Chapter 4. Requirements for HA CEP client and server code

When developing client and server code for high-availability CEP, you must follow certain additional requirements.

kie-remote API

The client code must use the kie-remote API and not the kie API. The kie-remote API is specified in the org.kie:kie-remote Maven artifact. You can find the source code in the kie-remote Maven module.

Explicit timestamps

The decision engine needs to determine the sequence in which events happen. For this reason, every event must have an associated timestamp assigned to it. In a high-availability environment, make this timestamp a property of the JavaBean that models the event. You must then annotate the event class with the @Timestamp annotation, where the name of the timestamp attribute itself is the parameter, as in the following example:

@Role(Role.Type.EVENT)
@Timestamp("myTime")
public class StockTickEvent implements Serializable {

    private String company;
    private double price;
    private long myTime;
}

@Role(Role.Type.EVENT)
@Timestamp("myTime")
public class StockTickEvent implements Serializable {

    private String company;
    private double price;
    private long myTime;
}

If you do not provide a timestamp attribute, Drools assigns a timestamp to every event based on the time when the event is inserted by the client into a remote session. However, this mechanism depends on the clocks on the client machines. If clocks between different clients diverge, inconsistencies can occur between events inserted by these hosts.

Lambda expressions for non-memory actions

Working memory actions (actions to insert, modify, or delete information in the working memory of the decision engine) must be processed on every node of the cluster. Actions that are not memory actions must be executed only on the leader.

For example, the code might include the following rule:

rule FindAdult when
  $p : Person(age >= 18)
then
  modify($p) { setAdult(true) }; // working memory action
  sendEmailTo($p); // side effect
end

rule FindAdult when
  $p : Person(age >= 18)
then
  modify($p) { setAdult(true) }; // working memory action
  sendEmailTo($p); // side effect
end

When this rule is triggered, the person must be marked as an adult on every node. However, only the leader must send the email, so that only one copy of the email is sent.

Therefore, in your code, you must wrap the email action (called a side effect) in a lambda expression, as shown in the following example:

rule FindAdult when
  $p : Person(age >= 18)
then
  modify($p) { setAdult(true) };
  DroolsExecutor.getInstance().execute( () -> sendEmailTo($p) );
end

rule FindAdult when
  $p : Person(age >= 18)
then
  modify($p) { setAdult(true) };
  DroolsExecutor.getInstance().execute( () -> sendEmailTo($p) );
end