the semantic web crafting infrastructures for agency jan phần 10 ppsx

Canonical form is the usual or standard state or manner of something, and in this book it is used in the computer language sense of a standard way of expressing.. DHCP Dynamic Host Config

It is true that for such a system to be effective, the people participating in it need to agreeabout a set of common standards or rules to facilitate communication and cooperation In theWeb, these common rules are compliance to the core technologies, such as URI, HTTP, andTCP/IP, and basic rules of conduct The latter suggest policy restrictions on exploits andintrusion attempts, or ways to combat the spread of computer viruses and worms However,well-chosen rules increase rather than decrease freedom.

In actual fact, the bottom-line hard requirement is simply that whatever you implementmust be gracefully compliant with existing infrastructure Your overlaid protocols andapplications can be as strange as you want, as long as they do not break the transport orinterfere with the expected capabilities of others

Bit 11.11 Infrastructure compliance is a self-correcting requirement and ment

environ-There is no need for a tyranny of regulation and restriction as long as functionality is ineveryone’s self-interests It is only against purely destructive efforts that blockingmeasures are required

Become too distant from the consensus way of working and you will likely loseconnectivity or common functionality Become too intrusive and you bring down the ire

of the community, who might first flame you, then filter you out of it Become too strangeand obscure in your innovation and nobody will adopt it – you may then continue to play insplendid isolation The usual balance is mildly chaotic diversity, constantly evolving invarious directions

It is a state we often see in nature

Consider ants If ants would always follow the paths laid down by their fellow ants, andnever diverge to create paths of their own, then the colony would starve as soon as foodsources on the existing paths became exhausted So they have evolved to meander slightly,leaving the strongest scent trail, with occasional individuals striking out boldly where noant has gone before Some of these will perish, a few return Sometimes the action of theone becomes the path for many, returning with new-found food, and then ultimately themajority path shifts

Bit 11.12 Successful collective problem solving relies on a diversity in the dual approaches and different paths

indivi-Significant advances may then attract consensus attention, the chosen divergent pathbecome a dominant one in future, but it never becomes the only path

Since the same rules democratically apply to everyone, the net result is that otherwisedominant organizations, governments, or corporations have less power to censor or imposetheir rules on the people who use the Web The individual gains freedom

Who Controls It?

A distributed and partially autonomous system like the proposed Semantic Web, and likethe Web before it, is ultimately controlled by the people who make themselves part of it anduse it

Bit 11.13 The Internet is functionally a collective; a complex, self-organizingsystem

It is a direct result of many autonomous entities acting in a combination of self-interestand advocacy for the common good This collective is guided by informed butindependent bodies

If people stop using the network, then effectively it will cease to exist Then it no longermatters or has any relevance to the people, simply because it no longer connects to theirlives

This is not the same as saying a network, controlled by a central authority, with extensionsinto and controlling our physical environment, would not matter Some people, a very muchsmaller collective, are then still using the system when all the others have opted out andrelinquished their distributed and moderating control over it

The choice is ours, collectively – yet any one individual action can be pivotal

Part IV

Appendix Material

Appendix A

Technical Terms and References

This appendix provides a glossary of technical terms used in the book, along with theoccasional technical references or listings that do not fit into the flow of the body text

Agent, in the sweb context, is some piece of software that runs without direct humancontrol or constant supervision to accomplish goals provided by a user Agents maywork together with other agents to collect, filter, and process information found on theWeb

Agency is the functionality expressed by agents, enabling for example automation anddelegation on behalf of a user

API (Application Programming Interface) is a set of definitions of the ways in which onepiece of computer software communicates with another – protocols, procedures,functions, variables, etc Using an appropriate API abstraction level, applications canreuse standardized code and access or manipulate data in consistent ways

The Semantic Web: Crafting Infrastructure for Agency Bo Leuf

# 2006 John Wiley & Sons, Ltd

Architecture, a design map or model of a particular system, showing significantconceptual features.

Authentication, a procedure to determine that a user is entitled to use a particularidentity, commonly using login and password but might be tied much tighter tolocation, digital signatures or pass-code devices, or hard-to-spoof personal propertiesusing various analytic methods

Bandwidth, a measure of the capacity a given connection has to transmit data, typically

in some power of bits per second or bytes per second Extra framing bits mean that therelationship between the two is around a factor 10 rather than 8

Broker, a component (with business logic) that can negotiate for instance procurementand sales of network resources

Canonical form is the usual or standard state or manner of something, and in this book it

is used in the computer language sense of a standard way of expressing

ccTLD (country-code Top Level Domain) designates the Internet domains registered witheach country and administered by that country’s NIC database The country codes arebased on the ISO3166 standard, but the list is maintained by IANA along withinformation about applicable registrar – for example, uk for the United Kingdom, sefor Sweden, and us for U.S.A Also see gTLD

CGI (Common Gateway Interface) is in essence an agreement between HTTP serverimplementers about how to integrate gateway scripts and programs to access existingbodies of documents or existing database applications A CGI program is executed inreal-time when invoked by an external client request, and it can output dynamicinformation, unlike traditional static Web page content

Client-Server, the traditional division between simpler user applications and centralfunctionality or content providers, sometimes written server-client – a seen variant is

‘cC-S’ for centralized client-server, though ‘cS-C’ would strictly speaking have beenmore logical to avoid thinking the clients are centralized

Content classification system is a formal way to index content by subject to make iteasier to find related content Examples mentioned in the metadata context of thisbook are DDC (Dewey Decimal Classification Number, for U.S libraries), LCC(Library of Congress Classification Number), LCSH (Library of Congress SubjectHeading), and MESH (Medical Subject Headings) Also see identifier

CSS (Cascading Style Sheets) is a systematic approach to designing (HTML) Web pages,where visual (or any device-specific) markup is specified separately from the content’sstructural markup Although applicable to XML as well, the corresponding andextended concept there is XSL

DAV or WebDAV (Distributed Authoring and Versioning), a proposed new Internetprotocol that includes built-in functionality to facilitate remote collaboration andcontent management Current, similar functionality is provided only by add-on server

or client applications

Dereferencing is the process required to access something referenced by a pointer – that

is, to follow the pointer In the Web, for example, the URL is the pointer, and HTTP is

a dereferencing protocol that uses DNS to convert the protocol into a usable IP address

to a physical server hosting the referenced resource

DHCP (Dynamic Host Configuration Protocol) is a method of automatically assigning IPnumbers to machines that join a server-administrated network.

Directory or Index services translate between abstraction names and actual location.DNS (Domain Name Service) is a directory service for translating Internet domain names

to actual IP addresses It is based on 13 root servers and a hierarchy of cachingnameservers emanating from registrar databases that can respond to client queries.DOM (Document Object Model) is a model in which the document or Web page containsobjects (elements, links, etc.) that can be manipulated It provides a tree-like structure

in which to find and change defined elements, or their class-defined subsets The DOMAPI provides a standardized, versatile view of a document’s contents that can beaccessed by any application

DTD (Document Type Definition) is a declaration in an SGML or XML document thatspecifies constraints on the structure of an SGML or XML document, usually interms of allowable elements and their attributes It is written in a discrete ascii-textfile Defining a DTD specifies the syntax of a language such as HTML, XHTML, orXSL

DS (Distributed Service) is when a Web Service is implemented as across many differentphysical servers working together

End-user, the person who actually uses an implementation

Encryption, opaquely encoding information so that only someone with a secret key candecrypt and read or use it In some cases, nobody can decrypt it, only confirm correctinput by the fact it gives the same encrypted result (used for password management inUnix/Linux, for example)

Gateway (also see proxy), a computer system that acts as bridge between differentnetworks, usually a local subnet and an external network It can also be a computerthat functions as a portal between a physical network and a virtual one on the samephysical machine that use different protocols

gTLD (generic or global Top Level Domain ) designates the Internet domains that wereoriginally intended not to be reserved for any single country – for example, theinternational and well-known com, org, net Also see ccTLD

Governance is the control of data and resources and who wields this control

Hash, a mathematical method for creating a numeric signature based on content; thesedays, often unique and based on public key encryption technology

HTML (HypeText Markup Language) is the language used to encode the logical structure

of Web content Especially in older versions, it also specifies visual formatting andvisual features now deprecated and consigned to stylesheet markup HTML usesstandardized ‘tags’ whose meaning and interpretation is set by the W3C

HTTP (HyperText Transfer Protocol) is the common protocol for communicationbetween Web server and browser client The current implementation is v l.1.HTTPS (HTTP over SSL) is a secure Web protocol that is based on transaction-generatedpublic keys exchanged between client and server and used to encrypt the messages.The method is commonly used in e-commerce (credit card information) and wheneverWeb pages require secure identity and password login

Hyperlink is a special kind of pointer defined as an embedded key-value pair that enables

a simple point-and-click transition to a new location or document in a reader client It

is the core enabling technology for Web browsing, defined in HTTP-space as a markuptag

IANA (Internet Assigned Numbers Authority, www.iana.org) maintains central registries

of assigned IP number groups and other assigned-number or code lists Domaincountry codes, protocols, schemas, and MIME type lists are included, although manyearlier responsibilities have been transferred to ICANN (whose motto is ‘Dedicated topreserving the central coordinating functions of the global Internet for the publicgood’)

ICANN (The Internet Corporation for Assigned Names and Numbers, www.icann.org)was formed as an international NGO to assume responsibility for the IP address spaceallocation, protocol parameter assignment, domain name system management, androot server system management functions previously performed under U.S Govern-ment contract by IANA and other entities

Identifier, generally refers in metadata context to some formal identification system forpublished content Examples of standard systems mentioned in the text are govdoc(Government document number), ISBN (International Standard Book Number), ISSN(International Standard Serial Number), SICI (Serial Item and Contribution Identifier),and ISMN (International Standard Music Number)

IETF (Internet Engineering Task Force, www.ietf.org) is the body that oversees work ontechnical specifications (such as the RFC)

Implementation, a practical construction that realizes a particular design

IP (Internet Protocol) is the basis for current Internet addressing, using allocated IPnumbers (such as 18.29.0.27), usually dereferenced with more human-readabledomain names (in this example, w3c.org)

IP (Intellectual Property) is a catch-all term for legal claims of ownership associated withany creative or derivative work, whether distributed in physical form (such as book orCD) or as electronic files, or as a published description of some component or system

of technology The former is legally protected by copyright laws, the latter by patentlaws Related claims for names and symbols are covered by trademark registrationlaws

Living document means a dynamic presentation that adapts on-the-fly to varying andunforeseen requirements by both producer and consumer of the raw data

MARC (MAchine-Readable Cataloging) project defines a data format which emergedfrom an initiative begun in the 1970s, led by the U.S Library of Congress MARCbecame USMARC in the 1980s and MARC 21 in the late 1990s It provides themechanism by which computers exchange, use and interpret bibliographic informationand its data elements make up the foundation of most library catalogs used today.Message, a higher logical unit of data, comprising one or more network packets, anddefined by the implementation protocol

Metadata is additional information that describes the data with which it is associated.Middleware, a third-party layer between applications and infrastructure

MIME (Multipurpose Internet Mail Extensions) extends the format of Internet mail toallow non-US-ASCII textual messages, non-textual messages, multi-part messagebodies, and non-US-ASCII information in message headers MIME is also widelyused in Web contexts to content-declare client-server exchanges and similarly extendthe capability of what was also originally ASCII-only MIME is specified in RFC 2045through 2049 (replacing 1521 and 1522).

Namespace is the abstract set of all names defined by a particular naming scheme – forexample, all the possible names in a defined top level Internet domain, as constrained

by allowable characters and name length

NIC (Network Information Center) is the common term used in connection with a domainname database owner or primary registrar – for example, the original InterNIC(a registered service mark of the U.S Department of Commerce, licensed toICANN, which operates the general information Web site www.internic.net), aparticular gTLD database owner (such as www.nic.info), or a national ccTLDadministrator (such as NIC-SE, www.nic-se.se, for Sweden)

NIC (Network Interface Card) is a common abbreviation for the ethernet adapter card thatconnects a computer or device with the network cable on the hardware level.Ontology is a collection of statements (written in a semantic language such as RDF) thatdefine the relations between concepts and specify logical rules for reasoning aboutthem Computers can thus ‘understand’ the meaning of semantic data in Web content

by following links to the specified ontologies

Open protocol, the specifications are published and can be used by anyone

Open source, opposite of proprietary ‘closed’ source ‘Open’ means that the source code

to applications and the related documentation is public and freely available Often,runnable software itself is readily available for free

OSI reference model (Open Systems Interconnect protocol layers), see Figure A.1, withreference to the OSI diagrams in Chapter 1 and 2, and to the native implementationexamples (.NET usually runs at the Application layer.)

OWL is the W3C recommendation for Sweb ontology work

Figure A.1 An indication of what kind of communication occurs at particular levels in the OSImodel, and some examples of relevant technologies that function at the respective levels The top fourare ‘message based’

p2p (peer-to-peer) designates an architecture where nodes function as equals, showingboth server and client functionality, depending on context The Internet was originallyp2p in design, and it is increasingly becoming so again.

P3P (Platform for Privacy Preferences) is a W3C recommendation for managing Website human-policy issues (usually user privacy preferences)

Packet, a smallest logical unit of data transported by a network, which includes extraheader information that identifies its place in a larger stream managed by a higherprotocol level

Persistency, the property of stored data remaining available and accessible indefinitely or

at least for a very long time, in some contexts despite active efforts to remove it.PIM, Personal Information Manager

Platform, shorthand for a specific mix of hardware, software, and possibly environmentthat determines which software can run In this sense, even the Internet as a whole

is a ‘platform’ for the (possibly distributed) applications and services that runthere

Protocol, specifies how various components in a system interact in a standardized way.Each implementation is defined by both model (as a static design) and protocol (as aspecified dynamic behavior) A protocol typically defines the acceptable states, thepossible outcomes, their causal relations, their meaning, and so on

Provenance is the audit trail of knowing where data originate, and who owns them.Proxy (also see gateway), an entity acting on behalf of another, often a server acting as alocal gateway from a LAN to the Internet

PURL (Persistent Uniform Resource Locator) is a temporary workaround to transitionfrom existing location-bound URL notation to the more general URI superset.Push, a Web (or any) technology that effectively broadcasts or streams content, as distinctfrom ‘pull’ that responds only to discrete, specific user requests

QoS (Quality of Service) is a metric for quantifying desired or delivered degree of servicereliability, priority, and other measures of interest for its quality

RDF (Resource Description Framework) is a model for defining information on the Web,

by expressing the meaning of terms and concepts in a form that computers can readilyprocess RDF can use XML for its syntax and URIs to specify entities, concepts,properties, and relations

RDFS (RDF Schema) is a language for defining a conceptual map of RDF vocabularies,which also specifies how to handle and label the elements

Reliable and unreliable packet transport methods are distinguished by the fact thatreliable transport requires that each and every message/packet is acknowledged whenreceived; otherwise, it will be re-sent until it is acknowledged, or a time-out value ortermination condition is reached

Representational, when some abstraction is used for indirect reference instead of theactual thing – a name, for example

Reputability is a metric of trust, a measure of known history (reputation)

Resource is Web jargon for any entity or collection of information, and includes Webpages, parts of a Web page, devices, people and more

RFC (Request For Comment) in the Internet context designates a body of technicalspecifications overseen by the IETF which encompasses both proposals and consensusstandards that define protocols and other aspects of the functional network.

RPC, remote procedure call, a protocol extension that enables remote software to directlyinvoke a host’s local API (application program interface) functionality

RSS is a common term for several related protocols for summary syndication of content

It is a simple way for clients to ‘subscribe’ to change notification

Schema is a term widely used to designate a kind of relationship table between terms andtheir meanings in a given context Such tables can be used to map the meaning ofparticular terms to corresponding terms in different logical structures and differentapplication contexts

Semantic Web (sweb) is the proper name for the ‘third-generation’ Web effort ofembedding meaning (semantics) in Web functionality

Service discovery is the term for the process of locating an agent or automated Web-basedservice that will perform a required function Semantics enable agents to describe to oneanother precisely what function they carry out and what input data are needed.SGML (Standard Generalized Markup Language) is an ISO standard (ISO 8879:1986)which defines tag-building rules for description encoding of text HTML, XML, andmost other markup languages are subsets of SGML

SSL (Secure Socket Layer) is a protocol for securely encrypting a connection usingexchanged public keys between the endpoints, usually seen in but not limited to theHTTPS Web document request

Swarm distribution, when peers adaptively source downloaded content to other peersrequesting the same material Random offsets ensure quick fulfillment Swarmservices in general are network services implemented by cooperating nodes, oftenself-organizing in adaptive ways

Swarm storage, when content is fragmented and distributed (with redundancy) to manydifferent nodes On retrieval, swarms adaptively cooperate to source

Sweb (Semantic Web, SW) is a common abbreviation used to qualify technologiesassociated with the Semantic Web effort

SWS (Semantic Web Service) is to Web Service what the Semantic Web is to the Web.TLD (Top Level Domain) is the root abstraction for HTTP namespaces, dereferenced byInternet DNS Also see gTLD and ccTLD

Triple is a subject-predicate-object expression of three terms that underlies RDF.UDDI (Universal Description, Discovery and Integration) is a specification that enablesbusinesses to find and transact dynamically with one another UDDI encompassesdescribing a business and its services, discovering other businesses that offer desiredservices, and integrating with them

URI (Uniform Resource Identifier) is a complete and unique scheme for identifyingarbitrary entities, defined in RFC 2396 (www.ietf.org/rfc/rfc2396.txt)

URI persistence is the desired characteristic that URI addresses remain valid indefinitely.Its opposite is ‘link-rot’ expressed as resource-not-found

URL (Uniform Resource Locator) is a standard way to specify the location of a resourceavailable electronically, as a representation of its primary access mechanism – it isthe addressing notation we are used to from Web and other Internet clients (includinge-mail) URLs are a subset of the URI model and are defined in RFC 1738.

URN (Uniform Resource Name) is another subset of URI, and refers to resource specifiersthat are required to remain globally unique and persistent even when the resourceceases to exist or becomes unavailable It is thus a representation based on resourcename, instead of location as in the familiar URL It is defined in RFC 2141.W3C (World Wide Web Consortium, www.w3c.org) was created in October 1994 todevelop interoperable technologies (specifications, guidelines, software, and tools) tolead the Web to its full potential W3C is a forum for information, commerce,communication, and collective understanding, and is the custodian of numerous openprotocols and APIs

WebDAV, see DAV

Web Services (WS) is a common name applied to functionality accessed through anyURI, as opposed to static data in stored documents

WSDL (Web Services Description Language), is a modular interface specification to WebServices

WUM (Web Usage Mining) describes technologies to profile how users utilize the Weband its different resources

XML (eXtensible Markup Language) is a markup language intended to supplant HTML,transitionally by way of an intermediate markup called XHTML (which is HTML 4.2expressed in XML syntax) XML lets individuals define and use their own tags It has

no built-in mechanism to convey the meaning of the user’s new tags to other users.XMLP (eXtensible Markup Language Protocol) defines an XML-message-based messageprotocol to encapsulate XML data for transfer in an interoperable manner betweendistributed services

XLink (XML Linking Language) defines constructs that may be inserted into XMLresources to describe links between objects, similar to but more powerful thanhyperlinks XLink also uses XPath

XPath (XML Path Language) is an expression language used by XSLT and XLink toaccess or refer to internal parts of an XML document

XPointer (XML Pointer Language), is based on the XML Path Language (XPath), andsupports addressing into the internal structures of XML documents It allows fortraversals of a document tree and choice of its internal parts based on various properties,such as element types, attribute values, character content, and relative position.XSL (Extensible Stylesheet Language) is a language, or more properly a family of W3Crecommendations, for expressing stylesheets in XML (see CSS) It consists of threeparts: XSL Transformations (XSLT, a language for transforming XML documents);XPath; and XSL Formatting Objects (XSLFO, an XML vocabulary for specifyingformatting semantics) An XSL stylesheet specifies the presentation of a class of XMLdocuments by describing how an instance of the class is transformed into an XMLdocument that uses the formatting vocabulary

The following sections complement the Chapter 5 descriptions of RDF and RDF-Schema

RDF Schema Example Listing

An RDF schema for defining book-related properties is referenced in Chapter 5, aproposRDF schema chaining The chosen example is from a collection of Dublin Core draft baseschemas (at www.ukoln.ac.uk/metadata/dcmi/dcxml/examples.html ) It spreads over severalpages in this book, so it is not suitable for inclusion in the body text The example schema isreferenced by name as dc.xsd

XML Schema 2001-12-18 by Pete Johnston

Based on Andy Powell,

Guidelines for Implementing Dublin Core in XML, 9th draft.

This XML Schema is for information only

</xs:documentation>

</xs:annotation)

<xs: import namespace¼"http://www.w3.org/XML/1998/namespace" schemaLocation ¼

"http://www.ukoln.ac.uk/metadata/dcmi/dcxml/xmls/xml.xsd" >

</xs:import>

<!

<xs:import namespace¼"http://www.w3.org/XML/1998/namespace" schemaLocation ¼"http://www.w3.org/2001/xml.xsd">

<xs:element name¼"description" type¼"descriptionType"/>

<xs:element name¼"publisher" type¼"agentType"/>

<xs:element name¼"contributor" type¼"agentType"/>

The process requires at least two components:

an RDF description file of you as the content creator;

a metadata block on a relevant HTML or XML page

The Creator Description File

The RDF description is stored as a flat-text file on your server, in any suitable public-weblocation that can be accessed using a normal URL (URI) We use a W3C-defmed ‘example’domain

<rdf:RDF

xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#" xmlns:rdfs=http://www.w3.org/2000/01/rdf-schema#"

xmlns:wn="http://xmlns.com/wordnet/1.6/"

http://example.com/ sem/about.xrdf#sw

The Metadata Block

Metadata blocks are inserted in the head-block of normal HTML pages, describing contentmetadata in Dublin Core terms In this example, we place a description with reference to theRDF description file just created:

<rdf:RDF ns#"

xmlns:rdf¼"http://www.w3.org/1999/02/22-rdf-syntax-xmlns:dc ¼"http://purl.org/dc/elements/1.1/"

xmlns:wot ¼"http://xmlns.com/wot/0.1/">

<rdf:Description rdf:about¼""

dc:title ¼ "My document"

dc:description ¼ "Experiments with sweb and rdf."

dc:date ¼ "2003-10-12" >

<dc:creator rdf:resource¼"http://example.com/sem/about.

xrdf#sw" />

<wot:assurance rdf:resource¼"http://example.com/sem/pagel asc" />

</rdf:Description>

</rdf:RDF>

Again, replace highlighted variable values with your own

At this point, your page is sweb-compliant in terms of RDF Obviously, further metadatamay be added to better describe the content than in this minimal example

The optional web-of-trust (wot) ‘assurance’ entry refers to a digital signature created ofthe completed page (source) using your private key – for example using the GnuPGcommand ‘gpg -ba page1.html’ if that is the program you use A user client can thusvalidate that the received document is identical to the page you created and signed

Appendix B

Semantic Web Resources

This appendix collects references for further reading, which in some cases goes beyond what

is mentioned in the text For the many URI links, you might prefer to visit the book’s Website where these links are published in active form

At a Glance

Further Reading summarizes resources of interest

 Book Resources lists some current sweb resources in print, sorted into overview, technicaland other/AI groups

 Other Publications notes significant periodical publications

 Web Resources summarizes important online sweb resources, grouped by main ogy or focus area

technol- Implementation Resources has pointers mainly to How-to tutorials for core technologies

 Miscellaneous collects supplementary or slightly off-topic resources

Further Reading

It is in the nature of hot new subjects that most resource material is in the form of scattereddocuments and resources on the Web Much of this material is both very specific and narrow,dealing with only one or another implementation This fact was one motivation to write thisbook, to try and collect useful information in one place for people who are looking for aconcise technology overview

Book Resources

When this book was started in 2002, few books were published specifically about SemanticWeb technologies or how they function This situation improved over the following twoyears, and relevant titles that seem worth pursuing are listed here

The Semantic Web: Crafting Infrastructure for Agency Bo Leuf

# 2006 John Wiley & Sons, Ltd