How does transactions in firestore error out?

Question

I'm trying to produce an error in transaction and my question is when does transaction produces error ? I tried inserting random non existent collection and document, go offline, still it doesn't catch any error.

In what conditions does transaction error out ?

var db = Firestore.firestore()
let ref = db.collection("foo").document("bar")

        db.runTransaction({ transaction, errorPointer -> Any? in

            var document: DocumentSnapshot

            do {

                try document = transaction.getDocument(ref)

            } catch let fetchError as NSError {
                // no Error here even if ref doesn't exist or I go offline

                errorPointer?.pointee = fetchError
                return nil
            }

            return nil
        }) { _, error in
            if let error = error {
                print("Transcation Completion Error: \(error)")
            } else {
                print("Transaction Succeeded!")
            }
        }

score 0 · Answer 1 · answered Mar 18 '22 at 19:07

If you just want to test error handling then just throw your own error from within the transaction. The transaction closure has two arguments, the transaction object and an error pointer. Assign the error pointer an NSError and handle it in the completion block.

errorPointer?.pointee = NSError(domain: "yourDomain", code: 0, userInfo: nil)

Beyond this, a transaction could fail for a number of reasons, such as performing a read operation after a write operation, a network error, exceeding the allotted data-usage limit, or too many failed attempts at retrying the transaction (because the underlying documents were modified outside the transaction). Further reading at link below.

https://firebase.google.com/docs/firestore/manage-data/transactions#transaction_failure

score 0 · Answer 2 · answered Mar 18 '22 at 19:22

According to Firebase documentation, a transaction can fail following the next options:

After the write operations, the transaction contains read operations. Before any write operations, read operations must always occur first.
The transaction read a document that had been changed outside of it. In this instance, the transaction is restarted automatically. A certain number of times the transaction is retried.
The transaction's request size exceeds the 10 MiB limit.
The size of a transaction is determined by the size of the documents and index items that it modifies. This contains the size of the target document and the sizes of the index items eliminated due to the operation.

When a transaction fails, it generates an error and does not write any data to the database. You don't have to roll back the transaction because Cloud Firestore does it for you.

Also, I would like to suggest you to check the NSError for Swift, here is a Github Repository that provides information of NSError, as well as this documentation that also describes the NSError of how to handle failed transactions and give appropriate error messages to the user as a feedback.

How does transactions in firestore error out?

2 Answers2

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