计算的测量

本页详细介绍计算的测量值。

参数：您想要的参数，以及如何根据输入的测量进行计算。
标签：哪些标签可让您区分源自不同传感器的计算的测量值。您可以重复使用输入测量中的标签，也可以根据计算出的测量结果来指定不同的标签。
时间戳：是使用输入测量的时间戳，还是输出计算的测量的时间。

实施

要生成计算的测量值，您需要编写一个可以处理所选的输入测量值，执行任何必要的计算或逻辑并输出计算出的测量数据的脚本。

JavaScript是用于编写计算的测量脚本的脚本语言。

计算的测量脚本包含两个函数：

初始化：设置脚本
计算：处理来自输入测量值的数据，执行计算并输出您计算出的测量值

脚本中必须同时包含初始化和计算函数。

init（）：初始化函数

脚本初始化函数可用于初始化脚本逻辑和状态管理所需的任何数据结构。在创建，修改脚本或系统维护之后重新启动服务，该脚本仅执行一次。

这是初始化函数的脚本模板：

/**
 * @name: init
 * @description: Perform one-time initialization of your script
 * param {string}
 **/
async function init() {
  // Perform initialization of script here
  // TODO
}

如果您在计算的测量不需要跟踪多个或连续输入测量的数据，例如测量值的总和或检查一对输入度量之间的差异，则可以选择将此函数保留为空。

async function init() {
  // Initialization not required
}

如果您在计算的测量需要跟踪多个或连续输入测量的数据，则应在此处创建适当的数据结构。

您也可以声明全局变量。例如：

// Declare global variable for a summation value
let sum;

async function init() {
  // Initialize sum to zero
  sum = 0;
}

compute（）：计算函数

每当来自指定输入测量的新测量数据或一批新测量数据到达时，都会执行脚本计算函数。在此函数中，您将从这些输入测量中读取数据，执行所需的任何逻辑或计算，然后输出计算出的测量数据。

这是计算函数的脚本模板：

/**
 * @name: compute
 * @description: Perform computation on input measurements. This function is called when new measurements arrive
 * @param {string} measurement - The input measurement name
 * @param {Array.<object>} data - The measurement array data
 **/
function compute(measurement, data) {
  // Perform calculation on data here
  // TODO

  // Example output
  const outputData = {
    metric1: 1,
    metric2: true,
    metric3: 1,
  };

  // Output computed measurement. See API docs below
  output(outputData);
}
 
// Output API function
/**
 * @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.lsid - [Example] lsid tag (for lifts)
 * @param {string} outputData.country - [Example] country tag
 * @param {number} outputData.metric1 - [Example] metric1
 * @param {boolean} outputData.metric2 - [Example] metric2
 * @param {integer} outputData.metric3 - [Example] metric3
 **/

请注意，因为使用的数据是测量数据的数组，因此通常您需要遍历输入的数据，才能对每个变量执行计算任务，如下所示：

function compute(measurement, data) {    
  // Loop through the array of incoming measurements
  for (let i = 0; i < data.length; i++) {
    const inputMeasurement = data[i];
 
    // Perform calculation and output
    // ...
    
    // Output
    output(...); 
  }
}

At the end of your computations, once you have constructed the output data for the computed measurement, you should call output(), with the output data, to output your computed measurement. This will inform the script engine to ingest your computed measurement into Senfi.

Note: The computation function does not return or output a computed measurement. You must call output() in order to output a computed measurement data to Senfi.

output(): Script Output

The script output function is a pre-defined function call that you should invoke whenever you want to output your computed measurement.

You must invoke the output function with a valid measurement data (e.g. all declared metrics and tags) for your computed measurement. Note the output measurement will automatically be attributed to the computed measurement name you have chosen in the CMS. It is not possible for the script to output another computed measurement.

Testing

You can test your computed measurement in the CMS when you are creating or editing the computed measurement.

Your testing data should be identical in format and values to the actual data from your input measurements. You can test with single or multiple sets of data.

The test data format is in JSON, as an Array of Objects of the form:

{
  "measurement":
  "data": []
}

where measurement is the name of the input measurement, and data is a list of one or more measurement data from the input measurement following the Senfi data message format.

Example of sending 1 set of measurement data from the measurement temperature_v1. The script computation function will be executed once during the test:

[
  {
    "measurement": "temperature_v1",
    "data": [{
      "tm_source": xxxxxxxxxx,
      "site_id" xxxxxxxx,
      "tag1": "xxxxxxxx",
      "tag2": "xxxxxxxx",
      "temp": xxxxxxxx,
    }]
  }
]

Abbreviated example of sending a batch of 3 sets of measurement data from the measurement temperature_v1. The script computation function will be executed once during the test:

[
  {
    "measurement": "temperature_v1",
    "data": [{
      ...
      "temp": xxxxxxxx,
    },{
      ...
      "temp": xxxxxxxx,
    },{
      ...
      "temp": xxxxxxxx,
    }]
  }
]

Abbreviated example of sending 3 sets of measurement data from the measurement temperature_v1. The script computation function will be executed 3 times during the test:

[
  {
    "measurement": "temperature_v1",
    "data": [{
      ...
      "temp": xxxxxxxx,
    }]
  },
  {
    "measurement": "temperature_v1",
    "data": [{
      ...
      "temp": xxxxxxxx,
    }]
  },
  {
    "measurement": "temperature_v1",
    "data": [{
      ...
      "temp": xxxxxxxx,
    }]
  }
]

If your script performs computation with multiple input measurements, you can specify multiple measurements. In this example, the computation function will be executed twice, once for temperature_v1 and once for humidity_v1:

[
  {
    "measurement": "temperature_v1",
    "data": [{
      ...
      "temp": xxxxxxxx,
    }]
  },
  {
    "measurement": "humidity_v1",
    "data": [{
      ...
      "humidity": xxxxxxxx,
    }]
  }
]

Execution

After you create or edit your computed measurement in the CMS, your computed measurement script will be compiled and the initialization and computation functions called:

When you finish creating the script: The initialization function will be run
When you finish editing the script: The initialization function will be run
When data from each of your input measurements arrives: The computation function will be run.

Note: If many measurements from the same input measurement arrive within a very short period of time, the measurements from that input measurement may be batched together. For example, if your input measurement input_measurement_1 sends data 10 times per second, you may receive 30 sets of measurements from input_measurement_1 every 3 seconds.

Note: If you have multiple input measurements, each invocation of the computation function will be for only one input measurement. For example, if you have the following input measurements input_measurement_1 and input_measurement_2, expect the computation function to be run twice, once for input_measurement_1 and another for input_measurement_1, for each set of data.

Errors and Debugging

Errors that occur during the initialization or execution of your script will show up in the Debugger in the Developer tab in the CMS.

Console messages logged by your script also appear in the Debugger.

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 Fahrenheit (℉) 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, where temperatureF is the temperature data in ℉:

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

And this is the computed measurement that you want to output, where temperatureC is the temperature metric in °C. You will retain the input measurement's tags in order to correctly attribute the computed measurement to the original temperature sensor:

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

To convert the temperatureF temperature metric in ℉ into temperatureC in °C, you would use the ℉ to °C formula:

temperatureC = (temperatureF − 32) * 5/9;

As you are just performing conversion when data arrives, there is no need to perform any initialization tasks or declare any global variables. So you can leave the initialization function empty:

async function init() {
  // No initialization tasks required for this simple example
}

Putting everything together, the full script would be similar to:

/**
 * @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 input measurements. This function is called when new measurements arrive
 * @param {string} measurement - The input 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: Lift Total Floors Moved

This example shows a more complex usage of computed measurement.

Imagine you have a lift controller that sends floor position values (i.e. 1 when the lift car is at the 1st Floor; 2 when it is at the 2nd floor, etc). You want to use the lift car position data to calculate the cumulative total number of floors it has moved.

For example, if the lift was previously reported to be at the 1st Floor and is now reported to be at the 5th Floor, the total number of floors moved is 5 - 1 = 4. If it is next reported to be at the 7th Floor, the new total number of floors moved is (7 - 5) + 4 = 6. Etc

This is the (abbreviated) measurement lift_controller_v1 from the lift controller that you have as input, where lsid is the lift ID tag, and position is the lift car position data:

 {
   "tm_source": xxxxxxxxxx,
   "site_id" xxxxxxxx,
   "lsid": "xxxxxxxx",
   "position": xxxxxxxxx
 }

And this is the computed measurement that you want to output, where floorsMoved is to cumulative total number of floors moved. You will retain the input measurement's tags in order to correctly attribute the computed measurement to the original lift:

 {
   "tm_source": xxxxxxxxxx,
   "site_id" xxxxxxxx,
   "lsid": "xxxxxxxx",
   "floorsMoved": xxxxxxxxx
 }

As you want to accumulate the total number of floors moved over time, and not just performing per-measurement-data calculations, you will declare a global variable totalFloorsMoved for this purpose:

let totalFloorsMoved;

Your computation function will perform calculation across consecutive lift_controller_v1 input measurements in the computation function. You will declare a global variable prevFloors to store the previous value of the floorsMoved metric, so the next time the computation function is executed on the next lift_controller_v1 input measurement, you can perform the calculation:

let prevFloors;

You can also use the API functions getStoredValue and setStoredValue to store accumulated values to persistent storage. This is so you will not lose the accumulated data if the Senfi undergoes system maintenance where computed measurements are terminated and reinitialized.

For example, to read previously calculated total floors moved:

let prevTotalFloorsMoved = await getStoredValue('floorsMoved');

Thus, the completed initialization function would be:

let totalFloorsMoved;
let prevFloor;
 
/**
 * @name: init
 * @description: Perform one-time initialization of your script
 * param {string}
 **/
async function init() {
  // Perform initialization of script here
  let _totalFloorsMovedVal = await getStoredValue('floorsMoved');
  let _prevFloorVal = await getStoredValue('prevFloor');
  
  // Return value from getStoredValue is a JSON string. 
  // We need to convert it back to the actual values (an integer) 
  // or null if no value was previously stored
  let _totalFloorsMoved = JSON.parse(_totalFloorsMovedVal);
  let _prevFloor = JSON.parse(_prevFloorVal);  
 
  if (_totalFloorsMoved == null) {
    totalFloorsMoved = 0; // This is the first time we run the script. Set to 0
  } else {
    totalFloorsMoved= _totalFloorsMoved ; // Set to the previously stored value
  }
 
  if (_prevFloor == null) {
    prevFloor = 0; // This is the first time we run the script. Set to 0
  } else {
    prevFloor = _prevFloor ; // Set to the previously stored value
  }
}

Now we need to write the computation logic:

Upon a new batch input measurements, for each individual measurement, calculate the absolute number of floors moved since the last measurement: floors moved = abs(current position - previous position)
Add the absolute number of floors moved to the cumulative total number of floors moved: total floors moved += floors moved
Construct and output the computed measurement
Store current position as previous position, for the next iteration

The computation function would be:

/**
 * @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) {
  // Perform calculation on data here
  for (let i = 0; i < data.length; i++) {
    const d = data[i];

    // On the very first measurement where _prevFloor is 0, set _prevFloor to current position
    if (_prevFloor == 0) {
      _prevFloor = d.positon;
    }

    // Calculate the difference in floors moved
    const floorsMoved = Math.abs(d.position - _prevFloor) + _totalFloorsMoved;
 
    // Constuct the output measurement
    const outputMeasurement = {
      site_id: d.site_id,
      country: d.country,
      lsid: d.lsid,
      floorsMoved: floorsMoved 
    }; 
 
    // Output computed measurement
    output(outputMeasurement );

    // Store current values
    _prevFloor = d.position;
    _totalFloorsMoved = floorsMoved;
  }
 
  // Persist global values, to survive script restart
  await setStoredValue('floorsMoved', _totalFloorsMoved);
  await setStoredValue('prevFloor', _prevFloor);
}

Computed Measurement Script API

Output API function

output

/**
 * @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.lsid - [Example] lsid tag (for lifts)
 * @param {string} outputData.country - [Example] country tag
 * @param {number} outputData.metric1 - [Example] metric1
 * @param {boolean} outputData.metric2 - [Example] metric2
 * @param {integer} outputData.metric3 - [Example] metric3
 **/

Persistent Storage API functions

setStoredValue

/**
 * @async - Use await to wait for the function to complete
 * @function setStoredValue
 * @description Persist a value to senfi database. You may retrieve this value using 'getStoreValue'
 * @param {string} key
 * @param {any} value
 **/

getStoredValue

/**
 * @async - Use await to wait for the function to complete
 * @function getStoredValue
 * @description Retrieve a value previously stored in senfi database.
 * @param {string} key
 * @return {Promise.<string>} stringified value. Use JSON.parse() to restore the value to original form
 **/

Timing API functions

setInterval

/**
 * @function setInterval
 * @param {function} fn
 * @param {integer} interval value, in miliseconds
 * @returns {string} token
 **/

clearInterval

/**
 * @function clearInterval
 * @param {string} token - The return value from setInterval()
 **/

setTimeout

/**
 * @function setTimeout
 * @param {function} fn
 * @param {integer} timeout value, in miliseconds
 * @returns {string} token
 **/

clearTimeout

/**
 * @function clearTimeout
 * @param {string} token - The return value from setTimeout()
 **/

Logging API functions

The following functions allow you to log messages which will show up in the Debugger in the CMS Developer Tab.

console.log
console.error
console.warn
console.info
console.debug

Next: Sending data to Senfi