Resource Preview defines a technique to get a minimal HTML representation of a resource identified by a URL. Applications often use this capability to render or display links with more appropriate icons and labels, or display a preview when a user mouses over a link.

Introduction

This specification describes how a client application can display links and embed rich previews for resources from other applications. Links may have a label and an icon, and previews are HTML markup provided by a server and displayed directly inside the client application.

Preview for a Link

Previews often appear as a pop-up when a user mouses over a link as in . However, a client may wish to display a preview differently depending on the kind of application, the size of the screen, and the capabilities of the device. For example, a desktop application on a PC might handle previews differently than a mobile application running on a small touchscreen.

Servers can provide both small and large previews. A client might show a small preview first, then if the user gestures, show additional details from the large preview. Servers suggest sizes for previews, and previews can ask to be resized after they are displayed.

A client only needs to know the URI of a resource to display a link and a preview. It doesn't need to know anything else about the resource. Clients don't need to copy, synchronize, or cache any data.

Terminology

Terminology uses and extends the terminology and capabilities of OSLC Core Overview [[!OSLCCore3]], W3C Linked Data Platform [[!LDP]], W3C's Architecture of the World Wide Web [[WEBARCH]], and Hyper-text Transfer Protocol [[!HTTP11]].

The following terms are used in discussions of previews:

Compact resource
A resource describing how to display a link and preview for another, associated resource.
Preview
An HTML representation of a resource that can be embedded in another user interface.

References

The namespace for OSLC Core is http://open-services.net/ns/core#.

Commonly used namespace prefixes:

    @prefix dcterms: <http://purl.org/dc/terms/>.
    @prefix oslc:    <http://open-services.net/ns/core#>.
    @prefix rdf:     <http://www.w3.org/1999/02/22-rdf-syntax-ns#>.
    @prefix xsd:     <http://www.w3.org/2001/XMLSchema#>.

Motivation

Applications often display links to related resources from other applications. For example, a quality management application might show a link to a related defect when displaying a test result. In some cases, the quality management application only has a URI pointing to the defect. The application needs a way to display the link with an appropriate label and icon so testers find out the status of the defect without leaving the quality management application.

Compact resources and resource preview provide these capabilities. Clients can discover the link label and a resource icon given only the resource URI. The client can then use the link label and icon as a means of displaying the link in a meaningful way. Previews allow users to see see information about related resources quickly without leaving the application they're in, even when the resources are from another server.

Working with Previews

To enable previews of a resource, servers supply an associated Compact resource describing the preview. The Compact resource can contain a link label, icon, and small and/or large previews of the resource. Compact resources always have a JSON representation [[!RFC4627]], but they can also have other representations such as XML, Turtle [[!turtle]] or JSON-LD [[!JSON-LD]]. Here is a simple example of a Compact resource:

{
    "title": "Screenshot of the problem",
    "icon": "http://example.com/icons/attachment.jpg",
    "smallPreview": {
        "document": "http://example.com/bugs/324/screenshot?preview=small"
    }
}

To display a link with a label or a preview of a resource, clients request its Compact resource. The URI of the Compact resource is found through an HTTP Link header [[!RFC5988]] in HTTP responses to the resource URI. Alternatively, a client can use a Prefer request header in requests for the resource to ask for its Compact resource. The server can then inline the Compact resource directly in the response, saving an HTTP request.

The Compact resource may contain small and/or large previews. Each preview can have size hints and a link to an HTML representation designed to be embedded in other user interfaces. To display the preview, the client creates an HTML iframe element in its user interface and sets the iframe element's src to the preview document URI. This sandboxes the preview from other content in the client application and allows the preview to use its own stylesheets and scripts.

Getting the Compact Resource

Clients can discover Compact resources in three ways:

  1. Discovering Compact Resources Using the HTTP Accept Header
  2. Discovering Compact Resources Using the HTTP Link Header
  3. Discovering Compact Resources Using the HTTP Prefer Header

The HTTP Accept header allows clients to access previews using the application/x-oslc-compact+xml MIME type extension to request the Compact resource representation for a URI instead of the content of the resource itself. This usage is considered deprecated and is included only to maintain compatibility with [[!OSLCUIPreview20]].

The HTTP Link header allows servers to offer previews for any resource, even binary resources with no RDF or JSON representations. To discover previews using the Link header, a client typically performs an HTTP HEAD or OPTIONS request on the resource URI. The response contains a Link header with the URI of the Compact resource. The client then performs a GET request on the Compact URI to retrieve the Compact resource.

Clients might instead use the HTTP Prefer header to get the Compact resource in a single HTTP request. If the client is confident that the resource has a preview, it makes an HTTP GET request on the resource URI using the return=representation preference [[!RFC7240]] and an include parameter [[!LDP]] asking for the Compact resource to be included in the response. The provider responds with a minimal representation of the resource and the Compact resource in the HTTP response body.

Discovering Compact Resources Using the HTTP Accept Header

[[!OSLCUIPreview20]] utilizes a MIME type extension, application/x-oslc-compact+xml to allow servers to use content negotiation with the HTTP Accept header to access the Compact representation of a resource instead of the content of the resource itself.

GET /bugs/324/screenshot HTTP/1.1
Host: example.com
Accept: application/x-oslc-compact+xml

This returns a Compact resource with Content-Type: application/x-oslc-compact+xml as defined in [[!OSLCUIPreview20]].

Discovering Compact Resources Using the HTTP Link Header

In responses to HTTP requests for resources that have a preview, servers include a Link header [[!RFC5988]] where the link relation is http://open-services.net/ns/core#Compact and the target URI is the URI of the Compact resource.

HEAD /bugs/324/screenshot HTTP/1.1
Host: example.com

Clients can request the Compact resource using the target URI of the Link header.

GET /bugs/324/screenshot?compact HTTP/1.1
Host: example.com
Accept: application/json
HTTP/1.1 200 OK
Content-Length: 192
Content-Type: application/json; charset=UTF-8
ETag: "3bf6fbc90e11b3c2cc30acb534b452ea"
Vary: Accept,Accept-Language

{
    "title": "Screenshot of the problem",
    "shortTitle": "screenshot.png",
    "smallPreview": {
        "document": "http://example.com/bugs/324/screenshot?preview=small"
    }
}

Servers can also return a Link header in response to successful requests that create resources. This allows the client to get the Compact resource URI for new resources without making additional requests. Note that servers set the Link context using an anchor parameter if the request URI is not the same as the newly-created resource URI.

POST /bugs HTTP/1.1
Host: example.com
Content-Length: 153
Content-Type: text/turtle

@prefix dcterms: <http://purl.org/dc/terms/> .

<>      a              <http://example.com/ns#Bug> ;
        dcterms:title  "Something went wrong" .
HTTP/1.1 201 Created
Content-Length: 0
Link: <http://example.com/bugs/478?compact>; rel="http://open-services.net/ns/core#Compact"; anchor="http://example.com/bugs/478"
Location: http://example.com/bugs/478

Discovering Compact Resources Using the HTTP Prefer Header

Clients can request a Compact resource by making an HTTP GET request to the target resource's URI using the return=representation preference of the HTTP Prefer request header [[!RFC7240]] and include parameter [[!LDP]] value http://open-services.net/ns/core#PreferCompact. Servers supporting resource preview must support this method of discovery for resources with RDF or JSON representations.

GET /bugs/324 HTTP/1.1
Host: example.com
Accept: application/json
Prefer: return=representation; include="http://open-services.net/ns/core#PreferCompact"

If the provider supports a preview for this resource and the request is successful, the response includes the Compact resource in its body.

The response content type is negotiated using the HTTP Accept request header. If the negotiated content type is application/json, the response body is a JSON object. The top-level JSON object has a compact property whose value is the JSON object describing the Compact resource in . The JSON may include other properties.

HTTP/1.1 200 OK
Content-Length: 315
Content-Type: application/json; charset=UTF-8
ETag: "f9d76afe5fbed1655c5768906db8958a"
Preference-Applied: return=representation
Vary: Accept,Accept-Language,Prefer

{
    "compact": {
        "title": "324: Need a fix <em>NOW</em>",
        "icon": "http://example.com/icons/defect.jpg",
        "largePreview": {
            "document": "http://example.com/bugs/324?preview=large",
            "hintHeight": "250px",
            "hintWidth": "400px"
        }
    }
}

Services may include a JSON-LD context in an application/json response. Clients who prefer RDF should request text/turtle or application/ld+json using the HTTP Accept request header, rather than application/json.

HTTP/1.1 200 OK
Content-Length: 788
Content-Type: application/json; charset=UTF-8
ETag: "d53d19be87fa9c61043c70bd91413dab"
Preference-Applied: return=representation
Vary: Accept,Accept-Language,Prefer

{
    "@id": "http://example.com/bugs/324",
    "@type": "http://example.com/ns#Bug",
    "@context": "https://tools.oasis-open.org/version-control/svn/oslc-core/trunk/specs/contexts/preview.jsonld",
    "compact": {
        "@id": "http://example.com/bugs/324?compact",
        "@type": "http://open-services.net/ns/core#Compact",
        "title": "324: Need a fix <em>NOW</em>",
        "shortTitle": "324",
        "icon": "http://example.com/icons/defect.jpg",
        "iconTitle": "Defect",
        "iconAltLabel": "Defect",
        "largePreview": {
            "document": "http://example.com/bugs/324?preview=large",
            "hintHeight": "250px",
            "hintWidth": "400px"
        },
        "smallPreview": {
            "document": "http://example.com/bugs/324?preview=small"
        }
    }
}

Displaying Previews

When displaying a preview inside another HTML-based presentation, clients create an iframe element, setting its src attribute to the preview's document URI. Previews can contain HTML, stylesheets, and scripts and might not render properly outside of an iframe. Using the iframe element also sandboxes the preview, avoiding cross-site scripting vulnerabilities.

Preview iframe

Typically, the server does not include the icon or title in the preview itself. The client can display these as needed. In , the preview document is the content inside the yellow, dashed lines.

Resources can have both a small and large preview, and clients can choose which to display. Some clients display the small preview initially and let users expand to the large preview using a button or "Show more" link.

The Compact resource can include oslc:hintHeight and oslc:hintWidth properties for each preview. Values for oslc:hintWidth and oslc:hintHeight are expressed in length units specified in [[!CSS21]].

Servers can also request dynamic resizing for small and large previews. The communication of dynamic size information happens within a web browser. JavaScript code running in the preview iframe sends a message that is received and acted upon by JavaScript code running in the iframe's parenting window. This cross-frame communication is done using HTML 5 postMessage. Dynamic resizing will not work inside browsers that do not support postMessage.

A resize message begins with oslc-resize: followed by a JSON object describing the dimensions using properties oslc:hintHeight and oslc:hintWidth.

JSON Property Name Type Occurs Description
oslc:hintHeight String Zero-or-one Preferred height of the preview. Values are expressed using length units as specified in [[!CSS21]].
oslc:hintWidth String Zero-or-one Preferred width of the preview. Values are expressed using length units as specified in [[!CSS21]].

For example, the following message requests that the preview be resized to height of 277 pixels and a width of 400 pixels.

    oslc-resize:{"oslc:hintHeight": "277px", "oslc:hintWidth": "400px"}

The following JavaScript example sends a resize request to the parent window.

var size = {
    'oslc:hintHeight': '277px',
    'oslc:hintWidth': '400px'
};

if (window.postMessage && window.parent)  {
    window.parent.postMessage('oslc-resize:' + JSON.stringify(size), "*");
}

Clients can ignore a server's size hints and use other values. For instance, a client might choose another size if the preview is too large for the window.

Preview Guidance

The following guidance is suggested for Client display of a resource link and rich previews. For purposes of this discussion, assume source resource A has a URI property that refers to target resource B:

Using the Compact representation title, short title, and icon

A Compact representation of the resource may contain a more accurate and slightly richer label for a target resource (dcterms:title element), a short-form title for the resource (oslc:shortTitle), and a corresponding image (oslc:icon), possibly chosen from a set with different sizes (oslc:iconSrcSet), all of which may be based on the current state of the target resource. If this becomes known to the Client, the client should assume that this information is better and use it in forming the hyperlink that is displayed to the user. When available, oslc:shortTitle may be used instead of dcterms:title in presentations where visual space is severely limited.

Clients should not fetch Compact representations when there is perfectly usable default display information available. When designing a Client application, consideration should be given to the potential benefits of obtaining an improved title and icon for the target resource against the costs of preemptively fetching the Compact representation in terms of added load on servers and networks (which might only be apparent to users on slow networks or heavily loaded servers).

Clients should be wary of material obtained from non-trusted sources; in particular, the Client should check that the dcterms:title and oslc:shortTitle property values do not contain HTML markup beyond the specified set of simple elements.

Preview sizing

When the resource does not supply preferred sizing for a preview, the Client should default to a reasonably generous value. The default value a Client uses may be different for different window and screen sizes.

Displaying a small preview

If a user mouses or hovers over a displayed link, the Client should determine whether the target resource has a small preview (oslc:smallPreview). If it does, the Client should present the small preview document in a hovering widget. Since preview documents may contain arbitrary content, including HTML and scripts, Client must use iframes if embedding the preview document inside another HTML-based presentation.

The Client should not attempt to prefetch a Compact representation just to have the preview URIs in hand so that the hover can come up faster. There is a low chance that the user will make a gesture that would call for the display of a small preview. It would generally be a poor trade-off to increase overall system load just to decrease UI latency for low probability eventualities. Clients may wish to utilize lazy loading techniques to only access preview dialogs that users actually view, and can cache those dialogs for subsequent uses if needed to improve performance.

Displaying a large preview

If the user then gestures that they want to see a bit more of the resource, the Client should determine whether the target resource has a large preview (oslc:largePreview). If it does, the Client should present the large preview document in a stationary widget that permits further user interaction. Again, since preview documents may contain arbitrary content, including HTML and scripts, Clients must use iframes if embedding the preview document inside another HTML-based presentation.

Implementation Conformance

General

Servers MAY choose to provide Compact resources for some resources and not others.

Resources with previews MUST support the HTTP OPTIONS method.

In responses to HTTP GET requests targeting resources that have a Compact resource, servers SHOULD either include a Vary response header with at least Accept and Prefer field values or a Cache-Control header value no-store.

Servers MAY consult the Accept-Language header on requests for a Compact resource (and on requests for preview documents) to return a resource for the requested natural language.

Clients SHOULD gracefully handle unsuccessful attempts to use previews. Previews may not be supported for all resources or may not work due to security or service availability issues.

Accept Header

In response to OPTIONS, HEAD or GET on resource, servers MAY include an Accept: application/x-oslc-compact+xml header to indicate the resource provides a compact representation.

Servers MAY support the application/x-oslc-compact+xml MIME type extension to allow clients to use content negotiation to access the Compact resource. However, this usage should be considered deprecated and is only provided for OSLC 2.0 compatibility.

Servers that do support the application/x-oslc-compact+xml MIME type extension MUST return the Compact resource using Content-Type: application/x-oslc-compact+xml as specified in [[!OSLCUIPreview20]].

Prefer Headers

Clients MAY request that the Compact resource is returned inline with the target resource using the Prefer request header [[!RFC7240]] and
  • Preference return, value representation
  • Parameter include [[!LDP]], value http://open-services.net/ns/core#PreferCompact.
Prefer: return=representation; include="http://open-services.net/ns/core#PreferCompact"

Servers MUST honor a client's request to in-line the Compact resource in JSON and RDF representations if the target resource has a Compact resource and the request is successful.

If the target resource does not exist or is not accessible to the requesting client, servers MUST return the same status code that it would have returned had the client not included the Prefer header in the request.

When servers in-line the Compact resource with the target resource in an application/json [[!RFC4627]] response, the response entity MUST be a JSON object with a compact property where the value is the Compact resource JSON as described in .

Servers MAY choose to return only a subset of the target resource properties when a client requests that the Compact resource is returned in-line.

Clients SHOULD inspect the response body for the Compact resource even if the server omits the Preference-Applied header [[!RFC7240]].

Compact Resources

Servers MUST support the application/json [[!RFC4627]] and text/turtle [[!turtle]] media types for the Compact resource.

Servers SHOULD support the application/ld+json media type [[!JSON-LD]] for the Compact resource.

The application/json Compact resource representation MUST be in the JSON format described in .

Servers MAY respond with JSON-LD in compacted document form when clients request the application/json representation of the Compact resource as long as the response meets the requirements for the JSON representation.

RDF representations of the Compact resource MUST conform to the shape specified in .

Content-Type application/x-oslc-compact+xml representations of the Compact resource MUST conform to the shape specified in XML Representation Format.

The Compact resource MAY have additional server-specific properties.

Previews

When displaying a preview inside another HTML presentation, clients MUST use an iframe element, setting its src attribute to the preview's document URI.

Servers MUST express the oslc:hintWidth and oslc:hintHeight properties of an oslc:Preview in length units as specified in [[!CSS21]].

Servers MAY use dynamic resizing for small and large previews in addition to oslc:Preview oslc:hintHeight and oslc:hintWidth properties.

Servers that support dynamic sizing MUST send resize messages to the parent window using the Window.postMessage method [[!webmessaging]] where the source of the event is the preview's window.

A resize message MUST consist of the characters oslc-resize: followed by a JSON object with at least one of the following properties:
  • oslc:hintHeight or
  • oslc:hintWidth.
oslc-resize:{"oslc:hintHeight": "277px", "oslc:hintWidth": "400px"}

A resize message MAY also also consist of the characters oslc-preview-height: followed by the desired height in pixels.
oslc-preview-height:277
This postMessage format should be considered deprecated and is only provided for OSLC 2.0 compatibility.

The oslc:hintHeight and oslc:hintWidth property values in the resize message JSON MUST be length units as specified in [[!CSS21]].

Servers MAY send more than one dynamic resize request.

Clients MAY choose to ignore the size hints in oslc:Preview resources or in dynamic resize messages.

Resource Constraints

This document applies the following constraints to the [[!OSLCCoreVocab]] vocabulary terms.

Resource: Compact

Resource: Preview

JSON Representation Format

A Compact resource JSON representation might look like the following for the target resource http://example.com/bugs/324:

{
    "title": "324: Need a fix <em>NOW</em>",
    "shortTitle": "324",
    "icon": "http://example.com/icons/defect.jpg",
    "iconSrcSet": "http://example.com/icons/smallIcon.png 16w, http://example.com/icons/largeIcon.png 64w",
    "iconTitle": "Defect",
    "iconAltLabel": "Defect",
    "largePreview": {
        "document": "http://example.com/bugs/324?preview=large",
        "hintHeight": "250px",
        "hintWidth": "400px"
    },
    "smallPreview": {
        "document": "http://example.com/bugs/324?preview=small"
    }
}

The JSON representation has the following constraints:

JSON-LD Context

The following JSON-LD Context may be used with JSON Compact representations.


JSON Schema

The following JSON Schema [[!JSONSchema]] describes the Compact JSON representation.


Change History

Revision Date Editor Changes Made
01 07/06/2016 Jim Amsden CSD was approved and published.