What data (if any) persists across web-requests in Ruby on Rails?

Programmer Question

I decided to use the singleton design pattern while creating a view helper class. This got me thinking; will the singleton instance survive across requests? This led to another question, Which variables (if any) survive across web requests and does that change depending on deployment? (Fastcgi, Mongrel, Passenger, ...)

I know that Controller instance variables aren't persisted. I know Constants are persisted (or reloaded?). But I don't know about class variables, instance variables on a class, Eigenclasses, ...

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