Parse.com push notifications coming delayed or only after Android app restart

Question

When testing parse.com push notifications (sent from CloudCode on afterSave), there is something weird.

Sometimes the Android app gets the notification immediately (below 1 second), but other times, it comes after a multi-second delay.
Restarting the app seems to cause the not-yet-received notifications to appear immediately.

What could be the cause?
Could this be a bug in, for example, the parse.com service?

Is there any limit of how many notifications can be sent or received (per unit of time) ?

The problem happens both with custom BroadcastReceiver and with the default system bar notification.

Server-side javascript CloudCode:

Parse.Cloud.afterSave("Timer", function(request) {
  // from https://www.parse.com/docs/js/guide#cloud-code
  console.log("Before Parse.Push.send -- without alert");
  var query = new Parse.Query(Parse.Installation);

  // http://blog.parse.com/announcements/pushing-from-the-javascript-sdk-and-cloud-code/ :
  Parse.Push.send({
    where: query,
    data: {
       //alert: "afterSave on a Timer -- Parse.Push.send"
    }
  });

  console.log("After Parse.Push.send -- without alert");
});

Custom broadcast receiver in Kotlin (but the problem happens also without the custom BroadcastReceiver) :

override fun onCreate(savedInstanceState: Bundle?) {
    super<BaseActivity>.onCreate(savedInstanceState)

    setContentView(R.layout.main_activity)
    // ...

    registerReceiver()
}

private fun registerReceiver() {
    val intentFilter = IntentFilter()
    intentFilter.addAction("com.parse.push.intent.RECEIVE")

    registerReceiver(MyBroadcastReceiver(), intentFilter)
}

inner class MyBroadcastReceiver : BroadcastReceiver() {
    override fun onReceive(context: Context, intent: Intent) {
        Toast.makeText(context, "MyBroadcastReceiver 2: onReceive: "
                + context + ";" + intent, Toast.LENGTH_SHORT).show()

        loadTimers()
    }

}

We are using a non-paid parse.com account for now. Could that affect the timeliness of the reaction to the push notifications?

Edit: if you think, that using push notifications to trigger near-realtime item updates/sync, is not a good idea (either in general or in parse.com), that would also be a valuable answer, especially if an alternative is proposed...

Answer 1

At karolvrn's suggestion, here's my answer:

I don't think there's any guarantee of immediate delivery of push notifications.

https://developers.google.com/cloud-messaging/concept-options#setting-the-priority-of-a-message

You have two options for assigning delivery priority to downstream messages: normal and high priority. Delivery of high and normal priority messages works like this:

• High priority. GCM attempts to deliver high priority messages immediately, allowing the GCM service to wake a sleeping device when possible and open a network connection to your app server...
• Normal priority. This is the default priority for message delivery...

I emphasized "attempts", which implies that it does not guarantee that the message will be delivered immediately.

Here's another developer's experience with reliability issues with GCM:

https://eladnava.com/google-cloud-messaging-extremely-unreliable/