Proyectos de Subversion Moodle

<?php

// This file is part of Moodle - http://moodle.org/

//

// Moodle is free software: you can redistribute it and/or modify

// it under the terms of the GNU General Public License as published by

// the Free Software Foundation, either version 3 of the License, or

// (at your option) any later version.

//

// Moodle is distributed in the hope that it will be useful,

// but WITHOUT ANY WARRANTY; without even the implied warranty of

// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the

// GNU General Public License for more details.

//

// You should have received a copy of the GNU General Public License

// along with Moodle.  If not, see <http://www.gnu.org/licenses/>.

/**

 * Cache definition class

 *

 * This file is part of Moodle's cache API, affectionately called MUC.

 * It contains the components that are requried in order to use caching.

 *

 * @package    core

 * @category   cache

 * @copyright  2012 Sam Hemelryk

 * @license    http://www.gnu.org/copyleft/gpl.html GNU GPL v3 or later

 */

defined('MOODLE_INTERNAL') || die();

/**

 * The cache definition class.

 *

 * Cache definitions need to be defined in db/caches.php files.

 * They can be constructed with the following options.

 *

 * Required settings:

 *     + mode

 *          [int] Sets the mode for the definition. Must be one of cache_store::MODE_*

 *

 * Optional settings:

 *     + simplekeys

 *          [bool] Set to true if your cache will only use simple keys for its items.

 *          Simple keys consist of digits, underscores and the 26 chars of the english language. a-zA-Z0-9_

 *          If true the keys won't be hashed before being passed to the cache store for gets/sets/deletes. It will be

 *          better for performance and possible only becase we know the keys are safe.

 *     + simpledata

 *          [bool] If set to true we know that the data is scalar or array of scalar.

 *     + requireidentifiers

 *          [array] An array of identifiers that must be provided to the cache when it is created.

 *     + requiredataguarantee

 *          [bool] If set to true then only stores that can guarantee data will remain available once set will be used.

 *     + requiremultipleidentifiers

 *          [bool] If set to true then only stores that support multiple identifiers will be used.

 *     + requirelockingbeforewrite

 *          [bool] If set to true then the system will throw an exception if you try to write to

 *          the cache without having a lock on the relevant keys.

 *     + maxsize

 *          [int] If set this will be used as the maximum number of entries within the cache store for this definition.

 *          Its important to note that cache stores don't actually have to acknowledge this setting or maintain it as a hard limit.

 *     + overrideclass

 *          [string] A class to use as the loader for this cache. This is an advanced setting and will allow the developer of the

 *          definition to take 100% control of the caching solution.

 *          Any class used here must inherit the cache_loader interface and must extend default cache loader for the mode they are

 *          using.

 *     + overrideclassfile

 *          [string] Suplements the above setting indicated the file containing the class to be used. This file is included when

 *          required.

 *     + datasource

 *          [string] A class to use as the data loader for this definition.

 *          Any class used here must inherit the cache_data_loader interface.

 *     + datasourcefile

 *          [string] Supplements the above setting indicating the file containing the class to be used. This file is included when

 *          required.

 *     + staticacceleration

 *          The cache loader will keep an array of the items set and retrieved to the cache during the request.

 *          Consider using this setting when you know that there are going to be many calls to the cache for the same information.

 *          Requests for data in this array will be ultra fast, but it will cost memory.

 *     + staticaccelerationsize

 *          [int] This supplements the above setting by limiting the number of items in the static acceleration array.

 *          Tweaking this setting lower will allow you to minimise the memory implications above while hopefully still managing to

 *          offset calls to the cache store.

 *     + ttl

 *          [int] A time to live for the data (in seconds). It is strongly recommended that you don't make use of this and

 *          instead try to create an event driven invalidation system.

 *          Not all cache stores will support this natively and there are undesired performance impacts if the cache store does not.

 *     + mappingsonly

 *          [bool] If set to true only the mapped cache store(s) will be used and the default mode store will not. This is a super

 *          advanced setting and should not be used unless absolutely required. It allows you to avoid the default stores for one

 *          reason or another.

 *     + invalidationevents

 *          [array] An array of events that should cause this cache to invalidate some or all of the items within it.

 *     + sharingoptions

 *          [int] The sharing options that are appropriate for this definition. Should be the sum of the possible options.

 *     + defaultsharing

 *          [int] The default sharing option to use. It's highly recommended that you don't set this unless there is a very

 *          specific reason not to use the system default.

 *     + canuselocalstore

 *          [bool] The cache is able to safely run with multiple copies on different webservers without any need for administrator

 *                 intervention to ensure that data stays in sync across nodes.  This is usually managed by a revision

 *                 system as seen in modinfo cache or language cache.  Requiring purge on upgrade is not sufficient as

 *                 it requires administrator intervention on each node to make it work.

 *

 * For examples take a look at lib/db/caches.php

 *

 * @package    core

 * @category   cache

 * @copyright  2012 Sam Hemelryk

 * @license    http://www.gnu.org/copyleft/gpl.html GNU GPL v3 or later

 */

class cache_definition {

    /** The cache can be shared with everyone */

    const SHARING_ALL = 1;

    /** The cache can be shared with other sites using the same siteid. */

    const SHARING_SITEID = 2;

    /** The cache can be shared with other sites of the same version. */

    const SHARING_VERSION = 4;

    /** The cache can be shared with other sites using the same key */

    const SHARING_INPUT = 8;

    /**

     * The default sharing options available.

     * All + SiteID + Version + Input.

     */

    const SHARING_DEFAULTOPTIONS = 15;

    /**

     * The default sharing option that gets used if none have been selected.

     * SiteID. It is the most restrictive.

     */

    const SHARING_DEFAULT = 2;

    /**

     * The identifier for the definition

     * @var string

     */

    protected $id;

    /**

     * The mode for the defintion. One of cache_store::MODE_*

     * @var int

     */

    protected $mode;

    /**

     * The component this definition is associated with.

     * @var string

     */

    protected $component;

    /**

     * The area this definition is associated with.

     * @var string

     */

    protected $area;

    /**

     * If set to true we know the keys are simple. a-zA-Z0-9_

     * @var bool

     */

    protected $simplekeys = false;

    /**

     * Set to true if we know the data is scalar or array of scalar.

     * @var bool

     */

    protected $simpledata = false;

    /**

     * An array of identifiers that must be provided when the definition is used to create a cache.

     * @var array

     */

    protected $requireidentifiers = array();

    /**

     * If set to true then only stores that guarantee data may be used with this definition.

     * @var bool

     */

    protected $requiredataguarantee = false;

    /**

     * If set to true then only stores that support multple identifiers may be used with this definition.

     * @var bool

     */

    protected $requiremultipleidentifiers = false;

    /**

     * If set to true then we know that this definition requires the locking functionality.

     * This gets set during construction based upon the setting requirelockingbeforewrite.

     * @var bool

     */

    protected $requirelocking = false;

    /**

     * Gets set to true if this definition requires a lock to be acquired before a write is attempted.

     * @var bool

     */

    protected $requirelockingbeforewrite = false;

    /**

     * Gets set to true if this definition requires searchable stores.

     * @since Moodle 2.4.4

     * @var bool

     */

    protected $requiresearchable = false;

    /**

     * Sets the maximum number of items that can exist in the cache.

     * Please note this isn't a hard limit, and doesn't need to be enforced by the caches. They can choose to do so optionally.

     * @var int

     */

    protected $maxsize = null;

    /**

     * The class to use as the cache loader for this definition.

     * @var string

     */

    protected $overrideclass = null;

    /**

     * The file in which the override class exists. This will be included if required.

     * @var string Absolute path

     */

    protected $overrideclassfile = null;

    /**

     * The data source class to use with this definition.

     * @var string

     */

    protected $datasource = null;

    /**

     * The file in which the data source class exists. This will be included if required.

     * @var string

     */

    protected $datasourcefile = null;

    /**

     * Set to true if the cache should hold onto items passing through it to speed up subsequent requests.

     * @var bool

     */

    protected $staticacceleration = false;

    /**

     * The maximum number of items that static acceleration cache should hold onto.

     * @var int

     */

    protected $staticaccelerationsize = false;

    /**

     * The TTL for data in this cache. Please don't use this, instead use event driven invalidation.

     * @var int

     */

    protected $ttl = 0;

    /**

     * Set to true if this cache should only use mapped cache stores and not the default mode cache store.

     * @var bool

     */

    protected $mappingsonly = false;

    /**

     * An array of events that should cause this cache to invalidate.

     * @var array

     */

    protected $invalidationevents = array();

    /**

     * An array of identifiers provided to this cache when it was initialised.

     * @var array

     */

    protected $identifiers = null;

    /**

     * Key prefix for use with single key cache stores

     * @var string

     */

    protected $keyprefixsingle = null;

    /**

     * Key prefix to use with cache stores that support multi keys.

     * @var array

     */

    protected $keyprefixmulti = null;

    /**

     * A hash identifier of this definition.

     * @var string

     */

    protected $definitionhash = null;

    /**

     * The selected sharing mode for this definition.

     * @var int

     */

    protected $sharingoptions;

    /**

     * Whether this cache supports local storages.

     * @var bool

     */

    protected $canuselocalstore = false;

    /**

     * The selected sharing option.

     * @var int One of self::SHARING_*

     */

    protected $selectedsharingoption = self::SHARING_DEFAULT;

    /**

     * The user input key to use if the SHARING_INPUT option has been selected.

     * @var string Must be ALPHANUMEXT

     */

    protected $userinputsharingkey = '';

    /**

     * Creates a cache definition given a definition from the cache configuration or from a caches.php file.

     *

     * @param string $id

     * @param array $definition

     * @param string $unused Used to be datasourceaggregate but that was removed and this is now unused.

     * @return cache_definition

     * @throws coding_exception

     */

    public static function load($id, array $definition, $unused = null) {

        global $CFG;

        if (!array_key_exists('mode', $definition)) {

            throw new coding_exception('You must provide a mode when creating a cache definition');

        }

        if (!array_key_exists('component', $definition)) {

            throw new coding_exception('You must provide a component when creating a cache definition');

        }

        if (!array_key_exists('area', $definition)) {

            throw new coding_exception('You must provide an area when creating a cache definition');

        }

        $mode = (int)$definition['mode'];

        $component = (string)$definition['component'];

        $area = (string)$definition['area'];

        // Set the defaults.

        $simplekeys = false;

        $simpledata = false;

        $requireidentifiers = array();

        $requiredataguarantee = false;

        $requiremultipleidentifiers = false;

        $requirelockingbeforewrite = false;

        $requiresearchable = ($mode === cache_store::MODE_SESSION) ? true : false;

        $maxsize = null;

        $overrideclass = null;

        $overrideclassfile = null;

        $datasource = null;

        $datasourcefile = null;

        $staticacceleration = false;

        $staticaccelerationsize = false;

        $ttl = 0;

        $mappingsonly = false;

        $invalidationevents = array();

        $sharingoptions = self::SHARING_DEFAULT;

        $selectedsharingoption = self::SHARING_DEFAULT;

        $userinputsharingkey = '';

        $canuselocalstore = false;

        if (array_key_exists('simplekeys', $definition)) {

            $simplekeys = (bool)$definition['simplekeys'];

        }

        if (array_key_exists('simpledata', $definition)) {

            $simpledata = (bool)$definition['simpledata'];

        }

        if (array_key_exists('requireidentifiers', $definition)) {

            $requireidentifiers = (array)$definition['requireidentifiers'];

        }

        if (array_key_exists('requiredataguarantee', $definition)) {

            $requiredataguarantee = (bool)$definition['requiredataguarantee'];

        }

        if (array_key_exists('requiremultipleidentifiers', $definition)) {

            $requiremultipleidentifiers = (bool)$definition['requiremultipleidentifiers'];

        }

        if (array_key_exists('requirelockingread', $definition)) {

            debugging('The cache option requirelockingread is deprecated and now has no effect.',

                    DEBUG_DEVELOPER);

        }

        if (array_key_exists('requirelockingwrite', $definition)) {

            debugging('The cache option requirelockingwrite is deprecated and now has no effect. ' .

                    "Consider removing the option, or using requirelockingbeforewrite for the $component:$area definition",

                    DEBUG_DEVELOPER);

        }

        if (array_key_exists('requirelockingbeforewrite', $definition)) {

            $requirelockingbeforewrite = (bool)$definition['requirelockingbeforewrite'];

        }

        // This generic $requirelocking variable is kept in code in case we ever add

        // another locking option, most obviously requirelockingbeforeread.

        $requirelocking = $requirelockingbeforewrite;

        if (array_key_exists('requiresearchable', $definition)) {

            $requiresearchable = (bool)$definition['requiresearchable'];

        }

        if (array_key_exists('maxsize', $definition)) {

            $maxsize = (int)$definition['maxsize'];

        }

        if (array_key_exists('overrideclass', $definition)) {

            $overrideclass = $definition['overrideclass'];

        }

        if (array_key_exists('overrideclassfile', $definition)) {

            $overrideclassfile = $definition['overrideclassfile'];

        }

        if (array_key_exists('datasource', $definition)) {

            $datasource = $definition['datasource'];

        }

        if (array_key_exists('datasourcefile', $definition)) {

            $datasourcefile = $definition['datasourcefile'];

        }

        if (array_key_exists('persistent', $definition)) {

            // Ahhh this is the legacy persistent option.

            $staticacceleration = (bool)$definition['persistent'];

        }

        if (array_key_exists('staticacceleration', $definition)) {

            $staticacceleration = (bool)$definition['staticacceleration'];

        }

        if (array_key_exists('persistentmaxsize', $definition)) {

            // Ahhh this is the legacy persistentmaxsize option.

            $staticaccelerationsize = (int)$definition['persistentmaxsize'];

        }

        if (array_key_exists('staticaccelerationsize', $definition)) {

            $staticaccelerationsize = (int)$definition['staticaccelerationsize'];

        }

        if (array_key_exists('ttl', $definition)) {

            $ttl = (int)$definition['ttl'];

        }

        if (array_key_exists('mappingsonly', $definition)) {

            $mappingsonly = (bool)$definition['mappingsonly'];

        }

        if (array_key_exists('invalidationevents', $definition)) {

            $invalidationevents = (array)$definition['invalidationevents'];

        }

        if (array_key_exists('sharingoptions', $definition)) {

            $sharingoptions = (int)$definition['sharingoptions'];

        }

        if (array_key_exists('selectedsharingoption', $definition)) {

            $selectedsharingoption = (int)$definition['selectedsharingoption'];

        } else if (array_key_exists('defaultsharing', $definition)) {

            $selectedsharingoption = (int)$definition['defaultsharing'];

        } else if ($sharingoptions ^ $selectedsharingoption) {

            if ($sharingoptions & self::SHARING_SITEID) {

                $selectedsharingoption = self::SHARING_SITEID;

            } else if ($sharingoptions & self::SHARING_VERSION) {

                $selectedsharingoption = self::SHARING_VERSION;

            } else {

                $selectedsharingoption = self::SHARING_ALL;

            }

        }

        if (array_key_exists('canuselocalstore', $definition)) {

            $canuselocalstore = (bool)$definition['canuselocalstore'];

        }

        if (array_key_exists('userinputsharingkey', $definition) && !empty($definition['userinputsharingkey'])) {

            $userinputsharingkey = (string)$definition['userinputsharingkey'];

        }

        if (!is_null($overrideclass)) {

            if (!is_null($overrideclassfile)) {

                if (strpos($overrideclassfile, $CFG->dirroot) !== 0) {

                    $overrideclassfile = $CFG->dirroot.'/'.$overrideclassfile;

                }

                if (strpos($overrideclassfile, '../') !== false) {

                    throw new coding_exception('No path craziness allowed within override class file path.');

                }

                if (!file_exists($overrideclassfile)) {

                    throw new coding_exception('The override class file does not exist.');

                }

                require_once($overrideclassfile);

            }

            if (!class_exists($overrideclass)) {

                throw new coding_exception('The override class does not exist.');

            }

            // Make sure that the provided class extends the default class for the mode.

            if (get_parent_class($overrideclass) !== cache_helper::get_class_for_mode($mode)) {

                throw new coding_exception('The override class does not immediately extend the relevant cache class.');

            }

        }

        if (!is_null($datasource)) {

            if (!is_null($datasourcefile)) {

                if (strpos($datasourcefile, $CFG->dirroot) !== 0) {

                    $datasourcefile = $CFG->dirroot.'/'.$datasourcefile;

                }

                if (strpos($datasourcefile, '../') !== false) {

                    throw new coding_exception('No path craziness allowed within data source file path.');

                }

                if (!file_exists($datasourcefile)) {

                    throw new coding_exception('The data source class file does not exist.');

                }

                require_once($datasourcefile);

            }

            if (!class_exists($datasource)) {

                throw new coding_exception('The data source class does not exist.');

            }

            if (!array_key_exists('cache_data_source', class_implements($datasource))) {

                throw new coding_exception('Cache data source classes must implement the cache_data_source interface');

            }

        }

        $cachedefinition = new cache_definition();

        $cachedefinition->id = $id;

        $cachedefinition->mode = $mode;

        $cachedefinition->component = $component;

        $cachedefinition->area = $area;

        $cachedefinition->simplekeys = $simplekeys;

        $cachedefinition->simpledata = $simpledata;

        $cachedefinition->requireidentifiers = $requireidentifiers;

        $cachedefinition->requiredataguarantee = $requiredataguarantee;

        $cachedefinition->requiremultipleidentifiers = $requiremultipleidentifiers;

        $cachedefinition->requirelocking = $requirelocking;

        $cachedefinition->requirelockingbeforewrite = $requirelockingbeforewrite;

        $cachedefinition->requiresearchable = $requiresearchable;

        $cachedefinition->maxsize = $maxsize;

        $cachedefinition->overrideclass = $overrideclass;

        $cachedefinition->overrideclassfile = $overrideclassfile;

        $cachedefinition->datasource = $datasource;

        $cachedefinition->datasourcefile = $datasourcefile;

        $cachedefinition->staticacceleration = $staticacceleration;

        $cachedefinition->staticaccelerationsize = $staticaccelerationsize;

        $cachedefinition->ttl = $ttl;

        $cachedefinition->mappingsonly = $mappingsonly;

        $cachedefinition->invalidationevents = $invalidationevents;

        $cachedefinition->sharingoptions = $sharingoptions;

        $cachedefinition->selectedsharingoption = $selectedsharingoption;

        $cachedefinition->userinputsharingkey = $userinputsharingkey;

        $cachedefinition->canuselocalstore = $canuselocalstore;

        return $cachedefinition;

    }

    /**

     * Creates an ah-hoc cache definition given the required params.

     *

     * Please note that when using an adhoc definition you cannot set any of the optional params.

     * This is because we cannot guarantee consistent access and we don't want to mislead people into thinking that.

     *

     * @param int $mode One of cache_store::MODE_*

     * @param string $component The component this definition relates to.

     * @param string $area The area this definition relates to.

     * @param array $options An array of options, available options are:

     *   - simplekeys : Set to true if the keys you will use are a-zA-Z0-9_

     *   - simpledata : Set to true if the type of the data you are going to store is scalar, or an array of scalar vars

     *   - overrideclass : The class to use as the loader.

     *   - staticacceleration : If set to true the cache will hold onto data passing through it.

     *   - staticaccelerationsize : Set it to an int to limit the size of the staticacceleration cache.

     * @return cache_application|cache_session|cache_request

     */

    public static function load_adhoc($mode, $component, $area, array $options = array()) {

        $id = 'adhoc/'.$component.'_'.$area;

        $definition = array(

            'mode' => $mode,

            'component' => $component,

            'area' => $area,

        );

        if (!empty($options['simplekeys'])) {

            $definition['simplekeys'] = $options['simplekeys'];

        }

        if (!empty($options['simpledata'])) {

            $definition['simpledata'] = $options['simpledata'];

        }

        if (!empty($options['persistent'])) {

            // Ahhh this is the legacy persistent option.

            $definition['staticacceleration'] = (bool)$options['persistent'];

        }

        if (!empty($options['staticacceleration'])) {

            $definition['staticacceleration'] = (bool)$options['staticacceleration'];

        }

        if (!empty($options['staticaccelerationsize'])) {

            $definition['staticaccelerationsize'] = (int)$options['staticaccelerationsize'];

        }

        if (!empty($options['overrideclass'])) {

            $definition['overrideclass'] = $options['overrideclass'];

        }

        if (!empty($options['sharingoptions'])) {

            $definition['sharingoptions'] = $options['sharingoptions'];

        }

        return self::load($id, $definition, null);

    }

    /**

     * Returns the cache loader class that should be used for this definition.

     * @return string

     */

    public function get_cache_class() {

        if (!is_null($this->overrideclass)) {

            return $this->overrideclass;

        }

        return cache_helper::get_class_for_mode($this->mode);

    }

    /**

     * Returns the id of this definition.

     * @return string

     */

    public function get_id() {

        return $this->id;

    }

    /**

     * Returns the name for this definition

     * @return string

     */

    public function get_name() {

        $identifier = 'cachedef_'.clean_param($this->area, PARAM_STRINGID);

        $component = $this->component;

        if ($component === 'core') {

            $component = 'cache';

        }

        return new lang_string($identifier, $component);

    }

    /**

     * Returns the mode of this definition

     * @return int One more cache_store::MODE_

     */

    public function get_mode() {

        return $this->mode;

    }

    /**

     * Returns the area this definition is associated with.

     * @return string

     */

    public function get_area() {

        return $this->area;

    }

    /**

     * Returns the component this definition is associated with.

     * @return string

     */

    public function get_component() {

        return $this->component;

    }

    /**

     * Returns true if this definition is using simple keys.

     *

     * Simple keys contain only a-zA-Z0-9_

     *

     * @return bool

     */

    public function uses_simple_keys() {

        return $this->simplekeys;

    }

    /**

     * Returns the identifiers that are being used for this definition.

     * @return array

     */

    public function get_identifiers() {

        if (!isset($this->identifiers)) {

            return array();

        }

        return $this->identifiers;

    }

    /**

     * Returns the ttl in seconds for this definition if there is one, or null if not.

     * @return int|null

     */

    public function get_ttl() {

        return $this->ttl;

    }

    /**

     * Returns the maximum number of items allowed in this cache.

     * @return int

     */

    public function get_maxsize() {

        return $this->maxsize;

    }

    /**

     * Returns true if this definition should only be used with mappings.

     * @return bool

     */

    public function is_for_mappings_only() {

        return $this->mappingsonly;

    }

    /**

     * Returns true if the data is known to be scalar or array of scalar.

     * @return bool

     */

    public function uses_simple_data() {

        return $this->simpledata;

    }

    /**

     * Returns true if this definition requires a data guarantee from the cache stores being used.

     * @return bool

     */

    public function require_data_guarantee() {

        return $this->requiredataguarantee;

    }

    /**

     * Returns true if this definition requires that the cache stores support multiple identifiers

     * @return bool

     */

    public function require_multiple_identifiers() {

        return $this->requiremultipleidentifiers;

    }

    /**

     * Returns true if this definition requires locking functionality. Either read or write locking.

     * @return bool

     */

    public function require_locking() {

        return $this->requirelocking;

    }

    /**

     * Returns true if this definition requires a lock to be aquired before a write is attempted.

     * @return bool

     */

    public function require_locking_before_write() {

        return $this->requirelockingbeforewrite;

    }

    /**

     * Returns true if this definition allows local storage to be used for caching.

     * @since Moodle 3.1.0

     * @return bool

     */

    public function can_use_localstore() {

        return $this->canuselocalstore;

    }

    /**

     * Returns true if this definition requires a searchable cache.

     * @since Moodle 2.4.4

     * @return bool

     */

    public function require_searchable() {

        return $this->requiresearchable;

    }

    /**

     * Returns true if this definition has an associated data source.

     * @return bool

     */

    public function has_data_source() {

        return !is_null($this->datasource);

    }

    /**

     * Returns an instance of the data source class used for this definition.

     *

     * @return cache_data_source

     * @throws coding_exception

     */

    public function get_data_source() {

        if (!$this->has_data_source()) {

            throw new coding_exception('This cache does not use a data source.');

        }

        return forward_static_call(array($this->datasource, 'get_instance_for_cache'), $this);

    }

    /**

     * Sets the identifiers for this definition, or updates them if they have already been set.

     *

     * @param array $identifiers

     * @return bool false if no identifiers where changed, true otherwise.

     * @throws coding_exception

     */

    public function set_identifiers(array $identifiers = array()) {

        if ($this->identifiers !== null) {

            throw new coding_exception("You can only set identifiers on initial definition creation." .

                " Define a new cache to set different identifiers.");

        }

        if (!empty($identifiers) && !empty($this->invalidationevents)) {

            throw new coding_exception("You cannot use event invalidation and identifiers at the same time.");

        }

        foreach ($this->requireidentifiers as $identifier) {

            if (!isset($identifiers[$identifier])) {

                throw new coding_exception('Identifier required for cache has not been provided: '.$identifier);

            }

        }

        $this->identifiers = array();

        foreach ($identifiers as $name => $value) {

            $this->identifiers[$name] = (string)$value;

        }

        // Reset the key prefix's they need updating now.

        $this->keyprefixsingle = null;

        $this->keyprefixmulti = null;

        return true;

    }

    /**

     * Returns the requirements of this definition as a binary flag.

     * @return int

     */

    public function get_requirements_bin() {

        $requires = 0;

        if ($this->require_data_guarantee()) {

            $requires += cache_store::SUPPORTS_DATA_GUARANTEE;

        }

        if ($this->require_multiple_identifiers()) {

            $requires += cache_store::SUPPORTS_MULTIPLE_IDENTIFIERS;

        }

        if ($this->require_searchable()) {

            $requires += cache_store::IS_SEARCHABLE;

        }

        return $requires;

    }

    /**

     * Please call {@link cache_definition::use_static_acceleration()} instead.

     *

     * @see cache_definition::use_static_acceleration()

     * @deprecated since 2.6

     */

    public function should_be_persistent() {

        throw new coding_exception('cache_definition::should_be_persistent() can not be used anymore.' .

            ' Please use cache_definition::use_static_acceleration() instead.');

    }

    /**

     * Returns true if we should hold onto the data flowing through the cache.

     *

     * If set to true data flowing through the cache will be stored in a static variable

     * to make subsequent requests for the data much faster.

     *

     * @return bool

     */

    public function use_static_acceleration() {

        if ($this->mode === cache_store::MODE_REQUEST) {

            // Request caches should never use static acceleration - it just doesn't make sense.

            return false;

        }

        return $this->staticacceleration;

    }

    /**

     * Please call {@link cache_definition::get_static_acceleration_size()} instead.

     *

     * @see cache_definition::get_static_acceleration_size()

     * @deprecated since 2.6

     */

    public function get_persistent_max_size() {

        throw new coding_exception('cache_definition::get_persistent_max_size() can not be used anymore.' .

            ' Please use cache_definition::get_static_acceleration_size() instead.');

    }

    /**

     * Returns the max size for the static acceleration array.

     * @return int

     */

    public function get_static_acceleration_size() {

        return $this->staticaccelerationsize;

    }

    /**

     * Generates a hash of this definition and returns it.

     * @return string

     */

    public function generate_definition_hash() {

        if ($this->definitionhash === null) {

            $this->definitionhash = md5("{$this->mode} {$this->component} {$this->area}");

        }

        return $this->definitionhash;

    }

    /**

     * Generates a single key prefix for this definition

     *

     * @return string

     */

    public function generate_single_key_prefix() {

        if ($this->keyprefixsingle === null) {

            $this->keyprefixsingle = $this->mode.'/'.$this->component.'/'.$this->area;

            $this->keyprefixsingle .= '/'.$this->get_cache_identifier();

            $identifiers = $this->get_identifiers();

            if ($identifiers) {

                foreach ($identifiers as $key => $value) {

                    $this->keyprefixsingle .= '/'.$key.'='.$value;

                }

            }

            $this->keyprefixsingle = md5($this->keyprefixsingle);

        }

        return $this->keyprefixsingle;

    }

    /**

     * Generates a multi key prefix for this definition

     *

     * @return array

     */

    public function generate_multi_key_parts() {

        if ($this->keyprefixmulti === null) {

            $this->keyprefixmulti = array(

                'mode' => $this->mode,

                'component' => $this->component,

                'area' => $this->area,

                'siteidentifier' => $this->get_cache_identifier()

            );

            if (isset($this->identifiers) && !empty($this->identifiers)) {

                $identifiers = array();

                foreach ($this->identifiers as $key => $value) {

                    $identifiers[] = htmlentities($key, ENT_QUOTES, 'UTF-8').'='.htmlentities($value, ENT_QUOTES, 'UTF-8');

                }

                $this->keyprefixmulti['identifiers'] = join('&', $identifiers);

            }

        }

        return $this->keyprefixmulti;

    }

    /**

     * Check if this definition should invalidate on the given event.

     *

     * @param string $event

     * @return bool True if the definition should invalidate on the event. False otherwise.

     */

    public function invalidates_on_event($event) {

        return (in_array($event, $this->invalidationevents));

    }

    /**

     * Check if the definition has any invalidation events.

     *

     * @return bool True if it does, false otherwise

     */

    public function has_invalidation_events() {

        return !empty($this->invalidationevents);

    }

    /**

     * Returns all of the invalidation events for this definition.

     *

     * @return array

     */

    public function get_invalidation_events() {

        return $this->invalidationevents;

    }

    /**

     * Returns a cache identification string.

     *

     * @return string A string to be used as part of keys.

     */

    protected function get_cache_identifier() {

        $identifiers = array();

        if ($this->selectedsharingoption & self::SHARING_ALL) {

            // Nothing to do here.

        } else {

            if ($this->selectedsharingoption & self::SHARING_SITEID) {

                $identifiers[] = cache_helper::get_site_identifier();

            }

            if ($this->selectedsharingoption & self::SHARING_VERSION) {

                $identifiers[] = cache_helper::get_site_version();

            }

            if ($this->selectedsharingoption & self::SHARING_INPUT && !empty($this->userinputsharingkey)) {

                $identifiers[] = $this->userinputsharingkey;

            }

        }

        return join('/', $identifiers);

    }

    /**

     * Returns true if this definition requires identifiers.

     *

     * @param bool

     */

    public function has_required_identifiers() {

        return (count($this->requireidentifiers) > 0);

    }

    /**

     * Returns the possible sharing options that can be used with this defintion.

     *

     * @return int

     */

    public function get_sharing_options() {

        return $this->sharingoptions;

    }

    /**

     * Returns the user entered sharing key for this definition.

     *

     * @return string

     */

    public function get_user_input_sharing_key() {

        return $this->userinputsharingkey;

    }