Develop Reference

Do I need to migrate GCM to FCM on client side?

I have the old android app on Play Store which I am not going to update anymore because of making a new app.
Do I need to migrate GCM to FCM in my old android app?
Many sources say:
You have to change your client side... blabla... etc...
but GCM and FCM FAQ says:
GCM tokens will continue to work on client devices.

Existing installs of client-side apps receiving notifications will continue to work for the foreseeable future. You don't need to get new tokens for all users. The existing tokens from GCM will continue to work
But from mid-April:
You won't be able to compile code using the old client-side APIs on a newer SDK/Google Play Services anymore
You won't be able to call the GCM REST API to send messages anymore
New installations of your existing GCM-using app may not be able to get a token.
To compile against the new SDK/Google Play Services, you will need to make changes to your client app to migrate to Firebase Cloud Messaging from GCM. This involves changes to the build.gradle, to AndroidManifest.xml and to your code, as described in Migrate a GCM Client App for Android to Firebase Cloud Messaging.

You need to update your client code and migrate to FCM. Because it is officially announced to do migration and it is very likely that from a point, for example by one of the android updates, they stop supporting old api callbacks and if so you will lose part of your users.
So my advice is to do the migration as it has been long enough so far that they had supported GCM along with supporting FCM and they can't and won't do it forever.

How do I ensure that only my app is able to register and receive messages sent from FCM?

I am currently setting up a Firebase Cloud Messaging project. When adding apps to this project some credentials are automatically generated by FCM, e.g. the Server key as well as Android and iOS client keys.
For Android, I can download and use the google-services.json file to setup the client, i.e. registering for an FCM token and receiving push messages. But how do I restrict this so only my app can receive these messages?
I thought it would be the (auto created) Android client API key, so to test I've put an incorrect package name and SHA fingerprint in restrictions for the key. No effect, can still receive messages.
I then tried removing the API key as well as oauth client info completely from the google-services.json file, to confirm that the device then would not be able to receive messages. No effect, can still receive messages. All client info seems to be completely ignored, except for the app id (and the general project info).
So, I am really wondering what prevents reverse engineering of an app to extract the app id and general project info, and then receiving push messages in an entirely different app? I don't understand why the Android client API key, etc. is included in the google-services.json file, or why they are even created, if it is not used.

All the information in google-services.json is essentially configuration data that your app uses to find the Firebase services on Google's servers. You should not rely on knowing it, or lack of knowing it, as a security mechanism.
Instead you should send your messages in a way that ensures they are delivered only to the targeted recipients. A large part of this is sending messages to specific FCM Instance ID tokens. Firebase's Instance ID tokens are unguessable. Initially this registration token is only known on the client-side device where it is generated, and that device determines who to share it with.
By ensuring your client-side code only shares the token with your own server-side code, which then use it to target FCM messages, you can create a completely safe delivery environment where you fully control who can receive each specific message.
On the other side of this are FCM topics, which are a simpler mechanism for delivering messages to groups of users. Since any client can subscribe to any topic that it knows the ID of, topics should be used for delivering messages that require less securely targeted delivery.

I've been in contact with Firebase Support over the past week to shed light on this. It seems there is an important difference between the now deprecated Google Cloud Messaging (GCM) and Firebase Cloud Messaging (FCM), and it is currently not well documented.
With GCM it was possible to use and restrict e.g. the Android client API key with a package name and a SHA fingerprint. This is no longer possible with FCM. The keys are auto created when you add apps to your project in the Firebase console, and included in the google-services.json file, and you can add the restrictions to these under Credentials in APIs & Services, but it will not have any effect.
Please also note that the SHA fingerprint you can add in the Firebase project is only for Firebase Invites or Firebase Dynamic Links.
Be careful and have this in mind when you decide what to use the messaging service for, or when you migrate from GCM to FCM.
There is a feature request for this at Firebase but they currently have no timeline. There also a request now for an update of their documentation regarding this issue.

Unregister from GCM and register on FCM

My android app uses GCM and urbanairship for push notifications.
Now Google has deprecated GCM and is recommending to switch to FCM.
The problem is I don't have access to the Google account which was used to set up GCM as it was set up using one of the developers account who has left. So I can not migrate from GCM to FCM as told in the migration tutorials.
In this scenario, how can I switch to FCM using a different Google account keeping in mind that I use urbanairship?
I want to preserve the registration tokens, or get them replaced without asking customers to uninstall and reinstall the app.
Is it possible that the app registers itself with FCM when I roll down an update?

The problem is I don't have access to the Google account which was used to set up GCM as it was set up using one of the developers account who has left.
This is not an uncommon situation. What I would suggest is contacting Google Support about the GCM Project. It would be best for you to get access to current project, specially if you are the one managing/using it.
After getting access, you could then migrate it to Firebase as needed.
In this scenario, how can I switch to FCM using a different Google account keeping in mind that I use urbanairship?
You can't switch to a different account. However, you could modify your app to receive messages from a different project. I haven't used UrbanAirship before, but AFAIK, if they are using GCM underneath their service, then they too should be handling the FCM migration part.
Is it possible that the app registers itself with FCM when I roll down an update?
In order to register an app is for it to call getToken(). So technically, yes. But you would have to handle pairing/saving the token to it's corresponding user.
With all that said, I would strongly suggest you contact Google Support first.

Here is how I solved the problem:
1. Created a new Google FCM account.
2. Created a new project in Urbanairship console using keys from new Google FCM account.
3. Updated the backend code to send notifications twice - once for old urbanairship project and once for new project. Out of the two attempts, one will fail for sure and one will succeed for sure as customer will be using either of the two versions of the app.
4. Customers using old version of app will have registation tokens which are available with old urbanairship project. Customers using new version of app will have registation tokens which are available with new urbanairship project.
5. When customer updates the app, urbanairship takes care of registering the token with new FCM account server. I don't have to do anything here. This was not expected as token is updated upon fresh installation and not during updates.
So to summarize:
UA Project Old -> Google GCM -> Device with old app version
UA Project New -> Google FCM -> Device with new app version

Here is the FCM Urban Airship migration guide. Assuming you are able to recover your GCM account you only need to follow that guide with your existing sender ID. If you change accounts, you will have to configure the new FCM project API key in the Urban Airship dashboard. The SDK will automatically update all the channels with the new registration token. During the transition period, only devices that have updated will be able to receive push.

GCM without using Firebase

Is it OK to use GCM without involving Firebase?
As I saw from Google(https://developers.google.com/cloud-messaging/android/client) that, "New Cloud Messaging projects must create a Firebase project". Is it true?

TL;DR: Yes. You could still use the service without (technically) using Firebase.
Google Cloud Messaging is the old version for Firebase Cloud Messaging. During that time, it was only required to create a Google Developer Project in order to use the service.
GCM has since become part of Firebase and was rebranded as Firebase Cloud Messaging. Although the name adds in Firebase, FCM is still a service on it's own, where most of what it is right now, is still based from the old version (of course there are a lot of new stuff by now, but you get what I mean) -- in which case you can still use the service without using the other Firebase services.
Creating a Firebase Project doesn't mean you would have to use or it counts as using the other Firebase services. It's just the place where you would see most of the data related to your FCM service.
All users, even the old GCM users, are currently (strongly) encouraged to actually import their projects to Firebase Console.

Can I use both GCM and FCM in my app?

I have an android project in which GCM is implemented. Back-end (server) is already coded suitably with GCM.
Now is it possible to add firebase (FCM) in my project and work in parallel with GCM? I cant migrate to FCM because it is impossible to change my back-end code as it is a big project and too much code is there. Has anyone done something like this?
Please help. Thanks in advance.

Though my particular case might be a bit specific, because the project I am involved in has a very specific backend, it seems to me FCM interoperates well with GCM, i.e you can have both in your app, GCM and FCM broadcast receivers / receiver services, and they will both work as well with no conflicts.
This has a huge advantage because you don't have to decide with migration at once, you can do this more gradually.
Well done, Google!

No, you can not use both FCM and GCM at same time. GCM is the old version and that it is strongly recommended to use FCM.
Why is Google Cloud Messaging (GCM) part of Firebase? How does it relate to Firebase Cloud Messaging (FCM)?
Google is investing in Firebase, making it our unified mobile
platform.
Over the last few years, we have expanded GCM to send messages to
multiple platforms beyond Android: iOS and Chrome. Firebase is known
for being cross platform, so FCM now makes a natural fit in the
Firebase suite of features designed for Android, iOS, and mobile web.
Another core value of Firebase is cross-feature integration. We are
adding the ability for other Firebase features to easily send messages
via FCM. For example, you can use Firebase Notifications to send
reengagement messages to your users.
We think that GCM will be even more useful to developers as an
integrated part of Firebase. You can still get the same great product,
now under a new name: "Firebase Cloud Messaging," or FCM for short.
Is GCM going to be deprecated?
We will continue to support the current version of GCM Android and iOS
SDKs because we know a lot of developers are using GCM SDKs today to
handle notifications, and client app upgrade takes time.
But all new client-side features will be added to FCM SDKs only moving
forward. You are strongly encouraged to upgrade to FCM SDKs.
You can get more details here

As per FAQ you don't need additional server configuration when you're migrating your client app from GCM to FCM, so just use FCM in your android app.
Here's the docs on how to migrate your app

I find the problem to use both GCM and FCM together. You will get to different deviceToken and one of these token get NotRegistered error.
For my FCM token always get NotRegistered error but the app is active and not uninstalled and my GCM token is valid and gets a push notification.