Firebase cloud functions - update a different object within OnUpdate cloud trigger - firebase

Question

Assume there is a collection of users and each user is associated with accounts, which are kept in a separate collection. For each account there is a balance which is updated periodically by some external means (e.g. the http trigger below). I need to be able to query for the user's total balance across all of her accounts.
I added onUpdate trigger which gets called everytime an account changes and updates the total accordingly. However, it seems that there is some race condition e.g. when two accounts get updated around the same time: after onUpdate is called for the first account and updates the total balance, it is still not updated when onUpdate is called for the second account. I'm guessing I need to somehow use "transaction" for the bookkeeping but not sure how.
const data = {
'users/XXX': {
email: "a#b.com",
balance: 0
},
"accounts/YYY": {
title: "Acc1",
userID: "XXX"
balance: 0
},
"accounts/ZZZ": {
title: "Acc2",
userID: "XXX"
balance: 0
}
};
exports.updateAccounts = functions.https.onRequest((request, response) => {
admin.firestore().collection('accounts').get().then((accounts) => {
accounts.forEach((account) => {
return admin.firestore().collection('accounts').doc(account.id).update({balance:
WHATEVER});
})
response.send("Done");
});
exports.updateAccount = functions.firestore
.document('accounts/{accountID}')
.onUpdate((change, context) => {
const userID = change.after.data().userID;
admin.firestore().doc("users/"+userID).get().then((user) => {
const new_balance = change.after.data().balance;
const old_balance = change.before.data().balance;
var user_balance = user.data().balance + new_balance - old_balance;
admin.firestore().doc("users/"+userID).update({balance: user_balance});
});
});

Answer 1

By looking at your code we can see several parts of it that could lead to incorrect results. It is not possible, without thoroughly testing and reproducing your problem, to be sure at 100% that correcting them will totally solve your problem but it is most probably the cause of the problems.
HTTP Cloud Function:
With the forEach() loop you are calling several asynchronous operations (the update() method) but you don't wait that all these asynchronous operations are completed before sending back the response. You should do as follows, using Promise.all() to wait all the asynchronous methods are completed before sending the response:
exports.updateAccounts = functions.https.onRequest((request, response) => {
const promises = [];
admin.firestore().collection('accounts').get()
.then(accounts => {
accounts.forEach((account) => {
promises.push(admin.firestore().collection('accounts').doc(account.id).update({balance: WHATEVER}));
return Promise.all(promises);
})
.then(() => {
response.send("Done");
})
.catch(error => {....});
});
onUpdate background triggered Cloud Function
There you need to correctly return the Promises chain in order to indicate to the platform when the Cloud Function is complete. The following should do the trick:
exports.updateAccount = functions.firestore
.document('accounts/{accountID}')
.onUpdate((change, context) => {
const userID = change.after.data().userID;
return admin.firestore().doc("users/"+userID).get() //Note the return here. (Note that in the HTTP Cloud Function we don't need it! see the link to the video series below)
.then(user => {
const new_balance = change.after.data().balance;
const old_balance = change.before.data().balance;
var user_balance = user.data().balance + new_balance - old_balance;
return admin.firestore().doc("users/"+userID).update({balance: user_balance}); //Note the return here.
});
});
I would suggest that you watch the 3 videos about "JavaScript Promises" from the Firebase video series: https://firebase.google.com/docs/functions/video-series/. They explain all the key points that were corrected above.
At first sight, it seems that if you modify, in the updateAccounts Cloud Function, several account documents that share the same user you will indeed need to implement the user balance update in a transaction, as several instances of the updateAccount Cloud Function may be triggered in parallel. The doc on Transactions is here.
Update:
You could implement a Transaction as follows in the updateAccounts Cloud Function (untested):
exports.updateAccount = functions.firestore
.document('accounts/{accountID}')
.onUpdate((change, context) => {
const userID = change.after.data().userID;
const userRef = admin.firestore().doc("users/" + userID);
return admin.firestore().runTransaction(transaction => {
// This code may get re-run multiple times if there are conflicts.
return transaction.get(userRef).then(userDoc => {
if (!userDoc.exists) {
throw "Document does not exist!";
}
const new_balance = change.after.data().balance;
const old_balance = change.before.data().balance;
var user_balance = userDoc.data().balance + new_balance - old_balance;
transaction.update(userRef, {balance: user_balance});
});
}).catch(error => {
console.log("Transaction failed: ", error);
return null;
});
});

Answer 2

In addition to what #Renaud Tarnec covered in their answer, you may also want to consider the following approaches:
Batched Write
In your updateAccounts function, you are writing many pieces of data at once, if any one of these fail, you may end up with a database that contains a mix of correctly updated data and data that had failed to be updated.
To solve this, you can use a batched write to write the data atomically where all new data is updated successfully or none of your data is written leaving your database in a known state.
exports.updateAccounts = functions.https.onRequest((request, response) => {
const db = admin.firestore();
db.collection('accounts')
.get()
.then((qsAccounts) => { // qs -> QuerySnapshot
const batch = db.batch();
qsAccounts.forEach((accountSnap) => {
batch.update(accountSnap.ref, {balance: WHATEVER});
})
return batch.commit();
})
.then(() => response.send("Done"))
.catch((err) => {
console.log("Error whilst updating balances via HTTP Request:", err);
response.status(500).send("Error: " + err.message)
});
});
Splitting the counters
Instead of storing a single "balance" in your document, it may instead be desirable (based on what you are trying to do) to store each account's balance in the user's document.
"users/someUser": {
...,
"balances": {
"accountId1": 10,
"accountId4": -20,
"accountId23": 5
}
}
If you need the cumulative balance, just add them together on the client. If you need to remove a balance, simply delete it's entry in the user document.
exports.updateAccount = functions.firestore
.document('accounts/{accountID}')
.onUpdate((change, context) => {
const db = admin.firestore();
const accountID = context.params.accountID;
const newData = change.after.data();
const accountBalance = newData.balance;
const userID = newData.userID;
return db.doc("users/"+userID)
.get()
.then((userSnap) => {
return db.doc("users/"+userID).update({["balances." + accountID]: accountBalance});
})
.then(() => console.log(`Successfully updated account #${accountID} balance for user #${userID}`))
.catch((err) => {
console.log(`Error whilst updating account #${accountID} balance for user #${userID}`, err);
throw err;
});
});