Cloud IoT Core device
privateKey	REQUIRED - String or buffer; device private key in PEM format; must be consistent with the selected tokenAlgorithm
tokenAlgorithm	OPTIONAL - String with RS256 as default value; cryptographic algorithm for signing the token used as MQTT client password; Google Cloud IoT Core currently supports choice between RS256 (2048-bit RSA key) and ES256 (P-256 EC key). When choosing between the two algorithms, consider that generating ES256 signatures is way faster than generating RS256 signatures
tokenLifecycle	OPTIONAL - Integer with 3600 as default value and 86400 as maximum allowed value; specifies the time validity of the device token in seconds; Google Cloud IoT Core automatically disconnects devices after their tokens have expired; a grace period of approximately 10 minutes is observed to compensate for possible clock skews
onConfiguration	OPTIONAL - Callback invoked whenever Google Cloud IoT Core publishes configuration information for the device. The MQTT client subscribes to configuration updates only if onConfiguration is specified. At every configuration update the MQTT client invokes the onConfiguration callback with (configuration) as argument; configuration is the received configuration passed as buffer
qosConfiguration	OPTIONAL - Integer equal to either 0 (at most once) or 1 (at least once, default value); determines the QoS with which the client subscribes to configuration updates
onCommand	OPTIONAL - Callback invoked whenever Google Cloud IoT Core sends a command to the device. The MQTT client subscribes to commands only if onCommand is specified. At every command the MQTT client invokes the onCommand callback with (command, subfolder) as arguments; command is the received command passed as buffer; subfolder is null if the command was published to /devices/{deviceId}/commands or a string if the command was published to /devices/{deviceId}/commands/{subfolder}
qosCommands	OPTIONAL - Integer equal to either 0 (at most once, default value) or 1 (at least once); determines the QoS with which the client subscribes to commands
host	OPTIONAL - String with mqtt.googleapis.com as default; identifies the Google Cloud IoT Core MQTT bridge host
port	OPTIONAL - Integer with 8883 as default; identifies the Google Cloud IoT Core MQTT bridge port number
keepalive	OPTIONAL - Integer with 60 as default value; specifies the interval in seconds at which the MQTT client sends PINGREQ packets. The keepalive value should be tuned considering the trade-off between rapid detection of client disconnections vs amount of background traffic generated. Note that PINGREQ packets count against Google Cloud IoT Core billing
reschedulePings	OPTIONAL - Boolean with true as default value; controls whether the transmission of PINGREQ packets is rescheduled after sending other packets so that outgoing traffic is minimized
reconnectPeriod	OPTIONAL - Integer with 1000 as default value; specifies the time interval in milliseconds between consecutive reconnection attempts
connectTimeout	OPTIONAL - Integer with 30000 as default value; specifies the maximum time in milliseconds waited before a CONNACK packet is received
queueQoSZero	OPTIONAL - Boolean with true as default value; controls whether outgoing messages with QoS=0 are queued or not during periods of disconnection
localAddress	OPTIONAL - String; identifies the local network address the MQTT client should connect from; when omitted, no binding is established and the MQTT client is allowed to use any available network interface
incomingStore	OPTIONAL - Support for alternative implementations of the message store; refer to the documentation of the MQTT.js library for details
outgoingStore	OPTIONAL - Same as above

A typical options object will be something like the following:

const options = {
  projectId: 'my-project',
  registryId: 'my-registry',
  deviceId: 'my-device',
  cloudRegion: 'us-central1',
  tokenAlgorithm: 'ES256',
  tokenLifecycle: 86400, // 24 hours
  privateKey: fs.readFileSync('./ec_private.pem'),
  keepalive: 300, // 5 minutes
  onConfiguration: processConfiguration, // callback to be declared
  onCommand: processCommand // callback to be declared
};

Once the MQTT client has been created, all the methods and events supported by the MQTT.js library are accessible. In addition, this helper provides support for the following methods and events:

client.publishEvent(event[, qos][, subfolder])

Used to publish device telemetry events. Arguments are defined as follows:

• event - event data, either as string or as buffer;
• qos - either 0 (at most once) or 1 (at least once), with 0 as default;
• subfolder - subfolder to which the event should be published; if subfolder is omitted, then the event is published to /devices/{deviceId}/events; if subfolder is specified, then the event is published to /devices/{deviceId}/events/subfolder.

Example:

const data = { 'sensor1': 'motion_detected' };

// Publish to 'alerts' subfolder with QoS=1
mqtt.publishEvent(JSON.stringify(data), 1, 'alerts');

Google Cloud IoT Core includes the subfolder name when mapping events from the MQTT bridge to Google Cloud Pub/Sub.

client.publishState(state[, qos])

Used to publish device state. Arguments are defined as follows:

• state - state data, either as string or as buffer;
• qos - either 0 (at most once) or 1 (at least once), with 0 as default.

Example:

const state = {
  'fan1_target_rpm': 1000,
  'fan2_target_rpm': 1200,
  'firmware_version': '1.2.3b'
};

// Publish with QoS=0
client.publishState(JSON.stringify(state));

client.changePrivateKey(newKey)

Used to replace the private key used to sign tokens. The newKey must be in PEM format, passed either as string or as buffer, and must be consistent with the tokenAlgorithm selected at client creation time (i.e. of the same type as the key being replaced).

The key replacement can be initiated at any time, but takes effect at the next token auto-renewal.

Example:

const newKey = fs.readFileSync('./ec_private2.pem');

client.changePrivateKey(newKey);

Google Cloud IoT Core enables key rotation by allowing up to three public keys or certificates to be associated with an individual device at any given time.

client.on('disconnect', (tokenExpired) => { });

The disconnect event is intended to be used in lieu of the offline event supported by the MQTT.js library. The disconnect event is emitted every time an offline event is emitted, but the disconnect event comes with the indication of whether the token was found expired or not (tokenExpired boolean flag).

In general, a disconnection reported with tokenExpired equal to true may simply be the symptom of a periodic token expiration, and be automatically solved by the token auto-renewal function supported by this helper module. However, please consider that Google Cloud IoT Core observes a relatively long grace period (approximately 10 minutes) before disconnecting a device whose token has expired. This creates a time window in which the disconnect event is reported with tokenExpired equal to true even if the actual cause of disconnection was not the token expiration itself. Additional insight may be obtained by listening to error events in addition to disconnect events.

client.on('token_renewal', (expires) => { });

The token_renewal event is emitted whenever the token auto-renewal function supported by this helper module takes care of generating a new token. The expires argument is an integer number that indicates the expiration time of the new token expressed as UNIX timestamp in seconds.

The following example summarizes the set of MQTT client events that may be reasonable to monitor. The definition of the corresponding event handlers is application specific.

client.on('connect', () => {
  console.log('Connected!');
  // Connected!
});

client.on('disconnect', (tokenExpired) => {
  console.log('Disconnected; token has expired:', tokenExpired);
  // Disconnected; token has expired: true
});

client.on('error', (error) => {
  console.log(error.message);
  // Connection refused: Not authorized
});

client.on('token_renewal', (expires) => {
  console.log('New token generated; new expiration time:', expires);
  // New token generated; new expiration time: 1507002199
});

Token auto-renewal

Each device that connects to the Google Cloud IoT Core MQTT bridge uses a JSON Web Token as MQTT password. This helper module supports a token auto-renewal function intended to ensure prompt token replacement and device reconnection whenever a device gets disconnected because its token has expired. Specifically, whenever the device experiences a disconnection - if one of the following conditions holds true - a new token is generated before attempting reconnection:

• The current token has expired;
• The remaining validity time of the current token is less than half of the configured token tokenLifecycle period.

The second point above has the intent to proactively replace the token when a disconnection due to causes such as network problems is experienced, thus moving farther in the future the need to disconnect/reconnect to replace the token. The criterion of waiting until at least half of the token lifecycle has been consumed protects against repeated token replacements in presence frequent disconnections, e.g. because of unstable network conditions.