Question

I have implemented c2dm and it works fine that the device receives the message and everything.
There is however one problem that I can’t find a solution for.

When a certain event occurs I want to push the same message to a few thousand devices quickly, preferably within a minute. I know that there is no certainty that the message will ever be delivered and that Google may delay the message due to different circumstances.

My implementation for iOS is about the same except of course the sending part. That takes about 5 sec for 15000 messages. For 5000 messages to Android it takes over an hour which is way to long.

Does anyone know a way to speed this up?
Or does Google stop this kind of mass push to stop spam?

The C2DM part of the code is below. This is in PHP but I have no problem reading most other programming languages.
I use a script to connect() once and then loop all tokens and use sendMessage() to send each message. After all messages is sent, it disconnect().

<?php
class C2DMclient
{
    private $authKey = NULL;
    private $ch = NULL;

    function connect() {
        $post_params = array(
            'Email'       => 'C2DM_USER',
            'Passwd'      => 'C2DM_PWD',
            'accountType' => 'HOSTED_OR_GOOGLE',
            'source'      => 'appname',
            'service'     => 'ac2dm',
        );
        $first = true;
        $data_msg = '';

        foreach($post_params as $key => $value) {
            if($first)
                $first = false;
            else
                $data_msg .= '&';

            $data_msg .= urlencode($key).'='.urlencode($value);
        }

        $x = curl_init('C2DM_CLIENTLOGIN');
        curl_setopt($x, CURLOPT_HEADER, true);
        curl_setopt($x, CURLOPT_POST, true);
        curl_setopt($x, CURLOPT_POSTFIELDS, $data_msg);
        curl_setopt($x, CURLOPT_RETURNTRANSFER, true);
        $response = curl_exec($x);
        curl_close($x);

        $pos = strpos($response, 'Auth=');
        $this->authKey = trim(substr($response, 5 + $pos));
        $this->ch = curl_init();
        curl_setopt($this->ch, CURLOPT_URL, 'C2DM_SERVER');
    }

    function disconnect() {
        curl_close($this->ch);
        $this->authKey = NULL;
    }

    function sendMessage($deviceToken, $message) {
        $data = array(
            'registration_id' => $deviceToken,
            'collapse_key'    => 'ck_type',
            'data.type'       => 'TYPE',
            'data.message'    => $message,
            'data.title'      => 'Title'
        );
        $headers = array('Authorization: GoogleLogin auth='.$this->authKey);

        if($headers)
            curl_setopt($this->ch, CURLOPT_HTTPHEADER, $headers);

        curl_setopt($this->ch, CURLOPT_SSL_VERIFYPEER, 0);
        curl_setopt($this->ch, CURLOPT_POST, true);
        curl_setopt($this->ch, CURLOPT_RETURNTRANSFER, true);
        curl_setopt($this->ch, CURLOPT_POSTFIELDS, $data);

        $messagedata = curl_exec($this->ch);
            return TRUE;
    }
}
?>

EDIT:
New solution is based on curl_multi_exec.

Curl handles are collected while looping through the database query results. After a few hundred curl handles are gathered, sendMessages($chs) are called to send all those messages. At the moment I collect 700 messages before sending and seems to have a good delivery rate and fast enough, ~10s to send 5000 messages. A higher number seem to affect delivery rate.

<?php
class C2DMclient
{
    private $authKey = NULL;
    private $ch = NULL;

    function connect() {
        $post_params = array(
            'Email'       => 'C2DM_USER',
            'Passwd'      => 'C2DM_PWD',
            'accountType' => 'HOSTED_OR_GOOGLE',
            'source'      => 'appname',
            'service'     => 'ac2dm',
        );
        $first = true;
        $data_msg = '';

        foreach($post_params as $key => $value) {
            if($first)
                $first = false;
            else
                $data_msg .= '&';

            $data_msg .= urlencode($key).'='.urlencode($value);
        }

        $x = curl_init('C2DM_CLIENTLOGIN');
        curl_setopt($x, CURLOPT_HEADER, true);
        curl_setopt($x, CURLOPT_POST, true);
        curl_setopt($x, CURLOPT_POSTFIELDS, $data_msg);
        curl_setopt($x, CURLOPT_RETURNTRANSFER, true);
        $response = curl_exec($x);
        curl_close($x);

        $pos = strpos($response, 'Auth=');
        $this->authKey = trim(substr($response, 5 + $pos));
    }

    function getMessageCurlHandle($deviceToken, $message) {
        $ch = curl_init();
        curl_setopt($ch, CURLOPT_URL, 'C2DM_SERVER');
        $data = array(
            'registration_id' => $deviceToken,
            'collapse_key'    => 'ck_type',
            'data.type'       => 'TYPE',
            'data.message'    => $message,
            'data.title'      => 'Title'
        );
        $headers = array('Authorization: GoogleLogin auth='.$this->authKey);

        if($headers)
            curl_setopt($ch, CURLOPT_HTTPHEADER, $headers);
        curl_setopt($ch, CURLOPT_SSL_VERIFYPEER, 0);
        curl_setopt($ch, CURLOPT_POST, true);
        curl_setopt($ch, CURLOPT_POSTFIELDS, $data);
        return $ch;
    }

    function sendMessages($chs) {
        $mh = curl_multi_init();
        foreach($chs as $ch) {
            curl_multi_add_handle($mh, $ch);
        }
        $active = null;
        do {
            $mrc = curl_multi_exec($mh, $active);
        } while($mrc == CURLM_CALL_MULTI_PERFORM);

        while($active && $mrc == CURLM_OK) {
            if(curl_multi_select($mh) != -1) {
                do {
                    $mrc = curl_multi_exec($mh, $active);
                } while($mrc == CURLM_CALL_MULTI_PERFORM);
            }
        }
        curl_multi_close($mh);
    }
}
?>

Answer 1

[Update] C2DM is now deprecated. It’s successor Google Cloud Messaging (GCM) supports multiple receivers aka batch sending:

{ "data": {
  "score": "5x1",
  "time": "15:10"
  },
  "registration_ids": ["4", "8", "15", "16", "23", "42"]
}

[/Update]

C2DM doesn’t support batch sending yet.

However you could send several POST request to the C2DM server at the same time,
unfortunately PHP has itself no support for multi-threading.

Have a look at curl_multi_exec which gives the possiblity to make several cURL requests at the same time.