How to run a sub-task only on one Worker Role instance

Question

Hi,

I have two instances of a worker role.

I want to run a sub-task (on a Thread Pool thread) only on one of the Worker Role instances.

My initial idea was to do something like this:

ThreadPool.QueueUserWorkItem((o) =>
{
    if (RoleEnvironment.CurrentRoleInstance.Id == RoleEnvironment.Roles[RoleEnvironment.CurrentRoleInstance.Role.Name].Instances.First().Id)
    {
        emailWorker.Start();
    }
});

However, the above code relies on Role.Instances collection always returning the instances in the same order. Is this the case? or can the items be returned in any order?

Is there another approved way of running a task on one role instance only?

Thanks,

Answer 1

Joe, the solution you are looking for typically rely on:

• either acquiring on lease (similar to a lock, but with an expiration) on a specific blob using the Blob Storage as a synchronization point between your role instances.
• or queuing / dequeuing a message from the Queue Storage, which is usually the suggested pattern to delay long running operations such as sending an email.

Either ways, you need to go through the Azure Storage to make it work. I suggest to have a look at Lokad.Cloud, as we have designed this open-source framework precisely to handle this sort of situations.

Answer 2

If they need to be doing different things, then it sounds to me like you don't have 2 instances of a single worker role. In reality you have 2 different worker roles.

Especially when looking at the scalability of your application, processes need to be able to run on more than one instance. What happens when that task that you only want to run on one role gets large enough that it needs to scale to 2 or more role instances?

One of the benefits of developing for Azure is that you get scalability automatically if you design your app properly. If makes you work extra to get something that's not scalable, which is what you're trying to do.

What triggers this task to be started? If you use a message on Queue Storage (as suggested by Joannes) then only one worker role will pick up the message and process it and it doesn't matter which instance of your worker role does that.

So for now if you've got one worker role that's doing the sub task and another worker role that's doing everything else, just add 2 worker roles to your Azure solution. However, even if you do that, the worker role that processes the sub task should be written in such a way that if you ever scale it to run more than a single instance that it will run properly. In that case, you might as well stick with the single worker role and code for processing messages off the Queue to start your sub task.