Firebase synchronisation of locally-modified data: handling errors & global status

I have two related questions regarding the Firebase web platform's synchronisation of locally-modified data to the server:

Every client sharing a Firebase database maintains its own internal version of any active data. When data is updated or saved, it is written to this local version of the database. The Firebase client then synchronizes that data with the Firebase servers and with other clients on a 'best-effort' basis.


1. Handling sync errors

The data-modification methods (set(), remove(), etc) can take an onComplete callback parameter:

A callback function that will be called when synchronization to the Firebase servers has completed. The callback will be passed an Error object on failure; else null.

var onComplete = function(error) {
  if (error) {
    console.log('Synchronization failed');
  } else {
    console.log('Synchronization succeeded');
  }
};

fredRef.remove(onComplete);

In the example above, what kind of errors should the fredRef.remove() callback expect to receive?

  • Temporary errors?
    • Client is offline (network connection lost) ?
    • Firebase server is temporarily overloaded or down for maintenance, but will be available again soon?
  • Permanent errors?
    • Permission denied (due to security rules) ?
    • Database location does not exist?

Is there a way to distinguish between temporary and permanent errors?

How should we handle / recover from these errors?

For temporary errors, do we need to call fredRef.remove() again after a short period of time, to retry the operation?


2. Global sync status

I realise that each call to set() and remove() will receive an individual sync success/failure result in the onComplete callback.  But I'm looking for a way to determine the global sync status of the whole Firebase client.

I'd like to use a beforeunload event listener to warn the user when they attempt to leave the page before all modified data has been synced to the server, and I'm looking for some function like firebase.isAllModifiedDataSynced().  Something like this:

window.addEventListener('beforeunload', function (event) {
    if (!firebase.isAllModifiedDataSynced()) {
        event.returnValue = 'Some changes have not yet been saved. If you ' +
                            'leave this page, your changes will be lost.';
    }
});

Here's an example of the same functionality in Google Drive:

Google Drive screenshot

I'm aware of the special /.info/connected location:

it is useful for a client to know when it is online or offline. Firebase clients provide a special location at /.info/connected which is updated every time the client's connection state changes. Here is an example:

var connectedRef = new Firebase("https://<YOUR-FIREBASE-APP>.firebaseio.com/.info/connected");
connectedRef.on("value", function(snap) {
  if (snap.val() === true) {
    alert("connected");
  } else {
    alert("not connected");
  }
});

The special /.info/connected location can be connected to a beforeunload event listener like this:

var connectedRef = new Firebase('https://myapp.firebaseio.com/.info/connected');
var isConnected  = true;

connectedRef.on('value', function (snap) {
    isConnected = snap.val();
});

window.addEventListener('beforeunload', function (event) {
    if (!isConnected) {
        event.returnValue = 'Some changes have not yet been saved. If you ' +
                            'leave this page, your changes will be lost.';
    }
});

My question is:

  • If isConnected is true, does this also mean that all modified data has been synced to the server?
  • i.e. Does "connected" also mean "synced"?

If not, how can the app determine the global sync status of the whole Firebase client?

  • Is there a special /.info/synchronized location?
  • Does the app need to manually keep track of the sync success/failure result of every onComplete callback?

Answers:

Answer

In the example above, what kind of errors should the fredRef.remove() callback expect to receive?

Client is offline (network connection lost) ?

No, this will not cause an error to be passed to the completion listener. It will simply cause the completion listener to not be called (yet).

Firebase server is temporarily overloaded or down for maintenance, but will be available again soon?

No. This is essentially the same as being without a network connection.

Permission denied (due to security rules) ?

Yes, this is will indeed cause an error to be passed to the completion handler.

Database location does not exist?

No, this will not cause an error to be caused to the completion listener.

If isConnected is true, does this also mean that all modified data has been synced to the server? i.e. Does "connected" also mean "synced"?

No it does not. .info/connected will fire with true when a connection is made to the database.

If not, how can the app determine the global sync status of the whole Firebase client?

There is currently no way to determine whether your local data is up to date with the server.

Is there a special /.info/synchronized location?

No, such a location doesn't exist.

Does the app need to manually keep track of the sync success/failure result of every onComplete callback?

That depends on the use-case. But if you want to simply know when all your writes are executed, push a dummy value and wait for that to complete. Since Firebase executes the writes in order, you can be certain at that stage that you've gotten the other events.

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