webthing

Implementation of an HTTP Web Thing.

Installation

webthing can be installed via npm, as such:

$ npm install webthing

Example

In this example we will set up a dimmable light and a humidity sensor (both using fake data, of course). Both working examples can be found in here.

Dimmable Light

Imagine you have a dimmable light that you want to expose via the web of things API. The light can be turned on/off and the brightness can be set from 0% to 100%. Besides the name, description, and type, a Light is required to expose two properties:

• on: the state of the light, whether it is turned on or off
  • Setting this property via a PUT {"on": true/false} call to the REST API toggles the light.
• brightness: the brightness level of the light from 0-100%
  • Setting this property via a PUT call to the REST API sets the brightness level of this light.

First we create a new Thing:

const light = new Thing('urn:dev:ops:my-lamp-1234',
                        'My Lamp',
                        ['OnOffSwitch', 'Light'],
                        'A web connected lamp');

Now we can add the required properties.

The on property reports and sets the on/off state of the light. For this, we need to have a Value object which holds the actual state and also a method to turn the light on/off. For our purposes, we just want to log the new state if the light is switched on/off.

light.addProperty(
  new Property(
    light,
    'on',
    new Value(true, (v) => console.log('On-State is now', v)),
    {
      '@type': 'OnOffProperty',
      title: 'On/Off',
      type: 'boolean',
      description: 'Whether the lamp is turned on',
    }));

The brightness property reports the brightness level of the light and sets the level. Like before, instead of actually setting the level of a light, we just log the level.

light.addProperty(
  new Property(
    light,
    'brightness',
    new Value(50, v => console.log('Brightness is now', v)),
    {
      '@type': 'BrightnessProperty',
      title: 'Brightness',
      type: 'number',
      description: 'The level of light from 0-100',
      minimum: 0,
      maximum: 100,
      unit: 'percent',
    }));

Now we can add our newly created thing to the server and start it:

// If adding more than one thing, use MultipleThings() with a name.
// In the single thing case, the thing's name will be broadcast.
const server = new WebThingServer(SingleThing(light), 8888);

process.on('SIGINT', () => {
  server.stop().then(() => process.exit()).catch(() => process.exit());
});

server.start().catch(console.error);

This will start the server, making the light available via the WoT REST API and announcing it as a discoverable resource on your local network via mDNS.

Sensor

Let's now also connect a humidity sensor to the server we set up for our light.

A MultiLevelSensor (a sensor that returns a level instead of just on/off) has one required property (besides the name, type, and optional description): level. We want to monitor this property and get notified if the value changes.

First we create a new Thing:

const sensor = new Thing('urn:dev:ops:my-humidity-sensor-1234',
                         'My Humidity Sensor',
                         ['MultiLevelSensor'],
                         'A web connected humidity sensor');

Then we create and add the appropriate property:

• level: tells us what the sensor is actually reading

  • Contrary to the light, the value cannot be set via an API call, as it wouldn't make much sense, to SET what a sensor is reading. Therefore, we are creating a readOnly property.

  const level = new Value(0.0);
  
  sensor.addProperty(
    new Property(
      sensor,
      'level',
      level,
      {
        '@type': 'LevelProperty',
        title: 'Humidity',
        type: 'number',
        description: 'The current humidity in %',
        minimum: 0,
        maximum: 100,
        unit: 'percent',
        readOnly: true,
      }));

Now we have a sensor that constantly reports 0%. To make it usable, we need a thread or some kind of input when the sensor has a new reading available. For this purpose we start a thread that queries the physical sensor every few seconds. For our purposes, it just calls a fake method.

// Poll the sensor reading every 3 seconds
setInterval(() => {
  // Update the underlying value, which in turn notifies all listeners
  level.notifyOfExternalUpdate(readFromGPIO());
}, 3000);

This will update our Value object with the sensor readings via the this.level.notifyOfExternalUpdate(readFromGPIO()); call. The Value object now notifies the property and the thing that the value has changed, which in turn notifies all websocket listeners.

Adding to Gateway

To add your web thing to the WebThings Gateway, install the "Web Thing" add-on and follow the instructions here.

Resources

• https://iot.mozilla.org/things/
• https://hacks.mozilla.org/2018/05/creating-web-things-with-python-node-js-and-java/
• https://nodejs.org/en/
• https://github.com/rzr/webthing-iotjs/wiki
• https://youtu.be/Z-oiFl6gwGw