XML Interview Questions and Answers

What is server-side XPointer?
The XPointer Framework provides an authoritative and extensible interpretation of the semantics of fragment identifiers for XML media types. However, HTTP does NOT transmit the fragment identifier as part of the HTTP request. Therefore XPointer is generally applied by the client, not by the server.
For example, assuming that http://www.myorg.org/myTripleStore identifies a resource that is willing to negotiate for RDF/XML, then the following is typical of an HTTP request for an RDF/XML representation of that resource and the server's response.
Request:

GET /myTripleStore HTTP/1.1
Host: www.myorg.org
Accept: application/rdf+xml

Response:
HTTP/1.1 200 Ok
Content-Type: application/rdf+xml

<rdf:RDF />
This request asks for the entire triple store, serialized as RDF/XML.
Server-side XPointer uses the HTTP "Range" header to transmit the XPointer expression to the server. For example, let's assume that the URI of the triple store is the same, but we want to select the subresources identified by the following RDQL query:
SELECT (?x foaf:mbox ?mbox)
WHERE (?x foaf:name "John Smith") (?x foaf:mbox ?mbox)
USING foaf FOR<http://xmlns.com/foaf/0.1/>
)

In that case the HTTP request, including a copy of the RDQL query wrapped up as an XPointer expression, looks as follows. Note that we have added a range-unit whose value is xpointer to indicate that the value of the Range header should be interpreted by an XPointer processor. Also note the use of the XPointer xmlns() scheme to set bind the namespace URI for the rdql() XPointer scheme. This is necessary since this scheme has not been standardized by the W3C.
GET /myTripleStore HTTP/1.1
Host: www.myorg.org
Accept: application/rdf+xml
Range: xpointer = xmlns(x:http://www.mindswap.org)x:rdql(
SELECT (?x foaf:mbox ?mbox)
WHERE (?x foaf:name "John Smith") (?x foaf:mbox ?mbox)
USING foaf FOR <http://xmlns.com/foaf/0.1/>
)

The response looks as follows. The HTTP 206 (Partial Content) status code is used to indicate that the server recognized and processed the Range header and that the response entity includes only the identified logical range of the addressed resource.
HTTP/1.1 206 Partial Content
Content-Type: application/rdf+xml

 <rdf:RDF />

What about non-XML resources?
You can use the XPointer Framework with non-XML resources. This is especially effective when your resource is backed by some kind of a DBMS, or when you want to query a data model, such as RDF, and not the XML syntax of a representation of that data model.
However, please note that the authoratitive interpretation of the fragment identifier is determined by the Internet Media Type. If you want to opt-in for XPointer, then you can always create publish your own Internet Media Type with IANA and specify that it supports the XPointer Framework for some kind of non-XML resource. In this case, you are going to need to declare your own XPointer schemes as well.

What XPointer schemes are supported in this release?
The XPointer integration distributions support shorthand pointers. In addition, they bundle support for at last the following XPointer schemes:
* xmlns()
* element()
* xpath() - This is not a W3C defined XPointer scheme since W3C has not published an XPointer sheme for XPath. The namespace URI for this scheme is http://www.cogweb.org/xml/namespace/xpointer . It provides for addressing XML subresources using a XPath 1.0 expressions.

How do I configure an XPointer processor?
There is no required configuration for the XPointer Framework. The uberjar command line utility provides some configuration options. Applications configure individual XPointer processors when they obtain an instance from an appropriate XPointerProcessor factory method.

How do integrate XPointer into my application?
There are several ways to do this. The easiest is to use the uberjar release, which can be directly executed on any Java enabled platform. This makes it trivial to test and develop XPointer support in your applications, including server-side XPointer. The uberjar release contains a Java class org.CognitiveWeb.xpointer.XPointerDriver that provides a simple but flexible command line utility that exposes an XPointer processor. The XPointer is provided as a command line argument and the XML resource is read from stdin. The results are written on stdout by default as a set of null-terminated XML fragments. See XPointerDriver in the XPointer JavaDoc for more information.
If you already have a Java application, then it is straight-forward to integrate XPointer support using: org.CognitiveWeb.xpointer.XPointerProcessor You can see an example integration by looking at the XPointerDriver in the source code release.

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