Difference between revisions of "ContextId"

Revision as of 22:59, 31 January 2008

Introduction

This page describes and defines a Higgins ContextId. See also: Context Discovery and Context Discovery Components.

Definition

• A ContextId is a kind of Context Correlation that identifies an instance of a Context.
• A ContextId MUST have a single value
• The single value of a ContextId MUST be a ContextIdDatatype

Details

• The value of a ContextId must directly or indirectly (e.g. by resolution) provide sufficient information to identify a Context. See the Context Discovery with a ContextId heading below.
• Two different ContextIds may refer to the same Context.

ContextId and IdAS

• If two ContextId's values are equal, they refer to the same underlying Context, although not necessarily the same Context "container" instance returned by IdAS. This means that, as a requester of information, different Digital Identities may see different subsets of the Context based on the authorization passed in during authentication.
• A given ContextId may be used against multiple Context Providers to produce the same Context (although, as mentioned, different Context instances).

Details

• The Identity Attribute Service (IdAS) can be used as a service to access a Context given a ContextId

Some ContextIds are not resolvable to a Context instance. Most Contexts are not abstract. Using a network (or local) data access service their ContextIds are resolvable to data objects that are instantiated and managed by Context Provider plug-ins.

Context Discovery Rules

A key property of a ContextId is that it can be resolved into the metadata necessary open a Context. The complete process for doing this is described in Context Discovery. Following are the discovery rules that apply to each form of ContextId.

contextURI Discovery

Since URIs do not offer the structured identifier features of XRIs, they must resolve directly to an XRDS or XDI document containing the Higgins context configuration metadata for a specific context type.

contextfileURI

1. Open the identified file.
2. For an XRDS file, select the highest priority service endpoint that conforms to the Higgins XRDS Service Endpoint requirements.
3. For an XDI X3 file, parse the Higgins context configuration metadata from the X3 document as described in Higgins XDI Context Reference.

contexthttpURI

1. Resolve the URI to a discovery document as specified in Section 6 of XRI Resolution 2.0.
   1. If the contexthttpURI ends in the context descriptor extension .xrds for an XRDS document, use the MIME type application/xrds+xml.
   2. If the contexthttpURI ends in the context descriptor extension .xdi for an XDI document, use the MIME type text/x3.
   3. If the contexthttpURI does not end in a context descriptor extension, use either or both of these MIME types as preferred in content negotiation with the server.
2. If the MIME type of the discovery document is application/xrds+xml, select the highest priority service endpoint that conforms to the Higgins XRDS Service Endpoint requirements.
3. If the MIME type is text/x3, parse the Higgins context configuration metadata from the X3 document as described in Higgins XDI Context Reference.

contextXRI Discovery

Since XRIs are abstract structured identifiers intended designed explicitly for resource discovery and description, they offer more flexiblity in the discovery process.

Using XRDS

1. Resolve the authority component of the XRI as specified in XRI Resolution 2.0. Trusted resolution is RECOMMENDED.
2. From this XRDS document, select the Higgins service endpoint as defined in Section 13 of XRI Resolution 2.0. Note that this provides a number of ways for ContextXRI authors to control service endpoint selection. Using the path component of the XRI to match a Higgins XRDS Service Endpoint is RECOMMENDED.
3. Parse the Higgins context metadata as defined in Higgins XRDS Service Endpoint.

Using XDI

1. Resolution of the authority component of the XRI may use either XRDS resolution or XDI context discovery.
   1. For XRDS resolution, follow the same steps as described above, except that in the final XRDS document, the highest priority SEP of type xri://$xdi MUST be selected, and an XDI X3 document MUST be requested from that service endpoint URI.
   2. For XDI context discovery, resolve the authority component of the XRI as specified in the XDI Contexts section of XDI RDF Model] document.
2. In both cases, from the final XDI document, parse the XDI context description as defined in Higgins XDI Context Reference.

Relationship to OpenID

Any OpenID 2.0 identifier as defined by OpenID Authentication 2.0 may function as a Higgins ContextId. OpenID 2.0 supports both URLs and XRIs. All of the examples of contexthttpURI and contextXRI above are valid OpenID 2.0 identifiers.

The only caveat to keep in mind is that if an OpenID 2.0 URL is used as a Higgins ContextId, the XRDS or XDI document to which it resolves must EITHER contain:

• Only one XRDS service endpoint or XDI context reference describing the target Context, OR
• The consuming application must know how to select the XRDS service endpoint or XDI context reference it requires (i.e., it must know which Higgins Context Type it is seeking).

Open Issues

1. Should the contextId ABNF support IRI (Internationalized Resource Identifiers) and not just URIs? Note that the ABNF above uses IRI rules; this needs to be changed if only URIs will be supported.

See Also

• EntityId
• LocalId
• Context Discovery
• Context Discovery Components
• Higgins XRDS Service Endpoint
• Higgins XDI Context Reference
• Concepts

Links

• Higgins Home