Question about Task's Fork Bomb

[BarrenTeam]

Hi, we intensly use tasks in our projects. In our last project we have several problems with task's fork bomb [a task A enqueue a task B then task A fails, when it's retried enqueue again task B and so on]

Suppose to have the following java code:

public class Task1 extends HttpServlet{

@Override

public void doPost(HttpServletRequest request, HttpServletResponse response){

//Datastore reads and writes

Queue queue= QueueFactory.getDefaultQueue();

queue.add(withUrl("/anothertask"));

}

}

We wonder if it's possible that a task's fork bomb can happen in this exemple.

If yes, we would like to know why it can happen.

P.s: In our project we have a lot of code like this above. We have encountered the fork bomb when the task throw a deadlineExceededException or a Datastore Exception

Thanks and sorry for our english :D

[Noah McIlraith]

You are supposed to design tasks with some level of idempotency, there are cases where tasks are executed more than once, which will also lead to a fork bomb if not coded properly.

You are supposed to use task names to prevent forks, as task names are unique, so a task can't be inserted twice (blocking a fork bomb).

[vlad]

Note that task naming only works for non-transactional tasks. For transactional tasks you have to come up with some kind of Datastore based "run-once" primitive. There is not a standard workaround for transactional task insertion.

[BarrenTeam]

Thanks for the answers. I already know the mechanism for avoid bomb fork. But my question is: WHY and HOW can task's bomb fork happen in the code above?

[Robert Kluin]

You've not given the code for what ever inserts task1, or the task it inserts.  So the isse could be in several spots.  But from your code, if task1 happens to run twice, either due to a failure or because it decided to run twice, then you get two of the "/anothertask" tasks running. If anothertask also inserts a task... Then you've got a setup with potential problems.

Robert

On Jun 18, 2011, at 5:38, BarrenTeam <barre...@gmail.com> wrote:

Thanks for the answers. I already know the mechanism for avoid bomb fork. But my question is: WHY and HOW can task's bomb fork happen in the code above?

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