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Google Cloud Messaging in IOSched

This app uses Google Cloud Messaging (GCM) to know when to synchronize the conference data and the user's schedule. For an introduction to GCM, see

On startup, the application registers the device with the GCM server (see the BaseActivity.registerGCMClient method). This registration sends two pieces of data: the device‘s GCM id and the user’s GCM Key. The GCM id is a core GCM concept, and essentially consists of a long number that identifies the device for the purposes of sending GCM messages. The GCM key, however, is something we use only in this app and is not part of the normal GCM protocol. More about this soon.

The server can send the client several different types of GCM messages. When a GCM message arrives, it is processed by GCMIntentService.onMessage. That method will take the appropriate action depending on the content of the message. There are several message verbs we recognize:

Message verbDescription
testPrints a message to logcat. Because debugging is fun.
announcement(deprecated, do not use)
sync_scheduleCauses a sync to happen, which will download new conference data and sync user data. This is normally sent by the server to indicate that there is new conference data available for download.
sync_userCauses a sync of user data only. This is sent by the server to indicate that the user‘s data (that is, their set of selected sessions) has been changed by an external agent (for example, the I/O website or another of the user’s devices), and therefore needs to be synced.
notificationShows a notification to the user, optionally with a dialog. Use sparingly. It can be annoying. See the Notification command syntax section below for more information about the format.

Notification command syntax

An example of the syntax for the payload in the ‘notification’ command is given below.

    "format": "1.0.00",
    "audience": "all",
    "expiry": "2016-07-14T19:40:00Z",
    "title": "Title Goes Here",
    "message": "Message goes here testing 1234 foo bar qux",
    "minVersion": "200",
    "maxVersion": "201",
    "url": "",
    "dialogTitle": "Dialog Title",
    "dialogText": "Hello! This is a test dialog. Lorem ipsum dolor sit amet."
    "dialogYes" : "Definitely!",
    "dialogNo" : "NO way!"
audienceyescan be “all”, “local” (in-person attendees only) or “remote” (remote attendees only)
expiryyesindicates when the message expires (if devices get it after that date, they ignore it)
titleyestitle to appear in the notification
messageyesmessage to appear in the notification
urlyesthe URL to direct the user to when they click the notification
maxVersion and minVersionnoallow you to filter what version of the app will receive the notification. Use integer version codes like 200, 201, etc. You can specify only one endpoint (min or max) or both. Both are interpreted as inclusive
dialogTitle, dialogText, dialogYes, dialogNonoIf the dialog fields are present, a dialog will be shown when the notification is clicked. That dialog will have the specified title and message. The message can contain newlines (use actual newlines, not \n), and will automaticaly linkify links. dialogYes can be ommitted for dialogs that do not have a positive action. For example “Dismiss” button only. The positive action (the YES button) will always launch the URL.

GCM key

The reason we use the concept of a GCM key is that the application has to synchronize the user's data (sessions selected to be in schedule) with the Google I/O website.

“GCM key” or “GCM group ID” is not part of the GCM jargon, it's something specific to this app.

Here is some background around the problem we were trying to solve, so that you can better understand our design decisions. First of all, there has to be a common data store that both the app and the website can access. Furthermore, the website needs to notify the application that the user's data has changed, triggering the application to do a sync that will fetch the fresh data. Another factor is that the user might have more than one Android device, so when they make a modification to their schedule on the website, the data should immediately update on ALL of their devices.

So these are the problems we had to solve:

  1. where to store the data so that both the website and the app could access it
  2. how to protect that data so that only the user could access it
  3. how to notify all of the user's devices when a change happens on their schedule
  4. how to ensure users can't abuse the system sending GCM messages to devices that are not theirs

Items 1) and 2) are solved in a very elegant fashion: we simply use the Google Drive AppData API. User data always stays in the user's own Google Drive account, and this solves the problem of access control to that data.

To tackle 3), one easy way would be to store the user‘s Google+ ID on the server, associated with the device IDs, so that we knew, given a Google+ ID, what are all the devices that the user has and thus can send a message to all of them. The website, on the other hand, knows the Google+ of the user who is logged in, so they can instruct the GCM server to notify those user’s devices when the schedule changes.

However, we wanted to minimize the amount of user information stored on the server (there is no such thing as too much security!), and by using the Google+ ID as key we would need an API on the GCM server that could send a GCM message to all devices belonging to that user. But since someone‘s Google+ ID is not a secret, a malicious user could, knowing an attendee’s Google+ ID, send an unending stream of GCM messages and drain their battery. Clearly that would not be satisfactory security, and fails to solve item 4).

So, to solve 4), instead of using Google+ IDs we use a randomly generated UUID which we call GCM key (aka GCM group ID). Each user has their own unique GCM key, but a user can‘t guess another user’s GCM key, and it's impossible to determine to what particular user a GCM key belongs. So the GCM server API takes GCM keys and not Google+ IDs.

And where is the GCM key for a user stored? Easy: in their Google Drive AppData folder, along with their selected sessions. That way both the app and the website know the user‘s GCM key, and can use that to signal the user’s devices.

So if you look at BaseActivity.registerGCMClient, you will see how this mechanism works: we first generate a random key locally; the first time we try to sync with Google Drive, we determine if there's already a key there. If there is, the existing key overrides the local key.

Note that the app also sends a sync_user message to the server when the user changes their schedule! This is done to cause a sync on all other user's devices, so they can reflect that latest change as soon as possible.

How to send a GCM message

If you‘ve set up your GCM server and enabled GCM on IOSched, you should be able to push GCM messages to all your users. To do so, you have to make an HTTP POST request to your GCM server (that is, your App Engine app that’s running the gcm-server/ code).

Here is a sample POST request:

POST /send/global/sync_schedule HTTP/1.1
Authorization: key=$ADMINKEY
Content-Type: application/octet-stream
Content-Length: 22


This would send the “sync_schedule” command to all users. (replace $ADMINKEY by the admin key you configured in the GCM server's class!)

To make this POST request, you can use one of the many command-line HTTP utilities such as curl, or use openssl directly. An example:

$ cat <<END | openssl s_client -connect
POST /send/global/sync_schedule HTTP/1.1
Authorization: key=$ADMINKEY
Content-Type: application/octet-stream
Content-Length: 22