C# - Parallel.Foreach() 用于服务调用

Question

I have a console application which does 3 steps as mentioned below,

1. Get pending notification records from db
2. Call service to send email (service returns email address as response)
3. After getting service response, update the db

For step 2, I am using Parallel.Foreach() and it is working far better than foreach() . I have gone through lot of articles and threads on stackoverflow which is causing more confusion on this topic. I have few questions

1. I am running this on a server, does it affect the performance and should I limit the number of threads? (The email count can be from 0-500 or 1000)
2. I ran into one issue, where in step 2, the service returned an email address as response but it was not available while updating the db. (email count here was 400) I am suspecting that the issue could be because of using parallel.foreach and that it did not add in notifList . If this is the case, can I add Thread.Sleep(1000) after Parallel.Foreach() loop ends, does it fix the issue?
3. In case of any exception, should I explicitly cancel the threads?

Appreciate your time and effort on helping me with this. Thank you!

public void notificationMethod()
{
    List<notify> notifList = new List<notify>();

    //step 1
    List<orders> orderList = GetNotifs();

    try
    {
        if (orderList.Count > 0)
        {
            Parallel.ForEach(orderList, (orderItem) =>
            {
               //step 2
               SendNotifs(orderItem);
                notifList.Add(new notify()
               {
                   //building list by adding email address along with other information
               });
            });
            if (notifList.Count > 0)
            {
                int index = 0; 
                int rows = 10; 
                int skipRows = index * rows;
                int updatedRows = 0;
                while (skipRows < notifList.Count)
                {
                    //pagination 
                    List<notify> subitem = notifList.Skip(index * rows).Take(rows).ToList<notify>();
                    updatedRows += subitem.Count;

                    //step 3
                    UpdateDatabase(subitem);

                    index++;
                    skipRows = index * rows;
                }
            }
        }
    }
    catch (ApplicationException ex)
    {
    }            
}

Answer 1

I also had a similar scenario regarding whether using Parallel.ForEach() would help improve the performance. But when I saw the below video from Microsoft, it gave me an idea to select Parallel.ForEach() only for CPU intensive workloads. In this case, your scenario will fall into I/O intensive workloads and could be handled better by async / await .

https://channel9.msdn.com/Series/Three-Essential-Tips-for-Async/Tip-2-Distinguish-CPU-Bound-work-from-IO-bound-work