How to display messages to the user after a POST + HTTP redirect

It depends on what platform you're running on.

Ruby on Rails calls this flash[:notice] = "Your action was performed", ASP.NET MVC calls this TempData["notice"] = "Your action was performed"...

Basically they just store data in HttpSession for a single round trip only. This way you can retrieve data on another web request.

Coming very late to this discussion..

You could use a combination of the two proposed options.

After the POST request the client is redirected (303) to an URL indicating that there could be a response message for this request:

Client: GET  http://example.com/foo.cgi
Server: 200  Ok

Client: POST http://example.com/bar.cgi
Server: 303  http://example.com/foo.cgi?msg=true

If the msg argument is true the message will be looked up in the session and (if found) included in the response to the client.
If the msg argument is ! true (or not present), the lookup step is skipped.

With this solution, you prevent the actual message being show in the URL, the URL only indicates there could be a message. Also, the message is only shown when needed (=when found in the session).

Another advantage is that this solution also allows for proper cash-controls to be included with HTTP responses.

If you don't want to rely on sessions for whatever reason you could use a Get variable / custom url and then if that variable is present, check the refer. If the refer is proper, then display the message. Yes, this adds reliance on refers being sent in order to show the confirmation message, but otherwise you're relying on sessions (which while reliable, aren't 100% perfect for all solutions either.)

And honestly, some large sites that are normally pretty good about this kind of thing just stick a "actiondone=true" in the url. (I've noticed Facebook does it in some places.)

There are several other stack overflow questions that touch on this, though I don't think any summarize the problem so clearly. Here are a few:

• How do I implement the Post/Redirect/Get pattern in asp.net WebForms?
• Response.Redirect with POST instead of Get?
• Both HTTP GET with request body and Can an HTTP GET legally contain content? approach the issue, and the discussions are worth reading.
• How do I maintain ModelState errors when using RedirectToAction?

Most of the convenient solutions are session-based or have more serious drawbacks (such as embedding the message in the querystring).

If you cannot guarantee that you'll have sessions, another (quite costly) method is to redirect to different views depending on the outcome of the form submission. For example, you might redirect to EditWidgetView, EditWidgetSaveSuccessfulView, or EditWidgetSaveErrorView (or perhaps you just don't redirect on errors). In some languages and frameworks, this is impractical to the point of making you give up on showing confirmation / error messages at all, but in others it may be worth it.