CODEWITHSUNDEEP
firebasegoogle-cloud-platformgoogle-cloud-firestoregoogle-cloud-functions
0x6f6b
0x6f6b

Reputation: 35

Cloud Firestore triggers run multiple times

In the cloud functions documentation for Firestore triggers, it mentions the following limitation: "Events are delivered at least once, but a single event may result in multiple function invocations. Avoid depending on exactly-once mechanics, and write idempotent functions."

I am writing a system that checks the length of an array, and when it reaches a certain length (100), it is cleared, and some processing is done on the data. I think this is what it means by an "exactly-once" mechanic.

My question is this, is checking:

if (change.after.data().array.length === change.before.data().array.length) {
  return;
}

A sufficient way to prevent multiple executions?

Upvotes: 0

Views: 790

Answers (1)

Renaud Tarnec
Renaud Tarnec

Reputation: 83191

Is checking the following a sufficient way to prevent multiple executions?

if (change.after.data().array.length === change.before.data().array.length) {   return; }

The answer is no. The Cloud Function could be ran multiple times in the case array length was 99 and is now 100 (= the single event mentioned in the doc you referred to).

There is a Blog article on Building idempotent functions which explains that a common way to make a Cloud Function idempotent is "to use the event ID which remains unchanged across function retries for the same event".

You could use this method, by saving the event ID when you do the "data processing" you mention in your question, and in case there is another execution, check if the ID was already saved.

Concretely, you can create a Firestore document with the CF event ID as Document ID and in your Cloud Function, you check if this document exists or not, with a Transaction. If it does not exist, you can proceed: the CF was never executed for this event ID.

An simpler solution for your case could be to ensure that if the Cloud Function is executed multiple times it results in the exact same situation in terms of data.

In other words, if the result of the "data processing" is exactly the same when running several times the Cloud Function for array.length == 100, then your function is idempotent ("the operation results remain unchanged when an operation is applied more than once").

Of course this highly depends on the business logic of your "data processing". If, for example, it involves the time of execution, obviously the end result could be different between two executions.

Upvotes: 2

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