Pros and cons of a multisite installation


(Christoph) #1

Given that a multisite installation of discourse uses about the same resources as an ordinary install with the same traffic , I wonder what exactly the downsides are, apart from a more complicated setup procedure.

An obvious one is that if one of your sites is down, all of your sites are down (right?). But what about support, for example? If I search meta for a solution to an issue and find something, will it automatically apply to my multisite install, even though the original topic doesn’t explicitly deal with multisite?


Does Discourse scale well with cores?
(Daan Seegers) #2

An obvious one is that if one of your sites is down, all of your sites are down (right?)

Yes


(Felix Freiberger) #3

Typically yes – however, for some steps, you’ll need to make sure you work on the correct instance. The key is to set an environment variable like RAILS_DB=myfancypantsinstance1 e.g. when entering the rails console.

This is why I recommend making the first instance on a multisite host a test instance: If you ever forget to specify the instance, you’ll target your test instance, which is probably safer than working with a production instance by accident :wink:


(Richard - DiscourseHosting.com) #4

That is not entirely true: for each Unicorn process there can a separate database connection for each site in the multisite. So 5 Unicorns, 1 Sidekiq and 5 sites = 30 Postgres server processes.

Some other disadvantages / things you need to be aware of with multisite:

  • Sidekiq is shared and /sidekiq is only available on the master site
  • All plugins are shared, there can not be separate plugin choices
  • discourse.conf is shared so there can only be one mail server
  • Some plugins are incompatible with multisite

(Christoph) #5

I don’t understand a word what you are saying, but that is entirely my fault. I understand, though, that you are counting software processes and I have no idea how these translate into hardware recourse requirements. Could you elaborate?

I understand that this can be a disadvantage from the perspective of a (commercial) hoster because your clients wont have access to /sidekiq on their respective sites and if you give them access to the mastersite they will possibly see more than they want or should see, right? But if all sites are mine, it probably doesn’t matter, right?

Very important point, thanks for bringing that up! However, provided that all installed plugins have a switch to turn them off, it doesn’t really matter, does it?

Wait, the only one of my instance where I did something with a discourse.conf file is the one where I have discourse behind a reverse proxy and there is nothing about email in that file.

How do I know which ones? Trial and error? Or, since this also about future compatibility, what type of plugins are likely to be incompatible?


(Richard - DiscourseHosting.com) #6

Very easily put: there is a (albeit small) memory penalty that is proportional to the amount of multisite forums.

Correct.

Most plugins just turn off their functionality but they are still loaded into memory. So there is a (small) memory and performance penalty there.

This is not about the nginx discourse.conf but about the Discourse one. I think it’s generated out of app.yml.

Trial and error…