How Google App Engine Java Task Queues can be used for mass scheduling for users?

I am focusing GAE-J for developing a Java web application.

I have a scenario where user will create his schedule for set of reminders. And I have to send emails on that particular date/time.

I can not create thread on GAE. So I have the solution of Task Queues.

So can I achieve this functionality with Task Queues. User will create tasks. And App Engine will execute it on specific date and time.

Thanks

Although using the task queue directly, as Chris suggests, will work, for longer reminder periods (eg, 30+ days) and in cases where the reminder might be modified, a more indirect approach is probably wise.

What I would recommend is storing reminders in the datastore, and then taking one of a few approaches, depending on your requirements:

• Run a regular cron job (say, hourly) that fetches a list of reminders coming up in the next interval, and schedules task queue tasks for each.
• Have a single task that you schedule to be run at the time the next reminder (system-wide) is due, which sends out the reminder(s) and then enqueues a new task for the next reminder that's due.
• Run a backend, as Chris suggests, which regularly scans the datastore for upcoming reminders.

In all the above cases, you'll probably need some special case code for when a user sets a reminder in less than the minimum polling interval you've set - probably enqueuing a task directly. You'll also want to consider batching up the sending of reminders, to minimize tasks and wallclock time consumed.

You can do this with Task Queues - basically when you receive the request 'remind me at date/time X by sending an email', you create a new task with the following basic structure:

if current time is close to or past the given date/time X:
  send the email
else
  fail this task

If the reminder time is far in the future, the first few times the task is scheduled, it will fail and be scheduled for later. The downside of this approach is that it doesn't guarantee that the task will run exactly when the reminder is supposed to be sent - it may be a little while before or afterwards. You could slim down this window by taking into account that your task can run for 10 minutes, so if you're within 10 minutes of the reminder time, sleep until the right time and then send the e-mail.

If the reminders have to be sent out as close in time as possible then just use a Backend - keep an instance running forever and dispatch all reminders to it, and it can continuously look at all reminders it has to send out and send them out at exactly the right time.