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GraalVM™ Filter Component

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The Filter Component provides scripting functionalities in Kura Wires using the GraalVM™ JavaScript engine:

  • The script execution is triggered when a wire envelope is received by the filter component.
  • It is possible to access the received envelope and inspect the wire records contained in it from the script.
  • The script can optionally emit a wire envelope containing one or more wire records for each wire envelope received.
  • The script context is persisted across multiple executions, allowing to perform stateful computations like running a counter, performing time averages etc.
  • A slf4j Logger is available to the script for debugging purposes.
  • The script context is restricted to allow only Wires-related processing. Any attempt to load additional Java classes will fail.
  • The default configuration contains an example script describing the component usage, it can be executed connecting a Timer and a Logger component.

Usage

The following global variables are available to the script:

  • input: an object that represents the received wire envelope.
  • output: an object that allows to emit wire records.
  • logger: a slf4j logger.

The following utility functions are available:

  • newWireRecord(Map<String, TypedValue<?>) -> WireRecord
  • newByteArray(int) -> byte[]
  • newBooleanValue(boolean) -> TypedValue
  • newByteArrayValue(byte[]) -> TypedValue
  • newDoubleValue(number) -> TypedValue
  • newIntegerValue(number) -> TypedValue
  • newLongValue(number) -> TypedValue
  • newStringValue(object) -> TypedValue

The following global constants expose the org.eclipse.kura.type.DataType enum variants:

  • BOOLEAN
  • BYTE_ARRAY
  • DOUBLE
  • FLOAT
  • INTEGER
  • LONG
  • STRING

Received envelope

The received envelope is represented by the input global variable and is mapped in Javascript as a WireEnvelope object.

GraalVM Javascript Engine allows a 1:1 mapping of Java Objects to JavaScript ones. Hence, it is possible to access the WireRecords of the envelope and the emitter pid using the methods specified in the WireEnvelope Kura API:

logger.info('Emitter pid is {}', input.getEmitterPid())
var records = input.getRecords()

The records array can be iterated to extract the WireRecord properties (reference the WireRecord Kura API):

for(let i=0; i<records.length; i++) {
 // WireRecord.getProperties() returns a map of String - TypedValue
 for (const [keyString, typedValue] of records[i].getProperties()) {
 logger.info('The {}-th record contains:'\, String(i))
 logger.info('{}={}\n', keyString, typedValue.getValue())
 }
}

As in the example above, wireRecord.getProperties returns a map of String, TypedValue. Refer to the TypedValue Kura API for the accessible public methods list.

Creating and emitting wire records

New mutable WireRecord instances can be created using the newWireRecord(Map<String, TypedValue<?>) function. The properties of a mutable WireRecord can be modified by setting Javascript object properties. The properties of a WireRecord object must be instances of the TypedValue class created using the new<type>Value() family of functions. Setting different kind of objects as properties of a WireRecord will result in an exception.

The output global variable is an object that can be used for emitting WireRecords. This object contains a list of WireRecords that will be emitted when the script execution finishes if no exceptions are thrown. The following code is an example of how to emit a list containing a single WireRecord:

var output = new Array()
var outputMap = new Object()

var byteArray = newByteArray(4)
byteArray[0] = 1
byteArray[1] = 2
byteArray[2] = 0xaa
byteArray[3] = 0xbb

outputMap['example.integer'] = newIntegerValue(10)
outputMap['example.long'] = newLongValue(100)
outputMap['example.float'] = newFloatValue(1.014)
outputMap['example.double'] = newDoubleValue(10.12)
outputMap['example.boolean'] = newBooleanValue(true)
outputMap['example.string'] = newStringValue('Hello World!')
outputMap['example.byte.array'] = newByteArrayValue(byteArray)

output[0] = newWireRecord(outputMap)

Script context

The script.context.drop option allows to reset the script context. If set to true the script context will be dropped every time the component configuration is updated, resetting the value of any persisted variable.

In the example below, with script.context.drop=false the following script will preserve the value of counter across executions. Setting script.context.drop=true will cause counter to be undefined every time the component is triggered.

counter = typeof(counter) === 'undefined'
 ? 0 // counter is undefined, initialise it to zero
 : counter; // counter is already defined, keep the previous value

Updated 7 months ago