Modelling Data on the Web note 5: More error handling & RDF, a graph-based DM

Robustness

In computer science, robustness is the ability of a computer system to cope with errors during execution and cope with erroneous input.

Formats for External Represent of data

A format consists of

1. a core data model (csv, table, XML, JSON,…)
2. a conceptual model (显性或隐性，如ER-Diagram)
3. schema(s) formalising/describing the format
   • documents describing(some aspects of our)design
   • e.g.,occupancy.rnc,occupancy.sch,…
4. the set of conforming ExtReps(e.g., XML documents)
   • concrete embodiments(具体体现) of our design

2 formats F1 = <DS1, CM1, S1, D1>, F2 = <DS2, CM2, S2, D2> it may be that

• S1 only captures some aspects of D1
• S1 is only a description in English
• D1=D2 but S1≠S2
• DS1=DS2 andCM1=CM2 but S1≠S2 andD1≠D2
• …and that F1 makes better use of DS1’s features than DS2

When you design a format, you design each of its aspect and

• how much you make explicit
• how you formalise CM, S

Schematron

The essence of Schematron

• finer control (validate parts of a document)
• have high expressivity
• Essentially, code based validation!

• little declarative

• Friendly: combine Schematron with RelaxNG. (has two phase validation)

• Powerful: 在context report中支持Xpath

• 需要 well formed XML
• work on DOM

what is Schematron

• Rule-based XML schema
• test oriented(有利于测试文档)
• Complimentary to other schema languages
• conceptually simple: a rule sets a context and contains
  • asserts (As) - act “when test is false”
  • reports (Rs) - act “when test is true”
• 可以做些别的模式不允许做的事情
• 要么passes all rules in a schema S 要么fails some of the rules in S

shcematron example

<pattern>
    <rule context="person">
        <let name="F" value="@FirstName"/>
        <let name="L" value="@LastName"/>
        <assert test="count(//person[@FirstName = $F and @LastName = $L]) = 1">
            There can be only one person with a given name,
            but there is <value-of select="$F"/> <value-of select="$L"/> at least twice!
        </assert>
    </rule>
</pattern>

core concepts of data model

data models

shape: none, tables, trees, graphs,…

core data modes for these above

• [tables] csv files, SQL tables
• [trees] sets of feature-value pairs, XML, JSON
• [graphs] RDF

schema language for for these above

• [SQL tables] SQL
• [XML] RelaxNG, XSD, Schematron,…
• [JSON] JSON Schema

manipulation mechanisms

• [SQL tables] SQL
• [XML] DOM, SAX, XQuery,…
• [JSON] JSON API,…

Graph shaped Data Models

External Representation for Graph

• Pictures are a bad external representation for graphs. Because it captures loads of irrelevant information like colour, location.
• turtle
• N3
• N-triples
• JSON-LD

RDF: a data structure formalisms for graphs

RDFS: a schema language for RDF

(subject, predicate, object)

• si ∈ U∪B
• pi ∈ U
• oi ∈ U∪B∪L U: URIs (for resources), B: Blank nodes, L: Literals

RDFS a schema language for RDF

• RDFS does not describe/constrain structure
• RDFS can’t be used to “validate” documents/graphs schemas describe ExtReps

What do schemas usually do

• schemas that describe ExtReps
• RDFS例外，它只能知道 what’s assumed/known, and thus – what can be inferred

SPARQL: a query language for graphs

• Ask return a boolean
• Select return a table
• CONSTRUCT return graphs
• DESCRIBE return graphs

example

@prefix rdf: <http://www.w3.org/1999/02/22-rdf-syntax-ns#> .
@prefix foaf: <http://xmlns.com/foaf/0.1/> .
@prefix ex: <http://www.cs.man.ac.uk/> .

ex:bobthebuilder
    foaf:firstName "Bob";
    foaf:lastName "Builder"; foaf:knows ex:wendy ; foaf:knows ex:farmerpickles; foaf:knows ex:bijanparsia.
ex:wendy
    foaf:firstName "wendy";
    foaf:knows ex:farmerpickles.
ex:farmerpickles
    foaf:firstName "Farmer";
    foaf:lastName "Pickles";
    foaf:knows ex:bobthebuilder.
ex:bijanparsia
    foaf:firstName "Bijan";
    foaf:lastName "Parsia".

1. How many friends does Bob Builder have?

SELECT DISTINCT COUNT(?friend)
WHERE {
    ex:bobthebuilder
    foaf:firstName "Bob";
    foaf:lastName "Builder";
    foaf:knows ?friend };

1. Give Bob Builder’s friends’ friends?

 SELECT ?first, ?last
 WHERE {
    ex:bobthebuilder
    foaf:firstName "Bob";
    foaf:lastName "Builder";
    foaf:knows ?x.
    ?x foaf:knows ?y.
    ?y foaf:firstName ?first;
    foaf:lastName ?last}

1. Bob Builder’s friends’ friends

SELECT ?first, ?last
WHERE {ex:bobthebuilder
        foaf:firstName "Bob";
        foaf:lastName "Builder";
        foaf:knows+ ?friend.
        ?friend foaf:firstName ?first;
        foaf:lastName ?last}