abstract class BackdropCacheArray

Provides a caching wrapper to be used in place of large array structures.

This class should be extended by systems that need to cache large amounts of data and have it represented as an array to calling functions. These arrays can become very large, so ArrayAccess is used to allow different strategies to be used for caching internally (lazy loading, building caches over time etc.). This can dramatically reduce the amount of data that needs to be loaded from cache backends on each request, and memory usage from static caches of that same data.

Note that array_* functions do not work with ArrayAccess. Systems using BackdropCacheArray should use this only internally. If providing API functions that return the full array, this can be cached separately or returned directly. However since BackdropCacheArray holds partial content by design, it should be a normal PHP array or otherwise contain the full structure.

Note also that due to limitations in PHP prior to 5.3.4, it is impossible to write directly to the contents of nested arrays contained in this object. Only writes to the top-level array elements are possible. So if you previously had set $object['foo'] = array(1, 2, 'bar' => 'baz'), but later want to change the value of 'bar' from 'baz' to 'foobar', you cannot do so a targeted write like $object['foo']['bar'] = 'foobar'. Instead, you must overwrite the entire top-level 'foo' array with the entire set of new values: $object['foo'] = array(1, 2, 'bar' => 'foobar'). Due to this same limitation, attempts to create references to any contained data, nested or otherwise, will fail silently. So $var = &$object['foo'] will not throw an error, and $var will be populated with the contents of $object['foo'], but that data will be passed by value, not reference. For more information on the PHP limitation, see the note in the official PHP documentation at· http://php.net/manual/arrayaccess.offsetget.php on ArrayAccess::offsetGet().

By default, the class accounts for caches where calling functions might request keys in the array that won't exist even after a cache rebuild. This prevents situations where a cache rebuild would be triggered over and over due to a 'missing' item. These cases are stored internally as a value of NULL. This means that the offsetGet() and offsetExists() methods must be overridden if caching an array where the top level values can legitimately be NULL, and where $object->offsetExists() needs to correctly return (equivalent to array_key_exists() vs. isset()). This should not be necessary in the majority of cases.

Classes extending this class must override at least the resolveCacheMiss() method to have a working implementation.

offsetSet() is not overridden by this class by default. In practice this means that assigning an offset via arrayAccess will only apply while the object is in scope and will not be written back to the persistent cache. This follows a similar pattern to static vs. persistent caching in procedural code. Extending classes may wish to alter this behavior, for example by overriding offsetSet() and adding an automatic call to persist().

Hierarchy

class BackdropCacheArray implements \ArrayAccess

Expanded class hierarchy of BackdropCacheArray

File

includes/bootstrap.inc, line 334: Functions that need to be loaded on every Backdrop request.

View source

abstract class BackdropCacheArray implements ArrayAccess {

  /**
   * A cid to pass to cache()->set() and cache()->get().
   */
  protected $cid;

  /**
   * A bin to pass to cache()->set() and cache()->get().
   */
  protected $bin;

  /**
   * An array of keys to add to the cache at the end of the request.
   */
  protected $keysToPersist = array();

  /**
   * Storage for the data itself.
   */
  protected $storage = array();

  /**
   * Constructs a BackdropCacheArray object.
   *
   * @param $cid
   *   The cid for the array being cached.
   * @param $bin
   *   The bin to cache the array.
   */
  public function __construct($cid, $bin) {
    $this->cid = $cid;
    $this->bin = $bin;

    if ($cached = cache($bin)->get($this->cid)) {
      $this->storage = $cached->data;
    }
  }

  /**
   * Implements ArrayAccess::offsetExists().
   */
  public function offsetExists($offset) {
    return $this->offsetGet($offset) !== NULL;
  }

  /**
   * Implements ArrayAccess::offsetGet().
   */
  public function offsetGet($offset) {
    if (isset($this->storage[$offset]) || array_key_exists($offset, $this->storage)) {
      return $this->storage[$offset];
    }
    else {
      return $this->resolveCacheMiss($offset);
    }
  }

  /**
   * Implements ArrayAccess::offsetSet().
   */
  public function offsetSet($offset, $value) {
    $this->storage[$offset] = $value;
  }

  /**
   * Implements ArrayAccess::offsetUnset().
   */
  public function offsetUnset($offset) {
    unset($this->storage[$offset]);
  }

  /**
   * Flags an offset value to be written to the persistent cache.
   *
   * If a value is assigned to a cache object with offsetSet(), by default it
   * will not be written to the persistent cache unless it is flagged with this
   * method. This allows items to be cached for the duration of a request,
   * without necessarily writing back to the persistent cache at the end.
   *
   * @param $offset
   *   The array offset that was requested.
   * @param $persist
   *   Optional boolean to specify whether the offset should be persisted or
   *   not, defaults to TRUE. When called with $persist = FALSE the offset will
   *   be unflagged so that it will not be written at the end of the request.
   */
  protected function persist($offset, $persist = TRUE) {
    $this->keysToPersist[$offset] = $persist;
  }

  /**
   * Resolves a cache miss.
   *
   * When an offset is not found in the object, this is treated as a cache
   * miss. This method allows classes implementing the interface to look up
   * the actual value and allow it to be cached.
   *
   * @param $offset
   *   The offset that was requested.
   *
   * @return
   *   The value of the offset, or NULL if no value was found.
   */
  abstract protected function resolveCacheMiss($offset);

  /**
   * Writes a value to the persistent cache immediately.
   *
   * @param $data
   *   The data to write to the persistent cache.
   * @param $lock
   *   Whether to acquire a lock before writing to cache.
   */
  protected function set($data, $lock = TRUE) {
    // Lock cache writes to help avoid stampedes.
    // To implement locking for cache misses, override __construct().
    $lock_name = $this->cid . ':' . $this->bin;
    if (!$lock || lock_acquire($lock_name)) {
      if ($cached = cache($this->bin)->get($this->cid)) {
        $data = $cached->data + $data;
      }
      cache($this->bin)->set($this->cid, $data);
      if ($lock) {
        lock_release($lock_name);
      }
    }
  }

  /**
   * Destructs the BackdropCacheArray object.
   */
  public function __destruct() {
    $data = array();
    foreach ($this->keysToPersist as $offset => $persist) {
      if ($persist) {
        $data[$offset] = $this->storage[$offset];
      }
    }
    if (!empty($data)) {
      $this->set($data);
    }
  }
}

Members

Name	Modifiers	Type	Description
BackdropCacheArray::$bin	protected	property	A bin to pass to cache()->set() and cache()->get().
BackdropCacheArray::$cid	protected	property	A cid to pass to cache()->set() and cache()->get().
BackdropCacheArray::$keysToPersist	protected	property	An array of keys to add to the cache at the end of the request.
BackdropCacheArray::$storage	protected	property	Storage for the data itself.
BackdropCacheArray::__construct	public	function	Constructs a BackdropCacheArray object.
BackdropCacheArray::__destruct	public	function	Destructs the BackdropCacheArray object.
BackdropCacheArray::offsetExists	public	function	Implements ArrayAccess::offsetExists().
BackdropCacheArray::offsetGet	public	function	Implements ArrayAccess::offsetGet().
BackdropCacheArray::offsetSet	public	function	Implements ArrayAccess::offsetSet().
BackdropCacheArray::offsetUnset	public	function	Implements ArrayAccess::offsetUnset().
BackdropCacheArray::persist	protected	function	Flags an offset value to be written to the persistent cache.
BackdropCacheArray::resolveCacheMiss	abstract protected	function	Resolves a cache miss.
BackdropCacheArray::set	protected	function	Writes a value to the persistent cache immediately.

abstract class BackdropCacheArray

Hierarchy

See also

File

Members

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