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.
 */

use Drupal\Core\Url;
use Drupal\Core\Form\FormStateInterface;
use Drupal\Core\Routing\RouteMatchInterface;
use Drupal\Core\StringTranslation\TranslatableMarkup;
use Drupal\views\ViewExecutable;

/**
 * Implements hook_help().
 */
function dblog_help($route_name, RouteMatchInterface $route_match) {
  switch ($route_name) {
    case 'help.page.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 <a href=":dblog">online documentation for the Database Logging module</a>.', [':dblog' => 'https://www.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.', [':dblog' => Url::fromRoute('dblog.overview')->toString()]) . '</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.', [':dblog' => Url::fromRoute('dblog.overview')->toString()]) . '</dd>';
      $output .= '<dt>' . t('This log is not persistent') . '</dt>';
      $output .= '<dd>' . t('The Database Logging module logs may be cleared by administrators and automated cron tasks, so they should not be used for <a href=":audit_trail_wiki">forensic logging</a>. For forensic purposes, use the Syslog module.', [':audit_trail_wiki' => 'https://en.wikipedia.org/wiki/Audit_trail']) . '</dd>';
      $output .= '</dl>';
      return $output;

    case 'dblog.overview':
      return '<p>' . t('The Database Logging module logs system events in the Drupal database. Monitor your site or debug site problems on this page.') . '</p>';
  }
}

/**
 * Implements hook_menu_links_discovered_alter().
 */
function dblog_menu_links_discovered_alter(&$links) {
  if (\Drupal::moduleHandler()->moduleExists('search')) {
    $links['dblog.search'] = [
      'title' => new TranslatableMarkup('Top search phrases'),
      'route_name' => 'dblog.search',
      'description' => new TranslatableMarkup('View most popular search phrases.'),
      'parent' => 'system.admin_reports',
    ];
  }

  return $links;
}

/**
 * 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 = \Drupal::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) {
    $connection = \Drupal::database();
    $min_row = $connection->select('watchdog', 'w')
      ->fields('w', ['wid'])
      ->orderBy('wid', 'DESC')
      ->range($row_limit - 1, 1)
      ->execute()->fetchField();

    // Delete all table entries older than the nth row, if nth row was found.
    if ($min_row) {
      $connection->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() {
  return \Drupal::database()->query('SELECT DISTINCT([type]) FROM {watchdog} ORDER BY [type]')
    ->fetchAllKeyed(0, 0);
}

/**
 * Implements hook_form_FORM_ID_alter() for system_logging_settings().
 */
function dblog_form_system_logging_settings_alter(&$form, FormStateInterface $form_state) {
  $row_limits = [100, 1000, 10000, 100000, 1000000];
  $form['dblog_row_limit'] = [
    '#type' => 'select',
    '#title' => t('Database log messages to keep'),
    '#default_value' => \Drupal::configFactory()->getEditable('dblog.settings')->get('row_limit'),
    '#options' => [0 => t('All')] + array_combine($row_limits, $row_limits),
    '#description' => t('The maximum number of messages to keep in the database log. Requires a <a href=":cron">cron maintenance task</a>.', [':cron' => Url::fromRoute('system.status')->toString()]),
  ];

  $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, FormStateInterface $form_state) {
  \Drupal::configFactory()->getEditable('dblog.settings')->set('row_limit', $form_state->getValue('dblog_row_limit'))->save();
}

/**
 * Implements hook_views_pre_render().
 */
function dblog_views_pre_render(ViewExecutable $view) {
  if (isset($view) && ($view->storage->get('base_table') == 'watchdog')) {
    $view->element['#attached']['library'][] = 'dblog/drupal.dblog';
  }
}