Firebase Cloud functions to Android application notifications

I need some hints for the following system:

• client app: Android
• backend: Firebase Cloud Funtions
• database: realtime database

One of the scenarios is as follows:

• All users are on android devices.

• Users are sign-in via OAuth2 (google accounts).

  1. User1 creates an offer and the offer is 'waiting' in active offers list.
  2. Other users are notified for the change is active offers list..
  3. User2 requests the offer and the offer is 'requested'
  4. Other users are notified for the change is active offers list..
  5. User 1 is notified for the User2's request

  6. User1 confirms/denies the User1's request:

     a) confirms and the offer is 'completed' and removed from the active offers list.

     b) denies and the offer is again 'waiting' in the active offers list.

  7. Other users are notified for the change is active offers list..
  8. User2 is notified for the User1's confirmation/denial

NOTE: System is specific and limited so the active offers list will be short enough and that's why currently I'm thinking about notifying users about list modifications and not about a single offer modification, but I'm open for advices.

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Currenty I've implemented the following:

1. User1 creates an offer and the offer is 'waiting':

   • android app calls createOffer(...) firebase cloud function
   • createOffer(...) adds a new offer to the offers list in realtime database.
   • onWrite(...) realtime database trigger is fired on active offers list level

2. here I need the hint

3. User2 requests the offer and the offer is 'requested':

   • android app calls requestOffer(...) firebase cloud function
   • requestOffer(...) modifies the offer in realtime database.
   • onWrite(...) realtime database trigger is fired on active offers list level

4. here I need the hint

5. here I need the hint

6. User1 confirms/denies the User1's request:

a) confirmation:

• android app calls confirnOfferRequest(...) firebase cloud function
• confirmOfferRequest(...) removes the offer from active offers list.
• onWrite(...) realtime database trigger is fired on active offers list level

b) denial:

• android app calls denyOfferRequest(...) firebase cloud function
• denyOfferRequest(...) modifies the offer in active offers list. onWrite(...) realtime database trigger is fired on active offers list level.

.

7. here I need the hint

8. here I need the hint

==================================

So I need hints about notifications to android application: steps 2, 4, 5, 7, 8

I guess I should use FCM, but:

1. Android tutorials I find are about the deprecated FirebaseInstanceId

2. I'm not sure what is my case:

   • topic messages
   • device groups
   • upstream messages

3. I'm not sure how to notify android app by onWrite(...) realtime database trigger.

4. Signed-in users only should be notified about active offers list changes. if a user is singed-out should not be notified and when sign-in should get actual state (active offers list).

=============

@James Poag, Thank you for the hints. I'll try and give feedback.

I have some questions:

When I said "notification" I didn't mean 'real' notification in device's 'status bar'. At least current plan is the application to show the info inside itself - this I called "notification". It seems simpler.

1. is there a reason to use onCreate() instead of onWrite() ?

Currently my plan is to perform same action on creating/updating an offer so I thought onWrite() would do the trick for both cases. Or not ?

3. thanks for transaction hint :)

4. I guess in my case I have "data" messages, not "real" notifications.

I have to read about the permissions. thanks.

Currenty I check for authorization this way:

if (!context.auth) {
    return Promise.reject(Error('User is not authenticated.'));
}