how to write a new processing class

Writing a Processing Class for GSN

In GSN, a processing class is a piece of code which acts in the final stage of data processing as sits between the wrapper and the data publishing engine. The processing class is the last processing stage on the data and its inputs are specified in the virtual sensor file.

All virtual sensors are subclass of the AbstractVirtualSensor (package gsn.vsensor). It requires its subclasses to implement the following three methods:

public boolean initialize();
public void dataAvailable(String inputStreamName, StreamElement se);
public void dispose();

initialize is the first method to be called after object creation. This method should configure the virtual sensor according to its parameters, if any, and return true in case of success, false otherwise. If this method returns false, GSN will generate an error message in the log and stops using the processing class hence stops loading the virtual sensor.

dispose is called when GSN destroys the virtual sensor. It should release all system resources in use by this virtual sensor. This method is typically called when we want to shutdown the GSN instance.

dataAvailable is called each time that GSN has data for this processing class, according to the virtual sensor file. If the processing class produces data, it should encapsulate this data in a StreamElement object and deliver it to GSN by calling dataProduced(StreamElement se) method.

Note that a processing class should always use the same StreamElement structure for delivering its output. Changing the structure type is not allowed and trying to do so will result in an error. However, a virtual sensor can be configured at initialization time with the kind of StreamElement it will produce (e.g., setting the output type to be the super set of all the possible outputs and providing null whenever the value is missing).

This allows to produce different types of StreamElements by the same VS depending on its usage. But one instance of the VS will still be limited to produce the same structure type. If a virtual sensor really needs to produce several different stream elements, user must provide the set of all possibilities in the stream elements and provide Null whenever the data item is not applicable.

The processing class can read the init parameters from the virtual sensor file: Example:

<class-name>gsn.vsensor.MyProcessor</class-name>
  <init-params>
    <param name="param1">DATA</param>
    <param name="param2">1234</param> 
  </init-params>		

And inside the processing class's initialization method:

String param1 = getVirtualSensorConfiguration().getMainClassInitialParams().get( "param1" );
String param2 = getVirtualSensorConfiguration().getMainClassInitialParams().get( "param2" );