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Resource Links are part of the [Cross-space references](https://www.contentful.com/help/references-across-spaces) feature set that allows you to link content across multiple spaces. It mainly uses `ResourceLink` links as a way to represent a relationship between entities from different spaces.

`ResourceLink` links are represented as a `sys` object containing `urn`, `linkType` and `type`:

| Property       | Description                                                                                                            |
| -------------- | ---------------------------------------------------------------------------------------------------------------------- |
| `sys.linkType` | Represents what kind of entity this resource links to. For cross-space entries the value is `Contentful:Entry`.        |
| `sys.type`     | The type of Link. `ResourceLink` is the default for cross-space resources.                                             |
| `sys.urn`      | The location of the resource. A [CRN](/references/content-management-api/overview) to a Contentful environment entity. |

As they are part of another space, resolving cross-space linked entities requires a special header to be passed in each request named `x-contentful-resource-resolution`.

#### Extra header for cross-space resolution

The `x-contentful-resource-resolution` header is a *base64 encoded JSON object* containing key-value pairs of spaceId/API Key. The plain JSON used to create the header should have the following shape:

```json
{
    "spaces": {
        "someSpaceId": "<access-token>",
        "anotherSpaceId": "<access-token>"
    }
}
```

> **Info**
>
> For the Content Preview API, the \<access-token> is \<cpa-token>. For the Content Delivery API, the \<access-token> is \<cda-token>.

That then needs to be *stringified* and encoded to base64. You can use `JSON.stringify` and `btoa` (JavaScript) to properly convert the JSON object to a stringified encoded version of it.

Here is a full example in JavaScript:

```js
const extraTokens = {
  "spaces": {
    "IdToR3s0lv3": "ND63YKcYBe335RWDnIuzv...",
    "4n0th3rSp4c3": "UuVe6icuBuXv..."
  }
}

// Converts object to string and uses base64 to encode the string
window.btoa(JSON.stringify(extraTokens)) // eyJzcGFjZXMiOnsiSWRUb1I[...]=
```

The value can then be passed to the `x-contentful-resource-resolution` header as-is. All cross-space references of matching space IDs will be returned within the `includes` property of the response.

#### Capabilities

* The `locale` query parameter is propagated to all extra spaces present in the new header.
* Only the first level of references of every extra space in the new header will be resolved. It applies to only one level of space. This is a behavior similar to `include=1`.
* The main space (space Id being requested in the URL) will still resolve up to 10 levels.
* Publishing new content in any of the extra spaces will cause the cache to be purged on every request that included that space Id.
* The `includes` property of the response will include both cross-space entries and assets.
* You can pass this new header in the Javascript SDK for automatic resolution.

#### Errors

Errors from cross-space requests are propagated back to the response payload in the `errors` property.
If a request to a single cross-space fails, the main space request and other spaces will still continue to process.

| HTTP Status Code | Description                                                                    |
| ---------------- | ------------------------------------------------------------------------------ |
| `401`            | Missing `Authorization` header                                                 |
| `400`            | Incorrect format for the `x-contentful-resource-resolution` header.            |
| `400`            | Number of extra spaces in the header exceeds the maximum allowed number (`20`) |

##### Errors property example

```json
{
    "errors": [
        {
            "details": {
                "urn": "crn:contentful:::content:spaces/<spaceId>/environments/<environmentId>/entries/<entryId>",
                "linkType": "Contentful:Entry",
                "type": "ResourceLink"
            },
            "sys": {
                "id": "notResolvable",
                "type": "error"
            }
        }

    ]
}
```

#### Limitations

* Up to `21` spaces to be resolved in a single request
* Only `20` extra space tokens are supported. You can make a single API call that resolves up to `21` spaces at the same time: `20` extra spaces and the entries from the space Id in the initial request.
* Up to `1000` references in the `includes` property in the response.
* Only the first level of cross-space references is resolved from the original spaceId in the URL
* The `Authorization` header is still required for every request and it should enable access to the main space Id in the URL.
* Errors from the extra space tokens will be returned in the `errors` property of the response.
* Using the new header will also consume the Rate Limiting from the space Ids present in it when the request is *uncached*.
* No [Advanced Caching](/platform/advanced-caching) support
* No [MRDI](/platform/mrdi) support

For more information, see the [Resource Links FAQs](https://www.contentful.com/help/resource-links-faqs).

## Retrieval of linked Resource Links

**Notes**:

* The `include` parameter resolves links between entries and assets within a space. Links between content types within a space are not included in the response.
* The `include` parameter is only available for the entry collection endpoint `/spaces/{space_id}/environments/{environment_id}/entries`.

To retrieve linked Resource Links, use the [Get all entries of a Space](/references/content-preview-api/entries/get-all-entries) endpoint.