Computed measurement

This page describes computed measurement in detail.

Overview

As described here, computed measurement is a type of data that is generated from other data sources.

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Design

The first thing to do when designing a computed measurement is to decide what are the inputs and what to output. A computed measurement can specify multiple measurements as input. Metrics and tags in those measurement will then be available to the function that is responsible for generating the output.

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Implementation

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 ... Junhao feedback that it is unclear that you must output a valid measurement. he thought he has to always output something, like NULL during initial invocation ...

Testing

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Execution

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Examples

Example 1: Temperature Scale Conversion

This example shows a simple usage of computed measurement.

Imagine you have a temperature sensor that sends raw temperature values in degrees Farenheit (℉) and you want to show the temperature in degrees Celsius (°C) instead. This is the temperature measurement from the temperature sensor that you have as input:

 {
   "tm_source": xxxxxxxxxx,
   "site_id" xxxxxxxx,
   "tag1": "xxxxxxxx",
   "tag2": "xxxxxxxx",
   "temperatureF": xxxxxxxxx
 }

And this is the computed measurement that you want to output:

 {
   "tm_source": xxxxxxxxxx,
   "site_id" xxxxxxxx,
   "tag1": "xxxxxxxx",
   "tag2": "xxxxxxxx",
   "temperatureC": xxxxxxxxx
 }

You can use the following script to perform the temperature metric conversion:

 /**
  * @name: init
  * @description: Perform one-time initialization of your script
  * param {string}
  **/
 async function init() {
   // No initialization tasks required for this simple example
 } 
 
 /**
  * @name: compute
  * @description: Perform computation on watching measurements. This function is called when new measurements arrive
  * @param {string} measurement - The source measurement name
  * @param {Array.<object>} data - The measurement array data
  **/
 function compute(measurement, data) {    
   // Loop through the array of incoming measurements,
   // and convert 'temperatureF' to 'temperatureC'
   for (let i = 0; i < data.length; i++) {
     const inputMeasurement = data[i];
     const temperatureF = inputMeasurement.temperatureF;
  
     // Calculate the temperature in C
     const temperatureC = (temperatureF − 32) * 5/9; 
  
     const outputMeasurement = {
       tm_source: inputMeasurement.tm_source,
       site_id: inputMeasurement.site_id,
       tag1: inputMeasurement.tag1,
       tag2: inputMeasurement.tag2,
       temperatureC: temperatureC,
     };  
   
     // Output computed measurement
     output(outputMeasurement);
   }
 }
 
 /**
  * @function output 
  * @description: User-invoked function to output a computed measurement
  * @param {Object} outputData - Object containing output metrics (required tags, optional tags, measurement metrics)
  * @param {string} outputData.tm_source - Timestamp (Same as input measurement)
  * @param {string} outputData.site_id - Site Id (Same as input measurement)
  * @param {string} outputData.tag1  - Tag1 (Same as input measurement)
  * @param {number} outputData.tag2 - Tag2 (Same as input measurement)
  * @param {boolean} outputData.temperatureC - Calculated temperature in C
 **/
               

Example 2

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