External proxies/caches

Recent Mollom support issues for sites running with Pressflow, Akamai, Varnish, or other external proxy/cache systems indicated that form submissions could not be completed due to missing Mollom session information. It is not sure yet whether the Mollom module is guilty here at all, or whether it is a configuration problem with those external systems.

Some initial background information:

• We only store data in the user's $_SESSION if we have a Mollom session id. For forms protected by textual analysis, we only have a Mollom session id upon first submission of a form. Effectively, this means that when the page containing the form is initially displayed, no user session is started (with regard to some of those systems).
• We only disable the page cache when a form contains a CAPTCHA. For CAPTCHA-only protected forms, this means that the page containing the form will never be cached. For forms protected by textual analysis, this means that the initial page containing the form will be cached. Only if Mollom is unsure about the form submission, a CAPTCHA will be added to the form. Effectively, this means that pages containing forms protected by textual analysis 1) are cached 2) but subsequent POSTs to the page/form are not.

Regarding page caching, users having issues with such external systems may want to try to move:

function mollom_pre_render_mollom($element) {
...
    // Prevent the page cache from storing a form containing a CAPTCHA element.
    $GLOBALS['conf']['cache'] = CACHE_DISABLED;
}

to the end of mollom_process_mollom().