The IoTAgent 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.
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}"
},
{
"object_id":"tt",
"name":"temperature",
"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).
The exact same syntax works for Configuration and Device provisioning.
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:
${@humidity * 10}
will give 500 as the result (i.e.: the value '50' is cast to number, to get 50, that is then multiplied by 10). If
this cast fails (because the value of the variable is not a number, e.g.: 'Fifty'), the overall result will be NaN.
Whenever a new measurement arrives to the IoTAgent for a device with declared expressions, all of the expressions for the device will be checked for execution: for all the defined active attributes containing expressions, the IoTAgent will check which ones contain expressions whose variables are present in the received measurement. For all of those whose variables are covered, their expressions will be executed with the received values, and their values updated in the Context Broker.
E.g.: if a device with the following provisioning information is provisioned in the IoTAgent:
{
"name":"location",
"type":"geo:point",
"expression": "${latitude}, ${longitude}"
},
{
"name":"fillingLevel",
"type":"Number",
"expression": "${@fillingLevel / 100}",
"cast": "Number"
},
and a measurement with the following values arrive to the IoTAgent:
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.
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 a 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 casted to Number), the measurement update will fail, and an error will be reported to the device.
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.
The expression language allows for two kinds of constants:
- Numbers (integer or real)
- Strings (marked with double quotes)
Current allowed characters are:
- All the alphanumerical characters
- Whitespaces
The following operations are currently available, divided by attribute type
- multiplication ('*')
- division ('/')
- addition ('+')
- substraction ('-' binary)
- negation ('-' unary)
- power ('^')
- 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 posisitonstartand ending at position<end>. - whitespace removal (
trim(<string>)): removes all the spaces surrounding the string passed as a parameter.
Parenthesis can be used to define precedence in the operations. Whitespaces between tokens are generally ignored.
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 |
|---|---|
| '5 * @value' | 30 |
| '(6 + @value) * 3' | 36 |
| '@value / 12 + 1' | 1.5 |
| '(5 + 2) * (@value + 7)' | 91 |
| '@value * 5.2' | 31.2 |
| '"Pruebas " + "De Strings"' | 'Pruebas De Strings' |
| '@name value is @value' | 'DevId629 value is 6' |