[[metadata]]
= Metadata

This section details the various forms of metadata provided by a Spring Data REST-based application.

[[metadata.alps]]
== Application-Level Profile Semantics (ALPS)

[quote, M. Admundsen / L. Richardson / M. Foster, https://tools.ietf.org/html/draft-amundsen-richardson-foster-alps-00]
http://alps.io/[ALPS] is a data format for defining simple descriptions of application-level semantics, similar in complexity to HTML microformats.  An ALPS document can be used as a profile to explain the application semantics of a document with an application-agnostic media type (such as HTML, HAL, Collection+JSON, Siren, etc.). This increases the reusability of profile documents across media types.

Spring Data REST provides an ALPS document for every exported repository. It contains information about both the RESTful transitions
and the attributes of each repository.

At the root of a Spring Data REST app is a profile link. Assuming you had an app with both `persons` and related `addresses`, the root
document would be as follows:

====
[source,javascript]
----
{
  "_links" : {
    "persons" : {
      "href" : "http://localhost:8080/persons"
    },
    "addresses" : {
      "href" : "http://localhost:8080/addresses"
    },
    "profile" : {
      "href" : "http://localhost:8080/profile"
    }
  }
}
----
====

A profile link, as defined in https://tools.ietf.org/html/rfc6906[RFC 6906], is a place to include application-level details. The
https://tools.ietf.org/html/draft-amundsen-richardson-foster-alps-00[ALPS draft spec] is meant to define a particular profile format,
which we explore later in this section.

If you navigate into the profile link at `localhost:8080/profile`, you see content resembling the following:

====
[source,javascript]
----
{
  "_links" : {
    "self" : {
      "href" : "http://localhost:8080/profile"
    },
    "persons" : {
      "href" : "http://localhost:8080/profile/persons"
    },
    "addresses" : {
      "href" : "http://localhost:8080/profile/addresses"
    }
  }
}
----
====

IMPORTANT: At the root level, `profile` is a single link and cannot serve up more than one application profile. That
is why you must navigate to `/profile` to find a link for each resource's metadata.

If you navigate to `/profile/persons` and look at the profile data for a `Person` resource, you see content resembling the following example:

====
[source,javascript]
----
{
  "version" : "1.0",
  "descriptors" : [ {
    "id" : "person-representation", <1>
    "descriptors" : [ {
      "name" : "firstName",
      "type" : "SEMANTIC"
    }, {
      "name" : "lastName",
      "type" : "SEMANTIC"
    }, {
      "name" : "id",
      "type" : "SEMANTIC"
    }, {
      "name" : "address",
      "type" : "SAFE",
      "rt" : "http://localhost:8080/profile/addresses#address"
    } ]
  }, {
    "id" : "create-persons", <2>
    "name" : "persons", <3>
    "type" : "UNSAFE", <4>
    "rt" : "#person-representation" <5>
  }, {
    "id" : "get-persons",
    "name" : "persons",
    "type" : "SAFE",
    "rt" : "#person-representation"
  }, {
    "id" : "delete-person",
    "name" : "person",
    "type" : "IDEMPOTENT",
    "rt" : "#person-representation"
  }, {
    "id" : "patch-person",
    "name" : "person",
    "type" : "UNSAFE",
    "rt" : "#person-representation"
  }, {
    "id" : "update-person",
    "name" : "person",
    "type" : "IDEMPOTENT",
    "rt" : "#person-representation"
  }, {
    "id" : "get-person",
    "name" : "person",
    "type" : "SAFE",
    "rt" : "#person-representation"
  } ]
}
----
<1> A detailed listing of the attributes of a `Person` resource, identified as `#person-representation`, lists the names
of the attributes.
<2> The supported operations. This one indicates how to create a new `Person`.
<3> The `name` is `persons`, which indicates (because it is plural) that a POST should be applied to the whole collection, not a single `person`.
<4> The `type` is `UNSAFE`, because this operation can alter the state of the system.
<5> The `rt` is `#person-representation`, which indicates that returned resource type will be `Person` resource.
====

NOTE: This JSON document has a media type of `application/alps+json`. This is different from the previous JSON document, which had
a media type of `application/hal+json`. These formats are different and governed by different specs.

You can also find a `profile` link in the collection of `_links` when you examine a collection resource, as the following example shows:

====
[source,javascript]
----
{
  "_links" : {
    "self" : {
      "href" : "http://localhost:8080/persons" <1>
    },
    ... other links ...
    "profile" : {
      "href" : "http://localhost:8080/profile/persons" <2>
    }
  },
  ...
}
----

<1> This HAL document respresents the `Person` collection.
<2> It has a *profile* link to the same URI for metadata.
====

Again, by default, the `profile` link serves up ALPS. However, if you use an https://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.1[`Accept` header], it can serve `application/alps+json`.

[[metadata.alps.control-types]]
=== Hypermedia Control Types

ALPS displays types for each hypermedia control. They include:

.ALPS types
[cols="1,5". options="header"]
|===
| Type | Description

| SEMANTIC | A state element (such as `HTML.SPAN`, `HTML.INPUT`, and others).
| SAFE | A hypermedia control that triggers a safe, idempotent state transition (such as `GET` or `HEAD`).
| IDEMPOTENT | A hypermedia control that triggers an unsafe, idempotent state transition (such as `PUT` or `DELETE`).
| UNSAFE | A hypermedia control that triggers an unsafe, non-idempotent state transition (such as `POST`).
|===

In the representation section shown earlier, bits of data from the application are marked as being `SEMANTIC`. The `address` field
is a link that involves a safe `GET` to retrieve. Consequently, it is marked as being `SAFE`. Hypermedia operations themselves map onto the types as
shown in the preceding table.

[[metadata.alps.projections]]
=== ALPS with Projections

If you define any projections, they are also listed in the ALPS metadata. Assuming we also defined `inlineAddress` and `noAddresses`, they
would appear inside the relevant operations. (See "`xref:projections-excerpts.adoc#projections-excerpts.projections[Projections]`" for the definitions and discussion of these two projections.) That is *GET* would appear in the operations for the whole collection, and *GET* would appear in the operations for a single resource. The following example shows
the alternate version of the `get-persons` subsection:

====
[source,javascript]
----
...
  {
    "id" : "get-persons",
    "name" : "persons",
    "type" : "SAFE",
    "rt" : "#person-representation",
    "descriptors" : [ { <1>
      "name" : "projection",
      "doc" : {
        "value" : "The projection that shall be applied when rendering the response. Acceptable values available in nested descriptors.",
        "format" : "TEXT"
      },
      "type" : "SEMANTIC",
      "descriptors" : [ {
        "name" : "inlineAddress", <2>
        "type" : "SEMANTIC",
        "descriptors" : [ {
          "name" : "address",
          "type" : "SEMANTIC"
        }, {
          "name" : "firstName",
          "type" : "SEMANTIC"
        }, {
          "name" : "lastName",
          "type" : "SEMANTIC"
        } ]
      }, {
        "name" : "noAddresses", <3>
        "type" : "SEMANTIC",
        "descriptors" : [ {
          "name" : "firstName",
          "type" : "SEMANTIC"
        }, {
          "name" : "lastName",
          "type" : "SEMANTIC"
        } ]
      } ]
    } ]
  }
...
----

<1> A new attribute, `descriptors`, appears, containing an array with one entry, `projection`.
<2> Inside the `projection.descriptors`, we can see `inLineAddress`. It render `address`, `firstName`, and `lastName`.
Relationships rendered inside a projection result in including the data fields inline.
<3> `noAddresses` serves up a subset that contains `firstName` and `lastName`.
====

With all this information, a client can deduce not only the available RESTful transitions but also, to some degree, the
data elements needed to interact with the back end.

[[metadata.alps.descriptions]]
=== Adding Custom Details to Your ALPS Descriptions

You can create custom messages that appear in your ALPS metadata. To do so, create `rest-messages.properties`, as follows:

====
[source,properties]
----
rest.description.person=A collection of people
rest.description.person.id=primary key used internally to store a person (not for RESTful usage)
rest.description.person.firstName=Person's first name
rest.description.person.lastName=Person's last name
rest.description.person.address=Person's address
----
====

These `rest.description.*` properties define details to display for a `Person` resource. They alter the ALPS format of the `person-representation`, as follows:

====
[source,javascript]
----
...
  {
    "id" : "person-representation",
    "doc" : {
      "value" : "A collection of people", <1>
      "format" : "TEXT"
    },
    "descriptors" : [ {
      "name" : "firstName",
      "doc" : {
        "value" : "Person's first name", <2>
        "format" : "TEXT"
      },
      "type" : "SEMANTIC"
    }, {
      "name" : "lastName",
      "doc" : {
        "value" : "Person's last name", <3>
        "format" : "TEXT"
      },
      "type" : "SEMANTIC"
    }, {
      "name" : "id",
      "doc" : {
        "value" : "primary key used internally to store a person (not for RESTful usage)", <4>
        "format" : "TEXT"
      },
      "type" : "SEMANTIC"
    }, {
      "name" : "address",
      "doc" : {
        "value" : "Person's address", <5>
        "format" : "TEXT"
      },
      "type" : "SAFE",
      "rt" : "http://localhost:8080/profile/addresses#address"
    } ]
  }
...
----

<1> The value of `rest.description.person` maps into the whole representation.
<2> The value of `rest.description.person.firstName` maps to the `firstName` attribute.
<3> The value of `rest.description.person.lastName` maps to the `lastName` attribute.
<4> The value of `rest.description.person.id` maps to the `id` attribute, a field not normally displayed.
<5> The value of `rest.description.person.address` maps to the `address` attribute.
====

Supplying these property settings causes each field to have an extra `doc` attribute.

NOTE: Spring MVC (which is the essence of a Spring Data REST application) supports locales, meaning you can bundle up multiple
properties files with different messages.


[[metadata.json-schema]]
== JSON Schema

https://json-schema.org/[JSON Schema] is another form of metadata supported by Spring Data REST. Per their website, JSON Schema has the following advantages:

* Describes your existing data format
* Clear, human- and machine-readable documentation
* Complete structural validation, useful for automated testing and validating client-submitted data

As shown in the xref:metadata.adoc#metadata.alps[previous section], you can reach this data by navigating from the root URI to the `profile` link.

====
[source,javascript]
----
{
  "_links" : {
    "self" : {
      "href" : "http://localhost:8080/profile"
    },
    "persons" : {
      "href" : "http://localhost:8080/profile/persons"
    },
    "addresses" : {
      "href" : "http://localhost:8080/profile/addresses"
    }
  }
}
----
====

These links are the same as shown earlier. To retrieve JSON Schema, you can invoke them with the following `Accept` header: `application/schema+json`.

In this case, if you ran `curl -H 'Accept:application/schema+json' http://localhost:8080/profile/persons`, you would see output resembling the following:

====
[source,javascript]
----
{
  "title" : "org.springframework.data.rest.webmvc.jpa.Person", <1>
  "properties" : { <2>
    "firstName" : {
      "readOnly" : false,
      "type" : "string"
    },
    "lastName" : {
      "readOnly" : false,
      "type" : "string"
    },
    "siblings" : {
      "readOnly" : false,
      "type" : "string",
      "format" : "uri"
    },
    "created" : {
      "readOnly" : false,
      "type" : "string",
      "format" : "date-time"
    },
    "father" : {
      "readOnly" : false,
      "type" : "string",
      "format" : "uri"
    },
    "weight" : {
      "readOnly" : false,
      "type" : "integer"
    },
    "height" : {
      "readOnly" : false,
      "type" : "integer"
    }
  },
  "descriptors" : { },
  "type" : "object",
  "$schema" : "https://json-schema.org/draft-04/schema#"
}
----

<1> The type that was exported
<2> A listing of properties
====

There are more details if your resources have links to other resources.

You can also find a `profile` link in the collection of `_links` when you examine a collection resource, as the following example shows:

====
[source,javascript]
----
{
  "_links" : {
    "self" : {
      "href" : "http://localhost:8080/persons" <1>
    },
    ... other links ...
    "profile" : {
      "href" : "http://localhost:8080/profile/persons" <2>
    }
  },
  ...
}
----

<1> This HAL document respresents the `Person` collection.
<2> It has a *profile* link to the same URI for metadata.
====

Again, the `profile` link serves xref:metadata.adoc#metadata.alps[ALPS] by default. If you supply it with an https://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.1[`Accept` header] of `application/schema+json`, it renders the JSON Schema representation.

//= JSON Patch

//TBD
