在 Flutter 的 Firebase Firestore 中的事务中保存数据的问题

Question

I have a problem with transactions in my web application created in Flutter. For database I use Firebase Firestore where I save documents via transaction.

Dependency:
cloud_firestore: 3.1.1

StudentGroup is my main document. It has 4 stages and each of them has 3-5 tasks. (Everything is in 1 document). I have to store game timer, so every 10 seconds I make an request to save time for current stage. (Every stage has different timer). I have a problem with saving task, because "Sometimes" when 2 requests are made in the same time, then I get some weird state manipulation.

1. Task is updated and "isFinished" is set to true
2. Timer is updated to correct value (with this update somehow previous task update is lost, "isFinished" is set to false

This is how I save task.

Future<Result> saveTask({required String sessionId, required String studentGroupId,
  required Task task}) async {
  print("trying to save task <$task>.");
  try {
    return await _firebaseFirestore.runTransaction((transaction) async {
      final studentGroupRef = _getStudentGroupDocumentReference(
          sessionId: sessionId,
          studentGroupId: studentGroupId
      );
      final sessionGroupDoc = await studentGroupRef.get();

      if (!sessionGroupDoc.exists) {
        return Result.error("student group not exists");
      }

      final sessionGroup = StudentGroup.fromSnapshot(sessionGroupDoc);
      sessionGroup.game.saveTask(task);
      transaction.set(studentGroupRef, sessionGroup.toJson());
    })
    .then((value) => taskFunction(true))
    .catchError((error) => taskFunction(false));
  } catch (error) {
    return Result.error("Error couldn't save task");
  }
}

This is how I save my time

Future<Result> updateTaskTimer({required String sessionId,
  required String studentGroupId, required Duration duration}) async {
  print("trying to update first task timer");
  try {
    return await _firebaseFirestore.runTransaction((transaction) async {
      final studentGroupRef = _getStudentGroupDocumentReference(
          sessionId: sessionId,
          studentGroupId: studentGroupId
      );
      final sessionGroupDoc = await studentGroupRef.get();

      if (!sessionGroupDoc.exists) {
        return Result.error("student group not exists");
      }

      final sessionGroup = StudentGroup.fromSnapshot(sessionGroupDoc);
      switch (sessionGroup.game.gameStage) {
        case GameStage.First:
          sessionGroup.game.stages.first.duration = duration.inSeconds;
          break;
        case GameStage.Second:
          sessionGroup.game.stages[1].duration = duration.inSeconds;
          break;
        case GameStage.Third:
          sessionGroup.game.stages[2].duration = duration.inSeconds;
          break;
        case GameStage.Fourth:
          sessionGroup.game.stages[3].duration = duration.inSeconds;
          break;
        case GameStage.Fifth:
          sessionGroup.game.stages[4].duration = duration.inSeconds;
          break;
      }
      transaction.set(
          studentGroupRef,
          sessionGroup.toJson(),
          SetOptions(merge: true)
      );

      print("Did I finish task 4? ${sessionGroup.game.stages.first.tasks[3].isFinished}");

    })
    .then((value) => timerFunction(true))
    .catchError((error) => timerFunction(false));
  } catch (error) {
    return Result.error("Error couldn't update task timer");
  }
}

timerFunction and taskFunction print some messages in console and return Result.error or Result.success (for now it returns bool)

I don't know If I am doing something wrong with Firebase Firestore Transaction. I would like to have atomic operations for reading and writing data.

Answer 1

Transactions ensure atomicity - which means that if the transaction succeeds then all the reads and writes occur in a non-overlapping way with other transactions. This prevents the type of problem you are describing.

But this doesn't work if you spread your reads and writes over multiple transactions. In particular, it looks to me like you are writing a task which was obtained from outside the transaction. Instead, you should use ids or similar to track which documents you need to update, then do a read and a write inside the transaction.Alternatively firebase also provides Batched Writes , which specify the specific properties you want to update. These will ensure that any other properties are not changed.For batch writes example you can refer to the link