Is Paid Content Syndication, with a Canonical tag, the same as selling links?

One of the advertising options that my site provides is paid content syndication. For a fee, we will republish articles from the advertiser's blog. We make sure to mark the article as sponsored and all of the links in the article as nofollow. We also point the canonical link back to the original article to avoid duplicate content issues from Google.

Since we are pointing the canonical URL back to the original article, all of the link juice that my version would get is not going to the…

Is Paid Content Syndication, with a Canonical tag, the same as selling links?

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What is the correct way of setting and then differentiating between paid and free user in Firebase / Firestore?

I am developing a mobile application that will have both free and paid versions.

The way I see how I could differentiate between what is a paid and free user is to set a property isPaid on their respective documents.

What I want to understand is how to set the isPaid property on user‘s document securely.

One option that I see is after successful payment directly access the document that belongs to the user and change the isPaid property. All of that would be done on the mobile application side.

Another option could be to use the Firebase Functions and trigger one to set the correct value on the isPaid property for the given user.

But what could prevent a malicious user from setting these properties directly OR how to ensure that there actually is a pending subscription from Google or Apple and the user really is a paying one.

Currently my Firestore structure looks like this:

{   "users": [     {          "uid": "123123123123123",         "name":"Bob",         "isPaid": false,         "messages": [             {                 "messageText":"hello"             }         ]     },     {          "uid": "456456456456456",         "name":"Alice",         "isPaid": true,         "messages": [             {                 "messageText":"hey"             }         ]     },      ... }