once("child_added") on non existing node does not fail but timeout

Question

First I'm not sure that there is a real problem but I guess I'll share my reasoning.

I use Firebase as a database / backend for the archiving of all the data from various sensors at home and an UI with cool graphs in hosting. So every 10 minutes I push various data (temperature, humidity, CO2 level, illumination, ...) coming from various rooms. I have almost 3 years of data available (so my base has a lots of nodes)

So my database structure is like that :

root
  readings
    room_id
      GUID
        time
        temp
        hum
        lum

For a few years I had a PHP script hosted at home that checked if the latest item inside each readings/room_id has a time value that is not too old (no more than 11 minutes old). I translated it to Firebase cloud function some days ago and I got something like this :

exports.monitor = functions.https.onRequest((req, res) => {
  const tstamp = Math.floor(Date.now() / 1000);
  var sensors = ["r01", "r02", "r03", "r04", "r05"];
  var promiseArray = [];
  var result = {};
  for (var i = 0; i < sensors.length; i++) {
    console.log('Adding promise for ' + sensors[i]);
    promiseArray.push(admin.database().ref('/readings/' + sensors[i]).limitToLast(1).once("child_added"));
  }
  Promise.all(promiseArray).then(snapshots => {
    console.log('All promises done : ' + snapshots.length);
    res.set('Cache-Control', 'private, max-age=300');
    for (var i = 0; i < snapshots.length; i++) {
      differenceInMinutes = (tstamp - snapshots[i].val().time) / 60;
      result[sensors[i]] = {current: tstamp,
                            sensor: snapshots[i].val().time,
                            diff: Math.round(differenceInMinutes * 10) / 10};
      if (differenceInMinutes < 11) {
        result[sensors[i]]['status'] = "OK";
      } else {
        result[sensors[i]]['status'] = "KO";
      }
    }
    return res.status(200).json(result);
  }).catch(error => {
    console.error('Error while getting sensors details', error.message);
    res.sendStatus(500);
  });
});

The code works well. So my question is : if I add another room ID in the sensors array that does not exists inside "readings" in my database, I thought I'll get an error (failed promise) instead I only got a huge timeout error, I don't want that kind of timeout on Firebase Cloud Functions (to avoid any unwanted cost).

Is that normal ? Is my code wrong ? Do I have to start by getting a shallow snapshot of "readings/room_id" check that it exists and check if has children ?

Thanks a lot for your help.

EDIT : With the help of Frank I fixed my code, here is the revised version :

exports.monitor = functions.https.onRequest((req, res) => {
  const tstamp = Math.floor(Date.now() / 1000);
  var sensors = ["r01", "r02", "r03", "r04", "r05"];
  var promiseArray = [];
  var result = {};
  for (var i = 0; i < sensors.length; i++) {
    console.log('Adding promise for ' + sensors[i]);
    promiseArray.push(admin.database().ref('/readings/' + sensors[i]).limitToLast(1).once("value"));
  }
  Promise.all(promiseArray).then(queryResults => {
    console.log('All promises done : ' + queryResults.length);
    res.set('Cache-Control', 'private, max-age=300');
    queryResults.forEach((snapshots, i) => {
      snapshots.forEach((snapshot) => {
        var currentData = snapshot.val();
        differenceInMinutes = (tstamp - currentData.time) / 60;
        result[sensors[i]] = {current: tstamp,
                              sensor: currentData.time,
                              diff: Math.round(differenceInMinutes * 10) / 10};
        if (differenceInMinutes < 11) {
          result[sensors[i]]['status'] = "OK";
        } else {
          result[sensors[i]]['status'] = "KO";
        }
      });
    });
    return res.status(200).json(result);
  }).catch(error => {
    console.error('Error while getting sensors details', error.message);
    res.sendStatus(500);
  });
});

Answer 1

a child_added event only fires when there is a child node. If there are not child nodes under the location (or matching the query) it will not fire.

To ensure you also get notified in the condition there are no children, you should listen to the value event:

for (var i = 0; i < sensors.length; i++) {
  console.log('Adding promise for ' + sensors[i]);
  var query = admin.database().ref('/readings/' + sensors[i]).limitToLast(1).once("value")
  promiseArray.push(query);
}

Since a value event may match multiple children in a single snapshot (despite your query only requesting a single child), you will need to loop over the children of the resulting snapshot:

Promise.all(promiseArray).then((queryResults) => {
  console.log('All promises done : ' + queryResults.length);
  res.set('Cache-Control', 'private, max-age=300');
  queryResults.forEach((snapshots) => {
    snapshots.forEach((snapshot) => {
      differenceInMinutes = (tstamp - snapshot.val().time) / 60;
      ...