Implementing a notification system for sending/accepting friend request using firebase cloud functions and firestore

Question

I am trying to implement a notification system for handling notifications similar to friend request notifications. For example, I write the email of the person I want to send a friend request to. then click on send request. I am confused as to after that, what exactly should happen?

Right now I am thinking that on clicking send request, I would create a document in cloud firestore in a 'notifications' collection, which would then invoke a cloud function that sends a push notification to the user with that email. The user now has the option to accept or deny the request. Choosing any of those actions would update the notification document which will again invoke a cloud function that would add both users to each other's friends list and notify the first user.

So my questions are: -

1. Is this method reasonably good to be implemented in a production app since it is involving many cloud function invocations and document read and writes? Or even better question - How would you implement something like this in a production-grade app?
2. Is there any other method that is secure (in the sense that no one should be able to wreck this process by changing the code on the frontend) and efficient (less read/writes/invocations)?
3. Can this be done without creating any document whatsoever? Purely by cloud functions? Then would it be able to capture other user's response to friend request and do the necessary changes?

Answer 1

For the problem you are describing I would approach it in the say way you are doing, and in fact there are not that many operations going on here, since you would have 2 Firestore writes and 2 invocations of cloud functions, not to mention that the second invocation could take a long time to be fired depending on the user's actions, so you don't need to be more efficient than that.

To you final question I would say that it would be difficult to have this implemented without information going through Firestore because you need to store the acceptance of the friend request, and for the same reason mentioned above, you need to store that information somewhere, even if temporarily.