Question

All i am using a quartz schedular for scheduling a job in an asp.net mvc application.This schedular schedules a job after fixed interval of time.

http://quartznet.sourceforge.net/

The service i have created basically runs every minute.It reads the message from the
message que(database in my case) every 1min , sends an email and updates the message sent status
to true.
I am having some problems though.TO be specific the problem is the service sends the same email twice because of the reasons mentioned below.

In some cases the service gets called as soon as an email is send before the db update happens.As The database update does not happen after sending email and service is invoked again,the processed message is again read from the database as unread message and gets resent.

The same message is read again from database.Thus the service ends of sending same message twice.

How do i handle this case in my code.

public void Execute(JobExecutionContext context)
        {
            List<QueuedEmail> lstQueuedEmail =
                _svcQueuedEmail.Filter((x => x.IsSent == false)).Take(NO_OF_MAILS_TO_SEND).ToList();
            if (lstQueuedEmail.Count > 0)
            {
                foreach (var queuedEmail in lstQueuedEmail)
                {
                    try
                    {
                        bool emailSendStatus = false;

                        emailSendStatus = EmailHelper.SendEmail(queuedEmail.From, queuedEmail.To, queuedEmail.Subject,
                                                                queuedEmail.Body, queuedEmail.FromName);
                        QueuedEmail objQueuedEmail =
                            _svcQueuedEmail.Filter(x => x.Id == queuedEmail.Id).FirstOrDefault();
                        if (emailSendStatus)
                        {
                            objQueuedEmail.IsSent = true;
                            objQueuedEmail.SentOnUtc = DateTime.UtcNow;
                        }
                        else
                        {
                            objQueuedEmail.IsSent = false;
                            if (objQueuedEmail.SentTries == null)
                            {
                                objQueuedEmail.SentTries = 1;
                            }
                            else
                            {
                                objQueuedEmail.SentTries += 1;
                            }
                        }
                        _svcQueuedEmail.Update(objQueuedEmail);
                    }
                    catch (Exception)
                    {
                        //log error 
                    }
                }
            }
        }

Answer 1

Assuming you have two states for an email: “Pending” and “Sent”.

You should add a third an intermediary state called “Sending” and as soon as you read the email from the Queue you should change it’s status to something like “Executing” so other threads/services won’t get it again.