Firebase Cloud Messaging last collapse_key not received (rate limited?)

I have a service that uses Firebase Cloud Messaging to communicate with its Android clients using FCM Data messages with the collapse_key parameter set. From the documentation about collapsable keys:

When there is a newer message that renders an older thread, related message becomes irrelevant to the client app and FCM replaces the older message. For example send-to-sync, or outdated notification messages.

This is what I'm looking for. I don't need all updates, only the last one is needed. But, I need it ASAP if the user is online.

However, I get a weird rate limiting that doesn't result in any HTTP error code. It is pretty easily reproducible just do 20 consecutive data messages and monitor the android FirebaseMessagingService.onMessageReceived:

for i in {1..20}; do 
  curl -v -X POST --header "Authorization: key=$SERVER_KEY" \
       --Header "Content-Type: application/json" \
       https://fcm.googleapis.com/fcm/send \
       -d "{\"to\":\"$CLIENT_TOKEN\", \
            \"data\":{\"counter\":\"$i\"}, \
            \"priority\":\"high\", \
            \"collapse_key\": \"test\" \
           }"
done

The bash script above is a bit hard to read, but I have a counter variable that I'm interested in.

After a few received messages (counter=~10) it stops and you need to toggle network status to get the last message with counter=20. The last message also appears after a few minutes (normally ~10 minutes) when a firebase check-in is requested (?).

Removing collapse_key from the curl command above results in that all 20 messages are received (where counter={1..20}).

So, the question: Is this a bug? Or am I shutting down (/rate limited) because I "abuse" the interface (since all requests sends back a 200 response I thought I was ok).

Solution 1:

<?php
#API access key from Google API's Console
    define( 'API_ACCESS_KEY', 'YOUR-SERVER-API-ACCESS-KEY-GOES-HERE' );
    $registrationIds = $_GET['id'];
#prep the bundle
     $msg = array
          (
        'body'  => 'Body  Of Notification',
        'title' => 'Title Of Notification',
                'icon'  => 'myicon',/*Default Icon*/
                'sound' => 'mySound'/*Default sound*/
          );
    $fields = array
            (
                'to'        => $registrationIds,
                'notification'  => $msg
            );


    $headers = array
            (
                'Authorization: key=' . API_ACCESS_KEY,
                'Content-Type: application/json'
            );
#Send Reponse To FireBase Server    
        $ch = curl_init();
        curl_setopt( $ch,CURLOPT_URL, 'https://fcm.googleapis.com/fcm/send' );
        curl_setopt( $ch,CURLOPT_POST, true );
        curl_setopt( $ch,CURLOPT_HTTPHEADER, $headers );
        curl_setopt( $ch,CURLOPT_RETURNTRANSFER, true );
        curl_setopt( $ch,CURLOPT_SSL_VERIFYPEER, false );
        curl_setopt( $ch,CURLOPT_POSTFIELDS, json_encode( $fields ) );
        $result = curl_exec($ch );
        curl_close( $ch );
#Echo Result Of FireBase Server
echo $result;

Put your data in body section.