drupal/core/modules/dblog/dblog.module

<?php

/**
 * @file
 * System monitoring and logging for administrators.
 *
 * The Database Logging module monitors your site and keeps a list of recorded
 * events containing usage and performance data, errors, warnings, and similar
 * operational information.
 *
 * @see watchdog()
 */

use Drupal\Core\Database\Database;

/**
 * Implements hook_help().
 */
function dblog_help($path, $arg) {
  switch ($path) {
    case 'admin/help#dblog':
      $output = '';
      $output .= '<h3>' . t('About') . '</h3>';
      $output .= '<p>' . t('The Database Logging module logs system events in the Drupal database. For more information, see the online handbook entry for the <a href="@dblog">Database Logging module</a>.', array('@dblog' => 'http://drupal.org/documentation/modules/dblog')) . '</p>';
      $output .= '<h3>' . t('Uses') . '</h3>';
      $output .= '<dl>';
      $output .= '<dt>' . t('Monitoring your site') . '</dt>';
      $output .= '<dd>' . t('The Database Logging module allows you to view an event log on the <a href="@dblog">Recent log messages</a> page. The log is a chronological list of recorded events containing usage data, performance data, errors, warnings and operational information. Administrators should check the log on a regular basis to ensure their site is working properly.', array('@dblog' => url('admin/reports/dblog'))) . '</dd>';
      $output .= '<dt>' . t('Debugging site problems') . '</dt>';
      $output .= '<dd>' . t('In case of errors or problems with the site, the <a href="@dblog">Recent log messages</a> page can be useful for debugging, since it shows the sequence of events. The log messages include usage information, warnings, and errors.', array('@dblog' => url('admin/reports/dblog'))) . '</dd>';
      $output .= '</dl>';
      return $output;
    case 'admin/reports/dblog':
      return '<p>' . t('The Database Logging module monitors your website, capturing system events in a log (shown here) to be reviewed by an authorized individual at a later time. This log is a list of recorded events containing usage data, performance data, errors, warnings and operational information. It is vital to check the Recent log messages report on a regular basis, as it is often the only way to tell what is going on.') . '</p>';
  }
}

/**
 * Implements hook_menu().
 */
function dblog_menu() {
  $items['admin/reports/dblog'] = array(
    'title' => 'Recent log messages',
    'description' => 'View events that have recently been logged.',
    'route_name' => 'dblog_overview',
    'weight' => -1,
  );
  $items['admin/reports/page-not-found'] = array(
    'title' => "Top 'page not found' errors",
    'description' => "View 'page not found' errors (404s).",
    'page callback' => 'dblog_top',
    'page arguments' => array('page not found'),
    'access arguments' => array('access site reports'),
    'file' => 'dblog.admin.inc',
  );
  $items['admin/reports/access-denied'] = array(
    'title' => "Top 'access denied' errors",
    'description' => "View 'access denied' errors (403s).",
    'page callback' => 'dblog_top',
    'page arguments' => array('access denied'),
    'access arguments' => array('access site reports'),
    'file' => 'dblog.admin.inc',
  );
  $items['admin/reports/event/%'] = array(
    'title' => 'Details',
    'page callback' => 'dblog_event',
    'page arguments' => array(3),
    'access arguments' => array('access site reports'),
    'file' => 'dblog.admin.inc',
  );

  if (module_exists('search')) {
    $items['admin/reports/search'] = array(
      'title' => 'Top search phrases',
      'description' => 'View most popular search phrases.',
      'page callback' => 'dblog_top',
      'page arguments' => array('search'),
      'access arguments' => array('access site reports'),
      'file' => 'dblog.admin.inc',
    );
  }

  return $items;
}

/**
 * Implements hook_page_build().
 */
function dblog_page_build(&$page) {
  if (arg(0) == 'admin' && arg(1) == 'reports') {
    $page['#attached']['css'][] = drupal_get_path('module', 'dblog') . '/css/dblog.module.css';
  }
}

/**
 * Implements hook_cron().
 *
 * Controls the size of the log table, paring it to 'dblog_row_limit' messages.
 */
function dblog_cron() {
  // Cleanup the watchdog table.
  $row_limit = config('dblog.settings')->get('row_limit');

  // For row limit n, get the wid of the nth row in descending wid order.
  // Counting the most recent n rows avoids issues with wid number sequences,
  // e.g. auto_increment value > 1 or rows deleted directly from the table.
  if ($row_limit > 0) {
    $min_row = db_select('watchdog', 'w')
      ->fields('w', array('wid'))
      ->orderBy('wid', 'DESC')
      ->range($row_limit - 1, 1)
      ->execute()->fetchField();

    // Delete all table entries older than the nth row, if nth row was found.
    if ($min_row) {
      db_delete('watchdog')
        ->condition('wid', $min_row, '<')
        ->execute();
    }
  }
}

/**
 * Gathers a list of uniquely defined database log message types.
 *
 * @return array
 *   List of uniquely defined database log message types.
 */
function _dblog_get_message_types() {
  $types = array();

  $result = db_query('SELECT DISTINCT(type) FROM {watchdog} ORDER BY type');
  foreach ($result as $object) {
    $types[] = $object->type;
  }

  return $types;
}

/**
 * Implements hook_watchdog().
 *
 * Note: Some values may be truncated to meet database column size restrictions.
 */
function dblog_watchdog(array $log_entry) {
  // Remove any backtraces since they may contain an unserializable variable.
  unset($log_entry['variables']['backtrace']);

  Database::getConnection('default', 'default')->insert('watchdog')
    ->fields(array(
      'uid' => $log_entry['uid'],
      'type' => substr($log_entry['type'], 0, 64),
      'message' => $log_entry['message'],
      'variables' => serialize($log_entry['variables']),
      'severity' => $log_entry['severity'],
      'link' => substr($log_entry['link'], 0, 255),
      'location' => $log_entry['request_uri'],
      'referer' => $log_entry['referer'],
      'hostname' => substr($log_entry['ip'], 0, 128),
      'timestamp' => $log_entry['timestamp'],
    ))
    ->execute();
}

/**
 * Implements hook_form_FORM_ID_alter() for system_logging_settings().
 */
function dblog_form_system_logging_settings_alter(&$form, $form_state) {
  $form['dblog_row_limit'] = array(
    '#type' => 'select',
    '#title' => t('Database log messages to keep'),
    '#default_value' => config('dblog.settings')->get('row_limit'),
    '#options' => array(0 => t('All')) + drupal_map_assoc(array(100, 1000, 10000, 100000, 1000000)),
    '#description' => t('The maximum number of messages to keep in the database log. Requires a <a href="@cron">cron maintenance task</a>.', array('@cron' => url('admin/reports/status')))
  );

  $form['#submit'][] = 'dblog_logging_settings_submit';
}

/**
 * Form submission handler for system_logging_settings().
 *
 * @see dblog_form_system_logging_settings_alter()
 */
function dblog_logging_settings_submit($form, &$form_state) {
  config('dblog.settings')->set('row_limit', $form_state['values']['dblog_row_limit'])->save();
}