GloServ: Global GloServ: Global Service Discovery Service Discovery ArchitectureArchitecture

Knarig Arabshian and Henning Schulzrinne

IRT internal talkApril 8, 2004

GloServ/Knarig 2

AgendaAgenda

Motivation Background

RDF and RQL Architecture

Hierarchy, registration, query Related work Conclusions and future work

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What is service discovery?What is service discovery? Where is “foo”? Who

is responsible for “bar”? What Italian

restaurants are near Columbia University?

Events + services Can be location-

based

…there is a classical music concert at 7pm across the street, near your hotel ...

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Why global services?Why global services? Ubiquitous computing is becoming prevalent in

today’s society Traveler visiting a new city wants to know all

classical music events. Doctor visiting a hospital wants to know

medical services in this hospital. Visitor in starbucks wants to know if it offers

local internet TV.

Service discovery should be global

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What are the challenges?What are the challenges? Service description and querying

Too specific: does not extend to a new service

Too broad: difficult to search or find Semantically describe services

Global scale Flat: does not scale Hierarchy: like DNS

Location based – restaurant.service.newyork.ny.us Service based – newyork.ny.us.service.restaurant

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Service description (RDF)Service description (RDF)

<?xml version=“1.0”?>

<RDF>

<Restaurant about=“http://…/PatsPizza.html”>

<Cuisine>Italian</Cuisine>

<Rating>6</Rating>

</Restaurant>

</RDF>

Resource Description Framework (RDF) XML-based by W3C Originally designed to represent metadata for web

resources Semantically defines web resources Identify things using URI Describe using properties and property values

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More RDF [2]More RDF [2]

(subject, predicate, object)

doc , title, My resumedoc, editor, <><>, homePage, http://…<>, fullName, Alice

Cueba

http://www.home.com/doc

http://www/~alice

My resume

Alice Cueba

titleeditor

homePage fullName

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More RDF [3]More RDF [3]<?xml version=“1.0”>

<rdf:RDF xmlns:rdf=“…” …>

<rdf:Description rdf:about=“http://www.home.com/doc”>

<dc:title>My resume</dc:title>

<a:editor rdf:nodeID=“editorInfo”/>

</rdf:Description>

<rdf:Description rdf:nodeID=“editorInfo”>

<a:fullName>Alice Cueba</a:fullName>

<a:homePage rdf:resource=“http://www/~alice”/>

</rdf:Description>

</rdf:RDF>

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RDF schemaRDF schema Vocabulary description language that provides a type

system for RDF. Classes are similar to object-oriented classes Properties are similar to members within a class (Subject, predicate, object) => (Class, property,

property value) Example:

Restaurant, rating, 6 Restaurant, cuisine, Italian

Rating = (domain=>Restaurant, range=>literal) Classproperty : can be many-to-many Classes can have subclasses, Properties can have

subproperties

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RDF schema [2]RDF schema [2]<RDF …>

<Class ID=“Restaurant”>

<Comment>A dining establishment

</Comment>

<label>Restaurant</label>

</Class>

<Property ID=“Rating”>

<domain resource=“#Restaurant”>

<range resource=“#Literal”>

</Property>

<Property ID=“Cuisine”>

<domain resource=“#Restaurant”>

<range resource=“#Literal”>

</Property>

</RDF>

<RDF …>

<Restaurant about=“http://…/PatsPizza.html”>

<Cuisine>Italian</Cuisine>

<Rating>6</Rating>

</Restaurant>

</RDF>

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Query in RDFQuery in RDF Different querying languages for RDF

RDF data query language (RDQL) RDF query language (RQL)

Chose RQL Allows schema query (why needed?)

Select Y from {X} rating {Y}

Where X=“http://…/PatsPizza”

Result: Y=6

Find me the rating of pat’s pizza

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Query in RDF [2]Query in RDF [2]

Querying the properties that belong to a particular domain

select @X from Property{@X} where @X in domain( Restaurant )

=>Result: @X = Rating, Cuisine

Querying subclasses of a particular class

select $X from Class{$X} where $X in subClassOf( Event )

=>Result: $X = Classical, Medical, Emergency

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Architecture: HierarchyArchitecture: Hierarchy

medical.event

serviceevent

concert.event

jazz.concert.event classical.concert.event

restaurant.service

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Determination of Service Determination of Service HierarchyHierarchy The GloServ architecture is similar to DNS

root name servers authoritative name servers that manage the information of

services.

Separate classification system similar to North American Industry Classification System (NAICS)

classifies the hierarchy of services establishes RDF schemas that describe each type of service The service categorization is similar to yellow pages directory.

Authority such as ICANN delegates the top level services

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RegistrationRegistration

gloserv:registrar

restaurant.service

2) Choose hierarchy level to register to

1)Present user with service hierarchy

3) Generate URI and contact correct server to obtain meta data for GUI

4)Create form (according to the meta data) for user to fill out

ServiceAgent Operator

5)Verify service agent and store RDF document in server

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User QueryUser Query

Local user agent

restaurant.service

2) Choose service to query to

1)Present user with services offered

3) Generate URI and contact correct server to obtain meta data for GUI

4)Create query form (according to the meta data) for user to fill out

User

5)Formulate RQL query and obtain list of services

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Generating URI to contact Generating URI to contact correct Authoritative correct Authoritative ServerServer Service Level

User chooses the level of the hierarchy it wants to either register to or query to

Registrar and Local User Agent: Concatenate the service hierarchy into a

URI: service->restaurant will evaluate to gloserv:restaurant.service

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Generating URI to contact Generating URI to contact correct Authoritative correct Authoritative ServerServer Location Level

Find out the nearest location-based servers by querying its cached RDF store of its location hierarchy: new_york.ny.us

Generate URI: gloserv:restaurant.service.new_york.ny.us

If this URI doesn’t exist, try the sibling location gloserv:restaurant.service.brooklyn.ny.us

May also try just the service URI itself in case it is not a location-based service (gloserv:restaurant.service)

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Query within Authoritative Query within Authoritative ServerServer restaurant.service authoritative server

will execute RQL query on its RDF store and return matches

User may also add additional text to search for

This evaluates to a heuristic that searches for (subject, predicate, objects) that match the text May not be as accurate Gives user opportunity to choose from a

broader list of services

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Related WorkRelated Work SLP (Service Location protocol)

User Agents (UA) perform service discovery on behalf of a client

Service Agents (SA) which advertise location and characteristics of the service on behalf of the service

Directory Agents (DA), records available services and also responds to service requests from UAs (optional).

Two modes of operation DA exists: UAs learn of services available by unicasting their

requests to the DA. DA doesn’t exist: UAs repeatedly multicast messages to Sas

that responds to the UAs via unicast.

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Related WorkRelated Work JINI

Built on top of the Java object and RMI system. Service registries, similar to SLP’s DAs The Java class hierarchy defines services and their attributes.

UPnP Doesn’t have a central service registry Services multicast their announcements to control points that are

listening Control points can also multicast discovery messages and search

for devices within the system. XML describes the services in greater detail.

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Differences/SimilaritiesDifferences/Similarities Scaling

SLP and Jini can cover small networks as well as

larger enterprise networks UPnP appropriate for home or small office networks.

Querying Simple text-based attribute-value query languages for SLP

and Jini UPnP provides more descriptive queries through XML.

Main Drawbacks do not cover a wide area network that spans the whole Internet SLP, Jini provide simpler querying mechanism which do not

give enough flexibility to the system.

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Conclusion and Future Conclusion and Future WorkWork

GloServ is a global service discovery architecture

Uses RDF/RQL to describe and query for services

Hierarchical definition of services Building a prototype

implementation of GloServ