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Cross-space references (formerly known as References across spaces) allow you to link content across multiple spaces. Using cross-space references, you can create reusable content and update multiple spaces with the same piece of content.
To accommodate referencing across multiple spaces, the `ResourceLink` field type has been added. The `ResourceLink` field carries references to entities in separate spaces. All `ResourceLink` fields are configured in the `ContentType` definitions as usual fields and accept the `allowedResources` validation rule. This validation restricts the entities that can be linked by the field.

Supported field properties: `localized`, `required`, `omitted` and `disabled`.

### Contentful Resource Names

With cross-space references, the Contentful Resource Name (`crn`) API identifier is used.

A `crn` is a formatted string used to describe an object in the API domain. It uses a shared syntax that can be resolved across Contentful services.
Currently, `crn` is only used in the context of cross-space references to recognize a Contentful [space](/references/content-management-api/spaces#space), [environment](/references/content-management-api/environments#environment), or [entry](/references/content-management-api/entries#entry).

The respective syntax templates are:

`crn:contentful:::content:spaces/<:spaceId>`
`crn:contentful:::content:spaces/<:spaceId>/environments/<:environmentId>`
`crn:contentful:::content:spaces/<:spaceId>/entries/<:entryId>`
`crn:contentful:::content:spaces/<:spaceId>/environments/<:environmentId>/entries/<:entryId>`

The `<:spaceId>`, `<:environmentId>`, and `<:entryId>` portions must contain standard [resource IDs](#/introduction/resource-ids). In the following snippets, the "\{crn}" placeholder will be used where one of these is needed.

> **Info**
>
> Omitting the environment in a `crn` is equivalent to referencing the master environment.

## Create a content type with a ResourceLink field

To create a content type with a ResourceLink field, use the [Create a content type with POST](/references/content-management-api/content-types/create-a-content-type-with-post) endpoint. The ResourceLink field is set in the request body, see example:

```
{
  "sys": {
    "type": "ContentType",
    "...": ""
  },
  "fields": [
    {
      "id": "myResourceLink",
      "type": "ResourceLink",
      "localized": true,
      "disabled": false,
      "omitted": false,
      "allowedResources": [
        {
          "type": "Contentful:Entry",
          "source": "{crn}",
          "contentTypes": [
            "foo",
            "bar"
          ]
        }
      ],
      "...": ""
    }
  ]
}
```

## Update a content type with a ResourceLink field

To update a content type with a ResourceLink field, use the [Create a content type with PUT](/references/content-management-api/content-types) endpoint. The ResourceLink field is set in the request body, see example:
Use this endpoint to add a `ResourceLink` field to a content type or update an existing `ResourceLink` field.

```
{
  "sys": {
    "type": "ContentType",
    "...": ""
  },
  "fields": [
    {
      "id": "myResourceLinks",
      "type": "Array",
      "localized": true,
      "disabled": false,
      "omitted": false,
      "validations": [
        {
          "size": {
            "min": "1",
            "max": "3"
          }
        }
      ],
      "allowedResources": [
        {
          "type": "Contentful:Entry",
          "source": "{crn}",
          "contentTypes": [
            "foo",
            "bar"
          ]
        }
      ],
      "...": ""
    }
  ]
}
```

#### Allowed resources

The `ResourceLink` field definition expects an `allowedResources` array as additional property, restricting which resources can be linked through the field.

> **Info**
>
> Only resources belonging to spaces within the same organization are accepted.

| Property                          | Description                                                                                             | Validation rule                                                                                        |
| --------------------------------- | ------------------------------------------------------------------------------------------------------- | ------------------------------------------------------------------------------------------------------ |
| `allowedResources`                | Array containing one item per distinct source.                                                          | Must contain at least one item.                                                                        |
| `allowedResources[].type`         | The type of resource that the rule contains.                                                            | Must be `Contentful:Entry`.                                                                            |
| `allowedResources[].source`       | The location of the allowed resources. A `crn` to a contentful environment for type `Contentful:Entry`. | Must be unique across `allowedResources` and does not refer to the same space owning the content type. |
| `allowedResources[].contentTypes` | The only entry types supported by Contentful.                                                           | Must be an array of strings.                                                                           |

#### Validations

For each field definition property, you can add or remove validations to the fields in the content type schema by specifying the `validations` property of a field.

| Property   | Description                       | Validation rule                                                             |
| ---------- | --------------------------------- | --------------------------------------------------------------------------- |
| `sys.crn`  | The identifier of the resource.   | Must correspond to a resource of type `sys.type` in the entry organization. |
| `sys.type` | The type of resource in the link. | Must be `Contentful:Entry`.                                                 |

## Create an entry with a ResourceLink field

To create an entry with a ResourceLink field, use the [Create an entry](/references/content-management-api/entries/create-an-entry) endpoint. The ResourceLink field is set in the request body, see example:

```
{
  "sys": {
    "type": "Entry"
  },
  "fields": {
    "myResourceLink": {
      "en-US": {
        "sys": {
          "type": "ResourceLink",
          "linkType": "Contentful:Entry",
          "urn": "{crn}"
        }
      }
    }
  }
}
```

## Update an entry with a ResourceLink field

To update an entry with a ResourceLink field, use the [Create an entry with ID](/references/content-management-api/entries/create-an-entry-with-a-specified-id) endpoint. The ResourceLink field is set in the request body, see example:

```
{
  "sys": {
    "type": "Entry"
  },
  "fields": {
    "myResourceLinks": {
      "en-US": [
        {
          "sys": {
            "type": "ResourceLink",
            "linkType": "Contentful:Entry",
            "urn": "{crn}"
          }
        }
      ]
    }
  }
}
```