Firestore BatchWrite vs Transactions

Question

After following this firestore document, it's pretty clear that both batch writes and transactions are a great for grouping multiple operations.

Although i couldn't figure what would be the best practice for my use case.

This is my batch write:

val batch = db.batch()

        val docRef = db.collection(CHATS_COLLECTION).document()
        chat.id = docRef.id

        return Observable.create { emitter: ObservableEmitter<String> ->
            batch.set(docRef, chat)
            batch.update(db.collection(USERS_COLLECTION).document(userId), "chatIds",
                FieldValue.arrayUnion(db.document("$CHATS_COLLECTION/${docRef.id}")))

            batch.commit()
                .addOnSuccessListener {
                    emitter.onNext(docRef.id)
                    emitter.onComplete()
                }
                .addOnFailureListener { e ->
                    emitter.onError(e)
                }
        }

I'm creating new chat document and at the same time updating my user document "chatsIds" array with the newly created document using batch write.

What could be drawbacks in replacing batch write with transaction?

Answer 1

It would get slower without any added benefit. Batched writes are perfect for your case: you want to update multiple places in your database without any reads.

If you had reads, then it would make sense to use a transaction. In your case, a batch write is the correct choice simply because it runs faster.

Answer 2

Both batch and transactions are will execute atomic manner.Batch write is suitable if you are not reading data from document while updating the document and transaction will be suitable if there is a reading before updating the document