Implementing Notifications in Rails

Question

In my application, I want to notify a user, when he/she is mentioned in a comment or a post.
The user handle is @user_name, similar to Facebook

Answer 1

Tumblr uses a Redis queuing system (like Resque) I believe to handle the volume.

Do a callback (as you mentioned) and hand it off to Resque. (There was a Railscasts about Resuqe recently)