# Ajax Failure Idiom **URL:** \ **Category:** Rails\ **Created:** [October 26, 2008, 1:33pm UTC](https://www.ruby-forum.com/t/ajax-failure-idiom/151828 "2008-10-26T13:33:10Z")\ **Posts on this page:** 4\ **Page:** 1
**Author:** ![corywilkerson](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/c/aca169/32.png) [@corywilkerson](https://www.ruby-forum.com/u/corywilkerson)\ **Post date:** [October 26, 2008, 1:33pm UTC](https://www.ruby-forum.com/t/ajax-failure-idiom/151828/1 "2008-10-26T13:33:10Z")
Folks, Curious what the current state of the union is with regard to ‘failure’ idioms for an AJAX call in 2.0 land. For example, let’s say I have a simple form\_remote\_for that posts to session\_controller.rb that validates user credentials and, upon successful validation, redirects that use to some landing page. That’s the happy path - now wondering what to do if the validation fails: - Return some sort of internal xml that indicates things have failed using respond\_to? and parse the XML/update the page accordingly? - Set the http response code to something in the 4XX range - invoke a failure callback? - Something else I’m not thinking of entirely? Haven’t been near this in a while and was wondering what the current practice du jour is… Thanks for any help/insight, CW ---
**Author:** ![corywilkerson](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/c/aca169/32.png) [@corywilkerson](https://www.ruby-forum.com/u/corywilkerson)\ **Post date:** [October 26, 2008, 1:45pm UTC](https://www.ruby-forum.com/t/ajax-failure-idiom/151828/2 "2008-10-26T13:45:41Z")
On 26 Oct 2008, at 12:33, Cory W. wrote: > the happy path - now wondering what to do if the validation fails: > > - Return some sort of internal xml that indicates things have failed > using respond\_to? and parse the XML/update the page accordingly? Well in a case like this i don’t think you need anything smart. In the successful case you page.redirect\_to somewhere, in the failure case you page.replace\_html or page.insert to add an appropriate error message somewhere. In the case where the client side code has more smarts things are a little different. Personally I use the X-JSON header quite a lot to convey meta data to the client and use RJS very little, ie the body of the response is nearly always a chunk of HTML or JSON, the json contain in the X-JSON header lets the client side code know what to do with it. > - Set the http response code to something in the 4XX range - invoke a > failure callback? That can also be appropriate, although I would save that for actual errors (eg I couldn’t talk to some server) rather than for something like a failed validation. You can use Ajax reponders if you just want a generic something bad action (eg show a message to the user). Fred ---
**Author:** ![corywilkerson](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/c/aca169/32.png) [@corywilkerson](https://www.ruby-forum.com/u/corywilkerson)\ **Post date:** [October 27, 2008, 9:25pm UTC](https://www.ruby-forum.com/t/ajax-failure-idiom/151828/3 "2008-10-27T21:25:41Z")
I dont know of a commonly used pattern for this, all of your suggestions seem okay. Of course you don’t need to parse xml and then update an element, in a lot of situations just returning an html fragment and update some element directly will suffice. In an application I developed recently, which excessively uses the ExtJs framework, I use JSON responses like Frederick suggests. (Be aware of Javascript Hijacking if you use JSON for exchanging data, though it’s not neccesarily a problem when it only contains error messages.) ---
**Author:** ![corywilkerson](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/c/aca169/32.png) [@corywilkerson](https://www.ruby-forum.com/u/corywilkerson)\ **Post date:** [October 27, 2008, 9:35pm UTC](https://www.ruby-forum.com/t/ajax-failure-idiom/151828/4 "2008-10-27T21:35:56Z")
Sjoerd A. wrote: > I dont know of a commonly used pattern for this, all of your suggestions > seem okay. Of course you don’t need to parse xml and then update an > element, in a lot of situations just returning an html fragment and > update some element directly will suffice. In an application I developed > recently, which excessively uses the ExtJs framework, I use JSON > responses like Frederick suggests. (Be aware of Javascript Hijacking if > you use JSON for exchanging data, though it’s not neccesarily a problem > when it only contains error messages.) To make this a little more tangible; this is similar to my approach: rescue\_from Exception do |e| respond\_to do |format| #…handle different formats format.js { render :json =\> {:error =\> e.to\_s}.to\_json, :status =\> 500 } end end Then in my Ajax callback method when a 500 is returned I know there’s an ‘error’ property on the returned JSON object, of which the value can then be inserted into an element, or whatever else you want to do with it.