499 errors in IE9 causing order duplicates

[Maxim Dounin]

Hello!

On Tue, Oct 26, 2010 at 09:48:23AM -0400, Paleo Tek wrote:

> We use nginx to reverse proxy to a farm of apache servers.  It
> rocks.  But we've noticed that Internet Explorer 9 has been
> generating duplicate orders.  It does not happen every time, but
> often enough.  It appears to be an IE 9 issue, but I'm hoping the
> experts here can confirm my theory:
> 
> A request comes from an IE 9 client.  Nginx passes the request to an
> upstream apache server.  For some reason, the IE 9 browser closes
> the connection before the apache server returns results with return
> code 200.  Nginx sees a closed connection, and generates a 499 error
> (client problem, connection closed).  IE 9 sees that, and
> immediately re-sends the request.  Nginx forwards to apache,
> receives the response, and forward to the client, logging a 200.
> Log results are below

IE can't "see" 499 as it's logged after client already closed 
connection.  It's IE who closed connection (which is logged with 
499 fake code) and re-issued request.  Most likely reason - user 
hit "Submit" button two times (so on second click IE aborted first 
request and issued another one).

This problems isn't nginx and/or IE specific, though may show in 
logs slightly differently depending on exact borwser and/or server 
behaviour.

Usual methods to prevent it from happening - either use javascript 
to disable submit button, or use some token in form and filter 
duplicates at server side, see e.g. [1].

[1] http://stackoverflow.com/questions/414835/avoid-race-conditions-in-php-on-submit-please-do-not-click-submit-more-than-once

Maxim Dounin