Measurement Transformation Expression Language

Overview

The IoT Agent Library provides an expression language for measurement transformation, that can be used to adapt the information coming from the South Bound APIs to the information reported to the Context Broker. Expressions in this language can be configured for provisioned attributes as explained in the Device Provisioning API section in the main README.md.

Measurement transformation

Expression definition

Expressions can be defined for Active attributes, either in the Device provisioning or in the Configuration provisioning. The following example shows a device provisioning payload with defined expressions:

{
    "devices": [
        {
            "device_id": "45",
            "protocol": "GENERIC_PROTO",
            "entity_name": "WasteContainer:WC45",
            "entity_type": "WasteContainer",
            "attributes": [
                {
                    "name": "location",
                    "type": "geo:point",
                    "expression": "${@latitude}, ${@longitude}"
                },
                {
                    "name": "fillingLevel",
                    "type": "Number",
                    "expression": "${@level / 100}"
                },
                {
                    "name": "level",
                    "type": "Number"
                },
                {
                    "name": "latitude",
                    "type": "Number"
                },
                {
                    "name": "longitude",
                    "type": "Number"
                }
            ]
        }
    ]
}

The value of the expression attribute is a string that can contain any number of expression patterns. Each expression pattern is marked with the following secuence: ${<expression>} where <expression> is a valid construction of the Expression Language (see definition below). In order for the complete expression to be evaluated, all the expression patterns must be evaluatable (there must be a value in the measurement for all the variables of all the expression patterns).

Note that you need to include in the provision operation all the attributes required as inputs for the expressions. In this example, they are level, latitude and longitude. Otherwise the device sending the measures will get {"name":"ATTRIBUTE_NOT_FOUND","message":"Some of the attributes does not exist"} when it sends some of these and the expression will not be calculated.

The exact same syntax works for Configuration and Device provisioning.

Variable values

Attribute expressions can contain values taken from the value of other attributes. Those values have, by default, the String type. For most arithmetic operations (*, /, etc...) if a variable is involved, its value will be cast to Number, regardless of the original type. For, example, if a variable @humidity has the value '50' (a String value), the following expression:

${@humidity * 10}

will give 500 as the result (i.e.: the value '50' is cast to number, to get 50, that is then multiplied by 10). If this cast fails (because the value of the variable is not a number, e.g.: 'Fifty'), the overall result will be NaN.

Expression execution

Whenever a new measurement arrives to the IoT Agent for a device with declared expressions, all of the expressions for the device will be checked for execution: for all the defined active attributes containing expressions, the IoT Agent will check which ones contain expressions whose variables are present in the received measurement. For all of those whose variables are covered, their expressions will be executed with the received values, and their values updated in the Context Broker.

E.g.: if a device with the following provisioning information is provisioned in the IoT Agent:

{
   "name":"location",
   "type":"geo:point",
   "expression": "${@latitude}, ${@longitude}"
},
{
   "name":"fillingLevel",
   "type":"Number",
   "expression": "${@level / 100}",
},

and a measurement with the following values arrive to the IoT Agent:

latitude: 1.9
level: 85.3

The only expression rule that will be executed will be that of the fillingLevel attribute. It will produce the value 0.853 that will be sent to the Context Broker.

Note that expressions are only applied if the attribute name (as received by the IoT Agent in the southbound interface) matches the expression variable. Otherwise, the southbound value is used directly. Let's illustrate with the following example:

"consumption": {
   "type": "String",
   "value": "${trim(@spaces)}"
}
  • Case 1: the following measure is received at the southbound interface:
consumption: "0.44"

As spaces attribute is not included, then the expression is not applied and the consumption measure value is directly used, so the following is sent to CB:

"consumption": {
   "type": "String",
   "value": "0.44"
}
  • Case 2: the following measure is received at the southbound interface:
consumption: "0.44"
spaces: "  foobar  "

As spaces attribute is included, then the expression is evaluated, so overriding the 0.44 value and sending the following to CB:

"consumption": {
    "type": "String",
    "value": "foobar"
}

Language description

Types

The way the parse() function works (at expressionParser.js) is as follows:

  • Expressions can have two return types: String or Number. This return type must be configured for each attribute that is going to be converted. Default value type is String.
  • Whenever an expression is executed without error, its result will be cast to the configured type. If the conversion fails (e.g.: if the expression is null or a String and is cast to Number), the measurement update will fail, and an error will be reported to the device.

However, the usage that the Expression Translation plugin does of that function is using always String type. That means that at the end, the result of the expression will be always cast to String. However, in NGSI v2 that String result could be re-cast to the right type (i.e. the one defined for the attribute in the provision operation). Have a look at the NGSI v2 support for more information on this.

Values

Variables

All the information reported in the measurement received by the IoT Agent is available for the expression to use. For every attribute coming from the South Bound, a variable with the syntax @<object_id> will be created for its use in the expression language.

Constants

The expression language allows for two kinds of constants:

  • Numbers (integer or float)
  • Strings (marked with double quotes)

Current allowed characters are:

  • All the alphanumerical characters
  • Whitespaces

Allowed operations

The following operations are currently available, divided by attribute type

Number operations

  • multiplication ('*')
  • division ('/')
  • addition ('+')
  • subtraction ('-' binary)
  • negation ('-' unary)
  • power ('^')

String operations

  • concatenation ('#'): returns the concatenation of the two values separated by #.
  • substring location (indexOf(<variable>, <substring>)): returns the index where the first occurrence of the substring <substring> can be found in the string value of <variable>.
  • substring (substr(<variable>, <start> <end>)): returns a substring of the string variable passed as a parameter, starting at posisiton start and ending at position <end>.
  • whitespace removal (trim(<string>)): removes all the spaces surrounding the string passed as a parameter.

Other available operators

Parenthesis can be used to define precedence in the operations. Whitespaces between tokens are generally ignored.

Examples of expressions

The following table shows expressions and their expected outcomes for a measure with two attributes: @value with value 6 and @name with value DevId629.

Expression Expected outcome Format
5 \* @value 30 Number
(6 + @value) \* 3 36 Number
@value / 12 + 1 1.5 Number
(5 + 2) \* (@value + 7) 91 Number
@value \* 5.2 31.2 Number
"Pruebas " + "De Strings" Pruebas De Strings String
@name value is @value DevId629 value is 6 String

NGSI v2 support

As it is explained in previous sections, expressions can have two return types: String or Number, being the former one the default. Whenever an expression is executed without error, its result will be cast to the configured type.

NGSI v2 and NGSI-LD fully supports all the types described in the JSON specification (string, number, boolean, object, array and null). Therefore, the result of an expression must be cast to the appropriate type (the type used to define the attribute) in order to avoid inconsistencies between the type field for an attribute and the type of the value that is being sent.

Currently, the expression parser does not support JSON Arrays and JSON document. A new issue has been created to address this aspect https://github.com/telefonicaid/iotagent-node-lib/issues/568. For the rest of types the workflow will be the following:

  1. Variables will be cast to String no matter the expression type (see comments above regarding this)
  2. The expression will be applied
  3. The output type will be cast again to the original attribute type.

  4. If attribute type is Number and the value is an Integer, then the value is casted to integer (JSON number)

  5. If attribute type is Number and the value is a Float, then the value is casted to float (JSON number)
  6. If attribute type is Boolean then the value is cast to boolean (JSON boolean). In order to do this conversion, only true or 1 are cast to true.
  7. If attribute type is None then the value is cast to null (JSON null)

Note. All the operations and castings described above are not performed using JELX, because the user has full control on the final value of the attributes, so there is no need of adding a layer of autocast that would interfere and makes things more complicated.

E.g.: if a device with the following provisioning information is provisioned in the IoT Agent:

{
    "name": "status",
    "type": "Boolean",
    "expression": "${@status *  20}"
}

and a measurement with the following values arrive to the IoT Agent:

status: true
  1. The expression * is a multiplication, so the expression type makes status to be casted to Number. The cast of true to number is 1.
  2. Expression is evaluated, resulting in 20
  3. 20 is cast to 20 since Expression Plugin always use String as Expression type.
  4. The attribute type is Boolean so the result is casted to Boolean before sending it to CB. The cast of 20 to boolean is false (only true or 1 are cast to true).

More examples of this workflow are presented below for the different types of attributes supported in NGSI v2 and the two possible types of expressions: Integer (arithmetic operations) or Strings.

  • pressure with value 52 (integer)
  • consumption with value 0.44 (float)
  • active with value null (None type)
Expression Expected outcome Format
${@pressure * 20} 1040 Integer
${trim(@pressure)} 52 Integer
${@consumption * 20} 8.8 Float
${trim(@consumption)} 0.44 Float
${@pressure * 20} 1040 Integer
${@active * 20} null None
${trim(@active) null None
${trim(@consumption)} 0.44 Float

To allow support for expressions in combination with multi entity plugin, where the same attribute is generated for different entities out of different incoming attribute values (i.e. object_id), we introduced support for object_id in the expression context.

For example, the following device:

"WeatherStation": {
    "commands": [],
    "type": "WeatherStation",
    "lazy": [],
    "active": [
        {
            "object_id": "v1",
            "name": "vol",
            "expression" : "${@v1*100}",
            "type": "Number",
            "entity_name": "WeatherStation1"
        },
        {
            "object_id": "v2",
            "name": "vol",
            "expression" : "${@v2*100}",
            "type": "Number",
            "entity_name": "WeatherStation2"
        },
        {
            "object_id": "v",
            "name": "vol",
            "expression" : "${@v*100}",
            "type": "Number"
        }
    ]
}

When receiving the attributes v, v1 and v2 in a payload from a message received in the southbound:

({
    "name": "v",
    "type": "Number",
    "value": 0
},
{
    "name": "v1",
    "type": "Number",
    "value": 1
},
{
    "name": "v2",
    "type": "Number",
    "value": 2
})

Will now generate the following NGSI v2 payload:

{
    "actionType": "append",
    "entities": [
        {
            "id": "ws9",
            "type": "WeatherStation",
            "vol": {
                "type": "Number",
                "value": 0
            }
        },
        {
            "vol": {
                "type": "Number",
                "value": 100
            },
            "type": "WeatherStation",
            "id": "WeatherStation1"
        },
        {
            "vol": {
                "type": "Number",
                "value": 200
            },
            "type": "WeatherStation",
            "id": "WeatherStation2"
        }
    ]
}

JEXL Based Transformations

As an alternative, the IoT Agent Library supports as well JEXL. To use JEXL, you will need to either configure it as default language using the defaultExpressionLanguage field to jexl (see configuration documentation) or configuring the usage of JEXL as expression language for a given device:

{
    "devices": [
        {
            "device_id": "45",
            "protocol": "GENERIC_PROTO",
            "entity_name": "WasteContainer:WC45",
            "entity_type": "WasteContainer",
            "expressionLanguage": "jexl",
            "attributes": [
                {
                    "name": "location",
                    "type": "geo:json",
                    "expression": "{coordinates: [longitude,latitude], type: 'Point'}"
                },
                {
                    "name": "fillingLevel",
                    "type": "Number",
                    "expression": "level / 100"
                },
                {
                    "name": "level",
                    "type": "Number"
                },
                {
                    "name": "latitude",
                    "type": "Number"
                },
                {
                    "name": "longitude",
                    "type": "Number"
                }
            ]
        }
    ]
}

In the following we provide examples of using JEXL to apply transformations.

Quick comparison to default language

  • JEXL supports the following types: Boolean, String, Number, Object, Array.
  • JEXL also supports the creation of GeoJSON Objects
  • JEXL allows to navigate and filter objects and arrays.
  • JEXL supports if..then...else... via ternary operator.
  • JEXL additionally supports the following operations: Divide and floor //, Modulus %, Logical AND && and Logical OR ||. Negation operator is !
  • JEXL supports comparisons.
  • JEXL does not cast or convert automatically the types as Legacy Expression Language as described here.

For more details, check JEXL language details here.

Examples of JEXL expressions

The following table shows expressions and their expected outcomes taking into account the following measures at southbound interface:

  • value with value 6 (number)
  • ts with value 1637245214901 (unix timestamp)
  • name with value "DevId629" (string)
  • object with value {name: "John", surname: "Doe"} (JSON object)
  • array with value [1, 3] (JSON Array)
Expression Expected outcome Format
5 * value 30 Integer
(6 + value) * 3 36 Integer
value / 12 + 1 1.5 Float
(5 + 2) * (value + 7) 91 Integer
value * 5.2 31.2 Float
"Pruebas " + "De Strings" "Pruebas De Strings" String
name + "value is " +value "DevId629 value is 6" String
{coordinates: [value,value], type: 'Point'} {"coordinates": [6,6], "type": "Point"} GeoJSON Object
ts|toisodate 2021-11-18T14:20:14.901Z ISO 8601 DateTime

Support for trim, length, substr and indexOf transformations was added.

Expression Expected outcome
" a "| trim a
name|length 8
name|indexOf("e") 1
name|substring(0,name|indexOf("e")+1) "De"

The following are some examples of JEXL expressions not supported by the legacy expression language:

Expression Expected outcome Format
value == 6? true : false true Boolean
value == 6 && name|indexOf("e")>0 true Boolean
array[1]+1 3 Number
object.name "John" String
{type:"Point",coordinates: [value,value]} {type:"Point",coordinates: [6,6]} Object

Available functions

There are several predefined JEXL transformations available to be used at any JEXL expression. The definition of those transformations and their JavaScript inmplementation can be found at jexlTransformsMap.js.

The library module also exports a method iotAgentLib.dataPlugins.expressionTransformation.setJEXLTransforms(Map) to be used by specific IoT Agent implementations in order to incorporate extra transformations to this set. It is important to remark that the lib jexlTransformsMap cannot be overwritten by the API additions. The idea behind this is to be able to incorporate new trasformations from the IoT Agent configuration file in a fast and tactical way.

Current common transformation set:

JEXL Transformation Equivalent JavaScript Function
jsonparse: (str) JSON.parse(str);
jsonstringify: (obj) JSON.stringify(obj);
indexOf: (val, char) String(val).indexOf(char);
length: (val) String(val).length;
trim: (val) String(val).trim();
substr: (val, int1, int2) String(val).substr(int1, int2);
addreduce: (arr) arr.reduce((i, v) | i + v));
lengtharray: (arr) arr.length;
typeof: (val) typeof val;
isarray: (arr) Array.isArray(arr);
isnan: (val) isNaN(val);
parseint: (val) parseInt(val);
parsefloat: (val) parseFloat(val);
toisodate: (val) new Date(val).toISOString();
timeoffset:(isostr) new Date(isostr).getTimezoneOffset();
tostring: (val) val.toString();
urlencode: (val) encodeURI(val);
urldecode: (val) decodeURI(val);
replacestr: (str, from, to) str.replace(from, to);
replaceregexp: (str, reg, to) str.replace(new RegExp(reg), to);
replaceallstr: (str, from, to) str.replaceAll(from, to);
replaceallregexp: (str, reg, to) str.replaceAll(new RegExp(reg,"g"), to);
split: (str, ch) str.split(ch);
mapper: (val, values, choices) choices[values.findIndex((target) | target == val)]);
thmapper: (val, values, choices) choices[values.reduce((acc,curr,i,arr) | (acc==0)||acc?acc:val<=curr?acc=i:acc=null,null)];
bitwisemask: (i,mask,op,shf) (op==="&"?parseInt(i)&mask: op==="|"?parseInt(i)|mask: op==="^"?parseInt(i)^mask:i)>>shf;
slice: (arr, init, end) arr.slice(init,end);