Cloud Functions for Firebase invoked many times

Question

I am just trying out Cloud Functions for Firebase in an effort to move my firebase-queue workers over to the cloud functions. I added a simple function to add a last updated timestamp whenever I create a new node at a given ref. The function looks like this:

var functions = require('firebase-functions');
const admin = require('firebase-admin');

admin.initializeApp(functions.config().firebase);

exports.setLastUpdatedTimestamp = functions.database.ref('/nodes/{nodeId}')
  .onWrite(event => {
    const original = event.data.val();
    console.log('Adding lastUpdatedTimestamp to node ', original.name);
    return event.data.ref.child('lastUpdatedAtFromFC').set(Date.now());
  });

I deployed this cloud function and added just one node from my app. I went to the Firebase Functions dashboard and see that the function has been invoked 169 times and I have no idea why. When I look at the logs, I see logs like attaching the function to all the past nodes as well.

Is it the case that the onWrite would behave kind of like child_added and run the function for all the existing entities as well?

Would this action be repeated every time I change and deploy the function again?

I was expecting it to run just once for the newly added node.

Answer 1

This is a common mistake when writing functions that deal with database writes. When you handle the event for the initial write at a location, then do a second write back to that same location, that second write will trigger yet another event that will run the function again, and so on, which will be an infinite loop.

You need some logic in your function that determines if the second write event should not re-write to the database. That will stop the loop. In your case, you don't need a function to set the last update time. You can do that with a special value on the client to tell the server to insert the current time into a field.

https://firebase.google.com/docs/reference/js/firebase.database.ServerValue#.TIMESTAMP

Answer 2

I had this exact issue. There are two questions here. How to stop an infinite loop once it's start. That's been answered. But the real question is how to add a lastUpdated date field to your objects using Firebase Cloud Function triggers.

Here's my attempt at handling the onWrite() loop issue.

const functions = require('firebase-functions');
const admin = require('firebase-admin');
admin.initializeApp();


exports.onWriteFeatherUpdateSummary = functions.database.ref('/messages/{id}')
    .onWrite((change, context) => {
      // Grab the current value of what was written to the Realtime Database.
      let updated = change.after.val();
      // Grab the previous value of what was written to the Realtime Database.
      const previous = change.before.val();
      let isChanged = true;
      let isCreated = (previous === null); // object 'created'

      // Only when the object gets updated
      if (!isCreated) {
        // An object should never directly change the lastUpdated value, this is for trhe trigger only
        isChanged = (updated.lastUpdated === previous.lastUpdated);
      }

      console.log(`isChanged: ${isChanged} ; isCreated: ${isCreated}`);

      if(isChanged) {
        // Insert whatever extra data you wnat on the update trigger
        const summary = `This is an update!`;

        // Add a 'createdDate' field on the first trigger
        if (isCreated) {
          // Make sure your object has a createdDate (Date) before the lastUpdated (Date)!
          Object.assign(updated,
              {
                createdDate : admin.database.ServerValue.TIMESTAMP
              }
            );
        }

        // Add lastUpdated Date field on very update trigger (but not when you just changed it with the trigger!)
        Object.assign(updated,
            {
              summary : summary,
              lastUpdated : admin.database.ServerValue.TIMESTAMP
            }
          );
      }

      return change.after.ref.set(updated);
    });