How can I share user sessions across multiple domains using Rails?
You can set the same session_key in both apps. In appA environment.rb change the session_key, like this
Rails::Initializer.run do |config| ... config.action_controller.session = { :session_key => '_portal_session', :secret => '72bf006c18d459acf51836d2aea01e0afd0388f860fe4b07a9a57dedd25c631749ba9b65083a85af38bd539cc810e81f559e76d6426c5e77b6064f42e14f7415' } ...endDo the same in AppB. (remember to use the very same secret)
Now you have shared sessions. Let's say you use restfull_authentication, wich sets a session variable called user_id. When you authenticate in appA it sets the user_id in the session. Now, in appB you just have to verify if user_id exists in the session.
This is the overall schema, you can elaborate more using this idea.
If you want to create single sign-on solution for your applications then I recommend to take a look at RubyCAS solution. It could be used also to provide single sign-on for other non-Rails applications as well as you can integrate authentication with LDAP or other authentication providers.