Node.js implementation of a Web Thing server
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webthing

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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('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',
      label: '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',
      label: '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('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.
    Map<String, Object> levelDescription = new HashMap<>();
    levelDescription.put('type', 'number');
    levelDescription.put('description', 'The current humidity in %');
    levelDescription.put('unit', '%');
    
    const level = new Value(0.0);
    
    sensor.addProperty(
      new Property(
        sensor,
        'level',
        level,
        {
          '@type': 'LevelProperty',
          label: '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.

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