When writing a custom function you simply have to
- Extend FunctionExecutor class.
- Create the corresponding .siddhiext extension mapping. Name of the extension mapping file should be in the format of <Namespace>.siddhiext. File should contain the function name and the full qualified class name of the extension class.
- Compile the class, build the jar containing the .class and .siddhiext files. Add them to the Siddhi class path
My custom function name is
plus and namespace is
custom.
package sample.query.extension;
public class PlusFunctionExtension extends FunctionExecutor {
@Override
protected void init(ExpressionExecutor[] attributeExpressionExecutors, ExecutionPlanContext executionPlanContext) {
for (ExpressionExecutor expressionExecutor : attributeExpressionExecutors) {
Attribute.Type attributeType = expressionExecutor.getReturnType();
if (attributeType == Attribute.Type.STRING || attributeType == Attribute.Type.BOOL || attributeType == Attribute.Type.OBJECT) {
throw new ExecutionPlanCreationException("Plus cannot have parameters with types String or Bool or object");
}
}
}
@Override
protected Object execute(Object[] data) {
double sum = 0;
for (Object obj:data) {
sum= sum+((Number) obj).doubleValue();
}
return sum;
}
@Override
protected Object execute(Object data) {
return data;
}
@Override
public void start() {
}
@Override
public void stop() {
}
@Override
public Attribute.Type getReturnType() {
return Attribute.Type.DOUBLE;
}
@Override
public Object[] currentState() {
return null;
}
@Override
public void restoreState(Object[] state) {
}
}
Name of my mapping file would be custom.siddhiext having the below entry
plus=sample.query.extension.PlusFunctionExtension
This extension can be referred in siddhi query as follows
from cseEventStream
select symbol, custom:plus(price, tax) as total
insert into outputStream;
Now you have seen a sample function extension I'll briefly explain the methods that have to be implemented when creating a custom function.
init : attributeExpressionExecutors related to the parameters of the function is passed here. In the above function, attributeExpressionExecutors related to price and tax attributes are passed to the init method.
execute : this method contains the actual functionality. Values of the attributes passed in the query are be passed as an object array.
start and stop : these methods are not necessary for this function. If you want to create any connections such as database connections you may need the start method and the stop can be used if you want to close the connection.
