Document deprecations

Author: dunglas

core/deprecations.md

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+# Deprecating Resources and Properties (Alternative to Versioning)
+
+A best practice regarding web APIs development is to apply [the evolution strategy](https://philsturgeon.uk/api/2018/05/02/api-evolution-for-rest-http-apis/)
+to indicate to client applications which resource types, operations and fields are deprecated and shouldn't be used anymore.
+
+While versioning an API requires modifying all clients to upgrade, even the ones not impacted by the changes.
+It's a tedious task that should be avoided as much as possible.
+
+On the other hand, the evolution strategy (also known as versionless APIs) consists of deprecating the fields, resources
+types or operations that will be removed at some point.
+
+Most modern API formats including [JSON-LD / Hydra](content-negotiation.md), [GraphQL](graphql.md) and [OpenAPI](swagger.md)
+allow to mark resources types, operations or fields as deprecated.
+
+## Deprecating Resource Classes, Operations and Properties
+
+When using API Platform, it's easy to mark a whole resource, a specific operation or a specific property as deprecated.
+All documentation formats mentioned in the introduction will then automatically take the deprecation into account.
+
+To deprecate a resource class, use the `deprecationReason` attribute:
+
+```php
+<?php
+// api/src/Entity/Parchment.php
+
+namespace App\Entity;
+
+use ApiPlatform\Core\Annotation\ApiResource;
+
+/**
+ * @ApiResource(deprecationReason="Create a Book instead")
+ */
+class Parchment
+{
+    // ...
+}
+```
+
+As you can see, to deprecate a resource, we just have to explain what the client should do to upgrade in the dedicated attribute.
+
+The deprecation will automatically be taken into account by clients supporting the previously mentioned format, including
+[API Platform Admin](../admin/index.md), [API Platform Client Generator](../client-generator/index.md) and the lower level
+[api-doc-parser library](https://github.com/api-platform/api-doc-parser).
+
+Here is how it renders for OpenAPI in the built-in Swagger UI shipped with the framework:
+
+![Deprecation shown in Swagger UI](images/deprecated-swagger-ui.png)
+
+And now in the built-in version of GraphiQL (for GraphQL APIs):
+
+![Deprecation shown in GraphiQL](images/deprecated-graphiql.png)
+
+You can also use this new `deprecationReason` attribute to deprecate specific [operations](operations.md):
+
+```php
+<?php
+// api/src/Entity/Review.php
+
+/**
+ * @ApiResource(itemOperations={
+ *     "get"={"deprecation_reason"="Retrieve a Book instead"}
+ * })
+ */
+class Parchment
+{
+    // ...
+}
+```
+
+It's also possible to deprecate a single property:
+
+```php
+<?php
+// api/src/Entity/Review.php
+
+namespace App\Entity;
+
+use ApiPlatform\Core\Annotation\ApiProperty;
+use ApiPlatform\Core\Annotation\ApiResource;
+
+/**
+ * @ApiResource
+ */
+class Review
+{
+    // ...
+
+    /**
+     * @ApiProperty(deprecationReason="Use the rating property instead")
+     */
+    public $letter;
+}
+```
+
+* With JSON-lD / Hydra, [an `owl:deprecated` annotation property](https://www.w3.org/TR/owl2-syntax/#Annotation_Properties) will be added to the appropriate data structure
+* With Swagger / OpenAPI, [a `deprecated` property](https://swagger.io/docs/specification/2-0/paths-and-operations/) will be added
+* With GraphQL, the [`isDeprecated` and `deprecationReason` properties](https://facebook.github.io/graphql/June2018/#sec-Deprecation) will be added to the schema