I would do it so that you just refresh some kind of "lastseen" value in the db, which you can later use to determine, how long it's been offline..

Set a timeout every time a ping is received. Cancel previous timeouts. Note... you might want to add a fudge factor to your timeout to reduce noise)

 var timerId;
 function ping(deviceId) {
     clearTimeout(timerId);
     timerId = setTimeout(function() {
         updateDb(deviceId);
     }, 15000 + 500);
 }

I'm doing something similar to you but will have 10,000 devices pinging every 15 minutes. I'll be using a different solution. I have a cron job (technically a Cloudwatch Event) that runs every 5 minutes to check which devices haven't pinged within the last 15 minutes.

that seams to be a job for a socket. 15 seconds no event means setTimeout gets triggered :)

or do you need a more complex mechanism ? for me it's the classic "delay event" principle.