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Question 1:

What is the scope of an instance variables in Rails actions (methods). Does each connection to the server form a new instance of a controller?

For example:

  • User_A loads a 'setter' page, causing a random instance variable called @randInt to be generated.
  • User_B (from another PC elsewhere) loads a 'getter' page, requesting @randInt.

Will User_B retrieve the @randInt set by User_A? Or is that instance variable unique to User_A's connection?

Question 2:

Question 2 is the same as question 1, but using @@randInt instead. If the answer to Question 2 is, "yes, both users can see this value," is it an acceptable practice to use global variables in Rails to store temporary data that you want to share among multiple users?

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Question 1: No, instance variables are shared only in the instance, where 'instance' refers to the instance of the controller, and so these variables only last one request (so User_B will receive a different @randInt).

Question 2:: @@ variables are not global variables, $ variables are. @@ are class variables. As the link explains, different machine instances (such as if you're using FCGI) will not share global ($) variables, so don't use them.

If you require global constants, set them in the config. Global variables are probably better left in the database (I can see a use for them, such as site settings, but the uses seem best suited to use with a database).

You could use class variables as persistant instance variables, but again you might be better of using a database to store such values as you're not guaranteed against your classes being reloaded (therefore resetting any class variables).

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2 Comments

I probably wouldn't use the term session as it provokes a confusion with the actual rails sessions. Actually, each request creates an new controller instance where the instance variables are valid for the duration of the request.
I considered it, but should of thought harder. You're absolutely right, updated the answer to clear up any confusion (hopefully).

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