Double Click Handling

Many users tend to click twice on links/and forms, causing therefore multiple identical requests, that can kill the performance of a server, especially when these are expensive requests. On one site, we saw users firing up to several hundred identical requests, probably via a “rapid fire” function available for some mice, coming from the gaming world.

There are essentially two generic ways to handle double clicks in OpenACS:

1. Server-side double click handling: The xotcl-request-monitor has since a long time the feature to block (optionally) multiple identical requests from the same client. The user will receive for subsequent identical requests the message “Repeated Operation, please resubmit”, which as some “educative” value. It has several heuristics built in to allow certain double requests (e.g., for embedded resources).
    
2. Client-side double click handling: Via HTML markup, one can disable the same requests for a certain time on the client side (i.e., in the browser). For this time, double-clicks are blocked (the button/link is disabled), such that the true double-click, or “rapid fire” functions are mitigated. The request is only one time executed. Potential problems are potential confusion, when a user wants to open the same page in rapid sequence in multiple tabs, windows. Furthermore, repeated requests from bots are not mitigated. This is a new feature which will part of the OpenACS 5.10.1 release.

The client-side double click handling is activated by a content developer for a link prone to this concern by adding the CSS class “prevent-double-click” to the HTML anchor or button. The double click prevention deactivates a button or an anchor element after clicking for a short time (per default for 2s) and ignores in this time window further clicks. The default time window can be specified via the package parameter DefaultPreventDoubleClickTimeoutMs of acs-templating. The time window can be specified for every occurrence via the data element “oacs-timeout”. When a double click occurs, the duplicate click is determined at the client side (in the browser) such that the first request will continue to run and will render its result when finished. Without double click prevention (or with the double click-prevention on the server side), the results of the first request are lost for the client, although these are still computed at the server side.

Usage Example:

    <p>text with a <a class="prevent-double-click" href="/slow.tcl?t=10s">link</a>
    <p>
    <form action="/slow.tcl">
    <input class="btn prevent-double-click" data-oacs-timeout="4000" type="submit" value="Submit">
    ...
    </form>