更好的选择

Question

I'm developing using Asp.net MVC 4, NHibernate and Session-per-request.

I have a service method which updates multiple databases so the work is wrapped in a TransactionScope. I have discovered that the NHibernate Session is not usable outside the TransactionScope due to it not being thread safe.

The code is similar to this:

public void ProcessItems()
{
  var items = itemService.GetAll();
  var mailMessages = new List<MailMessage>();
  using(var scope = new TransactionScope())
  {
    foreach(var item in items)
    {
      itemService.UpdateOne(item);
      itemService.UpdateTwo(item);
      try 
      {
        mailMessages.Add(itemService.GenerateMailMessage(item));
      }
      catch(Exception ex)
      {
        // we don't want exceptions caused be generating email to prevent DB work
        if (ex is InvalidOperationException
          || ex is NullReferenceException
          || ex is FormatException
          || ex is ArgumentException
          || ex is ItemNotFoundException)
        {
          LogError(String.Format("Unable to generate email alert for item.Id:{0} - {1}", item.Id, ex.Message), log);                                
        }
        else
        {
          // For exception types we don't know we can ignore rethrow
          throw;
        }
    }
    scope.Complete()
  }
  mailService.SendMail(mailMessages);
}

The database updates are critical to the success of the method. The email alerts are not. I don't want problems with the generation of the email alerts to prevent the database updates taking place.

My questions are:

1. Given the constraints does this look like a reasonable approach?
2. I'm worried that an exception I haven't handled may be thrown when generating the email message. This will cause the entire TransactionScope to be rolled back. It feels like I want any exception to be ignored if it happens in that try block of code. However I appreciate a catch-all is a no-no so any other suggestions for making this more robust are welcome.

EDIT

Just to clarify my question:

I know it would be better to generate and send the email after the TransactionScope. However I am unable to do this as GenerateMailMessage() makes use of the NHibernate Session which is not safe to use outside of the TransactionScope block.

I guess what I was really asking is would it be defensible to change the catch statement above to a geniune catch-all (still with logging taking place) in order to provide as much protection to the critical UpdateOne() and UpdateTwo() calls as possible?

Answer 1

Update

My advice would be to try to prevent the exception from occurring. Failing that, a catch-all is likely the only option you have remaining. Logging all exceptions is going to be critical here.

---

1st question: Your case isn't really a catch-all, you are catching all exceptions to query the type. My only advice is to log details for the exceptions you choose to consume.

2nd question: I would completely remove the generation of email from the scope if it is liable to fail. Once the transaction rolls back, all items will be rolled back too. Create and send all the emails on successful commit.

 public void ProcessItems() { var items = itemService.GetAll(); var mailMessages = new List<MailMessage>(); bool committed = false; using(var scope = new TransactionScope()) { foreach(var item in items) { itemService.UpdateOne(item); itemService.UpdateTwo(item); } scope.Complete() committed = true; } if (committed) { // Embed creation code and exception handling here. mailService.SendMail(mailMessages); } } 

Answer 2

I'd suggest changing this around. Instead of generating the email there and then... keep a list of the successfully processed items in a local List and then do all the mail sends at the end after you've committed.

public void ProcessItems()
{
  var items = itemService.GetAll();
  var successItems = new List<Item>();
  var mailMessages = new List<MailMessage>();
  using(var scope = new TransactionScope())
  {
    foreach(var item in items)
    {
      itemService.UpdateOne(item);
      itemService.UpdateTwo(item);
      successItems.Add(item);

// you still need try/catch handling for DB updates that fail... or maybe you want it all to fail.
    }
    scope.Complete()
  }

  mailMessages = successItems.Select(i => itemService.GenerateMailMessage).ToList();

  //Do stuff with mail messages

}