Question

I have read that TempData is backed by session in Asp.Net MVC, but only valid for the duration of the current request. This makes it appropriate for Redirect2Action etc.
Given that it’s only valid for the current request, does that mean that you won’t get into issues with load balancers and session on WebFarms. Basically because you’re only on one webserver per request.

Would it be an option to back it by the Request.Items collection instead?

Answer 1

TempData is only valid for the current and next request which is why it is stored in the session state. Storing it with the request would not work.

So any limitations that would break saving items in session state (e.g. load balances) will break TempData as well.

See the documentation for TempDataDictionary

You can use a TempDataDictionary object to pass data in the same way
that you use a ViewDataDictionary object. However, the data in a
TempDataDictionary object persists only from one request to the next,
unless you mark one or more keys for retention by using the Keep
method. If a key is marked for retention, the key is retained for the
next request.

A typical use for a TempDataDictionary object is to pass data from an
action method when it redirects to another action method. For example,
an action method might store information about an error in the
controller’s TempData property (which returns a TempDataDictionary
object) before it calls the RedirectToAction method. The next action
method can then handle the error and render a view that displays an
error message.