A Resource-Oriented Approach to the Repository OAI ORE Technical Committee Meeting, New York City

A Resource-Oriented Approach to the Repository The REST/Resource-Oriented Approach Repository Functional Requirements Delivering Functions using a Resource-Oriented Approach Caveat: All quite tentative! Needs more work etc etc!

The REST/Resource-Oriented Approach

Repository Functional Requirements

Delivering Functions using a Resource-Oriented Approach

Caveat: All quite tentative! Needs more work etc etc!

The REST/Resource-Oriented Approach “ Architectural style” Key principles Identifiable resources Uniform interface Stateless communication Exchange of representations of resources Hyperlinks in representations HTTP as one implementation of REST Actually, REST principles derived a posteriori from HTTP Roy Fielding, Architectural Styles and the Design of Network- based Software Architectures

“ Architectural style”

Key principles

Identifiable resources

Uniform interface

Stateless communication

Exchange of representations of resources

Hyperlinks in representations

HTTP as one implementation of REST

Actually, REST principles derived a posteriori from HTTP

Roy Fielding, Architectural Styles and the Design of Network- based Software Architectures

The REST/Resource-Oriented Approach Designing an application Decide on types/classes of resources Select/create formats for representations of resources Select methods to be supported for resources of different types Document likely response codes http://bitworking.org/news/How_to_create_a_REST_Protocol http://rest.blueoxen.net/cgi-bin/wiki.pl?RESTfulDesign

Designing an application

Decide on types/classes of resources

Select/create formats for representations of resources

Select methods to be supported for resources of different types

Document likely response codes

Repository Functional Requirements Enable agent to Harvest a batch of Representations, one per Object Reference An Object A Representation of an Object Obtain Representations of an Object A list of the available Representations of an Object Put An Object (into a Repository) A Representation of an Object (into a Repository)

Enable agent to

Harvest a batch of Representations, one per Object

Reference

An Object

A Representation of an Object

Obtain

Representations of an Object

A list of the available Representations of an Object

Put

An Object (into a Repository)

A Representation of an Object (into a Repository)

Essentially, concerned with two(?) types/classes of resource Instances identified by URIs Collection An aggregation of zero or more Objects Collection hasMember 0-n Objects Object isMemberOf 0-n Collections Specifying harvest criteria results in different sub-collections Object (definition) Object isComposedOf 0-n Objects Object isComponentOf 0-n Objects Object isDerivativeOf 0-n Objects Object hasDerivative 0-n Objects Any resource has zero or more representations hasComponent/ isComponentOf Collection Object hasMember/ isMemberOf 0…m 0…n hasDerivative/ isDerivativeOf 0…m 0…n 0…n 0…m Resources

Essentially, concerned with two(?) types/classes of resource

Instances identified by URIs

Collection

An aggregation of zero or more Objects

Collection hasMember 0-n Objects

Object isMemberOf 0-n Collections

Specifying harvest criteria results in different sub-collections

Object

(definition)

Object isComposedOf 0-n Objects

Object isComponentOf 0-n Objects

Object isDerivativeOf 0-n Objects

Object hasDerivative 0-n Objects

Any resource has zero or more representations

Resources and representations Components communicate by transferring representations of resources A representation is data that encodes information about resource state. Representations do not necessarily describe the resource, or portray a likeness of the resource, or represent the resource in other senses of the word "represent“ (AWWW) A URI owner may provide zero or more representations for a single resource via a single URI Basis for Content Negotiation A URI owner may provide different representations over time for a single resource via the same URI The same representation may be provided for multiple resources via different URIs A representation is a bitstream; two different bitstreams are two different representations

Components communicate by transferring representations of resources

A representation is data that encodes information about resource state. Representations do not necessarily describe the resource, or portray a likeness of the resource, or represent the resource in other senses of the word "represent“ (AWWW)

A URI owner may provide zero or more representations for a single resource via a single URI

Basis for Content Negotiation

A URI owner may provide different representations over time for a single resource via the same URI

The same representation may be provided for multiple resources via different URIs

A representation is a bitstream; two different bitstreams are two different representations

Resources and representations URIs are typically not assigned to representations But if it is useful/necessary to refer to the representation independently of the resource(s) it represents, a representation could be modelled/exposed as a resource in its own right could be identified by a URI May be for purposes of description rather than de-referencing However each different bitstream is a different representation, so each different bitstream would require a different URI Also isRepresentedBy is different relationship from hasComponent or isDerivedFrom hasComponent relationship is between Objects Do we really need to model representations as resources? Representations just of Objects? Or of Collections too? Or do we want to model some other “more specific” resource that is related to the Object but may still have multiple representations?

URIs are typically not assigned to representations

But if it is useful/necessary to refer to the representation independently of the resource(s) it represents, a representation

could be modelled/exposed as a resource in its own right

could be identified by a URI

May be for purposes of description rather than de-referencing

However

each different bitstream is a different representation, so each different bitstream would require a different URI

Also

isRepresentedBy is different relationship from hasComponent or isDerivedFrom

hasComponent relationship is between Objects

Do we really need to model representations as resources?

Representations just of Objects? Or of Collections too?

Or do we want to model some other “more specific” resource that is related to the Object but may still have multiple representations?

hasRepresentation/ isRepresentationOf If there is a requirement to refer to specific representations independently of the resource(s) they represent, then we need to introduce an additional relationship and an additional resource type/class Representation Resource isRepresentedBy 1-n Representations Representation isRepresentationOf 1-n Resources hasComponent/ isComponentOf Collection Object hasMember/ isMemberOf 0…m 0…n hasDerivative/ isDerivativeOf 0…m 0…n 0…n 0…m Resources Representation 1…n 1…m

If there is a requirement to refer to specific representations independently of the resource(s) they represent, then we need to introduce

an additional relationship and

an additional resource type/class

Representation

Resource isRepresentedBy 1-n Representations

Representation isRepresentationOf 1-n Resources

Formats XHTML Collection format (XML? XHTML plus embedded RDF/RDF-A/microformats?) “ Core” attributes of Collection maybe just type = Collection, label/title? List of references to (i.e. URIs of) Objects which are members of this Collection Object format (XML? XHTML plus embedded RDF/RDF-A/microformats?) “ Core” attributes of Object maybe just type = Object/other-type (i.e. “Semantics”), label/title? Datastream(s) (or references to datastreams) List of references to (i.e. URIs of) other Objects from which this Object derived (i.e. Lineage) List of references to (i.e. URIs of) Objects which are components of this Object (i.e. Composition) ? List representations format Either list of content-types of Representations of this Resource? Or list of content-types of and references to (i.e. URIs of) Resources which are Representations of this Resource? Does this require a separate format? Can this be done with Status Code 300 Multiple Choices? Could this data be part of Collection format & Object format? (Other representation formats for representations!)

XHTML

Collection format (XML? XHTML plus embedded RDF/RDF-A/microformats?)

“ Core” attributes of Collection

maybe just type = Collection, label/title?

List of references to (i.e. URIs of) Objects which are members of this Collection

Object format (XML? XHTML plus embedded RDF/RDF-A/microformats?)

“ Core” attributes of Object

maybe just type = Object/other-type (i.e. “Semantics”), label/title?

Datastream(s) (or references to datastreams)

List of references to (i.e. URIs of) other Objects from which this Object derived (i.e. Lineage)

List of references to (i.e. URIs of) Objects which are components of this Object (i.e. Composition)

? List representations format

Either list of content-types of Representations of this Resource?

Or list of content-types of and references to (i.e. URIs of) Resources which are Representations of this Resource?

Does this require a separate format?

Can this be done with Status Code 300 Multiple Choices?

Could this data be part of Collection format & Object format?

(Other representation formats for representations!)

Methods: Collection i.e. add Object to Collection (Note: also adds a Representation of the Object) Object Format Post i.e. get Collection details (including list of Objects in Collection) i.e. get list of content-types (and URIs?) of Representations of Collection XHTML Collection Format ?List Representations Format Get

Methods: Object Aside: Could be extended to cover creation of e.g. Object isComponentOf Object relations i.e. add Representation of Object (Representation Format) ?Post i.e. get Object details (including list of component Objects, list of Objects from which derived) i.e. get list of content-types (and URIs?) of Representations of Object XHTML Object Format ?List Representations Format Get

Methods: Representation i.e. get Representation of Object (Representation Format) ?Get

Response Codes: Collection 201 Created 400 Bad Request 401 Unauthorized 415 Unsupported Media Type Object Format Post 200 OK 200 OK XHTML Collection Format ?List Representations Format Get

Response Codes: Object 201 Created 400 Bad Request 401 Unauthorized ?415 Unsupported Media Type (Representation Format) ?Post 200 OK 200 OK XHTML Object Format ?List Representations Format Get

Response Codes: Representation 200 OK (Representation Format) ?Get

Repository Functional Requirements Enable agent to Harvest a batch of Representations, one per Object GET Collection URI For each member Object GET Object URI (Accept: Content-Type=application/listrep+xml) GET Object URI (Accept: Content-Type=xyz) or ?GET Representation URI

Enable agent to

Harvest a batch of Representations, one per Object

GET Collection URI

For each member Object

GET Object URI (Accept: Content-Type=application/listrep+xml)

GET Object URI (Accept: Content-Type=xyz) or ?GET Representation URI

Repository Functional Requirements Enable agent to Reference An Object A Representation of an Object Object URI Representation URI

Enable agent to

Reference

An Object

A Representation of an Object

Repository Functional Requirements Enable agent to Obtain A list of the available Representations of an Object Representations of an Object GET Object URI (Accept: Content-Type=application/listrep+xml) GET Object URI (Accept: Content-Type=xyz) or ?GET Representation URI

Enable agent to

Obtain

A list of the available Representations of an Object

Representations of an Object

Repository Functional Requirements Enable agent to Put An Object (into a Repository) A Representation of an Object (into a Repository) POST instance of Object Format to Collection URI ?POST instance of (Representation Format) to Object URI

Enable agent to

Put

An Object (into a Repository)

A Representation of an Object (into a Repository)

Summary Focus on resources of interest Collections Objects ?Representations (or “Views”) Exchange representations of resources Relationships between resources expressed using (typed) links/URIs in representations Server provides URIs, client consumes/follows them Uniform interface GET and POST probably sufficient for core functions? PUT and DELETE may be required for updates, removals etc Implementers can add functionality around core functionality Repository-as-service “disappears from view”

Focus on resources of interest

Collections

Objects

?Representations (or “Views”)

Exchange representations of resources

Relationships between resources expressed using (typed) links/URIs in representations

Server provides URIs, client consumes/follows them

Uniform interface

GET and POST probably sufficient for core functions?

PUT and DELETE may be required for updates, removals etc

Implementers can add functionality around core functionality

Repository-as-service “disappears from view”

isMoreGenericThan/ isMoreSpecificThan Leigh D: Web Arch takes care of resource-representation relations Maybe instead of modelling Representations and Object Resource isRepresentedBy Representation, should consider “specificity” i.e. an Object may be associated with zero or more “more specific” other Objects See Berners-Lee, Design Issues: Generic Resources http://www.w3.org/DesignIssues/Generic Object Object isMoreGenericThan 0-n Objects Object isMoreSpecificThan 0-n Objects Allows for recursion Allows a “more specific” Object to participate in a hasComponent relationship hasComponent/ isComponentOf Collection Object hasMember/ isMemberOf 0…m 0…n hasDerivative/ isDerivativeOf 0…m 0…n 0…n 0…m Resources 0…n 0…m

Leigh D: Web Arch takes care of resource-representation relations

Maybe instead of modelling Representations and Object Resource isRepresentedBy Representation, should consider “specificity”

i.e. an Object may be associated with zero or more “more specific” other Objects

See Berners-Lee, Design Issues: Generic Resources

http://www.w3.org/DesignIssues/Generic

Object

Object isMoreGenericThan 0-n Objects

Object isMoreSpecificThan 0-n Objects

Allows for recursion

Allows a “more specific” Object to participate in a hasComponent relationship

A Resource-Oriented Approach to the Repository OAI ORE Technical Committee Meeting, New York City