Expression Fn

Alarm rule condition TBEL script function

The expression() function is a user-defined script that enables custom condition expressions using TBEL on telemetry and attribute data. It receives arguments configured in the alarm rule setup, along with an additional ctx object that stores latestTs and provides access to all arguments.

Function signature

function expression(ctx, arg1, arg2, ...): boolean

Supported arguments

There are two types of arguments supported in the alarm rule configuration: attributes and latest telemetry.

These arguments are single values and may be of type: boolean, int64 (long), double, string, or JSON.

Usage

Example: Convert temperature from Fahrenheit to Celsius and raise the alarm if the Celsius value is greater than 36

var temperatureC = (temperatureF - 32) / 1.8;
return temperatureC > 36;

Alternatively, use ctx to access the argument as an object:

{
  "temperatureF": {
    "ts": 1740644636669,
    "value": 98.7
  }
}

You may notice that the object includes both the value of an argument and its timestamp as ts. The ctx object also includes the property latestTs, which represents the latest timestamp of the arguments telemetry in milliseconds.

Let's modify the expression that converts Fahrenheit to Celsius to also check if the temperature's timestamp is exactly at the start of an hour:

var temperatureC = (ctx.args.temperatureF.value - 32) / 1.8;
var temperatureTs = ctx.args.temperatureF.ts;
return temperatureC > 36 && ((temperatureTs / 1000) % 3600) == 0;