7.3. Clock Implementation in Complex Event Processing

7.3.1. Session Clock

Events have strong temporal constraints making it is necessary to use a reference clock. If a rule needs to determine the average price of a given stock over the last sixty minutes, it is necessary to compare the stock price event's timestamp with the current time. The reference clock provides the current time.
Because the rules engine can simultaneously run an array of different scenarios that require different clocks, multiple clock implementations can be used by the engine.
Scenarios that require different clocks include the following:
  • Rules testing: Testing always requires a controlled environment, and when the tests include rules with temporal constraints, it is necessary to control the input rules, facts, and the flow of time.
  • Regular execution: A rules engine that reacts to events in real time needs a real-time clock.
  • Special environments: Specific environments may have specific time control requirements. For instance, clustered environments may require clock synchronization or JEE environments may require you to use an application server-provided clock.
  • Rules replay or simulation: In order to replay or simulate scenarios, it is necessary that the application controls the flow of time.

7.3.2. Available Clock Implementations

JBoss BRMS Complex Event Processing comes equipped with two clock implementations:
Real-Time Clock
The real-time clock is the default implementation based on the system clock. The real-time clock uses the system clock to determine the current time for timestamps.
To explicitly configure the engine to use the real-time clock, set the session configuration parameter to realtime: 
import org.kie.api.KieServices.Factory;
import org.kie.api.runtime.KieSessionConfiguration;

KieSessionConfiguration config = KieServices.Factory.get().newKieSessionConfiguration()
   config.setOption( ClockTypeOption.get("realtime") );
Pseudo-Clock
The pseudo-clock is useful for testing temporal rules since it can be controlled by the application.
To explicitly configure the engine to use the pseudo-clock, set the session configuration parameter to pseudo: 
import org.kie.api.runtime.KieSessionConfiguration;
import org.kie.api.KieServices.Factory;

KieSessionConfiguration config = KieServices.Factory.get().newKieSessionConfiguration();
   config.setOption( ClockTypeOption.get("pseudo") );
This example shows how to control the pseudo-clock: 
import org.kie.api.runtime.KieSessionConfiguration;
import org.kie.api.KieServices.Factory;
import org.kie.api.runtime.KieSession;
import org.kie.api.time.SessionClock;
import org.kie.api.runtime.rule.FactHandle;

KieSessionConfiguration conf = KieServices.Factory.get().newKieSessionConfiguration();
   conf.setOption( ClockTypeOption.get( "pseudo" ) );
   KieSession session = kbase.newKieSession( conf, null );

   SessionPseudoClock clock = session.getSessionClock();

   // then, while inserting facts, advance the clock as necessary:
   FactHandle handle1 = session.insert( tick1 );
   clock.advanceTime( 10, TimeUnit.SECONDS );
   FactHandle handle2 = session.insert( tick2 );
   clock.advanceTime( 30, TimeUnit.SECONDS );
   FactHandle handle3 = session.insert( tick3 );