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I don't know if it's recommended but for simple cases where I just want to trigger some methods on multiple models I just use a concern: https://gist.github.com/byroot/5036110

They are still maintained, they've just moved to a separate repo/plugin: https://github.com/rails/rails-observers

One option would be to use a service object (e.g. UserCreator) which would encapsulate the action (saving a user) along with the action to be taken (delivering a signup email).

With external observers, it's not always obvious whether they are enabled or not. Service objects make this more explicit and are easier to test as well.