ALUMNI 2015_Hypermedia APIs, Andreas Harth

KIT – University of the State of Baden-Wuerttemberg and

National Research Center of the Helmholtz Association

Institute AIFB

www.kit.edu

Hypermedia APIs – Semantic Web Services Done Right

Andreas Harth

STI Alumni Meeting 2015

Institute AIFB 2 06.08.2015

Outline

Hyperlinks in Linked Data

Hyperlinks in Web APIs

Demo

Conclusion



HYPERLINKS IN LINKED DATA


Institute AIFB 4 06.08.2015 Hypermedia APIs – Semantic Web Services Done Right

Ted Nelson,

Computer Lib/

Dream machines,

published 1974,

republished 1987


Conway‘s Law

“organizations which design systems ... are constrained to produce

designs which are copies of the communication structures of these

organizations”

-- Melvin Conway

Hypermedia APIs – Semantic Web Services Done Right Institute AIFB 5 06.08.2015


WSMX Architecture



Doug Engelbart vs. Tim Berners-Lee

Daniel W. Connolly. An evaluation of the world wide web with respect

to Engelbart’s requirements. 2002. http://www.w3.org/Architecture/

NOTE-ioh-arch

…



The

Hyperdocument

„Back-Link

Capability“:

PASS


The

Hyperdocument

„Library

System“:

PASS


Random Surfer Model

1. URI u_t0 := entry point

2. Lookup URI u_t

3. With probability d, u_t+1 := URI from document at random

4. With propability 1-d, get bored and u_t+1 := URI from the visited

pages

5. Goto 2

Lawrence Page, Sergey Brin, Rajeev Motwani, Terry Winograd,

The PageRank Citation Ranking: Bringing Order to the Web, TR

1999.



Linked Data

HTTP URIs for things

Enable GET on URIs

Return links to other URIs



Specifying Surfer Behaviour


2. HTTP GET on URI u_t

3. u_t+1 := Follow URI as specified in program

4. Goto 2



Example

1. URI u_t0 := http://harth.org/andreas/foaf#ah

2. Lookup URI u_t

3. u_t+1 := Follow URI as specified in rules:

{ <http://harth.org/andreas/foaf#ah> foaf:knows ?x . } =>

GET (?x) .

4. Goto 2



Rule-based Forward-chaining?

Distinction between source schema and mediated schema blurs

No source descriptions of Linked Data sources available

Syntactic restrictions are difficult to enforce in linked environments

OWL DL vs. OWL Full (Nicolas Matentzoglu and Bijan Parsia, The OWL

Full/DL gap in the field, OWLED 2014), rdfs:label cannot

Distinction between edb/idb predicates difficult with a single triple relation

Rewriting algorithms like magic sets rely on separation between edb and

idb predicates



HYPERLINKS IN WEB APIS



There is a Third Way…


Specifying Surfer Behaviour


2. Perform request on URI u_t

3. u_t+1 := User specifies next URI, including request method and

additional data to send

4. Goto 2


The HTML Web as Hypermedia System

order123/step-address

Servers provide a list of possible transitions (URI,

HTTP method, message format)


Transition Systems

Labelled Transition System is a pair (S, /\, ->)

Set S of states (RDF description of resources)

Set /\ of labels (unsafe requests - PUT, POST, DELETE - with optional

request message)

Set -> of state transitions (subset of S x S)

Servers provide /\ incrementally, so that clients can reach the next state

Clients cannot view farther than one step


State Transitions vs. Activities

RPC (Activities) could have a duration

RPC is problematic in networked enviroments where network

connection drops can happen

Instantanious state transitions (GET/PUT/POST/DELETE) on

resources identified via URIs fit with Web Architecture


Hypermedia Agents as State Machines

Start state: initial set of URIs U

End state: query/goal Q

Transitions: set of rules that describe the next relation (given current

state, what transition to carry out/what should hold in next state)


World State

World state is based on a set of resources identified via URIs

Transition systems describe how the state of the world can evolve over

time

We assume discrete time (digital signals)

„Stroboscopic“ world in which we poll the world state at defined

intervals (e.g., 60 Hertz, 16 2/3 ms)


Main Loop Intuition

while goal not reached:

• get the state of the world (including descriptions of possible next steps) via HTTP GET

• based on world state, decide on next step

• carry out next HTTP {PUT|POST|DELETE} request


Catalogue vs. Links

AI planning requires access to all activities

SWS: registry with service descriptions

Web: no central registry, but link-following

Google is a registry of sorts, but the Googlebot crawler follows links to

collect data

SWS Composition: given a set of descriptions D (with IOPE),

coordinator assembles process (sequence of activities/service calls)

Hypermedia: servers provide possible next steps based on current

state (describing I only)


DEMO


The Quintessential 3D Application

Tracking service

Application

logic

Sensor service

Request

Response

Ressource

Ressource

Ressource

Ressource

Ressource

Ressource

Ressource

Ressource

Ressource

Data source 2

Processes

Data source 1

3D Assets Simulation service

Ressource Ressource Ressource Ressource Ressource

Querverweise/

Links

Vocabularies

Request

Response

Re

qu

est

Re

spo

nse

Visualisation service

Output service

Vocabularies

Vocabula

ries

In ARVIDA, the focus is on execution, not planning


Simple Application Logic


Worker Processes


CONCLUSION


Conclusion

Hyperlinks + APIs + Rules

= Semantic Web Services done

right


Image Credits

Title page: http://matthewjamestaylor.com/blog/experimenting-with-visited-links

Linking Open Data cloud diagram 2014, by Max Schmachtenberg, Christian

Bizer, Anja Jentzsch and Richard Cyganiak. http://lod-cloud.net/

CERN LHC image via http://www.xatakaciencia.com/fisica/el-lhc-descubre-dos-

nuevas-particulas-que-nunca-se-habian-visto-antes


Presentations & Public Speaking

ALUMNI 2015_Hypermedia APIs, Andreas Harth