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page.title=Making the Most of Google Cloud Messaging
parent.title=Syncing to the Cloud
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@jd:body

<div id="tb-wrapper">
  <div id="tb">
    <h2>This lesson teaches you to</h2>
    <ol>
      <li><a href="#multicast">Send Multicast Messages Efficiently</a></li>
      <li><a href="#collapse">Collapse Messages that can Be Replaced</a></li>
      <li><a href="#embed">Embed Data Directly in the GCM Message</a></li>
      <li><a href="#react">React Intelligently to GCM Messages</a></li>
    </ol>
    <h2>You should also read</h2>
    <ul>
      <li><a href="http://developer.android.com/google/gcm/index.html">Google
      Cloud Messaging for Android</a></li>
    </ul>
  </div>
</div>

<p>Google Cloud Messaging (GCM) is a free service for sending
messages to Android devices.  GCM messaging can greatly enhance the user
experience.  Your application can stay up to date without wasting battery power
on waking up the radio and polling the server when there are no updates.  Also,
GCM allows you to attach up to 1,000 recipients to a single message, letting you easily contact
large user bases quickly when appropriate, while minimizing the work load on
your server.</p>

<p>This lesson covers some of the best practices
for integrating GCM into your application, and assumes you are already familiar
with basic implementation of this service.  If this is not the case, you can read the <a
  href="{@docRoot}google/gcm/demo.html">GCM demo app tutorial</a>.</p>

<h2 id="multicast">Send Multicast Messages Efficiently</h2>
<p>One of the most useful features in GCM is support for up to 1,000 recipients for
a single message.  This capability makes it much easier to send out important messages to
your entire user base.  For instance, let's say you had a message that needed to
be sent to 1,000,000 of your users, and your server could handle sending out
about 500 messages per second.  If you send each message with only a single
recipient, it would take 1,000,000/500 = 2,000 seconds, or around half an hour.
However, attaching 1,000 recipients to each message, the total time required to
send a message out to 1,000,000 recipients becomes (1,000,000/1,000) / 500 = 2
seconds. This is not only useful, but important for timely data, such as natural
disaster alerts or sports scores, where a 30 minute interval might render the
information useless.</p>

<p>Taking advantage of this functionality is easy.  If you're using the <a
  href="{@docRoot}google/gcm/gs.html#libs">GCM helper
  library</a> for Java, simply provide a <code>List<String></code> collection of
registration IDs to the <code>send</code> or <code>sendNoRetry</code> method,
instead of a single registration ID.</p>

<pre>
// This method name is completely fabricated, but you get the idea.
List<String> regIds = whoShouldISendThisTo(message);

// If you want the SDK to automatically retry a certain number of times, use the
// standard send method.
MulticastResult result = sender.send(message, regIds, 5);

// Otherwise, use sendNoRetry.
MulticastResult result = sender.sendNoRetry(message, regIds);
</pre>

<p>For those implementing GCM support in a language other than Java, construct
an HTTP POST request with the following headers:</p>
<ul>
  <li><code>Authorization: key=YOUR_API_KEY</code></li>
  <li><code>Content-type: application/json</code></li>
</ul>

<p>Then encode the parameters you want into a JSON object, listing all the
registration IDs under the key <code>registration_ids</code>.  The snippet below
serves as an example.  All parameters except <code>registration_ids</code> are
optional, and the items nested in <code>data</code> represent the user-defined payload, not
GCM-defined parameters.  The endpoint for this HTTP POST message will be
<code>https://android.googleapis.com/gcm/send</code>.</p>

<pre>
{ "collapse_key": "score_update",
   "time_to_live": 108,
   "delay_while_idle": true,
   "data": {
       "score": "4 x 8",
       "time": "15:16.2342"
   },
   "registration_ids":["4", "8", "15", "16", "23", "42"]
}
</pre>

<p>For a more thorough overview of the format of multicast GCM messages, see the <a
  href="{@docRoot}google/gcm/gcm.html#send-msg">Sending
  Messages</a> section of the GCM guide.</pre>

<h2 id="collapse">Collapse Messages that Can Be Replaced</h2>
<p>GCM messages are often a tickle, telling the mobile application to
contact the server for fresh data.  In GCM, it's possible (and recommended) to
create collapsible messages for this situation, wherein new messages replace
older ones.  Let's take the example
of sports scores.  If you send out a message to all users following a certain
game with the updated score, and then 15 minutes later an updated score message
goes out, the earlier one no longer matters.  For any users who haven't received
the first message yet, there's no reason to send both, and force the device to
react (and possibly alert the user) twice when only one of the messages is still
important.</p>

<p>When you define a collapse key, when multiple messages are queued up in the GCM
servers for the same user, only the last one with any given collapse key is
delivered.  For a situation like with sports scores, this saves the device from
doing needless work and potentially over-notifying the user.  For situations
that involve a server sync (like checking email), this can cut down on the
number of syncs the device has to do.  For instance, if there are 10 emails
waiting on the server, and ten "new email" GCM tickles have been sent to the
device, it only needs one, since it should only sync once.</p>

<p>In order to use this feature, just add a collapse key to your outgoing
message.  If you're using the GCM helper library, use the Message class's <code>collapseKey(String key)</code> method.</p>

<pre>
Message message = new Message.Builder(regId)
    .collapseKey("game4_scores") // The key for game 4.
    .ttl(600) // Time in seconds to keep message queued if device offline.
    .delayWhileIdle(true) // Wait for device to become active before sending.
    .addPayload("key1", "value1")
    .addPayload("key2", "value2")
    .build();
</pre>

<p>If not using the helper library, simply add a variable to the
POST header you're constructing, with <code>collapse_key</code> as the field
name, and the string you're using for that set of updates as the value.</p>



<h2 id="embed">Embed Data Directly in the GCM Message</h2>
<p>Often, GCM messages are meant to be a tickle, or indication to the device
that there's fresh data waiting on a server somewhere.  However, a GCM message
can be up to 4kb in size, so sometimes it makes sense to simply send the
data within the GCM message itself, so that the device doesn't need to contact the
server at all.  Consider this approach for situations where all of the
following statements are true:
<ul>
  <li>The total data fits inside the 4kb limit.</li>
  <li>Each message is important, and should be preserved.</li>
  <li>It doesn't make sense to collapse multiple GCM messages into a single
  "new data on the server" tickle.</li>
</ul>

<p>For instance, short messages or encoded player moves
in a turn-based network game are examples of good use-cases for data to embed directly
into a GCM message. Email is an example of a bad use-case, since messages are
often larger than 4kb,
and users don't need a GCM message for each email waiting for them on
the server.</p>

<p>Also consider this approach when sending
multicast messages, so you don't tell every device across your user base to hit
your server for updates simultaneously.</p>
<p>This strategy isn't appropriate for sending large amounts of data, for a few
reasons:</p>
<ul>
  <li>Rate limits are in place to prevent malicious or poorly coded apps from spamming an
  individual device with messages.</li>
  <li>Messages aren't guaranteed to arrive in-order.</li>
  <li>Messages aren't guaranteed to arrive as fast as you send them out.  Even
  if the device receives one GCM message a second, at a max of 1K, that's 8kbps, or
  about the speed of home dial-up internet in the early 1990's.  Your app rating
  on Google Play will reflect having done that to your users.</p>
</ul>

<p>When used appropriately, directly embedding data in the GCM message can speed
up the perceived speediness of your application, by letting it skip a round trip
to the server.</p>

<h2 id="react">React Intelligently to GCM Messages</h2>
<p>Your application should not only react to incoming GCM messages, but react
<em>intelligently</em>.  How to react depends on the context.</p>

<h3>Don't be irritating</h3>
<p>When it comes to alerting your user of fresh data, it's easy to cross the line
from "useful" to "annoying".  If your application uses status bar notifications,
<a
  href="http://developer.android.com/guide/topics/ui/notifiers/notifications.html#Updating">update
  your existing notification</a> instead of creating a second one. If you
beep or vibrate to alert the user, consider setting up a timer.  Don't let the
application alert more than once a minute, lest users be tempted to uninstall
your application, turn the device off, or toss it in a nearby river.</p>

<h3>Sync smarter, not harder</h3>
<p>When using GCM as an indicator to the device that data needs to be downloaded
from the server, remember you have 4kb of metadata you can send along to
help your application be smart about it.  For instance, if you have a feed
reading app, and your user has 100 feeds that they follow, help the device be
smart about what it downloads from the server!  Look at the following examples
of what metadata is sent to your application in the GCM payload, and how the application
can react:</p>
<ul>
  <li><code>refresh</code> &mdash; Your app basically got told to request a dump of
  every feed it follows.  Your app would either need to send feed requests to 100 different servers, or
  if you have an aggregator on your server, send a request to retrieve, bundle
  and
  transmit recent data from 100 different feeds, every time one updates.</li>
  <li><code>refresh</code>, <code>feedID</code> &mdash; Better:  Your app knows to check
  a specific feed for updates.</li>
  <li><code>refresh</code>, <code>feedID</code>, <code>timestamp</code> &mdash;
  Best:  If the user happened to manually refresh before the GCM message
  arrived, the application can compare timestamps of the most recent post, and
  determine that it <em>doesn't need to do anything</em>.
</ul>