docs: Fix notification api guide issues (7461154, 12765600)

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page.title=GCM Advanced Topics
@jd:body

<div id="qv-wrapper">
<div id="qv">

<h2>Quickview</h2>

<ul>
<li>Learn more about GCM advanced features.</li>
</ul>


<h2>In this document</h2>

<ol>
<li><a href="#lifetime">Lifetime of a Message</a></li>
<li><a href="#throttling">Throttling</a></li>
<li><a href="#reg-state">Keeping the Registration State in Sync</a>
  <ol>
    <li><a href="#canonical">Canonical IDs</a></li>
  </ol>
</li>
<li><a href="#retry">Automatic Retry Using Exponential Back-Off</a></li>
<li><a href="#unreg">Unregistration</a>
  <ol>
    <li><a href="#unreg-why">Why you should rarely unregister</a></li>
    <li><a href="#unreg-how">How unregistration works</a></li>
  </ol>
</li>
<li><a href="#collapsible">Send-to-Sync vs. Messages with Payload</a>
  <ol>
    <li><a href="#s2s">Send-to-sync messages</a></li>
    <li><a href="#payload">Messages with payload</a></li>
<li><a href="#which">Which should I use?</a></li>
    </ol>
</li>
<li><a href="#ttl">Setting an Expiration Date for a Message</a> </li>
<li><a href="#throttling"></a><a href="#multi-senders">Receiving Messages from
Multiple Senders</a></li>
</ol>

</div>
</div>
<p>This document covers advanced topics for GCM.</p>




<h2 id="msg-lifetime">Lifetime of a Message</h2>
<p>When a 3rd-party server posts a message to GCM and receives a message ID back,
it does not mean that the message was already delivered to the device. Rather, it
means that it was accepted for delivery. What happens to the message after it is
accepted depends on many factors.</p>

<p>In the best-case scenario, if the device is connected to GCM, the screen is on,
and there are no throttling restrictions (see <a href="#throttling">Throttling</a>),
the message will be delivered right away.</p>

<p>If the device is connected but idle, the message will still be
delivered right away unless the <code>delay_while_idle</code> flag is set to true.
Otherwise, it will be stored in the GCM servers until the device is awake. And
that's where the <code>collapse_key</code> flag plays a role: if there is already
a message with the same collapse key (and registration ID) stored and waiting for
delivery, the old message will be discarded and the new message will take its place
(that is, the old message will be collapsed by the new one). However, if the collapse
key is not set, both the new and old messages are stored for future delivery.
Collapsible messages are also called <a href="#s2s">send-to-sync messages</a>.</p>

<p class="note"><strong>Note:</strong> There is a limit on how many messages can
be stored without collapsing. That limit is currently 100. If the limit is reached,
all stored messages are discarded. Then when the device is back online, it receives
a special message indicating that the limit was reached. The application can then
handle the situation properly, typically by requesting a full sync.
<br><br>
Likewise, there is a limit on how many <code>collapse_key</code>s you can have for
a particular device. GCM allows a maximum of 4 different collapse keys to be used
by the GCM server per device
any given time. In other words, the GCM server can simultaneously store 4 different
send-to-sync messages, each with a different collapse key. If you exceed this number
GCM will only keep 4 collapse keys, with no guarantees about which ones they will be.
See <a href="#s2s">Send-to-sync messages</a> for more information.
</p>

<p>If the device is not connected to GCM, the message will be stored until a
connection is established (again respecting the collapse key rules). When a connection
is established, GCM will deliver all pending messages to the device, regardless of
the <code>delay_while_idle</code> flag. If the device never gets connected again
(for instance, if it was factory reset), the message will eventually time out and
be discarded from GCM storage. The default timeout is 4 weeks, unless the
<code>time_to_live</code> flag is set.</p>

<p>Finally, when GCM attempts to deliver a message to the device and the
application was uninstalled, GCM will discard that message right away and
invalidate the registration ID. Future attempts to send a message to that device
will get a <code>NotRegistered</code> error. See <a href="#unreg">
How Unregistration Works</a> for more information.</p>
<p>Although is not possible to track the status of each individual message, the
Google Cloud Console stats are broken down by messages sent to device, messages
collapsed, and messages waiting for delivery.</p>

<h2 id="throttling">Throttling</h2>
<p>To prevent abuse (such as sending a flood of messages to a device) and
to optimize for the overall network efficiency and battery life of
devices, GCM implements throttling of messages using a token bucket
scheme. Messages are throttled on a per application and per <a href="#collapsible">collapse
key</a> basis (including non-collapsible messages). Each application
collapse key is granted some initial tokens, and new tokens are granted
periodically therefter. Each token is valid for a single message sent to
the device. If an application collapse key exhausts its supply of
available tokens, new messages are buffered in a pending queue until
new tokens become available at the time of the periodic grant. Thus
throttling in between periodic grant intervals may add to the latency
of message delivery for an application collapse key that sends a large
number of messages within a short period of time. Messages in the pending
queue of an application collapse key may be delivered before the time
of the next periodic grant, if they are piggybacked with messages
belonging to a non-throttled category by GCM for network and battery
efficiency reasons.</p>

<h2 id="reg-state">Keeping the Registration State in Sync</h2>
<p>Whenever the application registers as described in
<a href="{@docRoot}google/gcm/client.html">Implementing GCM Client</a>,
it should save the registration ID for future use, pass it to the
3rd-party server to complete the registration, and keep track of
whether the server completed the registration. If the server fails
to complete the registration, it should try again or unregister from GCM.</p>

<p>There are also two other scenarios that require special care:</p>
<ul>
  <li>Application update</li>
  <li>Backup and restore
  </li>
</ul>
<p>When an application is updated, it should invalidate its existing registration
ID, as it is not guaranteed to work with the new version.  Because there is no
lifecycle method called when the application is updated, the best way to achieve
this validation is by storing the current application version when a registration
ID is stored. Then when the application is started, compare the stored value with
the current application version. If they do not match, invalidate the stored data
and start the registration process again.</p>

<p>Similarly, you should not save the registration ID when an application is
backed up. This is because the registration ID could become invalid by the time
the application is restored, which would put the application in an invalid state
(that is, the application thinks it is registered, but the server and GCM do not
store that registration ID anymore&mdash;thus the application will not get more
messages).</p>
<h3 id="canonical">Canonical IDs</h3>
<p>On the server side, as long as the application is behaving well, everything
should work normally. However, if a bug in the application triggers multiple
registrations for the same device, it can be hard to reconcile state and you might
end up with duplicate messages.</p>
<p>GCM provides a facility called &quot;canonical registration IDs&quot; to easily
recover from these situations. A canonical registration ID is defined to be the ID
of the last registration requested by your application. This is the ID that the
server should use when sending messages to the device.</p>
<p>If later on you try to send a message using a different registration ID, GCM
will process the request as usual, but it will include the canonical registration
ID in the <code>registration_id</code> field of the response. Make sure to replace
the registration ID stored in your server with this canonical ID, as eventually
the ID you're using will stop working.</p>

<h2 id="retry">Automatic Retry Using Exponential Back-Off</h2>

<p>When registration or unregistration fails, the app should retry the failed operation.</p>
<p>In the simplest case, if your application attempts to register and GCM is not a
fundamental part of the application, the application could simply ignore the error
and try to register again the next time it starts. Otherwise, it should retry the
previous operation using exponential back-off. In exponential back-off, each time
there is a failure, it should wait twice the previous amount of time before trying
again. If the register (or unregister) operation was synchronous, it could be retried
in a simple loop. However, since it is asynchronous, the best approach is to schedule
a {@link android.app.PendingIntent} to retry the operation.

<h2 id="unreg">Unregistration</h2>

<p>This section explains when you should unregister in GCM and what happens
when you do.</p>

<h3 id="unreg-why">Why you should rarely unregister</h3>

<p>A registration ID (regID) represents a particular Android application running
on a particular device. You should only need to unregister in rare cases, such as
if you want an app to stop receiving messages, or if you suspect that the regID has
been compromised. In general, though, once an app has a regID, you shouldn't need
to change it.</p>

<p>In particular, you should never unregister your app as a mechanism for
logout or for switching between users, for the following reasons:</p>

<ul>
  <li>A regID maps an app to a device. It isn't associated with a particular
  logged in user. If you unregister and then re-register, GCM may return the same
  ID or a different ID&mdash;there's no guarantee either way.</li>

  <li>Unregistration may take up to 5 minutes to propagate.</li>
  <li>After unregistration, re-registration may again take up to 5 minutes to
propagate. During this time messages may be rejected due to the state of being
unregistered, and after all this, messages may still go to the wrong user.</li>
</ul>


<p>The solution is to manage your own mapping between users, the regID, and
individual messages:</p>

<ul>
  <li>Your app server should maintain a mapping between the current user
and the regID. This should include information about which user is supposed to
receive a particular message.</li>
  <li>The app running on the device should check to ensure that messages it
receives match the logged in user.</li>
</ul>


<h3 id="unreg-how">How unregistration works</h3>

<p>An application can be automatically unregistered after it is uninstalled from
the device. However, this process does not happens right away, as Android does not
provide an uninstall callback. What happens in this scenario is as follows:</p>
<ol>
  <li>The end user uninstalls the application.</li>
  <li>The 3rd-party server sends a message to GCM server.</li>
  <li>The GCM server sends the message to the device.</li>
  <li>The GCM client receives the message and queries Package Manager about
whether there are broadcast receivers configured to receive it, which returns
<code>false</code>.
</li>
  <li>The GCM client informs the GCM server that the application was uninstalled.</li>
  <li>The GCM server marks the registration ID for deletion.</li>
  <li>The 3rd-party server sends a message to  GCM.</li>
  <li>The GCM returns a <code>NotRegistered</code> error message to the 3rd-party server.</li>
  <li>The 3rd-party deletes the registration ID.
  </li>
</ol>

<p class ="note"><strong>Note:</strong> The GCM client is the Google Cloud
Messaging framework present on the device.</p>

<p>Note that it might take a while for the registration ID be completely removed
from GCM. Thus it is possible that messages sent during step 7 above gets a valid
message ID as response, even though the message will not be delivered to the device.
Eventually, the registration ID will be removed and the server will get a
<code>NotRegistered</code> error, without any further action being required from
the 3rd-party server (this scenario happens frequently while an application is
being developed and tested).</p>

<h2 id="collapsible">Send-to-Sync  vs. Messages with Payload</h2>

<p>Every message sent in GCM has the following characteristics:</p>
<ul>
  <li>It has a payload limit of 4096 bytes.</li>
  <li>By default, it is stored by GCM for 4 weeks.</li>
</ul>

<p>But despite these similarities, messages can behave very differently depending
on their particular settings. One major distinction between messages is whether
they are collapsed (where each new message replaces the preceding message) or not
collapsed (where each individual message is delivered). Every message sent in GCM
is either a &quot;send-to-sync&quot; (collapsible) message or a &quot;message with
payload&quot; (non-collapsible message). These concepts are described in more
detail in the following sections.</p>

<h3 id="s2s">Send-to-sync messages</h3>

<p>A send-to-sync (collapsible) message is often a &quot;tickle&quot; that tells
a mobile application to sync data from the server. For example, suppose you have
an email application. When a user receives new email on the server, the server
pings the mobile application with a &quot;New mail&quot; message. This tells the
application to sync to the server to pick up the new email. The server might send
this message multiple times as new mail continues to accumulate, before the application
has had a chance to sync. But if the user has received 25 new emails, there's no
need to preserve every &quot;New mail&quot; message. One is sufficient. Another
example would be a sports application that updates users with the latest score.
Only the most recent message is relevant, so it makes sense to have each new
message replace the preceding message. </p>

<p>The email and sports applications are cases where you would probably use the
GCM <code>collapse_key</code> parameter. A <em>collapse key</em> is an arbitrary
string that is used to collapse a group of like messages when the device is offline,
so that only the most recent message gets sent to the client. For example,
&quot;New mail,&quot; &quot;Updates available,&quot; and so on</p>
<p>GCM allows a maximum of 4 different collapse keys to be used by the GCM server
at any given time. In other words, the GCM server can simultaneously store 4
different send-to-sync messages per device, each with a different collapse key.
For example, Device A can have A1, A2, A3, and A4. Device B can have B1, B2, B3,
and B4, and so on. If you exceed this number GCM will only keep 4 collapse keys, with no
guarantees about which ones they will be.</p>

<h3 id="payload">Messages with payload</h3>

<p>Unlike a send-to-sync message, every &quot;message with payload&quot;
(non-collapsible message) is delivered. The payload the message contains can be
up to 4kb. For example, here is a JSON-formatted message in an IM application in
which spectators are discussing a sporting event:</p>

<pre class="prettyprint pretty-json">{
  "registration_id" : "APA91bHun4MxP5egoKMwt2KZFBaFUH-1RYqx...",
  "data" : {
    "Nick" : "Mario",
    "Text" : "great match!",
    "Room" : "PortugalVSDenmark",
  },
}</pre>

<p>A &quot;message with payload&quot; is not simply a &quot;ping&quot; to the
mobile application to contact the server to fetch data. In the aforementioned IM
application, for example, you would want to deliver every message, because every
message has different content. To specify a non-collapsible message, you simply
omit the <code>collapse_key</code> parameter. Thus GCM will send each message
individually. Note that the order of delivery is not guaranteed.</p>

<p>GCM will store up to 100 non-collapsible messages. After that, all messages
are discarded from GCM, and a new message is created that tells the client how
far behind it is. The message is delivered through a regular
<code>com.google.android.c2dm.intent.RECEIVE</code> intent with the
extra <code>message_type</code>, for which the value is always the string
&quot;deleted_messages&quot;.</p>

<p>The application should respond by syncing with the server to recover the
discarded messages. </p>

<h3 id="which">Which should I use?</h3>
  <p>If your application does not need to use non-collapsible messages, collapsible
messages are a better choice from a performance standpoint, because they put less
of a burden on the device battery. However, if you use collapsible messages, remember that
<strong>GCM only allows a maximum of 4 different collapse keys to be used by the GCM server
per device at any given time</strong>. You must not exceed this number, or it could cause
unpredictable consequences.</p>

<h2 dir="ltr" id="ttl">Setting an Expiration Date for a Message</h2>
<p>The Time to Live (TTL) feature lets  the sender  specify the maximum lifespan
of a message using the <code>time_to_live</code> parameter in the send request.
The value of this parameter must be a duration from 0 to 2,419,200 seconds, and
it corresponds to the maximum period of time for which GCM will store and try to
deliver the message. Requests that don't contain this field default to the maximum
period of 4 weeks.</p>
<p>Here are some possible uses for this feature:</p>
<ul>
  <li>Video chat incoming calls</li>
  <li>Expiring invitation events</li>
  <li>Calendar events</li>
</ul>
<h3 id="bg">Background </h3>
<p>GCM will usually deliver messages immediately after they are sent. However,
this might not always be possible. For example, the device could be turned off,
offline, or otherwise unavailable. In other cases, the sender itself might request
that messages not be delivered until the device becomes active by using the
<code>delay_while_idle</code> flag. Finally, GCM might intentionally delay messages
to prevent an application from consuming excessive resources and negatively
impacting battery life.</p>

<p>When this happens, GCM will store the message and deliver it as soon as it's
feasible. While this is fine in most cases, there are some applications for which
a late message might as well never be delivered. For example, if the message is
an incoming call or video chat notification, it will only be meaningful for a
small period of time before the call is terminated. Or if the message is an
invitation to an event, it will be useless if received after the event has ended.</p>

<p>Another advantage of specifying the expiration date for a message is that GCM
will never throttle messages with a <code>time_to_live</code> value of 0 seconds.
In other words, GCM will guarantee best effort for messages that must be delivered
&quot;now or never.&quot; Keep in mind that a <code>time_to_live</code> value of
0 means messages that can't be delivered immediately will be discarded. However,
because such messages are never stored, this provides the best latency for
sending notifications.</p>

<p>Here is an example of a JSON-formatted request that includes TTL:</p>
<pre class="prettyprint pretty-json">
{
  "collapse_key" : "demo",
  "delay_while_idle" : true,
  "registration_ids" : ["xyz"],
  "data" : {
    "key1" : "value1",
    "key2" : "value2",
  },
  "time_to_live" : 3
},
</pre>


<h2 id="multi-senders">Receiving Messages from Multiple Senders</h2>

<p>GCM allows multiple parties to send messages to the same application. For
example, suppose your application is an articles aggregator with multiple
contributors, and you want each of them to be able to send a message when they
publish a new article. This message might contain a URL so that the application
can download the article. Instead of having to centralize all sending activity in
one location, GCM gives you the ability to let each of these contributors send
its own messages.</p>

<p>To make this possible, all you need to do is have each sender generate its own
project number. Then include those IDs in the sender field, separated by commas,
when requesting a registration. Finally, share the registration ID with your
partners, and they'll be able to send messages to your application using their
own authentication keys.</p>
<p>This code snippet illustrates this feature. Senders are passed as an intent
extra in a comma-separated list:</p>

<pre class="prettyprint pretty-java">Intent intent = new Intent(GCMConstants.INTENT_TO_GCM_REGISTRATION);
intent.setPackage(GSF_PACKAGE);
intent.putExtra(GCMConstants.EXTRA_APPLICATION_PENDING_INTENT,
        PendingIntent.getBroadcast(context, 0, new Intent(), 0));
String senderIds = &quot;968350041068,652183961211&quot;;
intent.putExtra(GCMConstants.EXTRA_SENDER, senderIds);
ontext.startService(intent);
 </pre>

<p>Note that there is limit of 100 multiple senders.</p>