I have got a question about https://mercure.rocks/spec#subscribers.
The publisher could then take advantage of the previously described behavior by publishing a private update having https://example.com/books/1 as canonical topic and https://example.com/users/foo/?topic=https%3A%2F%2Fexample.com%2Fbooks%2F1 as alternate topic:
POST /.well-known/mercure HTTP/1.1
Host: example.com
Content-Type: application/x-www-form-urlencoded
Authorization: Bearer [snip]
topic=https://example.com/books/1&topic=https://example.com/users/foo/?topic=https%3A%2F%2Fexample.com%2Fbooks%2F1&private=on
Question: What if my application has over a million users – there may be a lot of “alternative topics” to (compute and) add to the updates, right? Is this practical in projects with a lot of users?