I need to create a notification service but can't decide the architecture

Question

I am creating a service in which i am asking user about a particular time of the day for example a user selects 7:15 PM , then user will be notified at this particular time everyday. Each user can select any time of the day and the time is stored in a NoSQL DB along with other user details, Now the problem is i need to scan database each minute to find out every user's specified time which is not ideal and very expensive. I cannot think of any other architecture,i cannot use local notifications of the phones, notifications has to be remote i.e. sent from a backend server.

Thanks for any help you could offer.

Answer 1

If you have a system where you can keep data in memory, you can implement a priority queue quite cheaply - such as what I've shown for Firebase Realtime Database in this example: https://stackblitz.com/edit/ttl

The key points for such a structure are:

• You keep all tasks in an ordered list, and then set a timer only for the first task in the list.
• If the timer expires, you execute the task and delete it.
• If an item comes in your add it in the right spot in the list.
• Either of the last two points can/will lead to a new first item in the list, so you set the timer for this new first item.

The main code is:

function checkFirstAndTrigger() {
  clearTimeout(timer); // clear any potentially pending timer, since we may have been interrupted
  if (first && first.exists()) {
    const deltaMs = first.child(ttlProp).val() - Date.now();
    // if the item has to be triggered now (or before now), do so
    if (deltaMs <= 0) {
      first.ref.remove(); // this will retrigger the child_added listener below
    }
    else {
      console.log(`Waiting for ${deltaMs}ms`)
      // we were triggered prematurely, wait until it's our time (or until we're interrupted)
      timer = setTimeout(checkFirstAndTrigger, deltaMs);
    }
  }
}

// Set a trigger for the first item/task in the list. This gets triggered 
// when a new first item is added, or when the current first item gets deleted.
queue.orderByChild(ttlProp).limitToFirst(1).on('child_added', (snapshot) => {
  // a new item became the first in the queue, either because a new one was added or because the previous first got deleted
  first = snapshot;
  checkFirstAndTrigger();
});