Explanation of the scheme (this is the maximum option that I can imagine):
2 or more Discourse multisite installations. Discourse 0 is a common SSO provider for other Discourse (in the same installation or another). It is also a SSO provider for the network “WP Multisite host 2” and individual network sites “WP Multisite host 1”. Some single WP or / and some Ghost websites.
Arrows in the diagram denote SSO
Is it possible with standard plugins? Any problems?
Discourse 0 in this scheme is essentially only an SSO provider. Is it possible to turn off the forum in this situation and leave only a few custom pages (for example, with a description of websites / forums to which authorization is given) and the login / registration form?
There will be different databases yes, but will the users in them be different if you delegate access control to a central SSO provider? At most, some users will not exist on some forums… but I don’t see a big difference between this and one forum with multiple categories.
So I’m just curious about your use case for this.
But… if you have multiple forums about completely different subjects, will they share (a large part of) the same users at all? If the answer is no, then the reasons for one userbase will only have a minimal impact.
No, I said related subjects. For example: cats owners website, dogs owners website, home pets forum, Livestock Forum, online shop with pet toys/food, veterinary directory, etc
Perhaps any 2 topics will not be related to each other, but they can be linked through a third. This is how networks are built. All this because of possible synergy. It may not be, but to miss the opportunity is stupid, if it can be done.
No, it’s just the most complex circuit that came to my mind. And I was just figuring out the limits of Discourse. In real life, I have not yet had such a difficult task with many discourses, and I now think that the allocation of a separate discourse only for the SSO task is overengineering. If you have come to this, then you are on the wrong path.
Now I adhere to the idea that you need to go from the general to the particular. That is, continuing the examples that I gave above: you first open a general forum dedicated to all pets, their treatment and food, and when it grows you see the need to separate several large categories into a separate forum (cat owners and dog owners, because these are the largest categories and they do not get along well together and in general they are not very interested in reading about each other’s problems). That’s when you can use your original forum as the SSO provider for the two new forums. They will all be in synergy, but will not suffer from too close proximity.
The second use case I see is when creating separate language forums, when the central forum is the main language and plus several forums in other languages connected to the topic via SSO are installed to it. But this should be done if the support in all used languages is equivalent. And perhaps you should also start with one forum, with separate categories for languages, and when it grows and becomes uncomfortable to communicate, then split into different forums.
The main point here should be automatic synchronization of user accounts between all forums and autologin to all forums / sites when entering one of them.