Drupal起動フェーズ2:ページキャッシュ

ページキャッシュとはどういう意味ですか?一部のページの閲覧量は非常に大きく、ステータスに関係なく、ページキャッシュ技術を使用することができます.ページの最初のリクエストが完了したら、レスポンス結果を保存します.次に同じページを再要求する場合は,最初から最後まで再実行する必要はなく,最初に実行した応答結果を取得し,そのまま利用者に返すだけでよい.
 
どのようなページリクエストをキャッシュできますか?Drupal使用関数drupal_page_is_Cacheable()は、キャッシュできるリクエストを区別します.

function drupal_page_is_cacheable($allow_caching = NULL) {
  $allow_caching_static = &drupal_static(__FUNCTION__, TRUE);
  if (isset($allow_caching)) {
    $allow_caching_static = $allow_caching;
  }

  return $allow_caching_static && ($_SERVER['REQUEST_METHOD'] == 'GET' || $_SERVER['REQUEST_METHOD'] == 'HEAD')
    && !drupal_is_cli();
}

デフォルトでは、キャッシュできるのはGETとHEADリクエストのみです.
 
最初のリクエスト応答結果を保存するにはどうすればいいですか?Drupal使用関数drupal_page_set_Cache()は応答結果を保存します.要求実行が完了すると、Drupalはこの関数を実行し、cache_を使用します.set()応答結果をcache_に保存Pageオブジェクト:

function drupal_page_set_cache() {
  global $base_root;

  if (drupal_page_is_cacheable()) {
    $cache = (object) array(
      'cid' => $base_root . request_uri(),
      'data' => array(
        'path' => $_GET['q'],
        'body' => ob_get_clean(),
        'title' => drupal_get_title(),
        'headers' => array(),
      ),
      'expire' => CACHE_TEMPORARY,
      'created' => REQUEST_TIME,
    );

    // Restore preferred header names based on the lower-case names returned
    // by drupal_get_http_header().
    $header_names = _drupal_set_preferred_header_name();
    foreach (drupal_get_http_header() as $name_lower => $value) {
      $cache->data['headers'][$header_names[$name_lower]] = $value;
      if ($name_lower == 'expires') {
        // Use the actual timestamp from an Expires header if available.
        $cache->expire = strtotime($value);
      }
    }

    if ($cache->data['body']) {
      if (variable_get('page_compression', TRUE) && extension_loaded('zlib')) {
        $cache->data['body'] = gzencode($cache->data['body'], 9, FORCE_GZIP);
      }
      cache_set($cache->cid, $cache->data, 'cache_page', $cache->expire);
    }
    return $cache;
  }
}

ここでDrupalがどのようにキャッシュされているかをよく見てみましょう.URLをkeyとし、キャッシュ内容はpath、body、title、headersを含む配列です./drupal/index.php?q=node/1および/drupal/index.php?q=node/2これは2つのキャッシュエントリになります.
 
同じページの後続リクエストをキャッシュから読み出すにはどうすればいいですか?これがDrupalの起動関数です.drupal_bootstrap_page_Cache()が完了した内容:

function _drupal_bootstrap_page_cache() {
  global $user;

  // Allow specifying special cache handlers in settings.php, like
  // using memcached or files for storing cache information.
  require_once DRUPAL_ROOT . '/includes/cache.inc';
  
  //       ？
  //     cache.inc             DrupalDatabaseCache,
  //    Memcached  (DrupalMemcachedCache)  File  (DrupalFileCache) ,
  //             settings.php  cache_backends  :
  //   $conf['cache_backends'][] = 'includes/cache.memcached.inc';
  //   $conf['cache_backends'][] = 'includes/cache.file.inc';
  foreach (variable_get('cache_backends', array()) as $include) {
    require_once DRUPAL_ROOT . '/' . $include;
  }
  
  // Check for a cache mode force from settings.php.
  if (variable_get('page_cache_without_database')) {
    $cache_enabled = TRUE;
  }
  else {
    drupal_bootstrap(DRUPAL_BOOTSTRAP_VARIABLES, FALSE);
    $cache_enabled = variable_get('cache');
  }
  
  drupal_block_denied(ip_address());
  
  // If there is no session cookie and cache is enabled (or forced), try
  // to serve a cached page.
  if (!isset($_COOKIE[session_name()]) && $cache_enabled) {
    // Make sure there is a user object because its timestamp will be
    // checked, hook_boot might check for anonymous user etc.
    $user = drupal_anonymous_user(); //    
    // Get the page from the cache.
    $cache = drupal_page_get_cache();
    // If there is a cached page, display it.
    if (is_object($cache)) {
      header('X-Drupal-Cache: HIT');
      // Restore the metadata cached with the page.
      $_GET['q'] = $cache->data['path'];
      drupal_set_title($cache->data['title'], PASS_THROUGH);
      date_default_timezone_set(drupal_get_user_timezone());
      // If the skipping of the bootstrap hooks is not enforced, call
      // hook_boot.
      if (variable_get('page_cache_invoke_hooks', TRUE)) {
        bootstrap_invoke_all('boot');
      }
      drupal_serve_page_from_cache($cache);
      // If the skipping of the bootstrap hooks is not enforced, call
      // hook_exit.
      if (variable_get('page_cache_invoke_hooks', TRUE)) {
        bootstrap_invoke_all('exit');
      }
      // We are done.
      exit;
    }
    else {
      header('X-Drupal-Cache: MISS');
    }
  }
}

 
まず、リクエスト者のIPアドレスが許可されているかどうかを確認します.

drupal_block_denied(ip_address());

要求者のIPアドレスが禁止されている場合、Drupalは要求者に403情報を送信する.

function drupal_block_denied($ip) {
  // Deny access to blocked IP addresses - t() is not yet available.
  if (drupal_is_denied($ip)) {
    header($_SERVER['SERVER_PROTOCOL'] . ' 403 Forbidden');
    print 'Sorry, ' . check_plain(ip_address()) . ' has been banned.';
    exit();
  }
}

 
次に、リクエストが匿名であるかどうかを確認します.匿名リクエストのみがキャッシュをチェックします.

if (!isset($_COOKIE[session_name()]) /*    COOKIE       */ && $cache_enabled) {
    // ......
}

匿名リクエストの場合、Drupalは関数drupal_を介してanonymous_user()$userグローバル変数を設定します.

function drupal_anonymous_user() {
  $user = new stdClass();
  $user->uid = 0;
  $user->hostname = ip_address();
  $user->roles = array();
  $user->roles[DRUPAL_ANONYMOUS_RID] = 'anonymous user';
  $user->cache = 0;
  return $user;
}

また、匿名リクエストは、現在のコンテンツがキャッシュされているかどうかを表す追加のHTTPヘッダ情報X-Drupal-Cacheを返します.

header('X-Drupal-Cache: HIT'); //     
header('X-Drupal-Cache: MISS'); //     

 
Drupalパス関数drupal_page_set_Cache()ページキャッシュを保存し、対応する、関数drupal_を使用page_get_Cache()ページキャッシュの取得:

function drupal_page_get_cache($check_only = FALSE) {
  global $base_root;
  static $cache_hit = FALSE;

  if ($check_only) {
    return $cache_hit;
  }

  if (drupal_page_is_cacheable()) {
    $cache = cache_get($base_root . request_uri(), 'cache_page');
    if ($cache !== FALSE) {
      $cache_hit = TRUE;
    }
    return $cache;
  }
}

取得したキャッシュ内容はdrupal_page_set_Cache()が保存する配列:

$cache = drupal_page_get_cache();

// $cache = array(
//     'path' => '...',
//   'body' => '...',
//   'title' => '...',
//   'headers' => array(...),
// );

 
最後に、関数drupal_を透過します.serve_page_from_Cache()はキャッシュ内容を返し,要求を終了する.

function drupal_serve_page_from_cache(stdClass $cache) {
  // Negotiate whether to use compression.
  $page_compression = variable_get('page_compression', TRUE) && extension_loaded('zlib');
  $return_compressed = $page_compression && isset($_SERVER['HTTP_ACCEPT_ENCODING']) && strpos($_SERVER['HTTP_ACCEPT_ENCODING'], 'gzip') !== FALSE;

  // Get headers set in hook_boot(). Keys are lower-case.
  $hook_boot_headers = drupal_get_http_header();

  // Headers generated in this function, that may be replaced or unset using
  // drupal_add_http_headers(). Keys are mixed-case.
  $default_headers = array();

  foreach ($cache->data['headers'] as $name => $value) {
    // In the case of a 304 response, certain headers must be sent, and the
    // remaining may not (see RFC 2616, section 10.3.5). Do not override
    // headers set in hook_boot().
    $name_lower = strtolower($name);
    if (in_array($name_lower, array('content-location', 'expires', 'cache-control', 'vary')) && !isset($hook_boot_headers[$name_lower])) {
      drupal_add_http_header($name, $value);
      unset($cache->data['headers'][$name]);
    }
  }

  // If the client sent a session cookie, a cached copy will only be served
  // to that one particular client due to Vary: Cookie. Thus, do not set
  // max-age > 0, allowing the page to be cached by external proxies, when a
  // session cookie is present unless the Vary header has been replaced or
  // unset in hook_boot().
  $max_age = !isset($_COOKIE[session_name()]) || isset($hook_boot_headers['vary']) ? variable_get('page_cache_maximum_age', 0) : 0;
  $default_headers['Cache-Control'] = 'public, max-age=' . $max_age;

  // Entity tag should change if the output changes.
  $etag = '"' . $cache->created . '-' . intval($return_compressed) . '"';
  header('Etag: ' . $etag);

  // See if the client has provided the required HTTP headers.
  $if_modified_since = isset($_SERVER['HTTP_IF_MODIFIED_SINCE']) ? strtotime($_SERVER['HTTP_IF_MODIFIED_SINCE']) : FALSE;
  $if_none_match = isset($_SERVER['HTTP_IF_NONE_MATCH']) ? stripslashes($_SERVER['HTTP_IF_NONE_MATCH']) : FALSE;

  if ($if_modified_since && $if_none_match
      && $if_none_match == $etag // etag must match
      && $if_modified_since == $cache->created) {  // if-modified-since must match
    header($_SERVER['SERVER_PROTOCOL'] . ' 304 Not Modified');
    drupal_send_headers($default_headers);
    return;
  }

  // Send the remaining headers.
  foreach ($cache->data['headers'] as $name => $value) {
    drupal_add_http_header($name, $value);
  }

  $default_headers['Last-Modified'] = gmdate(DATE_RFC1123, $cache->created);

  // HTTP/1.0 proxies does not support the Vary header, so prevent any caching
  // by sending an Expires date in the past. HTTP/1.1 clients ignores the
  // Expires header if a Cache-Control: max-age= directive is specified (see RFC
  // 2616, section 14.9.3).
  $default_headers['Expires'] = 'Sun, 19 Nov 1978 05:00:00 GMT';

  drupal_send_headers($default_headers);

  // Allow HTTP proxies to cache pages for anonymous users without a session
  // cookie. The Vary header is used to indicates the set of request-header
  // fields that fully determines whether a cache is permitted to use the
  // response to reply to a subsequent request for a given URL without
  // revalidation. If a Vary header has been set in hook_boot(), it is assumed
  // that the module knows how to cache the page.
  if (!isset($hook_boot_headers['vary']) && !variable_get('omit_vary_cookie')) {
    header('Vary: Cookie');
  }

  if ($page_compression) {
    header('Vary: Accept-Encoding', FALSE);
    // If page_compression is enabled, the cache contains gzipped data.
    if ($return_compressed) {
      // $cache->data['body'] is already gzip'ed, so make sure
      // zlib.output_compression does not compress it once more.
      ini_set('zlib.output_compression', '0');
      header('Content-Encoding: gzip');
    }
    else {
      // The client does not support compression, so unzip the data in the
      // cache. Strip the gzip header and run uncompress.
      $cache->data['body'] = gzinflate(substr(substr($cache->data['body'], 10), 0, -8));
    }
  }

  // Print the page.
  print $cache->data['body'];
}