Firebase: Run a query synchronously

Question

I am trying to set some user data depending on the no.of users already in my USERS COLLECTION. This even includes a userId which should be a number.

exports.setUserData = functions.firestore.document('/users/{documentId}')
  .onCreate(event => {
    return admin.firestore().collection('users')
      .orderBy('userId', 'desc').limit(1)
      .get().then(function(snapshot) {

      const user = snapshot.docs[0].data();
      var lastUserId = user.userId;
          var userObject = {
            userId: lastUserId + 1,... some other fields here
          };

        event.data.ref.set(userObject, {
          merge: true
        });

      });
  });

One issue I noticed here, quickly adding 2 users result in those documents having the same userId may be because the get() query is asynchronous?

Is there a way to make this whole setUserData method synchronous?

Answer 1

There is no way to make Cloud Functions run your function invocations sequentially. That would also be quite contrary to the serverless promise of auto-scaling to demands.

But in your case there's a much simpler, lower level primitive to get a sequential ID. You should store the last known user ID in the database and then use a transaction to read/update it.

var counterRef = admin.firestore().collection('counters').doc('userid');

return db.runTransaction(function(transaction) {
    // This code may get re-run multiple times if there are conflicts.
    return transaction.get(counterRef).then(function(counterDoc) {
        var newValue = (counterDoc.data() || 0) + 1;
        transaction.update(counterRef, newValue);
    });
});

Solution

var counterRef = admin.firestore().collection('counters').doc('userId');

return admin.firestore().runTransaction(function(transaction) {
  // This code may get re-run multiple times if there are conflicts.
  return transaction.get(counterRef).then(function(counterDoc) {
    var newValue = (counterDoc.data().value || 0) + 1;
    transaction.update(counterRef, {
      "value": newValue
    });
  });
}).then(t => {
  admin.firestore().runTransaction(function(transaction) {
    // This code may get re-run multiple times if there are conflicts.
    return transaction.get(counterRef).then(function(counterDoc) {
      var userIdCounter = counterDoc.data().value || 0;

      var userObject = {
        userId: userIdCounter
      };

      event.data.ref.set(userObject, {
        merge: true
      });
    });
  })
});