round robin scheduling java iterators

Question

I have a list of hosts in an array which represnt the servers available to do a particular job. Currently I simply iterate thru the list looking and establish comms with a host to check its not busy. If not I will send a job to it. This approach tends to mean that the first host in the list tends to get hot constanly with the load not balanced properly with the rest of the available hosts.

in pseudocode ..

for (Host h : hosts) {

    //checkstatus
    if status == job accepted break;

}

I'd like to balance this load properly between the hosts i.e first time host one is used 2nd time the method is used host 2. Just wondering that the most elegant solution to this is ??

Thanks W

Answer 1

You can create a new kind of Iterable that provides round-robin iteration:

public class RoundRobin<T> implements Iterable<T> {
  private List<T> coll;
  private int index = 0;

  public RoundRobin(List<T> coll) { this.coll = coll; }

  public Iterator<T> iterator() { 
     return new Iterator<T> {
        private T t;

        public T next() {
           T res = t;
           index = (index + 1) % coll.size();
           t = coll.get(index);
           return res;
        }
        public boolean hasNext() { return true; }
        public void remove() { throw new UnsopportedOperationException(); }
     };
  }

}

You need to define your hosts as RoundRobin<Host>

Answer 2

If the list is mutable and the cost of editing it is negligible compared to I/O with the hosts, you can just rotate it:

List<String> list = Arrays.asList("one", "two", "three");
Collections.rotate(list, -1);
System.out.println(list);

Answer 3

Google collections has a utility method Iterators.cycle(Iterable<T> iterable) that does what you want.

Answer 4

If you are creating an Iterator, best to create a defensive copy first and have the iterator work on that.

return new MyIterator(ImmutableList.<T>of(list));