[FIX] Cache: Add missing README.md

Author: mjansenDatabay

components/ILIAS/Cache/README.md

@@ -1 +1,107 @@
-# Cache
+ILIAS Caching Service
+=====================
+
+The ILIAS Caching Service replaces the old GlobalCache Service with ILIAS 9.
+Internally, the GlobalCache service currently also uses the new cache service,
+but will be removed in the medium term.
+
+# Cache data
+
+## When does it make sense to cache data?
+
+Data caching should be used when simple data does not change between two
+requests and is not user-independent. It must also be clear when the data in the
+cache becomes invalid and needs to be updated. Caching must also always include
+reading the data from a persistence layer (database, file system) as a fallback.
+
+## Differentiation from artifacts
+
+Artifacts are always suitable when the data not only does not change between two
+requests, but is always the same depending on the exact version of the ILIAS
+source code. Artifacts are generated when ILIAS is built, cache is filled at
+runtime. This can be easily explained using the following example:
+
+The translations in ILIAS (Language Service) would be a candidate for artifacts
+under the following circumstances:
+
+> The exact content of the translations in ILIAS is only available in the form
+> of language files. In this case, this content could be built as an Artifact
+> and
+> thus be available with high performance.
+
+But then why are translations not in Artifacts?
+
+> ILIAS offers administration in the GUI for the translations, i.e. the exact
+> translations do not necessarily have to come from the language files, but can
+> change through customization.
+
+Would the translations be a candidate for the cache?
+
+> Yes, under the following circumstances: It is known exactly when the content
+> of the translations changes (e.g. through changes to the language files
+> through
+> an ILIAS update or through manual adjustments in the administration. If these
+> situations are known, the language service can invalidate the cache at the
+> right
+> moment, read the correct content and rebuild the cache.
+
+## How to use the cache service
+
+The data is stored in a cache container. To obtain such a container, a cache
+container request is implemented. This is very simple and can also be
+implemented on existing classes, for example, which is often useful for
+repositories.
+
+```php
+use ILIAS\Cache\Container\Request;
+ 
+class MyRepository implements Request {
+    public function getContainerKey() : string{
+        return 'my_repository';
+    }
+    public function isForced() : bool{
+        return true;
+    } 
+}
+```
+
+The two methods to be implemented can be described as follows:
+
+`getContainerKey`: This is the namespace for your container. ILIAS will
+check that two components are not using the same namespace and can therefore
+overwrite each other's cache.
+
+`isForced`: Individual containers can be activated via the setup, see the README
+for the setup. However, this can be overwritten with isForces if the cotnaienr
+is to be active in any case. This should not be used in the core but only in
+plugins.
+
+The container can then be obtained via the cache service:
+
+```php
+global DIC;
+
+$container = $DIC->chache()->get(new MyRepository());
+```
+The following methods are then available in the container:
+
+```php
+use ILIAS\Cache\Container\Container;
+/** @var Container $container */
+$container = $DIC->globalCache()->get(new MyRepository());
+
+$container->has('key'); // Check if a key exists
+$container->get('key'); // Get a value
+$container->set('key', 'value'); // Set a value
+$container->delete('key'); // Delete a value
+$container->flush(); // Delete all values
+$container->lock(3.5); // Lock the container, see below
+```
+
+## Storable Values
+The service only supports simple scalar values for storage, e.g. no objects
+can be stored. The values can be stored individually or as an array.
+
+## Locking
+The COntainer can be locked for a certain period of time, i.e. `has` always  
+returns false during this time, `get` always returns null.