+++ /dev/null
-page.title=Syncing with App Engine
-parent.title=Syncing to the Cloud
-parent.link=index.html
-
-trainingnavtop=true
-next.title=Using the Backup API
-next.link=backupapi.html
-
-@jd:body
-
-<div id="tb-wrapper">
-<div id="tb">
-
-<!-- table of contents -->
-<h2>This lesson teaches you how to</h2>
-<ol>
- <li><a href="#prepare">Prepare Your Environment</a></li>
- <li><a href="#project">Create Your Project</a></li>
- <li><a href="#data">Create the Data Layer</a></li>
- <li><a href="#persistence">Create the Persistence Layer</a></li>
- <li><a href="#androidapp">Query and Update from the Android App</a></li>
- <li><a href="#serverc2dm">Configure the C2DM Server-Side</a></li>
- <li><a href="#clientc2dm">Configure the C2DM Client-Side</a></li>
-</ol>
-<h2>You should also read</h2>
- <ul>
- <li><a
- href="http://developers.google.com/appengine/">App Engine</a></li>
- <li><a href="http://code.google.com/android/c2dm/">Android Cloud to Device
- Messaging Framework</a></li>
- </ul>
-<h2>Try it out</h2>
-
-<p>This lesson uses the Cloud Tasks sample code, originally shown at the
-<a href="http://www.youtube.com/watch?v=M7SxNNC429U">Android + AppEngine: A Developer's Dream Combination</a>
-talk at Google I/O. You can use the sample application as a source of reusable code for your own
-application, or simply as a reference for how the Android and cloud pieces of the overall
-application fit together. You can also build the sample application and see how it runs
-on your own device or emulator.</p>
-<p>
- <a href="http://code.google.com/p/cloud-tasks-io/" class="button">Cloud Tasks
- App</a>
-</p>
-
-</div>
-</div>
-
-<p>Writing an app that syncs to the cloud can be a challenge. There are many little
-details to get right, like server-side auth, client-side auth, a shared data
-model, and an API. One way to make this much easier is to use the Google Plugin
-for Eclipse, which handles a lot of the plumbing for you when building Android
-and App Engine applications that talk to each other. This lesson walks you through building such a project.</p>
-
-<p>Following this lesson shows you how to:</p>
-<ul>
- <li>Build Android and Appengine apps that can communicate with each other</li>
- <li>Take advantage of Cloud to Device Messaging (C2DM) so your Android app doesn't have to poll for updates</li>
-</ul>
-
-<p>This lesson focuses on local development, and does not cover distribution
-(i.e, pushing your App Engine app live, or publishing your Android App to
-market), as those topics are covered extensively elsewhere.</p>
-
-<h2 id="prepare">Prepare Your Environment</h2>
-<p>If you want to follow along with the code example in this lesson, you must do
-the following to prepare your development environment:</p>
-<ul>
-<li>Install the <a href="http://code.google.com/eclipse/">Google Plugin for
- Eclipse.</a></li>
-<li>Install the <a
- href="http://code.google.com/webtoolkit/download.html">GWT SDK</a> and the <a
- href="http://code.google.com/appengine/">Java App Engine SDK</a>. The <a
- href="http://code.google.com/eclipse/docs/getting_started.html">Quick Start
- Guide</a> shows you how to install these components.</li>
-<li>Sign up for <a href="http://code.google.com/android/c2dm/signup.html">C2DM
- access</a>. We strongly recommend <a
- href="https://accounts.google.com/SignUp">creating a new Google account</a> specifically for
-connecting to C2DM. The server component in this lesson uses this <em>role
- account</em> repeatedly to authenticate with Google servers.
-</li>
-</ul>
-
-<h2 id="project">Create Your Projects</h2>
-<p>After installing the Google Plugin for Eclipse, notice that a new kind of Android project
-exists when you create a new Eclipse project: The <strong>App Engine Connected
- Android Project</strong> (under the <strong>Google</strong> project category).
-A wizard guides you through creating this project,
-during the course of which you are prompted to enter the account credentials for the role
-account you created.</p>
-
-<p class="note"><strong>Note:</strong> Remember to enter the credentials for
-your <i>role account</i> (the one you created to access C2DM services), not an
-account you'd log into as a user, or as an admin.</p>
-
-<p>Once you're done, you'll see two projects waiting for you in your
-workspace—An Android application and an App Engine application. Hooray!
-These two applications are already fully functional— the wizard has
-created a sample application which lets you authenticate to the App Engine
-application from your Android device using AccountManager (no need to type in
-your credentials), and an App Engine app that can send messages to any logged-in
-device using C2DM. In order to spin up your application and take it for a test
-drive, do the following:</p>
-
-<p>To spin up the Android application, make sure you have an AVD with a platform
-version of <em>at least</em> Android 2.2 (API Level 8). Right click on the Android project in
-Eclipse, and go to <strong>Debug As > Local App Engine Connected Android
- Application</strong>. This launches the emulator in such a way that it can
-test C2DM functionality (which typically works through Google Play). It'll
-also launch a local instance of App Engine containing your awesome
-application.</p>
-
-<h2 id="data">Create the Data Layer</h2>
-
-<p>At this point you have a fully functional sample application running. Now
-it's time to start changing the code to create your own application.</p>
-
-<p>First, create the data model that defines the data shared between
-the App Engine and Android applications. To start, open up the source folder of
-your App Engine project, and navigate down to the <strong>(yourApp)-AppEngine
- > src > (yourapp) > server</strong> package. Create a new class in there containing some data you want to
-store server-side. The code ends up looking something like this:</p>
-<pre>
-package com.cloudtasks.server;
-
-import javax.persistence.*;
-
-@Entity
-public class Task {
-
- private String emailAddress;
- private String name;
- private String userId;
- private String note;
-
- @Id
- @GeneratedValue(strategy = GenerationType.IDENTITY)
- private Long id;
-
- public Task() {
- }
-
- public String getEmailAddress() {
- return this.emailAddress;
- }
-
- public Long getId() {
- return this.id;
- }
- ...
-}
-</pre>
-<p>Note the use of annotations: <code>Entity</code>, <code>Id</code> and
-<code>GeneratedValue</code> are all part of the <a
- href="http://www.oracle.com/technetwork/articles/javaee/jpa-137156.html">Java
- Persistence API</a>. Essentially, the <code>Entity</code> annotation goes
-above the class declaration, and indicates that this class represents an entity
-in your data layer. The <code>Id</code> and <code>GeneratedValue</code>
-annotations, respectively, indicate the field used as a lookup key for this
-class, and how that id is generated (in this case,
-<code>GenerationType.IDENTITY</code> indicates that the is generated by
-the database). You can find more on this topic in the App Engine documentation,
-on the page <a
- href="http://code.google.com/appengine/docs/java/datastore/jpa/overview.html">Using
- JPA with App Engine</a>.</p>
-
-<p>Once you've written all the classes that represent entities in your data
-layer, you need a way for the Android and App Engine applications to communicate
-about this data. This communication is enabled by creating a Remote Procedure
-Call (RPC) service.
-Typically, this involves a lot of monotonous code. Fortunately, there's an easy way! Right
-click on the server project in your App Engine source folder, and in the context
-menu, navigate to <strong>New > Other</strong> and then, in the resulting
-screen, select <strong>Google > RPC Service.</strong> A wizard appears, pre-populated
-with all the Entities you created in the previous step,
-which it found by seeking out the <code>@Entity</code> annotation in the
-source files you added. Pretty neat, right? Click <strong>Finish</strong>, and the wizard
-creates a Service class with stub methods for the Create, Retrieve, Update and
-Delete (CRUD) operations of all your entities.</p>
-
-<h2 id="persistence">Create the Persistence Layer</h2>
-
-<p>The persistence layer is where your application data is stored
-long-term, so any information you want to keep for your users needs to go here.
-You have several options for writing your persistence layer, depending on
-what kind of data you want to store. A few of the options hosted by Google
-(though you don't have to use these services) include <a
- href="http://code.google.com/apis/storage/">Google Storage for Developers</a>
-and App Engine's built-in <a
- href="http://code.google.com/appengine/docs/java/gettingstarted/usingdatastore.html">Datastore</a>.
-The sample code for this lesson uses DataStore code.</p>
-
-<p>Create a class in your <code>com.cloudtasks.server</code> package to handle
-persistence layer input and output. In order to access the data store, use the <a
- href="http://db.apache.org/jdo/api20/apidocs/javax/jdo/PersistenceManager.html">PersistenceManager</a>
-class. You can generate an instance of this class using the PMF class in the
-<code>com.google.android.c2dm.server.PMF</code> package, and then use that to
-perform basic CRUD operations on your data store, like this:</p>
-<pre>
-/**
-* Remove this object from the data store.
-*/
-public void delete(Long id) {
- PersistenceManager pm = PMF.get().getPersistenceManager();
- try {
- Task item = pm.getObjectById(Task.class, id);
- pm.deletePersistent(item);
- } finally {
- pm.close();
- }
-}
-</pre>
-
-<p>You can also use <a
- href="http://code.google.com/appengine/docs/python/datastore/queryclass.html">Query</a>
-objects to retrieve data from your Datastore. Here's an example of a method
-that searches out an object by its ID.</p>
-
-<pre>
-public Task find(Long id) {
- if (id == null) {
- return null;
- }
-
- PersistenceManager pm = PMF.get().getPersistenceManager();
- try {
- Query query = pm.newQuery("select from " + Task.class.getName()
- + " where id==" + id.toString() + " && emailAddress=='" + getUserEmail() + "'");
- List<Task> list = (List<Task>) query.execute();
- return list.size() == 0 ? null : list.get(0);
- } catch (RuntimeException e) {
- System.out.println(e);
- throw e;
- } finally {
- pm.close();
- }
-}
-</pre>
-
-<p>For a good example of a class that encapsulates the persistence layer for
-you, check out the <a
- href="http://code.google.com/p/cloud-tasks-io/source/browse/trunk/CloudTasks-AppEngine/src/com/cloudtasks/server/DataStore.java">DataStore</a>
-class in the Cloud Tasks app.</p>
-
-
-
-<h2 id="androidapp">Query and Update from the Android App</h2>
-
-<p>In order to keep in sync with the App Engine application, your Android application
-needs to know how to do two things: Pull data from the cloud, and send data up
-to the cloud. Much of the plumbing for this is generated by the
-plugin, but you need to wire it up to your Android user interface yourself.</p>
-
-<p>Pop open the source code for the main Activity in your project and look for
-<code><YourProjectName> Activity.java</code>, then for the method
-<code>setHelloWorldScreenContent()</code>. Obviously you're not building a
-HelloWorld app, so delete this method entirely and replace it
-with something relevant. However, the boilerplate code has some very important
-characteristics. For one, the code that communicates with the cloud is wrapped
-in an {@link android.os.AsyncTask} and therefore <em>not</em> hitting the
-network on the UI thread. Also, it gives an easy template for how to access
-the cloud in your own code, using the <a
- href="http://code.google.com/webtoolkit/doc/latest/DevGuideRequestFactory.html">RequestFactory</a>
-class generated that was auto-generated for you by the Eclipse plugin (called
-MyRequestFactory in the example below), and various {@code Request} types.</p>
-
-<p>For instance, if your server-side data model included an object called {@code
-Task} when you generated an RPC layer it automatically created a
-{@code TaskRequest} class for you, as well as a {@code TaskProxy} representing the individual
-task. In code, requesting a list of all these tasks from the server looks
-like this:</p>
-
-<pre>
-public void fetchTasks (Long id) {
- // Request is wrapped in an AsyncTask to avoid making a network request
- // on the UI thread.
- new AsyncTask<Long, Void, List<TaskProxy>>() {
- @Override
- protected List<TaskProxy> doInBackground(Long... arguments) {
- final List<TaskProxy> list = new ArrayList<TaskProxy>();
- MyRequestFactory factory = Util.getRequestFactory(mContext,
- MyRequestFactory.class);
- TaskRequest taskRequest = factory.taskNinjaRequest();
-
- if (arguments.length == 0 || arguments[0] == -1) {
- factory.taskRequest().queryTasks().fire(new Receiver<List<TaskProxy>>() {
- @Override
- public void onSuccess(List<TaskProxy> arg0) {
- list.addAll(arg0);
- }
- });
- } else {
- newTask = true;
- factory.taskRequest().readTask(arguments[0]).fire(new Receiver<TaskProxy>() {
- @Override
- public void onSuccess(TaskProxy arg0) {
- list.add(arg0);
- }
- });
- }
- return list;
- }
-
- @Override
- protected void onPostExecute(List<TaskProxy> result) {
- TaskNinjaActivity.this.dump(result);
- }
-
- }.execute(id);
-}
-...
-
-public void dump (List<TaskProxy> tasks) {
- for (TaskProxy task : tasks) {
- Log.i("Task output", task.getName() + "\n" + task.getNote());
- }
-}
-</pre>
-
-<p>This {@link android.os.AsyncTask} returns a list of
-<code>TaskProxy</code> objects, and sends it to the debug {@code dump()} method
-upon completion. Note that if the argument list is empty, or the first argument
-is a -1, all tasks are retrieved from the server. Otherwise, only the ones with
-IDs in the supplied list are returned. All the fields you added to the task
-entity when building out the App Engine application are available via get/set
-methods in the <code>TaskProxy</code> class.</p>
-
-<p>In order to create new tasks and send them to the cloud, create a request
-object and use it to create a proxy object. Then populate the proxy object and
-call its update method. Once again, this should be done in an
-<code>AsyncTask</code> to avoid doing networking on the UI thread. The end
-result looks something like this.</p>
-
-<pre>
-new AsyncTask<Void, Void, Void>() {
- @Override
- protected Void doInBackground(Void... arg0) {
- MyRequestFactory factory = (MyRequestFactory)
- Util.getRequestFactory(TasksActivity.this,
- MyRequestFactory.class);
- TaskRequest request = factory.taskRequest();
-
- // Create your local proxy object, populate it
- TaskProxy task = request.create(TaskProxy.class);
- task.setName(taskName);
- task.setNote(taskDetails);
- task.setDueDate(dueDate);
-
- // To the cloud!
- request.updateTask(task).fire();
- return null;
- }
-}.execute();
-</pre>
-
-<h2 id="serverc2dm">Configure the C2DM Server-Side</h2>
-
-<p>In order to set up C2DM messages to be sent to your Android device, go back
-into your App Engine codebase, and open up the service class that was created
-when you generated your RPC layer. If the name of your project is Foo,
-this class is called FooService. Add a line to each of the methods for
-adding, deleting, or updating data so that a C2DM message is sent to the
-user's device. Here's an example of an update task:
-</p>
-
-<pre>
-public static Task updateTask(Task task) {
- task.setEmailAddress(DataStore.getUserEmail());
- task = db.update(task);
- DataStore.sendC2DMUpdate(TaskChange.UPDATE + TaskChange.SEPARATOR + task.getId());
- return task;
-}
-
-// Helper method. Given a String, send it to the current user's device via C2DM.
-public static void sendC2DMUpdate(String message) {
- UserService userService = UserServiceFactory.getUserService();
- User user = userService.getCurrentUser();
- ServletContext context = RequestFactoryServlet.getThreadLocalRequest().getSession().getServletContext();
- SendMessage.sendMessage(context, user.getEmail(), message);
-}
-</pre>
-
-<p>In the following example, a helper class, {@code TaskChange}, has been created with a few
-constants. Creating such a helper class makes managing the communication
-between App Engine and Android apps much easier. Just create it in the shared
-folder, define a few constants (flags for what kind of message you're sending
-and a seperator is typically enough), and you're done. By way of example,
-the above code works off of a {@code TaskChange} class defined as this:</p>
-
-<pre>
-public class TaskChange {
- public static String UPDATE = "Update";
- public static String DELETE = "Delete";
- public static String SEPARATOR = ":";
-}
-</pre>
-
-<h2 id="clientc2dm">Configure the C2DM Client-Side</h2>
-
-<p>In order to define the Android applications behavior when a C2DM is recieved,
-open up the <code>C2DMReceiver</code> class, and browse to the
-<code>onMessage()</code> method. Tweak this method to update based on the content
-of the message.</p>
-<pre>
-//In your C2DMReceiver class
-
-public void notifyListener(Intent intent) {
- if (listener != null) {
- Bundle extras = intent.getExtras();
- if (extras != null) {
- String message = (String) extras.get("message");
- String[] messages = message.split(Pattern.quote(TaskChange.SEPARATOR));
- listener.onTaskUpdated(messages[0], Long.parseLong(messages[1]));
- }
- }
-}
-</pre>
-
-<pre>
-// Elsewhere in your code, wherever it makes sense to perform local updates
-public void onTasksUpdated(String messageType, Long id) {
- if (messageType.equals(TaskChange.DELETE)) {
- // Delete this task from your local data store
- ...
- } else {
- // Call that monstrous Asynctask defined earlier.
- fetchTasks(id);
- }
-}
-</pre>
-<p>
-Once you have C2DM set up to trigger local updates, you're all done.
-Congratulations, you have a cloud-connected Android application!</p>
--- /dev/null
+page.title=Making the Most of Google Cloud Messaging
+parent.title=Syncing to the Cloud
+parent.link=index.html
+
+trainingnavtop=true
+
+previous.title=Using the Backup API
+previous.link=backupapi.html
+
+@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/guide/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="http://developer.google.com/android/gcm/demo">GCM
+ 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="http://developer.android.com/guide/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="http://developer.android.com/guide/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> — 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> — Better: Your app knows to check
+ a specific feed for updates.</li>
+ <li><code>refresh</code>, <code>feedID</code>, <code>timestamp</code> —
+ 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>
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