Least invasive way to uniquely identify Android user

Question

How can you uniquely identify a user who has installed your app so that:

1. You will know it is them if they delete and reinstall your app;
2. You will know it is them if they install your app on a second device they intend to use simultaneously?

Just as an example, I see that the Netflix app will automatically link to your desktop account without any user interaction. I'm guessing that they use accountManager.getAccounts() or similar method, because they also require the GET_ACCOUNTS permission. But of course that permission is marked as Protection level: dangerous. Is there any technique to do this that is less invasive or potentially alarming?

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The key to answering this is to be both simple (for the user) and minimally invasive. Android provides heaps of ways to identify users and many of those ways involve piercing a user's privacy, and if that is the only way, I will do what I do now (optional email registration). I just want a way for my app to know if a user already is registered in my system across installs without having to interview the user (username/password, email address, third-party OAuth, etc).

My main reasons are:

1. I don't want support requests from users who orphaned their content after a reinstall; and
2. I don't want to host lots of orphaned content.

Answer 1

Have a look at Firebase Authentication. It's quite seamless and does not require much effort to incorporate. Also it does not feel intrusive or cumbersome to the end user.

Here is a video tutorial by Google.

EDIT: In case your users are sure to have a cellular device with a phone number, you can use AccountKit. It is also what they call OTA (One Time Authentication). AccountKit uses just the users phone number to verify and validate users.

EDIT: Firebase Authentication now features 'Phone Verification' which is similar to AccountKit mentioned above. Both are good services. However, Firebase phone verification lets you make your own UI from scratch (which means a lot better control than AccountKit). Also, if you don't want to make your UI, you can always use FirebaseUI

Answer 2

i have implemented something that seems little similar to your thing by push notification , i can get error if user uninstalled my app(and from the registration id i get the user) , and if he re installed he obtain a new registration id , and try to get the user UUID for different devices

Answer 3

I think the simplest way would be using UUID and storing the hash on sharedPreferences. You should generate the UUID as earlier as possible in your app.

sharedPrefs = context.getSharedPreferences(APP_SHARED_PREFS,Activity.MODE_PRIVATE);
if (sharedPrefs.getString("YOUR-KEY-TO-THE-UUID") == null || "".equals(sharedPrefs.getString("YOUR-KEY-TO-THE-UUID"))){
    prefsEditor = sharedPrefs.edit();
    prefsEditor.putString("YOUR-KEY-TO-THE-UUID", UUID.randomUUID().toString());
    prefsEditor.commit();
}