Causes of duplicate apache POST requests, other than double submission of form?

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This might sound like a question that gets asked frequently but I am not looking for solutions to handle duplicate requests. I just want to know what could cause Apache to receive duplicate requests in the first place.

I have been running into a rather sporadic problem. I have a form that does a POST request on submit but the request can somehow get duplicated just a second later (according to access logs). This used to be a more frequent problem because we were not handling it as gracefully so I put in some client side code to disable the submit button during the form submit event. This prevents double submission (obviously as long as javascript is enabled), but the problem still persists in a very randomly manner. One thing I noticed from logs is clients that cause the issue are android phones running Chrome. Does mobile Chrome do funky things like retry POST requests on it's own? When testing it on my own, I could never get duplicate POST requests to occur, unless I remove the javascript code that disables the submit button.

Web server setup is super simple. No load balancing or anything, just a single server running Apache 2.2.15. We use PHP 5.6 but that probably has nothing to do with this.

apache
1 Answers

I guess it is users doubleclicking rather than clicking, and the application they use transforms every click into a new POST request. Here I'd look into the application design.

Usually I use frameworks that totally cover this and thus can only guess. Clicking the button should not only trigger the POST request but also disable the button while the action is in progress. So JavaScript code could look like

  • disable button
  • post the data
  • enable button

If, due to the POST, the browser navigates to another page this would not be harmful at all.

EDIT: Seeing you did exactly what I suggested, maybe there is another cause. Suppose users POST their data, and then the screen goes dark, or they switch applications. When they reactivate the browser, is it possible the browser reloads the page by repeating the last request?

I think frameworks cover such situations by responding with a redirect as response to POST, and the redirect would retrieve the data via GET. Since GET is idempotent, it can be run repeatedly without further damage.

user_42222060
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