For the successful use case, the Post/Redirect/Get (PRG) work flow is pretty simple: simply redirect (client-side) to the desired page. But what about cases when errors are encountered during server-side validation and we want to preserve the inputs when we display the input page again?
As far as I can tell, there are two approaches: simply re-render the input page after the form POST submission (i.e. no redirection) during errors (thus disregarding the PRG pattern); or, redirect to the input page, and store the previous inputs somewhere it can be retrieved later (e.g. session), during rendering. Both have drawbacks: in the first, we are presented with the problems PRG pattern helps us to avoid (e.g. bookmarkability, double submission); the second approach leads to inconsistent GETs (first GET will find the stored inputs, subsequent GETs might not). Are there other alternatives to those mentioned here? I am hoping for inputs from the community on how this case is best handled.
I typically do it the first way you describe—redirect only in the event of a successful submission. It's hard to see a real use case for bookmarking a form containing invalid data; on the other hand it often makes sense to bookmark a confirmation page (after successful submit).