Writing a Custom Transformer

To write a custom transformer, create a class extending "org.wso2.siddhi.core.query.processor.transform.TransformProcessor" add the SiddhiExtension Annotation, compile that class, and add the jar to the class path <CEP_HOME>/repository/components/lib. Then add the fully-qualified class name for the implementation class in a new line, to the siddhi.extension file located at <CEP_HOME>/repository/conf/siddhi.

For example, if you have created the extension with namespace debs and function name getVelocities, you can use that in a query as follows:

from debsStream#transform.debs:getVelocities(v,vx,vy,vz) 
select velocityX, velocityY, velocityZ 
insert into velocityStream;

Given below is a sample code that implements the above function.

package org.wso2.siddhi.extension;

import org.apache.log4j.Logger;
import org.wso2.siddhi.core.config.SiddhiContext;
import org.wso2.siddhi.core.event.Event;
import org.wso2.siddhi.core.event.in.InEvent;
import org.wso2.siddhi.core.event.in.InListEvent;
import org.wso2.siddhi.core.event.in.InStream;
import org.wso2.siddhi.core.executor.expression.ExpressionExecutor;
import org.wso2.siddhi.core.query.processor.transform.TransformProcessor;
import org.wso2.siddhi.query.api.definition.Attribute;
import org.wso2.siddhi.query.api.definition.StreamDefinition;
import org.wso2.siddhi.query.api.expression.Expression;
import org.wso2.siddhi.query.api.expression.Variable;
import org.wso2.siddhi.query.api.extension.annotation.SiddhiExtension;
import java.util.HashMap;
import java.util.List;
import java.util.Map;

@SiddhiExtension(namespace = "debs", function = "getVelocities")
public class VelocityTransformProcessor extends TransformProcessor {
    private Map<String, Integer> paramPositions = new HashMap<String, Integer>();
    public VelocityTransformProcessor() {
        this.outStreamDefinition = new StreamDefinition().name("velocityStream")
                .attribute("velocityX", Attribute.Type.DOUBLE)
                .attribute("velocityY", Attribute.Type.DOUBLE)
                .attribute("velocityZ", Attribute.Type.DOUBLE);
    }
    @Override
    protected InStream processEvent(InEvent inEvent) {
        double vMagnitude = (Double) inEvent.getData(paramPositions.get("v"));
        double vxComponent = (Double) inEvent.getData(paramPositions.get("vx"));
        double vyComponent = (Double) inEvent.getData(paramPositions.get("vy"));
        double vzComponent = (Double) inEvent.getData(paramPositions.get("vz"));
        Object[] data = new Object[]{vMagnitude * vxComponent / 10000000000L,
                vMagnitude * vyComponent / 10000000000L,
                vMagnitude * vzComponent / 10000000000L};
        return new InEvent(inEvent.getStreamId(), System.currentTimeMillis(), data);
    }
    @Override
    protected InStream processEvent(InListEvent inListEvent) {
        InListEvent transformedListEvent = new InListEvent();
        for (Event event : inListEvent.getEvents()) {
            if (event instanceof InEvent) {
                transformedListEvent.addEvent((Event) processEvent((InEvent) event));
            }
        }
        return transformedListEvent;
    }
    @Override
    protected Object[] currentState() {
        return new Object[]{paramPositions};
    }
    @Override
    protected void restoreState(Object[] objects) {
        if (objects.length > 0 && objects[0] instanceof Map) {
            paramPositions = (Map<String, Integer>) objects[0];
        }
    }
    @Override
    protected void init(Expression[] parameters, List<ExpressionExecutor> expressionExecutors, StreamDefinition inStreamDefinition, StreamDefinition outStreamDefinition, String elementId, SiddhiContext siddhiContext) {
        for (Expression parameter : parameters) {
            if (parameter instanceof Variable) {
                Variable var = (Variable) parameter;
                String attributeName = var.getAttributeName();
                paramPositions.put(attributeName, inStreamDefinition.getAttributePosition(attributeName));
            }
        }
    }
    @Override
    public void destroy() {
    }
}

To make CEP aware of the above extension, the following line should go into the <CEP_HOME>/repository/conf/siddhi/siddhi.extension file.

org.wso2.siddhi.extension.VelocityTransformProcessor