drupal/modules/throttle/throttle.module

<?php
// $Id$

/**
 * @file
 * Allows configuration of congestion control auto-throttle mechanism.
 */

function throttle_menu() {
  $items['admin/settings/throttle'] = array(
    'title' => 'Throttle',
    'description' => 'Control how your site cuts out content during heavy load.',
    'page callback' => 'drupal_get_form',
    'page arguments' => array('throttle_admin_settings'),
    'access arguments' => array('administer site configuration'),
    'file' => 'throttle.admin.inc',
  );
  return $items;
}

/**
 * Determine the current load on the site.
 *
 * Call the throttle_status() function from your own modules, themes, blocks,
 * etc. as follows:
 *
 *   $throttle = module_invoke('throttle', 'status');
 *
 * to determine the current throttle status. Use module_invoke() so the
 * call will still work if the throttle module is disabled. For example, in
 * your theme you might choose to disable pictures when your site is too busy
 * (reducing bandwidth), or in your modules you might choose to disable
 * some complicated logic when your site is too busy (reducing CPU utilization).
 *
 * @return
 *   0 or 1. 0 means that the throttle is currently disabled. 1 means that
 *   the throttle is currently enabled. When the throttle is enabled, CPU
 *   and bandwidth intensive functionality should be disabled.
 */
function throttle_status() {
  return variable_get('throttle_level', 0);
}

/**
 * Implementation of hook_exit().
 *
 * Changes the current throttle level based on page hits.
 */
function throttle_exit() {
  // The following logic determines what the current throttle level should
  //  be, and can be disabled by the admin. If enabled, the mt_rand() function
  //  returns a number between 0 and N, N being specified by the admin. If
  //  0 is returned, the throttle logic is run, adding two additional database
  //  queries. Otherwise, the following logic is skipped. This mechanism is
  //  referred to in the admin page as the 'probability limiter', roughly
  //  limiting throttle related database calls to 1 in N.
  if (!mt_rand(0, variable_get('throttle_probability_limiter', 9))) {

    // Count users with activity in the past n seconds.
    // This value is defined in the user module Who's Online block.
    $time_period = variable_get('user_block_seconds_online', 900);

    // When determining throttle status in your own module or theme, use
    // $throttle = module_invoke('throttle', 'status');
    // as that will still work when throttle.module is disabled.
    // Clearly here the module is enabled so we call throttle_status() directly.
    $throttle = throttle_status();

    if ($max_guests = variable_get('throttle_anonymous', 0)) {
      $guests = sess_count(time() - $time_period, TRUE);
    }
    else {
      $guests = 0;
    }
    if ($max_users = variable_get('throttle_user', 0)) {
      $users = sess_count(time() - $time_period, FALSE);
    }
    else {
      $users = 0;
    }

    // update the throttle status
    $message = '';
    if ($max_users && $users > $max_users) {
      if (!$throttle) {
        variable_set('throttle_level', 1);
        $message = format_plural($users,
                                 '1 user accessing site; throttle enabled.',
                                 '@count users accessing site; throttle enabled.');
      }
    }
    elseif ($max_guests && $guests > $max_guests) {
      if (!$throttle) {
        variable_set('throttle_level', 1);
        $message = format_plural($guests,
                                 '1 guest accessing site; throttle enabled.',
                                 '@count guests accessing site; throttle enabled.');
      }
    }
    else {
      if ($throttle) {
        variable_set('throttle_level', 0);
        // Note: unorthodox format_plural() usage due to Gettext plural limitations.
        $message = format_plural($users, '1 user', '@count users') .', ';
        $message .= format_plural($guests, '1 guest accessing site; throttle disabled', '@count guests accessing site; throttle disabled');
      }
    }
    if ($message) {
      cache_clear_all();
      watchdog('throttle', 'Throttle: %message', array('%message' => $message));
    }
  }
}

/**
 * Implementation of hook_help().
 */
function throttle_help($path, $arg) {
  switch ($path) {
    case 'admin/help#throttle':
      $output = '<p>'. t('The throttle module provides a congestion control mechanism that automatically adjusts to a surge in incoming traffic. If your site is referenced by a popular website, or experiences a "Denial of Service" (DoS) attack, your webserver might become overwhelmed. The throttle mechanism is utilized by modules to temporarily disable CPU-intensive functionality, increasing performance. For instance, via the throttle module, modules may choose to disable resource-intensive blocks or the code within the site theme may temporarily disable user pictures in posts.') .'</p>';
      $output .= '<p>'. t('The congestion control throttle can be automatically enabled when the number of anonymous or authenticated users currently visiting the site exceeds a specified threshold.') .'</p>';
      $output .= '<p>'. t('For more information please read the configuration and customization handbook <a href="@throttle">Throttle page</a>.', array('@throttle' => 'http://drupal.org/handbook/modules/throttle/')) .'</p>';
      return $output;
    case 'admin/settings/throttle':
      return '<p>'. t('The throttle module provides a congestion control mechanism that automatically adjusts to a surge in incoming traffic. If your site is referenced by a popular website, or experiences a "Denial of Service" (DoS) attack, your webserver might become overwhelmed. The throttle mechanism is utilized by modules to temporarily disable CPU-intensive functionality, increasing performance.') .'</p>';
  }
}