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.
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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.
Fig. 1Preview for a Link
Previews often appear as a pop-up when a user mouses over a link as in Fig. 1Preview for a Link. 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.
1.1 Terminology
Terminology uses and extends the terminology and capabilities of
OSLC Core Overview, 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.
1.3 Typographical Conventions and Use of RFC Terms
As well as sections marked as non-normative, all authoring guidelines, diagrams, examples,
and notes in this specification are non-normative. Everything else in this specification is
normative.
The key words MUST, MUST NOT, REQUIRED, SHOULD, SHOULD NOT, RECOMMENDED, MAY,
and OPTIONAL in this specification are to be interpreted as described in [RFC2119].
The namespace for OSLC Core is http://open-services.net/ns/core#.
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.
3. Working with Previews
This section is non-normative.
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:
Example 1: 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.
3.1 Getting the Compact Resource
This section is non-normative.
Clients can discover Compact resources in three ways:
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.
3.1.1 Discovering Compact Resources Using the HTTP Accept Header
This section is non-normative.
[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.
Example 2: Requesting the Compact Resource
GET /bugs/324/screenshot HTTP/1.1
Host: example.com
Accept: application/x-oslc-compact+xml
3.1.2 Discovering Compact Resources Using the HTTP Link Header
This section is non-normative.
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.
Example 3: Requesting the Resource Headers
HEAD /bugs/324/screenshot HTTP/1.1
Host: example.com
Clients can request the Compact resource using the target URI of the Link header.
Example 5: Requesting the Compact Resource
GET /bugs/324/screenshot?compact HTTP/1.1
Host: example.com
Accept: application/json
Example 6: JSON Compact Response
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.
Example 7: Creating a Resource
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" .
3.1.3 Discovering Compact Resources Using the HTTP Prefer Header
This section is non-normative.
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.
Example 9: Requesting the Compact Resource Using the Prefer Header
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
Appendix A.JSON Representation Format. The JSON may include other properties.
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.
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.
Fig. 2Preview iframe
Typically, the server does not include the icon or title in the preview itself. The client can display these
as needed. In Fig. 2Preview iframe, 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.
The following JavaScript example sends a resize request to the parent window.
Example 13: JavaScript postMessage Example
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.
4. Preview Guidance
This section is non-normative.
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:
4.1 Navigation to the target resource
This section is non-normative.
At any time, the user should be able to gesture that they would like to navigate the link to the target
resource, regardless of how the link to the target resource happens to be rendered.
4.2 Default way to display the link to the target resource
This section is non-normative.
The typical way for forming and displaying the hyperlink should be based on information that is stored locally
in source resource A (or that is generally known to the Client). Generally, link text can be derived from
property values of resource A, and potentially from a string or literal property value in the reference to the
target resource B, if such a property exists. Because these property values are part of the representation of
the source resource A, they are available without consulting the target resource, and will be available even
if the target resource B cannot be fetched. When available, Clients should display this string (as opposed to
the URI) when presenting resource A to indicate a potential navigation to resource B. This is the basic
presentation of a link to target resource B.
The default display of the link from A to B is visible to any user authorized to access resource A. The use of
Compact representation information described below is only viable for users who are also authorized to access
resource B.
4.3 Using the Compact representation title, short title, and icon
This section is non-normative.
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.
4.4 Preview sizing
This section is non-normative.
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.
4.5 Displaying a small preview
This section is non-normative.
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.
4.6 Displaying a large preview
This section is non-normative.
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.
5. Implementation Conformance
5.1 General
Servers MAY choose to provide Compact resources for some resources and not others.
[RP-1]
Resources with previews MUST support the HTTP OPTIONS method. [RP-2]
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.
[RP-3]
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.
[RP-4]
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.
[RP-5]
5.2 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.
[RP-6]
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.
[RP-7]
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].
[RP-8]
5.3 Link Headers
In responses to successful HTTP requests for a target resource that has a Compact resource, servers MUST
include a Link header [RFC5988] where:
The context URI is the effective request URI,
The link relation is http://open-services.net/ns/core#Compact, and
The target URI is the URI of the Compact resource.
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.
[RP-12]
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.
[RP-13]
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
Appendix A.JSON Representation Format.
[RP-14]
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.
[RP-15]
Clients SHOULD inspect the response body for the Compact resource even if the server omits the
Preference-Applied header [RFC7240].
[RP-16]
5.5 Compact Resources
Servers MUST support the
application/json [RFC4627] and text/turtle [turtle] media types for the
Compact resource.
[RP-17]
Servers SHOULD support the
application/ld+json media type [JSON-LD] for the Compact resource.
[RP-18]
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.
[RP-20]
RDF representations of the Compact resource MUST conform to the shape specified in
6.1Resource: Compact.
[RP-21]
Content-Type application/x-oslc-compact+xml representations of the Compact resource MUST conform
to the shape specified in Appendix B.XML Representation Format.
[RP-22]
The Compact resource MAY have additional server-specific properties.
[RP-23]
5.6 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.
[RP-24]
Servers MUST express the
oslc:hintWidth and oslc:hintHeight properties of an oslc:Preview in
length units as specified in [CSS21].
[RP-25]
Servers MAY use dynamic resizing for small and large previews in addition to oslc:Previewoslc:hintHeight and oslc:hintWidth
properties.
[RP-26]
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.
[RP-27]
A resize message MUST consist of the characters oslc-resize: followed by a JSON object with at
least one of the following properties:
Summary: Describes how to display a resource preview.
Compact Properties
Prefixed Name
Occurs
Read-only
Value-type
Representation
Range
Description
dcterms:title
Zero-or-one
true
string
N/A
Unspecified
Title that may be used in the display of a link to the resource. The value should include only content that is valid inside an HTML <span> element. Providers should include a dcterms:title property with an informative label for the resource. The title is typically shown to a user as a hyperlink. For a resource with no obvious title, Providers should omit the dcterms:title property. Providers must first HTML escape the contents of the dcterms:title before sending the response.
oslc:icon
Zero-or-one
true
Resource
Reference
Unspecified
URI of an image which may be used in the display of a link to the resource.
oslc:iconAltLabel
Zero-or-one
true
string
N/A
Unspecified
Alternative label used in association with the oslc:icon, such as HTML img tag's alt attribute.
oslc:iconSrcSet
Zero-or-one
true
string
N/A
Unspecified
Specification of a set of images of different sizes based on HTML img element srcset attribute.
oslc:iconTitle
Zero-or-one
true
string
N/A
Unspecified
Title used in association with the oslc:icon, such as HTML img tag's title attribute.
oslc:largePreview
Zero-or-one
true
AnyResource
Either
oslc:Preview
URI and sizing properties for an HTML document to be used for a large preview.
oslc:shortTitle
Zero-or-one
true
string
N/A
Unspecified
Abbreviated title which may be used in the display of a link to the resource. The value should include only content that is valid inside an HTML <span> element. Providers should include an abbreviated title for the resource when possible. The abbreviated title is typically shown to a user as a hyperlink in presentations where visual space is limited. As a general guideline, the length of the abbreviated title should be 5 characters or less. A user-visible identifier that ordinarily appears in the dcterms:title, such as a defect number, makes for a good oslc:shortTitle value. When a resource has no obvious identifier or handle, Providers should omit the oslc:shortTitle property. Providers must first HTML escape the contents of the oslc:shortTitle before sending the response.
oslc:smallPreview
Zero-or-one
true
AnyResource
Either
oslc:Preview
URI and sizing properties for an HTML document to be used for a small preview.
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.
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].
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
Appendix A.JSON Representation Format.
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.
Content-Type application/x-oslc-compact+xml representations of the Compact resource MUST conform
to the shape specified in Appendix B.XML Representation Format.
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 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.
The JSON representation has the following constraints:
The value of the title or shortTitle properties MUST, if present, only contain
markup valid in an HTML span element. To afford clients the greatest flexibility to style the
text to match the context in which it is being displayed, the titles SHOULD be plain text with HTML markup
used only to emphasize words or phrases or to convey additional information about the target resource.
icon, if present, MUST be the URI of an image. The image SHOULD be square. A client MAY scale the
image as needed.
iconSrcSet, if present, MUST provide a comma-separated list of image URIs along with image size
as specified for the HTML img srcset attribute.
iconAltLabel and iconTitle, if present, MUST only have string content with no
markup.
smallPreview and largePreview, if present, MAY have any number of properties,
including none.
The value of smallPreview and largePreview, if present, MUST be the JSON
representation of an
oslc:Preview.
A PreviewMUST include a document property. It MAY have any number of other properties.
documentMUST be the URI of a document containing a preview of the resource.
hintWidth and hintHeight, if present, MUST be expressed using length units
as specified in [CSS21].
A.1 JSON-LD Context
This section is non-normative.
The following JSON-LD Context may be used with JSON Compact representations.
Example 20: JSON-LD Context for a Compact Resource
The XML representation has the following syntactic and semantic constraints:
The root element MUST be rdf:RDF and SHOULD identify the
namespaces for RDF ( http://www.w3.org/1999/02/22-rdf-syntax-ns#), Dublin Core (
http://purl.org/dc/terms/), and OSLC ( http://open-services.net/ns/core#), as well
as namespaces for any additional Provider supplied properties.
The oslc:Compact element MUST be a child of the root element and
MUST specify an rdf:about attribute whose value is the URI of the target
resource.
The oslc:Compact element MAY have any number of child elements, including
none. Each child element specifies one property of the target resource. The order of child elements is not
significant.
Each of the dcterms:title, oslc:shortTitle, oslc:icon,
oslc:smallPreview, and oslc:largePreview child elements
MAY occur at most once.
The content of a dcterms:title or oslc:shortTitle element
MUST be limited to any XHTML <span> element. To afford clients the greatest
flexibility to style the text to match the context in which it is being displayed, the titles
SHOULD be plain text with XHTML markup used only to emphasize words or phrases or to
convey additional information about the target resource.
An oslc:icon element MUST have an rdf:resource attribute whose
value is a URI of an image. The image SHOULD be 16x16 pixels in size.
The oslc:iconAltLabel and oslc:iconTitle elements MUST only
have string content.
The oslc:smallPreview and oslc:largePreview elements MAY have
any number of child elements, including none.
The oslc:Preview element MUST be a child of an
oslc:smallPreview or oslc:largePreview element and
MAY occur at most once.
An oslc:Preview element MAY have any number of child elements,
including none. Each child element specifies one property of the Preview. The order of child
elements is not significant.
Each of the oslc:document, oslc:hintWidth, oslc:hintHeight,
and oslc:initialHeight child elements MAY occur at most once.
The oslc:document element MUST have an
rdf:resource attribute whose value is a URI of a document containing a preview of the
resource.
The content of an oslc:hintWidth, oslc:hintHeight, or
oslc:initialHeight element MUST be a valid relative length value.
All elements MAY have additional atttributes not specified here. All elements, other than
dcterms:title and oslc:shortTitle, MAY include additional child
elements not specified here. Consumers MUST quietly ignore unknown elements and attributes
encountered in a Compact representation.
