Writing a custom condition in WSO2 CEP with left and right argument

Question

I want to extend the Wso2 CEP product in our needs and try to write a custom condition as indicated in this official wso2 cep link.

I am able to write an extension class that extends "org.wso2.siddhi.core.executor.conditon.AbstractGenericConditionExecutor" and implement its abstract method as indicated below:

    @SiddhiExtension(namespace = "myext", function = "startswithA")
public class StringUtils extends
        org.wso2.siddhi.core.executor.conditon.AbstractGenericConditionExecutor {

    static Log log = LogFactory.getLog(StringUtils.class);

    @Override
    public boolean execute(AtomicEvent atomicEvent) {
        log.error("Entered the execute method");
        log.error("Atomic event to string: " + atomicEvent.toString());

        return true;
    }
}

when i use this extensioned method as:

from allEventsStream[myext:startswithA(name)]
insert into selectedEventsStream *;

In this situation, i want that startswithA method returns true if the name field has 'A' at the begining of it. However when i run this query in CEP the whole event drops into my execute function i.e. there is no sign to show that i send "name" field is sent to startswithA method as argument.

How can i understand which field of the stream is sent to my extended method as argument?

Also i want to write conditions like

from allEventsStream[myext:startswith('A', name)]
insert into selectedEventsStream *;

How can i achive this?

Answer 1

In 'AbstractGenericConditionExecutor' there's another method that gives you the set of expression executors that are included in the parameters when executor instantiates:

public void setExpressionExecutors(List<ExpressionExecutor> expressionExecutors)

You don't necessarily have to override this method and store the list, it is already stored there in the 'AbastractGenericConditionExecutor' as a list named expressionExecutors. You can pass the event to these executors to retrieve the relevant values from the event in order.

For an example, if you include a variable (like 'name') in the query (as a parameter at index 0), you'll get a 'VariableExpressionExecutor' in the list at index 0 that will fetch you the value of the variable from the event. Similarly for a constant like 'A', you'll get a different executor that will give you the value 'A' when called.

Answer 2

To add to Rajeev's answer, if you want to filter all the names that starts with 'A', you can override the execute method of your custom Siddhi extension similar to the following segment.

@Override
public boolean execute(AtomicEvent atomicEvent) {
    if(!this.expressionExecutors.isEmpty()) {
        String name = (String)this.expressionExecutors.get(0).execute(atomicEvent);
        if(name.startsWith("A")) {
            return true;
        }
    }
    return false;
}

When writing the query, it would be similar to

from allEventStream[myext:startsWithA(name)]
insert into filteredStream *;

You can extend this behaviour to achieve an extension that supports

from allEventsStream[myext:startswith('A', name)]

type queries as well.

HTH,

Lasantha