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<h1 id="gcm">Google Cloud Messaging</h1>

<p>Google Cloud Messaging (GCM) is a service for both Android-powered device and
Chrome instances to send and receive message data from servers. <a href="gcm">GCM Chrome
API</a> allows the Chrome apps or extensions to access the GCM
service for the signed-in Chrome users. The service works even if an app or
extension isn't currently running. For example, calendar updates could be pushed
to users even when their calendar app isn't open. </p>

<p>To use both API and service, you must agree to the <a href="https://developers.google.com/terms/">Google APIs Terms of
Service</a> and <a href="https://developers.google.com/cloud/terms/">Google Cloud Platform Terms
of Service</a>.</p>

<p>This document describes how to set up and use GCM. For additional information
see the reference documentation for the <a href="gcm">GCM Chrome API</a> and
the <a href="http://developer.android.com/google/gcm/index.html">GCM Service</a>. To get
help with GCM or to give us feedback, please see
<a href="#feedback">Feedback</a>.</p>

<p>Please note that GCM Chrome API is experimental. It is only available to Chrome
users on the dev channel.</p>

<p class="note"><strong>API Samples</strong>: Want to play with the code? Check out the
<a href="https://github.com/GoogleChrome/chrome-app-samples/tree/master/samples/gcm-notifications">gcm-notifications</a>
sample.</p>

<h2 id="enable_gcm">Enable GCM</h2>

<p>To use GCM for your app or extension, do the following:</p>

<ol>
<li>Upload your app or extension client to the <a href="https://developers.google.com/chrome/web-store/docs/publish">Chrome Web
Store</a>.</li>
<li>A user installs your app or extension.</li>
<li>Your app or extension client requests a registration ID using $(ref:gcm.register)
function and sends this ID to your server together with information
identifying the user. For more details, please see <a href="#obtain_registration_id">Obtain registration
ID</a>.</li>
</ol>

<h2 id="receive_a_message">Receive a message</h2>

<p>At a glance, receiving a message works like this:</p>

<ol>
<li>Your app or extension client should register a handler to receive the
$(ref:gcm.onMessage) event.</li>
<li>When your server sends a message to the user, it specifies all of the
registration IDs that are related to that user and passes the message to the
GCM service.</li>
<li>GCM servers route the message to all instances of Chrome running apps or
extensions with one of the registration IDs.</li>
<li>When the message arrives on the client, Chrome starts the app or extension,
if it is not already running, and calls the registered handler to process the
message.</li>
</ol>

<p>For more details, please see <a href="#receive_messages">Receive messages</a>.</p>

<h2 id="send_a_message">Send a message</h2>

<p>At a glance, sending a message works like this:</p>

<ol>
<li>Your app or extension generates a unique message ID, so that it is possible
to find out which message failed to be queued or delivered.</li>
<li>Your app or extension calls $(ref:gcm.send) function with an object containing
message ID, destination ID that identifies the server, and data that consist
of a list of key value pairs. In this step you can also provide an optional
time-to-live for the message.</li>
<li>The callback passed to $(ref:gcm.send) function will be called to notify the
result. Your app or extension should check $(ref:runtime.lastError) to find out if
the message has been successfully queued for delivery. Please refer to <a href="#error_reference">Error
reference</a> for possible error codes that could be
returned.</li>
<li>In cases that the queued message could not be delivered to the GCM server,
like network error, a $(ref:gcm.onSendError) event will be fired. Your app or
extension can listen to this event and react to it, e.g. by trying to resend
the message. Please refer to <a href="#error_reference">Error reference</a> for
possible error codes that could be returned.</li>
</ol>

<p>For more details, please see <a href="#send_messages">Send messages</a>.</p>

<h2 id="set_up_project">Set up project</h2>

<h3 id="create_google_api_project">Create Google API project</h3>

<ol>
<li>Login to the <a href="https://cloud.google.com/console/project">Google Developers
Console</a> using the same Google <br>
Account that you will use to upload your app or extension.</li>
<li>If you haven't created an API project yet, click <strong>Create Project</strong>.</li>
<li>Supply a project name and click <strong>Create</strong>.</li>
<li>Once the project has been created, a page appears that displays your project
number. For example, <strong>Project Number: 670330094152</strong>.</li>
<li>Copy down your project number. You will use it later on as the GCM sender ID.</li>
</ol>

<h3 id="enable_the_gcm_service">Enable the GCM service</h3>

<ol>
<li>In the sidebar on the left, select <strong>APIs &amp; auth</strong>.</li>
<li>In the displayed list of APIs, turn the <strong>Google Cloud Messaging for
Android</strong> toggle to ON.</li>
</ol>

<h2 id="set_up_chrome_app_or_extension">Set up Chrome App or Extension</h2>

<h3 id="add_permission_to_manifest">Add permission to manifest</h3>

<p>To use the gcm service, you must declare the <code>gcm</code> permission in
<code>manifest.json</code>.</p>

<pre data-filename="manifest.json"><code>
"permissions": [
  "gcm", "storage", ...
]
</code></pre>

<p>Please note that the <code>storage</code> permission is also provided here because the
sample codes below needs to persist some data via the
<a href="storage">chrome.storage</a> API.</p>

<h2 id="write_chrome_app_or_extension">Write Chrome App or Extension</h2>

<h3 id="obtain_registration_id">Obtain registration ID</h3>

<p>Your app or extension needs to register with GCM servers before it can receive
messages. When an app or extension registers, it receives a registration ID.
This is achieved by calling $(ref:gcm.register) and specifying a list of senders
identified by project numbers from Google Developers Console. Your app or
extension should pass a callback function to verify that the registration was
successful. If successful, the received registration ID should be sent back to
your application server in a secure way, for example, via https. Otherwise, your
app or extension should handle the error identified by
<code>chrome.runtime.lastError</code> and retry later.</p>

<p>If your app or extension wishes to receive messages from the different senders,
it can call $(ref:gcm.register) again with the new sender list and the new
registration ID will be returned.</p>

<pre data-filename="background.js"><code>
function registerCallback(registrationId) {
  if (chrome.runtime.lastError) {
    // When the registration fails, handle the error and retry the
    // registration later.
    return;
  }

  // Send the registration ID to your application server.
  sendRegistrationId(function(succeed) {
    // Once the registration ID is received by your server,
    // set the flag such that register will not be invoked
    // next time when the app starts up.
    if (succeed)
      chrome.storage.local.set({registered: true});
  });
}

function sendRegistrationId(callback) {
  // Send the registration ID to your application server
  // in a secure way.
}

chrome.runtime.onStartup.addListener(function() {
  chrome.storage.local.get("registered", function(result) {
    // If already registered, bail out.
    if (result["registered"])
      return;

    // Up to 100 senders are allowed.
    var senderIds = ["Your-Sender-ID"];
    chrome.gcm.register(senderIds, registerCallback);
  });
});
</code></pre>

<p>If your app or extension is installed in different profiles and/or in different
Chrome instances, each of them will receive a different registration ID.</p>

<p>Your app or extension could call $(ref:gcm.unregister) to revoke the registration ID.
The unregistration should only be done in rare cases, such as if your app or
extension does not want to receive further messages, or the registration ID is
suspected to be compromised.</p>

<pre data-filename="background.js"><code>
function unregisterCallback() {
  if (chrome.runtime.lastError) {
    // When the unregistration fails, handle the error and retry
    // the unregistration later.
    return;
  }
}

chrome.gcm.unregister(unregisterCallback);
</code></pre>

<p>Your app or extension is automatically unregistered from the GCM service when a
user uninstalls it.</p>

<h3 id="receive_messages">Receive messages</h3>

<p>When your server sends a message to the user, it specifies all of the
registration IDs that are related to that user and passes the message to the GCM
service. GCM servers route the message to all instances of Chrome running apps
or extensions with one of the registration IDs. If your app or extension has
been installed in more than one profiles in a single Chrome instance, all of
them can receive messages independently based on their unique registration IDs.</p>

<p>Messages from the server are delivered via the $(ref:gcm.onMessage) event. Your app
or extension must register a handler to receive this event.</p>

<pre data-filename="background.js"><code>
chrome.gcm.onMessage.addListener(function(message) {
  // A message is an object with a data property that
  // consists of key-value pairs.
});
</code></pre>

<p>As long as Chrome is running, even if the extension or app is not running, it is
woken up to deliver a message.</p>

<h3 id="send_messages">Send messages</h3>

<p>To send messages upstream, your app or extension should call $(ref:gcm.send) with an
object containing:</p>

<ul>
<li>Message ID that identifies the message when it fails to be queued or
delivered. The message ID can be any kind of string. However, it is
recommended to stay unique across the lifetime of your app or extension, even
after it restarts. If you use the same message ID, there may be a chance that
the previous message gets overridden. If an auto-increment counter is used to
create the message ID, your app or extension should persist the counter value
via <a href="storage">chrome.storage</a> API and restore it when the app
reloads.</li>
<li>Destination ID that identifies the server. This is the project number from the
Google Developers Console plus the suffix "@gcm.googleapis.com".</li>
<li>Data that consist of a list of string-to-string key value pairs (up to 4KB
total).</li>
<li>Time-to-live (TTL, optional). This property value must be a duration from 0 to
2,419,200 seconds (4 weeks) and it corresponds to the maximum period of time
for which GCM will store and try to deliver the message. If this property is
not set, it is default to the maximum value. When a TTL is set to 0, GCM will
try to deliver the message immediately. If the immediate effort fails, the
message will be discarded.</li>
</ul>

<p>When the callback passed in $(ref:gcm.send) is called without runtime error, it does
not mean that the message was already delivered to the GCM server. Rather, it
means that it was queued for delivery. If the message fails to reach the
destination within the specified TTL period, for example due to network error,
the $(ref:gcm.onSendError) will be fired.</p>

<pre data-filename="background.js"><code>
// Substitute your own sender ID here. This is the project
// number you got from the Google Developers Console.
var senderId = "Your-Sender-ID";

// Make the message ID unique across the lifetime of your app.
// One way to achieve this is to use the auto-increment counter
// that is persisted to local storage.

// Message ID is saved to and restored from local storage.
var messageId = 0;
chrome.storage.local.get("messageId", function(result) {
  if (chrome.runtime.lastError)
    return;
  messageId = parseInt(result["messageId"]);
  if (isNaN(messageId))
    messageId = 0;
});

// Sets up an event listener for send error.
chrome.gcm.onSendError.addListener(sendError);

// Returns a new ID to identify the message.
function getMessageId() {
  messageId++;
  chrome.storage.local.set({messageId: messageId});
  return messageId.toString();
}

function sendMessage() {
  var message = {
    messageId: getMessageId(),
    destinationId: senderId + "@gcm.googleapis.com",
    timeToLive: 86400,    // 1 day
    data: {
      "key1": "value1",
      "key2": "value2"
    }
  };
  chrome.gcm.send(message, function(messageId) {
    if (chrome.runtime.lastError) {
      // Some error occurred. Fail gracefully or try to send
      // again.
      return;
    }

    // The message has been accepted for delivery. If the message
    // can not reach the destination, onSendError event will be
    // fired.
  });
}

function sendError(error) {
  console.log("Message " + error.messageId +
      " failed to be sent: " + error.errorMessage);
}
</code></pre>

<h3 id="messages_deleted_event">Messages deleted event</h3>

<p>GCM will store up to 100 non-collapsible messages. After that, all messages are
discarded from GCM, and an event $(ref:gcm.onMessagesDeleted) will be fired, which
tells the client that it falls behind. Your app or extension should respond by
syncing with your application server to recover the discarded messages.</p>

<pre data-filename="background.js"><code>
chrome.gcm.onMessagesDeleted.addListener(messagesDeleted);

function messagesDeleted() {
  // All messages have been discarded from GCM. Sync with
  // your application server to recover from the situation.
}
</code></pre>

<h3 id="collapsible_messages">Collapsible messages</h3>

<p>GCM messages are often a tickle, telling the app or extension to contact the
server for fresh data. In GCM, it's possible to create collapsible messages for
this situation, wherein new messages replace older ones. When a collapse key is
provided and multiple messages are queued up in the GCM servers for the same
user, only the last one with any given collapse key is delivered to your app or
extension. As a result the <code>message</code> object passed to the $(ref:gcm.onMessage) event
will contain a <code>collapseKey</code>field. For more details about sending collapsible
messages, please refer to <a href="http://developer.android.com/google/gcm/adv.html#collapsible">GCM Advance
Topics</a>.</p>

<h2 id="publish_your_app">Publish your app</h2>

<p>To use the GCM service, you must publish your app in the <a href="https://developers.google.com/chrome/web-store/docs/publish">Chrome Web
Store</a>.</p>

<h2 id="error_reference">Error reference</h2>

<p>An error could occur when a gcm API function is called. Your app or extension
should check $(ref:runtime.lastError) for more information in your callback. The
error code will also be passed as a parameter to $(ref:gcm.onSendError) event.</p>

<p>Here's a brief summary of the gcm errors:</p>

<ul>
<li><strong>Function was called with invalid parameters</strong>: this could happen when gcm
functions are called with bad parameters.</li>
<li><strong>Profile was not signed in</strong>: this could happen when gcm functions are called
from a profile that was not signed in.</li>
<li><strong>Asynchronous operation is pending</strong>: this could happen when certain gcm
function is called again without waiting for the callback passed in previous
function to be called.</li>
<li><strong>Network error occurred</strong>: this could happen when GCM server fails to reach
due to network problem, like losing Internet connection.</li>
<li><strong>Server error occurred</strong>: this could happen when GCM server fails to reach
due to server problem, like server busy.</li>
<li><strong>Time-to-live exceeded</strong>: this could happen when a message could not be
delivered within the specific time-to-live period.</li>
<li><strong>Unknown error occurred</strong>: this could happen due to any other internal
errors.</li>
</ul>

<h2 id="feedback">Feedback</h2>

<p>You can provide feedback about Google Cloud Messaging and the gcm API through
the Google Group <a href="https://groups.google.com/forum/#!forum/gcm-for-chrome-feedback">GCM for Chrome
Feedback</a>.
Use this group to ask for help, file bug reports, and request features.</p>