core.api.php

<?php

/**
 * @file
 * Documentation landing page and topics, plus core library hooks.
 */

/**
 * @mainpage
 * Welcome to the Drupal API Documentation!
 *
 * This site is an API reference for Drupal, generated from comments embedded
 * in the source code. More in-depth documentation can be found at
 * https://www.drupal.org/developing/api.
 *
 * Here are some topics to help you get started developing with Drupal.
 *
 * @section essentials Essential background concepts
 *
 * - @link oo_conventions Object-oriented conventions used in Drupal @endlink
 * - @link extending Extending and altering Drupal @endlink
 * - @link best_practices Security and best practices @endlink
 * - @link info_types Types of information in Drupal @endlink
 *
 * @section interface User interface
 *
 * - @link menu Menu entries, local tasks, and other links @endlink
 * - @link routing Routing API and page controllers @endlink
 * - @link form_api Forms @endlink
 * - @link block_api Blocks @endlink
 * - @link ajax Ajax @endlink
 *
 * @section store_retrieve Storing and retrieving data
 *
 * - @link entity_api Entities @endlink
 * - @link field Fields @endlink
 * - @link config_api Configuration API @endlink
 * - @link state_api State API @endlink
 * - @link views_overview Views @endlink
 * - @link database Database abstraction layer @endlink
 *
 * @section other_essentials Other essential APIs
 *
 * - @link plugin_api Plugins @endlink
 * - @link container Services and the Dependency Injection Container @endlink
 * - @link events Events @endlink
 * - @link i18n Internationalization @endlink
 * - @link cache Caching @endlink
 * - @link utility Utility classes and functions @endlink
 * - @link user_api User accounts, permissions, and roles @endlink
 * - @link theme_render Render API @endlink
 * - @link themeable Theme system @endlink
 * - @link update_api Update API @endlink
 * - @link migration Migration @endlink
 *
 * @section additional Additional topics
 *
 * - @link batch Batch API @endlink
 * - @link queue Queue API @endlink
 * - @link typed_data Typed Data @endlink
 * - @link testing Automated tests @endlink
 * - @link php_assert PHP Runtime Assert Statements @endlink
 * - @link third_party Integrating third-party applications @endlink
 *
 * @section more_info Further information
 *
 * - @link https://api.drupal.org/api/drupal/groups/8 All topics @endlink
 * - @link https://www.drupal.org/project/examples Examples project (sample modules) @endlink
 * - @link https://www.drupal.org/list-changes API change notices @endlink
 * - @link https://www.drupal.org/developing/api/8 Drupal 8 API longer references @endlink
 */

/**
 * @defgroup third_party REST and Application Integration
 * @{
 * Integrating third-party applications using REST and related operations.
 *
 * @section sec_overview Overview of web services
 * Web services make it possible for applications and websites to read and
 * update information from other websites. There are several standard
 * techniques for providing web services, including:
 * - SOAP: http://wikipedia.org/wiki/SOAP
 * - XML-RPC: http://wikipedia.org/wiki/XML-RPC
 * - REST: http://wikipedia.org/wiki/Representational_state_transfer
 * Drupal sites can both provide web services and integrate third-party web
 * services.
 *
 * @section sec_rest_overview Overview of REST
 * The REST technique uses basic HTTP requests to obtain and update data, where
 * each web service defines a specific API (HTTP GET and/or POST parameters and
 * returned response) for its HTTP requests. REST requests are separated into
 * several types, known as methods, including:
 * - GET: Requests to obtain data.
 * - POST: Requests to update or create data.
 * - PUT: Requests to update or create data (limited support, currently unused
 *   by entity resources).
 * - PATCH: Requests to update a subset of data, such as one field.
 * - DELETE: Requests to delete data.
 * The Drupal Core REST module provides support for GET, POST, PATCH, and DELETE
 * quests on entities, GET requests on the database log from the Database
 * Logging module, and a plugin framework for providing REST support for other
 * data and other methods.
 *
 * REST requests can be authenticated. The Drupal Core Basic Auth module
 * provides authentication using the HTTP Basic protocol; the contributed module
 * OAuth (https://www.drupal.org/project/oauth) implements the OAuth
 * authentication protocol. You can also use cookie-based authentication, which
 * would require users to be logged into the Drupal site while using the
 * application on the third-party site that is using the REST service.
 *
 * @section sec_rest Enabling REST for entities and the log
 * Here are the steps to take to use the REST operations provided by Drupal
 * Core:
 * - Enable the REST module, plus Basic Auth (or another authentication method)
 *   and HAL.
 * - Node entity support is configured by default. If you would like to support
 *   other types of entities, you can copy
 *   core/modules/rest/config/install/rest.settings.yml to your sync
 *   configuration directory, appropriately modified for other entity types,
 *   and import it. Support for GET on the log from the Database Logging module
 *   can also be enabled in this way; in this case, the 'entity:node' line
 *   in the configuration would be replaced by the appropriate plugin ID,
 *   'dblog'.
 * - Set up permissions to allow the desired REST operations for a role, and set
 *   up one or more user accounts to perform the operations.
 * - To perform a REST operation, send a request to either the canonical URL
 *   for an entity (such as node/12345 for a node), or if the entity does not
 *   have a canonical URL, a URL like entity/(type)/(ID). The URL for a log
 *   entry is dblog/(ID). The request must have the following properties:
 *   - The request method must be set to the REST method you are using (POST,
 *     GET, PATCH, etc.).
 *   - The content type for the data you send, or the accept type for the
 *     data you are receiving, must be set to 'application/hal+json'.
 *   - If you are sending data, it must be JSON-encoded.
 *   - You'll also need to make sure the authentication information is sent
 *     with the request, unless you have allowed access to anonymous users.
 *
 * For more detailed information on setting up REST, see
 * https://www.drupal.org/documentation/modules/rest.
 *
 * @section sec_plugins Defining new REST plugins
 * The REST framework in the REST module has support built in for entities, but
 * it is also an extensible plugin-based system. REST plugins implement
 * interface \Drupal\rest\Plugin\ResourceInterface, and generally extend base
 * class \Drupal\rest\Plugin\ResourceBase. They are annotated with
 * \Drupal\rest\Annotation\RestResource annotation, and must be in plugin
 * namespace subdirectory Plugin\rest\resource. For more information on how to
 * create plugins, see the @link plugin_api Plugin API topic. @endlink
 *
 * If you create a new REST plugin, you will also need to enable it by
 * providing default configuration or configuration import, as outlined in
 * @ref sec_rest above.
 *
 * @section sec_integrate Integrating data from other sites into Drupal
 * If you want to integrate data from other websites into Drupal, here are
 * some notes:
 * - There are contributed modules available for integrating many third-party
 *   sites into Drupal. Search on https://www.drupal.org/project/project_module
 * - If there is not an existing module, you will need to find documentation on
 *   the specific web services API for the site you are trying to integrate.
 * - There are several classes and functions that are useful for interacting
 *   with web services:
 *   - You should make requests using the 'http_client' service, which
 *     implements \GuzzleHttp\ClientInterface. See the
 *     @link container Services topic @endlink for more information on
 *     services. If you cannot use dependency injection to retrieve this
 *     service, the \Drupal::httpClient() method is available. A good example
 *     of how to use this service can be found in
 *     \Drupal\aggregator\Plugin\aggregator\fetcher\DefaultFetcher
 *   - \Drupal\Component\Serialization\Json (JSON encoding and decoding).
 *   - PHP has functions and classes for parsing XML; see
 *     http://php.net/manual/refs.xml.php
 * @}
 */

/**
 * @defgroup state_api State API
 * @{
 * Information about the State API.
 *
 * The State API is one of several methods in Drupal for storing information.
 * See the @link info_types Information types topic @endlink for an
 * overview of the different types of information.
 *
 * The basic entry point into the State API is \Drupal::state(), which returns
 * an object of class \Drupal\Core\State\StateInterface. This class has
 * methods for storing and retrieving state information; each piece of state
 * information is associated with a string-valued key. Example:
 * @code
 * // Get the state class.
 * $state = \Drupal::state();
 * // Find out when cron was last run; the key is 'system.cron_last'.
 * $time = $state->get('system.cron_last');
 * // Set the cron run time to the current request time.
 * $state->set('system.cron_last', REQUEST_TIME);
 * @endcode
 *
 * For more on the State API, see https://www.drupal.org/developing/api/8/state
 * @}
 */

/**
 * @defgroup config_api Configuration API
 * @{
 * Information about the Configuration API.
 *
 * The Configuration API is one of several methods in Drupal for storing
 * information. See the @link info_types Information types topic @endlink for
 * an overview of the different types of information. The sections below have
 * more information about the configuration API; see
 * https://www.drupal.org/developing/api/8/configuration for more details.
 *
 * @section sec_storage Configuration storage
 * In Drupal, there is a concept of the "active" configuration, which is the
 * configuration that is currently in use for a site. The storage used for the
 * active configuration is configurable: it could be in the database, in files
 * in a particular directory, or in other storage backends; the default storage
 * is in the database. Module developers must use the configuration API to
 * access the active configuration, rather than being concerned about the
 * details of where and how it is stored.
 *
 * Configuration is divided into individual objects, each of which has a
 * unique name or key. Some modules will have only one configuration object,
 * typically called 'mymodule.settings'; some modules will have many. Within
 * a configuration object, configuration settings have data types (integer,
 * string, Boolean, etc.) and settings can also exist in a nested hierarchy,
 * known as a "mapping".
 *
 * Configuration can also be overridden on a global, per-language, or
 * per-module basis. See https://www.drupal.org/node/1928898 for more
 * information.
 *
 * @section sec_yaml Configuration YAML files
 * Whether or not configuration files are being used for the active
 * configuration storage on a particular site, configuration files are always
 * used for:
 * - Defining the default configuration for an extension (module, theme, or
 *   profile), which is imported to the active storage when the extension is
 *   enabled. These configuration items are located in the config/install
 *   sub-directory of the extension. Note that changes to this configuration
 *   after a module or theme is already enabled have no effect; to make a
 *   configuration change after a module or theme is enabled, you would need to
 *   uninstall/reinstall or use a hook_update_N() function.
 * - Defining optional configuration for a module or theme. Optional
 *   configuration items are located in the config/optional sub-directory of the
 *   extension. These configuration items have dependencies that are not
 *   explicit dependencies of the extension, so they are only installed if all
 *   dependencies are met. For example, in the scenario that module A defines a
 *   dependency which requires module B, but module A is installed first and
 *   module B some time later, then module A's config/optional directory will be
 *   scanned at that time for newly met dependencies, and the configuration will
 *   be installed then. If module B is never installed, the configuration item
 *   will not be installed either.
 * - Exporting and importing configuration.
 *
 * The file storage format for configuration information in Drupal is
 * @link http://wikipedia.org/wiki/YAML YAML files. @endlink Configuration is
 * divided into files, each containing one configuration object. The file name
 * for a configuration object is equal to the unique name of the configuration,
 * with a '.yml' extension. The default configuration files for each module are
 * placed in the config/install directory under the top-level module directory,
 * so look there in most Core modules for examples.
 *
 * @section sec_schema Configuration schema and translation
 * Each configuration file has a specific structure, which is expressed as a
 * YAML-based configuration schema. The configuration schema details the
 * structure of the configuration, its data types, and which of its values need
 * to be translatable. Each module needs to define its configuration schema in
 * files in the config/schema directory under the top-level module directory, so
 * look there in most Core modules for examples.
 *
 * Configuration can be internationalized; see the
 * @link i18n Internationalization topic @endlink for more information. Data
 * types label, text, and date_format in configuration schema are translatable;
 * string is non-translatable text (the 'translatable' property on a schema
 * data type definition indicates that it is translatable).
 *
 * @section sec_simple Simple configuration
 * The simple configuration API should be used for information that will always
 * have exactly one copy or version. For instance, if your module has a
 * setting that is either on or off, then this is only defined once, and it
 * would be a Boolean-valued simple configuration setting.
 *
 * The first task in using the simple configuration API is to define the
 * configuration file structure, file name, and schema of your settings (see
 * @ref sec_yaml above). Once you have done that, you can retrieve the active
 * configuration object that corresponds to configuration file mymodule.foo.yml
 * with a call to:
 * @code
 * $config = \Drupal::config('mymodule.foo');
 * @endcode
 *
 * This will be an object of class \Drupal\Core\Config\Config, which has methods
 * for getting configuration information. For instance, if your YAML file
 * structure looks like this:
 * @code
 * enabled: '0'
 * bar:
 *   baz: 'string1'
 *   boo: 34
 * @endcode
 * you can make calls such as:
 * @code
 * // Get a single value.
 * $enabled = $config->get('enabled');
 * // Get an associative array.
 * $bar = $config->get('bar');
 * // Get one element of the array.
 * $bar_baz = $config->get('bar.baz');
 * @endcode
 *
 * The Config object that was obtained and used in the previous examples does
 * not allow you to change configuration. If you want to change configuration,
 * you will instead need to get the Config object by making a call to
 * getEditable() on the config factory:
 * @code
 * $config =\Drupal::service('config.factory')->getEditable('mymodule.foo');
 * @endcode
 *
 * Individual configuration values can be changed or added using the set()
 * method and saved using the save() method:
 * @code
 * // Set a scalar value.
 * $config->set('enabled', 1);
 * // Save the configuration.
 * $config->save();
 * @endcode
 *
 * Configuration values can also be unset using the clear() method, which is
 * also chainable:
 * @code
 * $config->clear('bar.boo')->save();
 * $config_data = $config->get('bar');
 * @endcode
 * In this example $config_data would return an array with one key - 'baz' -
 * because 'boo' was unset.
 *
 * @section sec_entity Configuration entities
 * In contrast to the simple configuration settings described in the previous
 * section, if your module allows users to create zero or more items (where
 * "items" are things like content type definitions, view definitions, and the
 * like), then you need to define a configuration entity type to store your
 * configuration. Creating an entity type, loading entities, and querying them
 * are outlined in the @link entity_api Entity API topic. @endlink Here are a
 * few additional steps and notes specific to configuration entities:
 * - For examples, look for classes that implement
 *   \Drupal\Core\Config\Entity\ConfigEntityInterface -- one good example is
 *   the \Drupal\user\Entity\Role entity type.
 * - In the entity type annotation, you will need to define a 'config_prefix'
 *   string. When Drupal stores a configuration item, it will be given a name
 *   composed of your module name, your chosen config prefix, and the ID of
 *   the individual item, separated by '.'. For example, in the Role entity,
 *   the config prefix is 'role', so one configuration item might be named
 *   user.role.anonymous, with configuration file user.role.anonymous.yml.
 * - You will need to define the schema for your configuration in your
 *   modulename.schema.yml file, with an entry for 'modulename.config_prefix.*'.
 *   For example, for the Role entity, the file user.schema.yml has an entry
 *   user.role.*; see @ref sec_yaml above for more information.
 * - Your module can provide default/optional configuration entities in YAML
 *   files; see @ref sec_yaml above for more information.
 * - Some configuration entities have dependencies on other configuration
 *   entities, and module developers need to consider this so that configuration
 *   can be imported, uninstalled, and synchronized in the right order. For
 *   example, a field display configuration entity would need to depend on
 *   field configuration, which depends on field and bundle configuration.
 *   Configuration entity classes expose dependencies by overriding the
 *   \Drupal\Core\Config\Entity\ConfigEntityInterface::calculateDependencies()
 *   method.
 * - On routes for paths starting with '/admin' or otherwise designated as
 *   administration paths (such as node editing when it is set as an admin
 *   operation), if they have configuration entity placeholders, configuration
 *   entities are normally loaded in their original language, without
 *   translations or other overrides. This is usually desirable, because most
 *   admin paths are for editing configuration, and you need that to be in the
 *   source language and to lack possibly dynamic overrides. If for some reason
 *   you need to have your configuration entity loaded in the currently-selected
 *   language on an admin path (for instance, if you go to
 *   example.com/es/admin/your_path and you need the entity to be in Spanish),
 *   then you can add a 'with_config_overrides' parameter option to your route.
 *   The same applies if you need to load the entity with overrides (or
 *   translated) on an admin path like '/node/add/article' (when configured to
 *   be an admin path). Here's an example using the configurable_language config
 *   entity:
 *   @code
 *   my_module.my_route:
 *     path: '/admin/my-path/{configurable_language}'
 *     defaults:
 *       _controller: '\Drupal\my_module\MyController::myMethod'
 *     options:
 *       parameters:
 *         configurable_language:
 *           type: entity:configurable_language
 *           with_config_overrides: TRUE
 *   @endcode
 *   With the route defined this way, the $configurable_language parameter to
 *   your controller method will come in translated to the current language.
 *   Without the parameter options section, it would be in the original
 *   language, untranslated.
 *
 * @see i18n
 *
 * @}
 */

/**
 * @defgroup cache Cache API
 * @{
 * Information about the Drupal Cache API
 *
 * @section basics Basics
 *
 * Note: If not specified, all of the methods mentioned here belong to
 * \Drupal\Core\Cache\CacheBackendInterface.
 *
 * The Cache API is used to store data that takes a long time to compute.
 * Caching can either be permanent or valid only for a certain time span, and
 * the cache can contain any type of data.
 *
 * To use the Cache API:
 * - Request a cache object through \Drupal::cache() or by injecting a cache
 *   service.
 * - Define a Cache ID (cid) value for your data. A cid is a string, which must
 *   contain enough information to uniquely identify the data. For example, if
 *   your data contains translated strings, then your cid value must include the
 *   interface text language selected for page.
 * - Call the get() method to attempt a cache read, to see if the cache already
 *   contains your data.
 * - If your data is not already in the cache, compute it and add it to the
 *   cache using the set() method. The third argument of set() can be used to
 *   control the lifetime of your cache item.
 *
 * Example:
 * @code
 * $cid = 'mymodule_example:' . \Drupal::languageManager()->getCurrentLanguage()->getId();
 *
 * $data = NULL;
 * if ($cache = \Drupal::cache()->get($cid)) {
 *   $data = $cache->data;
 * }
 * else {
 *   $data = my_module_complicated_calculation();
 *   \Drupal::cache()->set($cid, $data);
 * }
 * @endcode
 *
 * Note the use of $data and $cache->data in the above example. Calls to
 * \Drupal::cache()->get() return a record that contains the information stored
 * by \Drupal::cache()->set() in the data property as well as additional meta
 * information about the cached data. In order to make use of the cached data
 * you can access it via $cache->data.
 *
 * @section bins Cache bins
 *
 * Cache storage is separated into "bins", each containing various cache items.
 * Each bin can be configured separately; see @ref configuration.
 *
 * When you request a cache object, you can specify the bin name in your call to
 * \Drupal::cache(). Alternatively, you can request a bin by getting service
 * "cache.nameofbin" from the container. The default bin is called "default", with
 * service name "cache.default", it is used to store common and frequently used
 * caches.
 *
 * Other common cache bins are the following:
 *   - bootstrap: Data needed from the beginning to the end of most requests,
 *     that has a very strict limit on variations and is invalidated rarely.
 *   - render: Contains cached HTML strings like cached pages and blocks, can
 *     grow to large size.
 *   - data: Contains data that can vary by path or similar context.
 *   - discovery: Contains cached discovery data for things such as plugins,
 *     views_data, or YAML discovered data such as library info.
 *
 * A module can define a cache bin by defining a service in its
 * modulename.services.yml file as follows (substituting the desired name for
 * "nameofbin"):
 * @code
 * cache.nameofbin:
 *   class: Drupal\Core\Cache\CacheBackendInterface
 *   tags:
 *     - { name: cache.bin }
 *   factory: ['@cache_factory', 'get']
 *   arguments: [nameofbin]
 * @endcode
 * See the @link container Services topic @endlink for more on defining
 * services.
 *
 * @section delete Deletion
 *
 * There are two ways to remove an item from the cache:
 * - Deletion (using delete(), deleteMultiple() or deleteAll()) permanently
 *   removes the item from the cache.
 * - Invalidation (using invalidate(), invalidateMultiple() or invalidateAll())
 *   is a "soft" delete that only marks items as "invalid", meaning "not fresh"
 *   or "not fresh enough". Invalid items are not usually returned from the
 *   cache, so in most ways they behave as if they have been deleted. However,
 *   it is possible to retrieve invalid items, if they have not yet been
 *   permanently removed by the garbage collector, by passing TRUE as the second
 *   argument for get($cid, $allow_invalid).
 *
 * Use deletion if a cache item is no longer useful; for instance, if the item
 * contains references to data that has been deleted. Use invalidation if the
 * cached item may still be useful to some callers until it has been updated
 * with fresh data. The fact that it was fresh a short while ago may often be
 * sufficient.
 *
 * Invalidation is particularly useful to protect against stampedes. Rather than
 * having multiple concurrent requests updating the same cache item when it
 * expires or is deleted, there can be one request updating the cache, while the
 * other requests can proceed using the stale value. As soon as the cache item
 * has been updated, all future requests will use the updated value.
 *
 * @section tags Cache Tags
 *
 * The fourth argument of the set() method can be used to specify cache tags,
 * which are used to identify which data is included in each cache item. A cache
 * item can have multiple cache tags (an array of cache tags), and each cache
 * tag is a string. The convention is to generate cache tags of the form
 * [prefix]:[suffix]. Usually, you'll want to associate the cache tags of
 * entities, or entity listings. You won't have to manually construct cache tags
 * for them — just get their cache tags via
 * \Drupal\Core\Cache\CacheableDependencyInterface::getCacheTags() and
 * \Drupal\Core\Entity\EntityTypeInterface::getListCacheTags().
 * Data that has been tagged can be invalidated as a group: no matter the Cache
 * ID (cid) of the cache item, no matter in which cache bin a cache item lives;
 * as long as it is tagged with a certain cache tag, it will be invalidated.
 *
 * Because of that, cache tags are a solution to the cache invalidation problem:
 * - For caching to be effective, each cache item must only be invalidated when
 *   absolutely necessary. (i.e. maximizing the cache hit ratio.)
 * - For caching to be correct, each cache item that depends on a certain thing
 *   must be invalidated whenever that certain thing is modified.
 *
 * A typical scenario: a user has modified a node that appears in two views,
 * three blocks and on twelve pages. Without cache tags, we couldn't possibly
 * know which cache items to invalidate, so we'd have to invalidate everything:
 * we had to sacrifice effectiveness to achieve correctness. With cache tags, we
 * can have both.
 *
 * Example:
 * @code
 * // A cache item with nodes, users, and some custom module data.
 * $tags = array(
 *   'my_custom_tag',
 *   'node:1',
 *   'node:3',
 *   'user:7',
 * );
 * \Drupal::cache()->set($cid, $data, CacheBackendInterface::CACHE_PERMANENT, $tags);
 *
 * // Invalidate all cache items with certain tags.
 * \Drupal\Core\Cache\Cache::invalidateTags(array('user:1'));
 * @endcode
 *
 * Drupal is a content management system, so naturally you want changes to your
 * content to be reflected everywhere, immediately. That's why we made sure that
 * every entity type in Drupal 8 automatically has support for cache tags: when
 * you save an entity, you can be sure that the cache items that have the
 * corresponding cache tags will be invalidated.
 * This also is the case when you define your own entity types: you'll get the
 * exact same cache tag invalidation as any of the built-in entity types, with
 * the ability to override any of the default behavior if needed.
 * See \Drupal\Core\Cache\CacheableDependencyInterface::getCacheTags(),
 * \Drupal\Core\Entity\EntityTypeInterface::getListCacheTags(),
 * \Drupal\Core\Entity\Entity::invalidateTagsOnSave() and
 * \Drupal\Core\Entity\Entity::invalidateTagsOnDelete().
 *
 * @section context Cache contexts
 *
 * Some computed data depends on contextual data, such as the user roles of the
 * logged-in user who is viewing a page, the language the page is being rendered
 * in, the theme being used, etc. When caching the output of such a calculation,
 * you must cache each variation separately, along with information about which
 * variation of the contextual data was used in the calculation. The next time
 * the computed data is needed, if the context matches that for an existing
 * cached data set, the cached data can be reused; if no context matches, a new
 * data set can be calculated and cached for later use.
 *
 * Cache contexts are services tagged with 'cache.context', whose classes
 * implement \Drupal\Core\Cache\Context\CacheContextInterface. See
 * https://www.drupal.org/developing/api/8/cache/contexts for more information
 * on cache contexts, including a list of the contexts that exist in Drupal
 * core, and information on how to define your own contexts. See the
 * @link container Services and the Dependency Injection Container @endlink
 * topic for more information about services.
 *
 * Typically, the cache context is specified as part of the #cache property
 * of a render array; see the Caching section of the
 * @link theme_render Render API overview topic @endlink for details.
 *
 * @section configuration Configuration
 *
 * By default cached data is stored in the database. This can be configured
 * though so that all cached data, or that of an individual cache bin, uses a
 * different cache backend, such as APCu or Memcache, for storage.
 *
 * In a settings.php file, you can override the service used for a particular
 * cache bin. For example, if your service implementation of
 * \Drupal\Core\Cache\CacheBackendInterface was called cache.custom, the
 * following line would make Drupal use it for the 'cache_render' bin:
 * @code
 *  $settings['cache']['bins']['render'] = 'cache.custom';
 * @endcode
 *
 * Additionally, you can register your cache implementation to be used by
 * default for all cache bins with:
 * @code
 *  $settings['cache']['default'] = 'cache.custom';
 * @endcode
 *
 * For cache bins that are stored in the database, the number of rows is limited
 * to 5000 by default. This can be changed for all database cache bins. For
 * example, to instead limit the number of rows to 50000:
 * @code
 * $settings['database_cache_max_rows']['default'] = 50000;
 * @endcode
 *
 * Or per bin (in this example we allow infinite entries):
 * @code
 * $settings['database_cache_max_rows']['bins']['dynamic_page_cache'] = -1;
 * @endcode
 *
 * For monitoring reasons it might be useful to figure out the amount of data
 * stored in tables. The following SQL snippet can be used for that:
 * @code
 * SELECT table_name AS `Table`, table_rows AS 'Num. of Rows',
 * ROUND(((data_length + index_length) / 1024 / 1024), 2) `Size in MB` FROM
 * information_schema.TABLES WHERE table_schema = '***DATABASE_NAME***' AND
 * table_name LIKE 'cache_%'  ORDER BY (data_length + index_length) DESC
 * LIMIT 10;
 * @endcode
 *
 * @see \Drupal\Core\Cache\DatabaseBackend
 *
 * Finally, you can chain multiple cache backends together, see
 * \Drupal\Core\Cache\ChainedFastBackend and \Drupal\Core\Cache\BackendChain.
 *
 * @see https://www.drupal.org/node/1884796
 * @}
 */

/**
 * @defgroup user_api User accounts, permissions, and roles
 * @{
 * API for user accounts, access checking, roles, and permissions.
 *
 * @section sec_overview Overview and terminology
 * Drupal's permission system is based on the concepts of accounts, roles,
 * and permissions.
 *
 * Users (site visitors) have accounts, which include a user name, an email
 * address, a password (or some other means of authentication), and possibly
 * other fields (if defined on the site). Anonymous users have an implicit
 * account that does not have a real user name or any account information.
 *
 * Each user account is assigned one or more roles. The anonymous user account
 * automatically has the anonymous user role; real user accounts
 * automatically have the authenticated user role, plus any roles defined on
 * the site that they have been assigned.
 *
 * Each role, including the special anonymous and authenticated user roles, is
 * granted one or more named permissions, which allow them to perform certain
 * tasks or view certain content on the site. It is possible to designate a
 * role to be the "administrator" role; if this is set up, this role is
 * automatically granted all available permissions whenever a module is
 * enabled that defines permissions.
 *
 * All code in Drupal that allows users to perform tasks or view content must
 * check that the current user has the correct permission before allowing the
 * action. In the standard case, access checking consists of answering the
 * question "Does the current user have permission 'foo'?", and allowing or
 * denying access based on the answer. Note that access checking should nearly
 * always be done at the permission level, not by checking for a particular role
 * or user ID, so that site administrators can set up user accounts and roles
 * appropriately for their particular sites.
 *
 * @section sec_define Defining permissions
 * Modules define permissions via a $module.permissions.yml file. See
 * \Drupal\user\PermissionHandler for documentation of permissions.yml files.
 *
 * @section sec_access Access permission checking
 * Depending on the situation, there are several methods for ensuring that
 * access checks are done properly in Drupal:
 * - Routes: When you register a route, include a 'requirements' section that
 *   either gives the machine name of the permission that is needed to visit the
 *   URL of the route, or tells Drupal to use an access check method or service
 *   to check access. See the @link menu Routing topic @endlink for more
 *   information.
 * - Entities: Access for various entity operations is designated either with
 *   simple permissions or access control handler classes in the entity
 *   annotation. See the @link entity_api Entity API topic @endlink for more
 *   information.
 * - Other code: There is a 'current_user' service, which can be injected into
 *   classes to provide access to the current user account (see the
 *   @link container Services and Dependency Injection topic @endlink for more
 *   information on dependency injection). In code that cannot use dependency
 *   injection, you can access this service and retrieve the current user
 *   account object by calling \Drupal::currentUser(). Once you have a user
 *   object for the current user (implementing \Drupal\user\UserInterface), you
 *   can call inherited method
 *   \Drupal\Core\Session\AccountInterface::hasPermission() to check
 *   permissions, or pass this object into other functions/methods.
 * - Forms: Each element of a form array can have a Boolean '#access' property,
 *   which determines whether that element is visible and/or usable. This is a
 *   common need in forms, so the current user service (described above) is
 *   injected into the form base class as method
 *   \Drupal\Core\Form\FormBase::currentUser().
 *
 * @section sec_entities User and role objects
 * User objects in Drupal are entity items, implementing
 * \Drupal\user\UserInterface. Role objects in Drupal are also entity items,
 * implementing \Drupal\user\RoleInterface. See the
 * @link entity_api Entity API topic @endlink for more information about
 * entities in general (including how to load, create, modify, and query them).
 *
 * Roles often need to be manipulated in automated test code, such as to add
 * permissions to them. Here's an example:
 * @code
 * $role = \Drupal\user\Entity\Role::load('authenticated');
 * $role->grantPermission('access comments');
 * $role->save();
 * @endcode
 *
 * Other important interfaces:
 * - \Drupal\Core\Session\AccountInterface: The part of UserInterface that
 *   deals with access checking. In writing code that checks access, your
 *   method parameters should use this interface, not UserInterface.
 * - \Drupal\Core\Session\AccountProxyInterface: The interface for the
 *   current_user service (described above).
 * @}
 */

/**
 * @defgroup container Services and Dependency Injection Container
 * @{
 * Overview of the Dependency Injection Container and Services.
 *
 * @section sec_overview Overview of container, injection, and services
 * The Services and Dependency Injection Container concepts have been adopted by
 * Drupal from the @link http://symfony.com/ Symfony framework. @endlink A
 * "service" (such as accessing the database, sending email, or translating user
 * interface text) is defined (given a name and an interface or at least a
 * class that defines the methods that may be called), and a default class is
 * designated to provide the service. These two steps must be done together, and
 * can be done by Drupal Core or a module. Other modules can then define
 * alternative classes to provide the same services, overriding the default
 * classes. Classes and functions that need to use the service should always
 * instantiate the class via the dependency injection container (also known
 * simply as the "container"), rather than instantiating a particular service
 * provider class directly, so that they get the correct class (default or
 * overridden).
 *
 * See https://www.drupal.org/node/2133171 for more detailed information on
 * services and the dependency injection container.
 *
 * @section sec_discover Discovering existing services
 * Drupal core defines many core services in the core.services.yml file (in the
 * top-level core directory). Some Drupal Core modules and contributed modules
 * also define services in modulename.services.yml files. API reference sites
 * (such as https://api.drupal.org) generate lists of all existing services from
 * these files. Look for the Services link in the API Navigation block.
 * Alternatively you can look through the individual files manually.
 *
 * A typical service definition in a *.services.yml file looks like this:
 * @code
 * path_alias.manager:
 *   class: Drupal\path_alias\AliasManager
 *   arguments: ['@path_alias.repository', '@path_alias.whitelist', '@language_manager']
 * @endcode
 * Some services use other services as factories; a typical service definition
 * is:
 * @code
 *   cache.entity:
 *     class: Drupal\Core\Cache\CacheBackendInterface
 *     tags:
 *       - { name: cache.bin }
 *     factory: ['@cache_factory', 'get']
 *     arguments: [entity]
 * @endcode
 *
 * The first line of a service definition gives the unique machine name of the
 * service. This is often prefixed by the module name if provided by a module;
 * however, by convention some service names are prefixed by a group name
 * instead, such as cache.* for cache bins and plugin.manager.* for plugin
 * managers.
 *
 * The class line either gives the default class that provides the service, or
 * if the service uses a factory class, the interface for the service. If the
 * class depends on other services, the arguments line lists the machine
 * names of the dependencies (preceded by '@'); objects for each of these
 * services are instantiated from the container and passed to the class
 * constructor when the service class is instantiated. Other arguments can also
 * be passed in; see the section at https://www.drupal.org/node/2133171 for more
 * detailed information.
 *
 * Services using factories can be defined as shown in the above example, if the
 * factory is itself a service. The factory can also be a class; details of how
 * to use service factories can be found in the section at
 * https://www.drupal.org/node/2133171.
 *
 * @section sec_container Accessing a service through the container
 * As noted above, if you need to use a service in your code, you should always
 * instantiate the service class via a call to the container, using the machine
 * name of the service, so that the default class can be overridden. There are
 * several ways to make sure this happens:
 * - For service-providing classes, see other sections of this documentation
 *   describing how to pass services as arguments to the constructor.
 * - Plugin classes, controllers, and similar classes have create() or
 *   createInstance() methods that are used to create an instance of the class.
 *   These methods come from different interfaces, and have different
 *   arguments, but they all include an argument $container of type
 *   \Symfony\Component\DependencyInjection\ContainerInterface.
 *   If you are defining one of these classes, in the create() or
 *   createInstance() method, call $container->get('my_service.name') to
 *   instantiate a service. The results of these calls are generally passed to
 *   the class constructor and saved as member variables in the class.
 * - For functions and class methods that do not have access to either of
 *   the above methods of dependency injection, you can use service location to
 *   access services, via a call to the global \Drupal class. This class has
 *   special methods for accessing commonly-used services, or you can call a
 *   generic method to access any service. Examples:
 *   @code
 *   // Retrieve the entity_type.manager service object (special method exists).
 *   $entity_type_manager = \Drupal::entityTypeManager();
 *   // Retrieve the service object for machine name 'foo.bar'.
 *   $foobar = \Drupal::service('foo.bar');
 *   @endcode
 *
 * As a note, you should always use dependency injection (via service arguments
 * or create()/createInstance() methods) if possible to instantiate services,
 * rather than service location (via the \Drupal class), because:
 * - Dependency injection facilitates writing unit tests, since the container
 *   argument can be mocked and the create() method can be bypassed by using
 *   the class constructor. If you use the \Drupal class, unit tests are much
 *   harder to write and your code has more dependencies.
 * - Having the service interfaces on the class constructor and member variables
 *   is useful for IDE auto-complete and self-documentation.
 *
 * @section sec_define Defining a service
 * If your module needs to define a new service, here are the steps:
 * - Choose a unique machine name for your service. Typically, this should
 *   start with your module name. Example: my_module.my_service.
 * - Create a PHP interface to define what your service does.
 * - Create a default class implementing your interface that provides your
 *   service. If your class needs to use existing services (such as database
 *   access), be sure to make these services arguments to your class
 *   constructor, and save them in member variables. Also, if the needed
 *   services are provided by other modules and not Drupal Core, you'll want
 *   these modules to be dependencies of your module.
 * - Add an entry to a modulename.services.yml file for the service. See
 *   @ref sec_discover above, or existing *.services.yml files in Core, for the
 *   syntax; it will start with your machine name, refer to your default class,
 *   and list the services that need to be passed into your constructor.
 *
 * Services can also be defined dynamically, as in the
 * \Drupal\Core\CoreServiceProvider class, but this is less common for modules.
 *
 * @section sec_tags Service tags
 * Some services have tags, which are defined in the service definition. See
 * @link service_tag Service Tags @endlink for usage.
 *
 * @section sec_injection Overriding the default service class
 * Modules can override the default classes used for services. Here are the
 * steps:
 * - Define a class in the top-level namespace for your module
 *   (Drupal\my_module), whose name is the camel-case version of your module's
 *   machine name followed by "ServiceProvider" (for example, if your module
 *   machine name is my_module, the class must be named
 *   MyModuleServiceProvider).
 * - The class needs to implement
 *   \Drupal\Core\DependencyInjection\ServiceModifierInterface, which is
 *   typically done by extending
 *   \Drupal\Core\DependencyInjection\ServiceProviderBase.
 * - The class needs to contain one method: alter(). This method does the
 *   actual work of telling Drupal to use your class instead of the default.
 *   Here's an example:
 *   @code
 *   public function alter(ContainerBuilder $container) {
 *     // Override the language_manager class with a new class.
 *     $definition = $container->getDefinition('language_manager');
 *     $definition->setClass('Drupal\my_module\MyLanguageManager');
 *   }
 *   @endcode
 *   Note that $container here is an instance of
 *   \Drupal\Core\DependencyInjection\ContainerBuilder.
 *
 * @see https://www.drupal.org/node/2133171
 * @see core.services.yml
 * @see \Drupal
 * @see \Symfony\Component\DependencyInjection\ContainerInterface
 * @see plugin_api
 * @see menu
 * @}
 */

/**
 * @defgroup listing_page_service Page header for Services page
 * @{
 * Introduction to services
 *
 * A "service" (such as accessing the database, sending email, or translating
 * user interface text) can be defined by a module or Drupal core. Defining a
 * service means giving it a name and designating a default class to provide the
 * service; ideally, there should also be an interface that defines the methods
 * that may be called. Services are collected into the Dependency Injection
 * Container, and can be overridden to use different classes or different
 * instantiation by modules. See the
 * @link container Services and Dependency Injection Container topic @endlink
 * for details.
 *
 * Some services have tags, which are defined in the service definition. Tags
 * are used to define a group of related services, or to specify some aspect of
 * how the service behaves. See the
 * @link service_tag Service Tags topic @endlink for more information.
 *
 * @see container
 * @see service_tag
 *
 * @}
 */

/**
 * @defgroup typed_data Typed Data API
 * @{
 * API for describing data based on a set of available data types.
 *
 * PHP has data types, such as int, string, float, array, etc., and it is an
 * object-oriented language that lets you define classes and interfaces.
 * However, in some cases, it is useful to be able to define an abstract
 * type (as in an interface, free of implementation details), that still has
 * properties (which an interface cannot) as well as meta-data. The Typed Data
 * API provides this abstraction.
 *
 * @section sec_overview Overview
 * Each data type in the Typed Data API is a plugin class (annotation class
 * example: \Drupal\Core\TypedData\Annotation\DataType); these plugins are
 * managed by the typed_data_manager service (by default
 * \Drupal\Core\TypedData\TypedDataManager). Each data object encapsulates a
 * single piece of data, provides access to the metadata, and provides
 * validation capability. Also, the typed data plugins have a shorthand
 * for easily accessing data values, described in @ref sec_tree.
 *
 * The metadata of a data object is defined by an object based on a class called
 * the definition class (see \Drupal\Core\TypedData\DataDefinitionInterface).
 * The class used can vary by data type and can be specified in the data type's
 * plugin definition, while the default is set in the $definition_class property
 * of the annotation class. The default class is
 * \Drupal\Core\TypedData\DataDefinition. For data types provided by a plugin
 * deriver, the plugin deriver can set the definition_class property too.
 * The metadata object provides information about the data, such as the data
 * type, whether it is translatable, the names of its properties (for complex
 * types), and who can access it.
 *
 * See https://www.drupal.org/node/1794140 for more information about the Typed
 * Data API.
 *
 * @section sec_varieties Varieties of typed data
 * There are three kinds of typed data: primitive, complex, and list.
 *
 * @subsection sub_primitive Primitive data types
 * Primitive data types wrap PHP data types and also serve as building blocks
 * for complex and list typed data. Each primitive data type has an interface
 * that extends \Drupal\Core\TypedData\PrimitiveInterface, with getValue()
 * and setValue() methods for accessing the data value, and a default plugin
 * implementation. Here's a list:
 * - \Drupal\Core\TypedData\Type\IntegerInterface: Plugin ID integer,
 *   corresponds to PHP type int.
 * - \Drupal\Core\TypedData\Type\StringInterface: Plugin ID string,
 *   corresponds to PHP type string.
 * - \Drupal\Core\TypedData\Type\FloatInterface: Plugin ID float,
 *   corresponds to PHP type float.
 * - \Drupal\Core\TypedData\Type\BooleanInterface: Plugin ID bool,
 *   corresponds to PHP type bool.
 * - \Drupal\Core\TypedData\Type\BinaryInterface: Plugin ID binary,
 *   corresponds to a PHP file resource.
 * - \Drupal\Core\TypedData\Type\UriInterface: Plugin ID uri.
 *
 * @subsection sec_complex Complex data
 * Complex data types, with interface
 * \Drupal\Core\TypedData\ComplexDataInterface, represent data with named
 * properties; the properties can be accessed with get() and set() methods.
 * The value of each property is itself a typed data object, which can be
 * primitive, complex, or list data.
 *
 * The base type for most complex data is the
 * \Drupal\Core\TypedData\Plugin\DataType\Map class, which represents an
 * associative array. Map provides its own definition class in the annotation,
 * \Drupal\Core\TypedData\MapDataDefinition, and most complex data classes
 * extend this class. The getValue() and setValue() methods on the Map class
 * enforce the data definition and its property structure.
 *
 * The Drupal Field API uses complex typed data for its field items, with
 * definition class \Drupal\Core\Field\TypedData\FieldItemDataDefinition.
 *
 * @section sec_list Lists
 * List data types, with interface \Drupal\Core\TypedData\ListInterface,
 * represent data that is an ordered list of typed data, all of the same type.
 * More precisely, the plugins in the list must have the same base plugin ID;
 * however, some types (for example field items and entities) are provided by
 * plugin derivatives and the sub IDs can be different.
 *
 * @section sec_tree Tree handling