Push notifications and the endless quest for "rightness"

I recently was tasked with getting a push notification system up and running (e.g. the "unseen" badge count and messaging you see in lots of mobile apps). We had a fairly simple polling-based notification system that worked quite well but we really wanted that next level of connectedness that you get from instantaneous notifications, even when the app is "closed".

My attempt to get this working basically extended the existing system which used a count and some Booleans. After countless hours of trying to get this sync right, I came to a realization:

Trying to keep state by passing Booleans back and forth is like flinging paint at a wall and expecting the Mona Lisa to appear.

I had seenLatest, I had hasChanges and even resorted to forceUpdate. There was always a corner-case or timing/sequencing condition that would trip it up. And then I realized that I needed to embrace timing. Each syncable thing has a lastUpdated time and each client has a lastSaw time.

Key for getting this working was remembering to think about this from a user's point of view - they may well be logged in on three devices (aka clients) but once they have seen a notification on any device they don't want to see it again.

The pseudocode for this came down to:

Polling Loop / When a thing changes

  userLastSawTimestamp = max(clientLastSawTimestamps)
  
  if (thing.lastUpdated > userLastSawTimestamp) {
    showNotification(thing)
  }

On User Viewing Thing (i.e. clearing the badge)

  clientLastSawTimestamp[clientId] = now

And now it works how it should.

And I see failed attempts to do it correctly everywhere I look ... :-)