[Rails] Re: storing values in session variable vs instance variable

Norm Scherer wrote in post #1102294:
> On 03/18/2013 10:37 AM, John Merlino wrote:
>> Is there a good practice when to use the session variable. Sometimes I
>> find myself using both the session variable and instance variable. For
>> example, I have a table that allows a user to select a date. I store
>> the date both in the session and in the instance variable. Should I
>> not bother to store it in session? In what situations should you use
>> the session variable? --
> The rails session persists across requests while instance variables do
> not. If you need something in a following request it (or a reference)
> must be in the session. If you do not need it later I always use an
> instance variable.

I wouldn't say that this is precisely accurate. You could continue
passing a variable from one request to another, and to third, and so on.

Session variables are useful when you really need a value to persist for
the entire session. I would also say that it's good practice to use
session variable sparingly. Session variables are somewhat akin to
global variables in the sense that they represent globally accessible
shared state (at least within the context of the current session).

Session variables are also long lived, taking up memory as long as the
session is kept alive. Although in a Rails application sessions are torn
down and recreate upon every request, but that might even be worse than
just leaving them in memory between requests.

Another thing to keep in mind is that by default Rails uses a cookie
based session storage mechanism, which means sessions have a hard 4K
limit (cookies are limited to 4K by spec). Another reason to avoid
putting large amounts of data in the session.

A typical use case for session variable are things like the "id" of the
current user. Notice I said the "id" not the user object itself. It's
better to load the user object when, and only when, necessary. There are
other great uses for session variables, but think twice about if it's
really necessary and try to keep the amount of data as small as
possible. Remember for every variable in the session is just one more
thing that has to be loaded from persistent storage on every single
request.

--
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