Beware of Drupal modules that disable the page cache

When doing performance assessment for large and complex sites to assess why they are not fast or scalable, we often run into cases where modules intentionally disable the Drupal page cache.

Depending on how often it happens and for which pages, disabling the page cache can negatively impact the site's performance, be that in scalability, or speed of serving pages.

How to inspect code for page cache disabling

If you want to inspect a module to see if it disables the page cache, search its code for something like the following:

// Recommended way of disabling the cache in Drupal 7.x
drupal_page_is_cacheable(FALSE);

Or:

$GLOBALS['conf']['cache'] = 0;

Or:

$GLOBALS['conf']['cache'] = CACHE_DISABLED;

Or:

$conf['cache'] = FALSE;

Modules that disable the page cache

We have found the following modules that disable the page cache in some cases:

Bibliography Module

Bibliography module

In biblio_init(), the module disables the page cache if someone is visiting a certain URL, such as "biblio/*" or "publications/*", depending on how the module is configured.

if ($user->uid === 0) { 
  // Prevent caching of biblio pages for anonymous users
  // so session variables work and thus filering works
  $base = variable_get('biblio_base', 'biblio');
  if (drupal_match_path($_GET['q'], "$base\n$base/*"))
    $conf['cache'] = FALSE;
}

Flag Module

Flag module

This code in flag/includes/flag_handler_relationships.inc

if (array_search(DRUPAL_ANONYMOUS_RID, $flag->roles['flag']) !== FALSE) {
  // Disable page caching for anonymous users.
  drupal_page_is_cacheable(FALSE);

Or in Drupal 6.x:

if (array_search(DRUPAL_ANONYMOUS_RID, $flag->roles['flag']) !== FALSE) {
  // Disable page caching for anonymous users.
  $GLOBALS['conf']['cache'] = 0;

Invite Module

Invite module

case 'user_register':
  // In order to prevent caching of the preset 
  // e-mail address, we have to disable caching 
  // for user/register.
  $GLOBALS['conf']['cache'] = CACHE_DISABLED;

CAPTCHA Module

CAPTCHA module

The CAPTCHA module disables the cache wherever a CAPTCHA form is displayed, be that in a comment or on the login form.

This is done via the hook_element_info() which sets a callback in the function captcha_element_process().

If you find other modules that are commonly used, please post a comment below about it.