Simulating Stochastic Models

Recently I’ve been tasked with working on an infrastructure system which comprises several computationally intensive tasks that can be scheduled on a series of host machines. Some tasks have dependencies on other tasks, and cannot be scheduled before its dependencies have finished. In a simplified setup we assume that any task can run on any machine, provided that machine is not busy with running another task.

The following picture illustrates the system

Host A runs task 1 and 3, while host B runs task 2 and 4. Task 4 has a dependency on task 1 and task 3 has one on task 2. When all tasks are completed the job is considered completed. It is evident from the picture that a task (task 3) may be delayed in its scheduling even though its dependencies are satisfied due to resource constraints.

Applying Model-Based Testing for Fault-Injection in cloud services – Part III

2-services model

Interestingly enough, when modeling more than one service, the model will also generate tests that verify services continue functioning even when others go down. Essentially what we are checking here is that we don’t have any unnecessary dependencies between the services, and that a service won’t hang because another one is down.

When designing models including more than one service the state-space can rapidly explode. Consider adding just another service: Payment Service into the model which we depend on upon check-out.

We extend our model by adding:

    public class PaymentService : ModelService

    {

        public override string Name { get { return "Payment"; } }

        public void ProcessPayment()

        {

            ConditionRunning();

        }

    }

And changing the model to:

    static class WebshopModelProgram

    {

        static ShoppingCartService cartService = new ShoppingCartService();

        static PaymentService paymentService = new PaymentService();

        [Rule(Action = "AddToCart()")]

        static void AddToCart()

        {

            cartService.AddItem();

        }

        [Rule(Action = "ProceedToCheckOut()/result")]

        static bool ProceedToCheckOut()

        {

            if(cartService.Count() == 0)

                return false;

            cartService.Empty();

            paymentService.ProcessPayment();

            return true;

        }