Documentation Version: 3.4 - Publication Date : Dec 24, 2014

Execution Cancellation

A task in the code you execute in a cluster might take longer than expected. If you cannot stop/cancel that task, it will keep eating your resources. The standard Java executor framework solves this problem with the cancel() API and by encouraging us to code and design for cancellations. That is a highly ignored part of software development.

public class Fibonacci<Long> implements Callable<Long>, Serializable {
  int input = 0; 

  public Fibonacci() { 
  } 

  public Fibonacci( int input ) { 
    this.input = input;
  } 

  public Long call() {
    return calculate( input );
  }

  private long calculate( int n ) {
    if ( Thread.currentThread().isInterrupted() ) {
      return 0;
    }
    if ( n <= 1 ) {
      return n;
    } else {
      return calculate( n - 1 ) + calculate( n - 2 );
    }
  }
}

The Fibanacci callable class above calculates the Fibonacci number for a given number. In the calculate method, we check if the current thread is interrupted so that the code can respond to cancellations once the execution is started. The fib() method below submits the Fibonacci calculation task for number 'n' and waits a maximum of 3 seconds for the result. If the execution does not completed in 3 seconds, future.get() will throw a TimeoutException and upon catching it, we cancel the execution, saving some CPU cycles.

long fib( int n ) throws Exception {
  HazelcastInstance hazelcastInstance = Hazelcast.newHazelcastInstance();
  IExecutorService es = hazelcastInstance.getExecutorService();
  Future future = es.submit( new Fibonacci( n ) );  
  try {
    return future.get( 3, TimeUnit.SECONDS );
  } catch ( TimeoutException e ) {
    future.cancel( true );            
  }
  return -1;
}

fib(20) will probably take less than 3 seconds. However, fib(50) will take much longer. (This is not an example for writing better Fibonacci calculation code, but for showing how to cancel a running execution that takes too long.) The method future.cancel(false) can only cancel execution before it is running (executing), but future.cancel(true) can interrupt running executions if your code is able to handle the interruption. If you are willing to cancel an already running task, then your task should be designed to handle interruptions. If the calculate (int n) method did not have the (Thread.currentThread().isInterrupted()) line, then you would not be able to cancel the execution after it is started.