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<section id='abstract'>
This primer provides an introduction to Linked Data Platform (LDP), including examples which explain the principal concepts, including the LDP resource, the contribution of the container and the associated affordances. Two sample scenarios show how an LDP client can interact with a LDP server in the context of read-write Linked Data application i.e. how to HTTP for accessing, updating, creating and deleting resources from servers that expose their resources as Linked Data.
</section>
<section id='sotd'>
<p>
This is the first draft of LDP Primer Note of W3C LDP WG.
</p>
</section>
<section id="intro-section">
<h1 id="intro">Introduction</h1>
<p>
Linked Data[[LINKED-DATA]] is a universal approach for handling data which has the idea of data entities and links between them, where the mechanisms and principles of the Web is used to give a data layer on top of applications are delivered, through the modification, processing, visualization and sharing of information.
</p>
<p>
To construct clients and servers that read and write Linked Data resources, LDP specifies standard HTTP and RDF techniques and best practices that you should use, and anti-patterns you should avoid, The Primer aims to provide introductory examples and guidance in the use of the LDP protocol. For a systematic account the reader should consult the normative LDP reference [[LDP]]. For an overview of the use cases for LDP and the elicited requirements that guided its design, the reader should consult the LDP Use Cases and Requirements [[LDP-UCR]] and for best practises and guidelines, the reader should consult the LDP LDP Best Practices and Guidelines document [[LDP-BP]].
</p>
<b id="conventions">Conventions Used in This Document</b>
<p>The examples in this guide are given as a serialization of RDF graphs using the Turtle [[TURTLE]] and JSON-LD [[JSON-LD]] syntaxes of RDF.</p>
<div class="syntaxmenu">
<p>The buttons below can be used to show or hide the available syntaxes.</p>
<form>
<p>
<input id="hide-ts" onclick="display('turtle', 'none'); set_display_by_id('hide-ts', 'none'); set_display_by_id('show-ts', ''); return false;" type="button" value="Hide Turtle Syntax" />
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<p>Commonly used namespace prefixes omitted from the Turtle serialisations:</p>
<pre style="word-wrap: break-word; white-space: pre-wrap;">
@prefix rdf: <http://www.w3.org/1999/02/22-rdf-syntax-ns#> .
@prefix rdfs: <http://www.w3.org/2000/01/rdf-schema#> .
@prefix owl: <http://www.w3.org/2002/07/owl#> .
@prefix ldp: <http://www.w3.org/ns/ldp#> .
@prefix xsd: <http://www.w3.org/2001/XMLSchema#> .
@prefix dcterms: <http://purl.org/dc/terms/> .
@prefix foaf: <http://xmlns.com/foaf/0.1/> .
@prefix wdrs: <http://www.w3.org/2007/05/powder-s#> .
@prefix bt: <http://example.org/vocab/bugtracker#> . </pre>
<p>The JSON-LD examples refer to the following (external) context document:</p>
<pre style="word-wrap: break-word; white-space: pre-wrap;">
{
"@context":
{
"rdf": "http://www.w3.org/1999/02/22-rdf-syntax-ns#",
"rdfs": "http://www.w3.org/2000/01/rdf-schema#",
"owl": "http://www.w3.org/2002/07/owl#",
"ldp": "http://www.w3.org/ns/ldp#",
"xsd": "http://www.w3.org/2001/XMLSchema#",
"dcterms": "http://purl.org/dc/terms/",
"foaf": "http://xmlns.com/foaf/0.1/",
"wdrs": "http://www.w3.org/2007/05/powder-s#",
"bt": "http://example.org/vocab/bugtracker#"
}
}
</pre>
<p>
</p>
<h2>LDP concepts in a glance</h2>
<p>
A server hosting Linked Data Platform Resources (LDPRs) may manage two kinds of LDPRs, those resources who whose state is represented using RDF called LDP RDF Sources (LDP-RS) and those using other formats called LDP Non-RDF Source (LDP-NR) such as html files, images, other binary files, etc. Resoures respond to retreival operations using HTTP GET. Often a description conveyed in the response document will describe a specific domain entity; Status, Friendship, Product, Order, Bug, etc. On the other hand, it might contain a description of a number of different concepts. The links contained in the descriptions lead to the subsequent discovery and processing of other resources. Affordances offered by the server make discoverable the forward paths in the application. Together the resources, links and associated affordances together specify what might be termed the API.
</p>
<p>
<img src="images/resources.png" />
</p>
<p>
The LDP protocols covers read and write interactions with Resources. Writable aspects include creation of new resources (using POST or PUT), updates (using PUT or PATCH), deletion of resources and importantly creation. Resource creation is an essential feature providing structured creation of resources. Affordances published by the server show that some Resources can be used to create other Resources. This common pattern is often seen in cases where one resource is be made up of a number of others, e.g. a Document Store consists of Documents, a Bug Tracker consists of Bugs, a Photo Album consists of Photos, a Networth of a person consists of Assets and Liabilities. This special kind of Resource is a Container (LDPC), and this is able to respond to requests to create new resources. During creation the created resource is appended to its Container and a containment link between the Container and the new entry is made.
</p>
<p>
Therefore a LDPC is a specialization of a LDP-RS representing a collection of links to LDPRs or information resources [[WEBARCH]] that responds to client requests for creation, modification, and/or enumeration of its linked members and documents. The simplest container is the Basic Container (LDP-BC). It defines the basic containment described using generic vocabularly. This can be used in a generic storage service to manage a containment hierarchy of arbitrary resources.
</p>
<figure id="fig-bc">
<img src="images/bc.png" alt=".." />
<figcaption>Generic document storage using a Basic Container.</figcaption>
</figure>
<p>
Such servers do not impose any restriction on LDPRs and generally act as storage systems without any domain specific application logic and vocabularies. The first scenarion in this document concerns a document storage system based on Basic Containers.
</p>
<p>
A Direct Container allows to use some domain specific vocabulary to relate the resources of the hierarchy. The additional assertion is called the membership property.
</p>
<figure id="fig-dc1">
<img src="images/dc1.png" alt=".." />
<figcaption>Using domain vocabularly with a Direct container.</figcaption>
</figure>
<p>
Direct Containers can also use domain specific vocabulary and link from a resource other than the Container resource to the new resource.
</p>
<figure id="fig-dc_photos">
<img src="images/dc_photos.png" alt=".." />
<figcaption>Membership triples with a non-Container subject.</figcaption>
</figure>
<p>
Different facets of a Resource can be managed using multiple Containers.
</p>
<figure id="fig-dc_bugs">
<img src="images/dc_bugs.png" alt=".." />
<figcaption>Managing multiple facets of a Bug with two Direct Containers.</figcaption>
</figure>
<p>
Existing applications with their own data model and business logic can expose their data using the LDP specification. These systems impose restrictions on LDPRs since the LDP interaction should be compliant with the underlying business logic and data model. The bug tracker example presented in the latter part of the primer is an example of an application specific LDP server.
</p>
<p class="note">Formal definitions of two terms LDPR and LDPC and other concepts introduced by LDP can be found in the 'Terminology' section of the Linked Data Platform 1.0 specification [[LDP]]</p>
<p>The following provide a set of examples to show the Linked Data Platform interactions. Note, this is a primer and should not be considered as a canonical example of ideal LDP modeling.</p>
</section>
<section id="photomanager">
<h1>Online document store example (LDP Basics)</h1>
<p>
This section provides a set of examples of using an online document store application. These examples will demonstrate the behaviour of both types of LDPRs and LDP Basic Containers. Registration with the online document store application by a user results in some data storage space (a root Basic Container) where web resources that are supported by Linked Data Platform can be stored. Using this root Basic Container a user can create new documents and also child containers to further organize her documents stored in this application.
</p>
<p>
APIs of web applications are commonly documented by listing valid operations which can operate on URLs, where the URLs are described as templates. A description of a LDP based document store is contained in the following table. We note with emphasis that it is important for servers to use links as the main mechanism to reveal the location of resources. If it would be necessary to encode such templates into client applications, this would be a strong indicator that the design breaches a number of good design principles. </p>
</p>
<table class="simple">
<thead>
<th>Path</th>
<th>Method</th>
<th>Description</th>
</thead>
<tbody>
<tr>
<td rowspan="5">/{username}/</td>
<td>GET</td>
<td>Lists all the documents in the root container.</td>
</tr>
<tr>
<td>POST</td>
<td>Create a new document under the root container.</td>
</tr>
<tr>
<td>PUT</td>
<td>Update the description and/or list of files of the root container.</td>
</tr>
<tr>
<td>PATCH</td>
<td>Update the description and/or list of files of the root container.</td>
</tr>
<tr>
<td>DELETE</td>
<td>Not allowed.</td>
</tr>
<tr>
<td class="col1" rowspan="5">
<div class='code'>/{username}/{{document}/}*</div>
</td>
<td>GET</td>
<td>Retrive the document.</td>
</tr>
<tr>
<td>POST</td>
<td>Discovered from the resource affordances.</td>
</tr>
<tr>
<td>PUT</td>
<td>Update the document.</td>
</tr>
<tr>
<td>PATCH</td>
<td>Partial update to the document if PATCH is supported.</td>
</tr>
<tr>
<td>DELETE</td>
<td>Delete the project description and associated bug reports.</td>
</tr>
<tr>
<td rowspan="2">
<div class='code'>/*/*</div>
</td>
<td>OPTIONS</td>
<td>Discover the allowed operations over a resource</td>
</tr>
<tr>
<td>HEAD</td>
<td>Only retrieve metainformation about a resource</td>
</tr>
</tbody>
</table>
<p>
In this example, we will see how Alice, a user of this system, does read / write management of documents using the LDP protocol. A typical interaction with the system would start with Alice registering as a user. It is likely that registration would be a LDP based interaction, but this aspect is out of scope of this example. A consequence of the registration is allocation of space for the storage of documents, and communication of this URL to the user, e.g. a basic container at http://data.example.org/alice/. This section describes a typical flow of interactions where Alice firsts reads the root document and discovers its affordances. This is followed by subsequent examples of creation, update and delete, and finishes with how the client is able to create nested structure from the containers.
</p>
<section id="filelookup">
<h2>Looking up a basic container (GET on an LDP-BC) </h2>
<p>First Alice looks up her storage by retrieving the LDP Basic Container assigned to her to hold her documents. Alice's LDP client does this by doing a GET request on the URI, http://data.example.org/alice/. </p>
<pre title="Request - basic container retreival" class='example' data-include='getbc.txt' data-oninclude='fixCode'></pre>
<p>As her document storage was just created, it is an empty container. </p>
<pre title="Response - basic container retreival" class='example' data-include='getbc_res.txt' data-oninclude='fixCode'></pre>
<p> As shown in the example, in addition to the RDF representation of the Basic Container using the requested media type the server provides an E-tag of the resource representation and Link headers advertising that the requested resource is indeed an LDP Basic Container and it will support the LDP interaction model. </p>
<p class="note">The Linked Data Platform 1.0 specification [[LDP]] says that all LDP servers MUST support Turtle media type for LDP-RS resources and SHOULD support JSON-LD media type.</p>
</section>
<section>
<h2> Discovering the affordances (OPTIONS on an LDP-BC) </h2>
<p>
Now, Alice wants to know what she can do in her document space. She can use the OPTIONS operation to learn of the permitted operations LDP-BC that she retrieved in the previous example.
</p>
<pre title="Request - retreiving OPTIONS of a basic container" class='example'>
OPTIONS /alice/ HTTP/1.1
Host: example.org
</pre>
<pre title="Response - retreiving OPTIONS of a basic container" class='example'>
HTTP/1.1 200 OK
Allow: OPTIONS,HEAD,GET,POST,PUT,PATCH
Accept-Post: text/turtle, application/ld+json, image/bmp, image/jpeg
Accept-Patch: example/patch
Link: <http://www.w3.org/ns/ldp#Resource>; rel='type'
Link: <http://www.w3.org/ns/ldp#BasicContainer>; rel='type'
Content-Length: 0
</pre>
<p>According to the response, HTTP operations {OPTIONS,HEAD, GET,POST,PUT,PATCH} are allowed on her root container. In addition to the allowed operations, Accept-Post and Accept-Patch provides which media types are supported by the respective operations. The rel="type" Link header advertises that this resource supports LDP protocol and it is an LDP Basic Container.</p>
<p>In this case, the response tells Alice's LDP client that this is an LDP-Basic Container and the container allows her to POST things of both RDF types (text/turtle, application/ld+json) and images (image/bmp and image/jpeg).</p>
</section>
<section>
<h2> Creating a RDF resource (POST an RDF resource to an LDP-BC) </h2>
<p>
Alice can uplaod a social profile document to her store, by POSTing her FOAF personal profile document to her LDP-BC at the root of her document store. Note, the Slug header offers the server a hint about URL of the resource to be created.
</p>
<pre title="Request - creating a RDF resource" class='example' data-include='bccreate.txt' data-oninclude='fixCode'></pre>
</pre>
<pre title="Response - creating a RDF resource" class="example">
HTTP/1.1 201 Created
Location: /alice/foaf
Link: <http://www.w3.org/ns/ldp#Resource>; rel='type'
Content-Length: 0
</pre>
<p> The response to the create request provides a Link to the newly created resource using the Location header. In this case, the server has honored the hint provided by the Slug header and created the new resource in the URL http://data.example.org </p>
<p>Knowing the URL of the newly created resource, Alice can check the container again to confirm that the container correctly contains the newly created resource.</p>
<pre title="Request - basic container retreival after resource created" class="example">
GET /alice/ HTTP/1.1
Host: example.org
Accept: text/turtle
</pre>
<pre title="Response - basic container retreival after resource created" class='example' data-include='bcget_res.txt' data-oninclude='fixCode'/>
<p>
The ldp:contains containment triple discloses the newly created resource in the response.
</p>
</section>
<section>
<h2> Creating a non-RDF (binary) resource (POST an image to an LDP-BC) </h2>
<p>Next, Alice wants to upload a photo of her to the document storage. She can create an image by POSTing it in the same way she created the RDF document.</p>
<pre title="Request - creating a non-RDF resource" class="example">
POST /alice/ HTTP/1.1
Host: data.example.org
Slug: avatar
Link: <http://www.w3.org/ns/ldp#Resource>; rel="type"
Content-Type: image/png
Content- Length: nnn
### binary data ###
</pre>
<pre title="Response - creating a non-RDF resource" class="example">
HTTP/1.1 201 Created
Location: /alice/avatar
Link: <http://www.w3.org/ns/ldp#Resource>; rel="type"
Link: <http://data.example.org/alice/avatar/meta>; rel="describedby"
Content-Length: 0
</pre>
<p> </p>
<p>The outcome of creating a non-RDF is similar to creating a RDF resource. If successful, the server will return a 201 success code with a Location header that points to the created resource. Furthermore, in the case of binary resources the server may create an additional file to maintain the metadata about the binary file. In the above example, the server creates a new LDP-RS to maintain metadata about the binary resource such as creation date, owner, etc. and this metadata resource is advised using a Link header with the relation "describedby". </p>
<p> Similar to creating a RDF resource (LDP-RS), a containment triple will be added to the container when a non-RDF (LDP-NR) is created. Thus, the representation of the LDP container after creating the image looks like the following. </p>
<pre title="Container representation after the non-RDF resource creation" class='example' data-include='bc_after_bin.txt' data-oninclude='fixCode'></pre>
</section>
<section>
<h2> Update a RDF LDP resource (PUT on an LDP-RS) </h2>
<!--
I MISSED THESE BITS OUT ... (??)
<pre title="A request for retrieving a RDF resource" class="example" data-include='getfoaf.txt' data-oninclude='fixCode'/>
<pre title="The response of retrieving a RDF resource" class='example' data-include='getfoaf_res.txt' data-oninclude='fixCode'/>
-->
<p>After creating the image as shown in the previous example, Alice now wants to update her FOAF profile with a link to the image. After retrieving her FOAF profile using a HTTP GET operation, she uses HTTP PUT to update the document by amending the RDF with a link to her photo.</p>
<p> In this example, Alice's LDP client send the E-tag of the resource representation that it retrieved previously to prevent any lost update problems. </p>
<pre title="Request - updating a RDF resource" class="example" data-include='foafupdate.txt' data-oninclude='fixCode'></pre>
<pre title="Response - updating a RDF resource" class="example">
HTTP/1.1 204 No Content
Link: <http://www.w3.org/ns/ldp#Resource>; rel="type"
ETag: W/"123454322"
</pre>
<p>If the operation is successful, the document will be updated with new information.</p>
<p class="note"> Alice can also use PATCH operation to update the resource.</p>
</section>
<section>
<h2> Deleting a resource (DELETE on an LDPR) </h2>
<p>If Alice decides to delete the image, she can do that with a delete operation.</p>
<pre title="Request - deleting a RDF resource" class="example">
DELETE /alice/avatar HTTP/1.1
Host: data.example.org
If-Match: W/"123454322"
</pre>
<pre title="Response - deleting a RDF resource" class="example">
HTTP/1.1 204 No Content
Link: <http://www.w3.org/ns/ldp#Resource>; rel="type"
ETag: W/"123454322"
</pre>
<p> Once the document is deleted, the containment triple will be removed from the container. For example, after the deleting the resource the container representation will be as the following. </p>
<pre title="Container representation after resource deletion" class="example" data-include='bc_after_del.txt' data-oninclude='fixCode'></pre>
<p> For any subsequent request on the resource after the deletion, the server will respond with the appropriate HTTP response code. </p>
<pre title="Request - after deletion" class="example">
GET /alice/avatar HTTP/1.1
Host: example.org
Accept: image/png
</pre>
<pre title="Response - after deletion" class="example">
HTTP/1.1 410 Gone
</pre>
</section>
<section>
<h3 id="meta-structure">Structural Manipulation (Child Containers)</h3>
<p>In order for the client to introduce hierachy to the management of documents, the document store allows creation of documents which are containers. That enables Alice can to create a container hierarchy to organise her documents. This can be done by POSTing (a child) container representation to a (parent) container. This enables Alice to create a child container which she intends to use for image storage.
</p>
<pre title="State of Alice's document store before creating the photo (child) container" class='example' data-include='create_cr_s1.txt' data-oninclude='fixCode'></pre>
<p>To create a new container for managing photos, Alice POSTs a representation of a container (LDP-BC) to the root container. Alice express her intention that the newly created resource should be an LDP Basic Container by including a Link header in the request with the relationship 'type'. </p>
<pre title="Request - creating a new container" class='example' data-include='create_cr_req.txt' data-oninclude='fixCode'></pre>
<p>If the create is successful, the server responds with location of the newly created container for the photos.</p>
<pre title="Response - creating the new container" class="example">
HTTP/1.1 201 Created
Location: /alice/photos/
Content-Length: 0
</pre>
<p>After creation of this new container, the parent container will look like</p>
<pre title="State of Alice's document store after creating the photo (child) container" class='example' data-include='create_cr_s2.txt' data-oninclude='fixCode'></pre>
<p>and the photo container will look like the following.</p>
<pre title="State of Alice's newly created photo container" class='example' data-include='create_cr_s3.txt' data-oninclude='fixCode'></pre>
</section>
</section>
<section id="bugtracker">
<h1>Bug Tracker Example (LDP Direct containers)</h1>
<p>The previous section provided practical examples of basic LDP interactions using LDP Basic Containers. One of the limitations of LDP Basic Containers is that fixed LDP vocabulary is used to assert the relations between a container and its contained resource. However, some scenarios require domain specific vocabulary to be used to list the members of a container. For example, an application which already had its own vocabulary would like continue using the same vocabulary when using LDP protocol. LDP Direct containers allow the domain-specific vocabulary to be used when listing members in a container. This is done using the ldp:hasMemberRelation or ldp:isMemberOfRelation predicate of the Direct Containers which will be explained in the following examples. </p>
<p>In addition, in some scenarios it is necessary to add relationships between the newly created resource and some other resource (which is not necessarily the container). This allows to define relationship between any other information resource or non-information resource (real world thing). This will be done using ldp:membershipResource predicate of the Direct Container which will be explained in the following example. </p>
<p class="note">For more information on information resources (documents) vs real world entities (things) separation please refer to <a href="http://www.w3.org/TR/webarch/#id-resources">Web Arch (Section 2.2. URI/Resource Relationships) </a>, , <a href="http://www.w3.org/TR/cooluris/#semweb">Cool URIs (Section 3. URIs for Real-World Objects)</a>, <a href="http://www.w3.org/TR/urls-in-data/#landing-pages">URLs in Data (Section 3. Landing Pages and Records)</a>.</p>
<p>
The examples in this section will revolve around a very simple Bug Tracker application. Bug Tracker application records the bugs of several products allowing reporting, updating and deleting bugs and products. In contrast to the online document store example, the bug tracker wants to use a simple domain vocabulary, e.g. has_bug, to express membership relationships in the containers. LDP provides the additional interaction capability in the protocol to add the domain specific triples based on the properties defined in the LDP Direct Container.
</p>
<p>A RESTful API for a simple Bug Tracker system might be described as follows.</p>
<table class="simple">
<thead>
<th>Path</th>
<th>Method</th>
<th>Description</th>
</thead>
<tbody>
<!--tr>
<td rowspan="5">/tracker/</td>
<td>GET</td>
<td>Lists all the product descriptions.</td>
</tr>
<tr>
<td>POST</td>
<td>Create a new product description.</td>
</tr>
<tr>
<td>PUT</td>
<td>Update the app description and/or list of product descriptions</td>
</tr>
<tr>
<td>PATCH</td>
<td>Update the app description and/or list of product descriptions</td>
</tr>
<tr>
<td>DELETE</td>
<td>Not allowed.</td>
</tr-->
<tr>
<td class="col1" rowspan="5">
<div class='code'>/tracker/{product-id}/</div>
</td>
<td>GET</td>
<td>Lists the bug reports associated with a product.</td>
</tr>
<tr>
<td>POST</td>
<td>Create a new bug report associated with a product.</td>
</tr>
<tr>
<td>PUT</td>
<td>Update the project description.</td>
</tr>
<tr>
<td>PATCH</td>
<td>Not supported.</td>
</tr>
<tr>
<td>DELETE</td>
<td>Delete the project description and associated bug reports.</td>
</tr>
<tr>
<td rowspan="5">
<div class='code'>/tracker/{product-id}/{bug-id}</div>
</td>
<td>GET</td>
<td>Gets the bug report.</td>
</tr>
<tr>
<td>POST</td>
<td>Not supported.</td>
</tr>
<tr>
<td>PUT</td>
<td>Update the bug report.</td>
</tr>
<tr>
<td>PATCH</td>
<td>Not supported.</td>
</tr>
<tr>
<td>DELETE</td>
<td>Delete the bug report.</td>
</tr>
<tr>
<td rowspan="2">
<div class='code'>/*/*</div>
</td>
<td>OPTIONS</td>
<td>Discover the allowed operations over a resource</td>
</tr>
<tr>
<td>HEAD</td>
<td>Only retrieve meta information about a resource</td>
</tr>
</tbody>
</table>
<p> In the examples in this section, we will only focus on the container representation, creation and deletion of resources because that is where the Basic Containers, Direct Containers, and Indirect Containers have their differences. </p>
<section id="navandret">
<h2>Navigation and Retreival</h2>
<p>One of the main use cases of the example bug tracker is to list of the bugs of a given product. Assuming that a user got a URL of a product by out of band means, she can look it up to get more information including the bugs associated with it. To get the description of the product, a user (or her LDP client) can do a GET request on the URI of the known product resource. LDPR servers must provide text/turtle representations of the requested LDPRs and may provide RDF format representations such as JSON-LD or RDF/XML using standard HTTP content negotiation.</p>
<pre class="example" title="Request - Product Lookup" data-include='product_lookup_req.txt' data-oninclude='fixCode'></pre>
<p>If the product resource is available, the server responds with the RDF representation of the Direct Container that corresponds to the given product using the requested media type,
<code>text/turtle</code> or <code>application/ld+json</code> in this case.</p>
<pre title="Response - Product Lookup" class='example' data-include='product_lookup_resp.txt' data-oninclude='fixCode'></pre>
<p>The representation of the product contains both information about the product such as the title and the information about members of the product LDPC, i.e. the bugs associated with the product. </p>
<p>As you can see from the Link Header that is returned and the RDF representation of the container, this example uses an LDP Direct Container allowing the application to use domain specific vocabulary in the container and allowing it to create relationships to resources other than the container itself upon resource creation. For example, it manages the member relationship, (<?productURI>, bt:hasBug, <?bugURI>), using the application-specific vocabulary term, bt:hasBug. This is done by defining the ldp:hasMemberRelation predicate of the Direct Container to bt:hasBug (<?directContainerURI>, ldp:hasMemberRelation, bt:hasbug). </p>
<p class="note"> In this example, we have shown the Direct Containers with the membership triple pattern ( membership-constant-URI , membership-predicate , member-derived-URI ) using ldp:hasMemberRelation where the constant membership is resource is in the subject of the triple and the newly created resources will be added as the object of the triple. It is also possible for the Direct Container to have the membership triple pattern ( member-derived-URI , membership-predicate , membership-constant-URI ) using ldp:isMemberOfRelation predicate where the constant member resource will be the object of the triple and the newly created resource will be added as the subject of the triple. </p>
<p> In addition, in this example the bugs are associated with the product (a non-information resource with a # URI) and not with the Direct Container itself. This is done by defining the ldp:membershipResource predicate of the LDP Direct Container to the product non-information resource URI (<?directContainerURI>, ldp:membershipResource, </tracker/product1/#it>). By doing so one can define the subject of the membership triple any resource of interest.
</p>
<p>The next example illustrates the behaviour of LDP Direct containers when new resources are created and how these predicates of the Direct Containers affect the interaction model of LDP.</p>
<!-- <p>Looking up a bug is similar to looking up a product. Based on links in the representation of the Product, the client uses GET to navigate to a known Bug resource.</p>
<pre class="example" title="Bug lookup request"
data-include='bug_look_up_req.txt' data-oninclude='fixCode'></pre>
<p>The server responds with the representation of the bug.</p>
<pre title="Bug lookup response"
class='example' data-include='bug_look_up_resp.txt'
data-oninclude='fixCode'></pre -->
</section>
<section>
<h3 id="BugCreate">Creation</h3>
<p>Continuing from the previous example, we can report a Bug against "LDP Demo" product by creating a Bug report (an LDPR representing the bug) under the "LDP Demo" product description LDPC by posting RDF representation of the Bug report to the LDPC associated with the product description. </p>
<pre title="A request for creating a bug" class='example' data-include='bug_create_req.txt' data-oninclude='fixCode'></pre>
<p>If the creation is successful, the server responds with location of the newly created resource.</p>
<pre title="A response of creating new a bug" class='example' data-oninclude='fixCode'>
HTTP/1.1 201 Created
Location: /tracker/ldp-demo/67
Content-Length: 0
</pre>
<p>If the creation fails, the server will respond with an appropriate status code depending on the error. If successful, the LDP Demo product description LDPC will have the following representation after the creation of new resource.</p>
<pre title="The state of the product LDPC after the bug creation" class='example' data-include='bug_create_s1.txt' data-oninclude='fixCode'></pre>
<p> As you can see two new triples are added to the container. That is (</tracker/ldp-demo/>, <ldp:contains>, </tracker/ldp-demo/bug67>) and
(</tracker/ldp-demo/#it>, <bt:hasbug>, </tracker/ldp-demo/bug67>). The former is added in any type of container and the latter is defined by the direct
container properties. </p>
<p>And the created Bug resource will have the following representation. Note that server has added a server managed property, creation date (dcterms:created), and a default value for the state (bt:isInState) to the Bug in addition to what was being POSTed.</p>
<pre title="The state of the bug LDPR" class='example' data-include='bug_create_s2.txt' data-oninclude='fixCode'></pre>
</section>
<section>
<h3 id="BugDelete">Deletion</h3>
This example illustrates the behaviour of Direct Container when a resource is deleted.
<pre class="example">
DELETE /tracker/ldp-demo/bug3 HTTP/1.1
Host: example.org
If-Match: W/"123454322"
</pre>
<p>If the delete is successful, the server will respond with a success status code.</p>
<pre class="example">
HTTP/1.1 204 No Content
ETag: W/"123454322"
</pre>
<p> After the deletion, the representation of the container will look like the following</p>
<pre title="The state of the product LDPC after the bug deletion" class='example' data-include='bug_delete_s1.txt' data-oninclude='fixCode'></pre>
<p> As seen from the LDP Direct Container representation above both containment triple (</tracker/ldp-demo/>, ldp:contains, </tracker/ldp-demo/bug3>) and the membership triple (</tracker/ldp-demo/#it>, bt:hasBug, </tracker/ldp-demo/bug3>) are removed from the container representation. </p>
</section>
</section>
<section id="bugtrackerextd">
<h1>Extended Bug Tracker Example (LDP Indirect containers)</h1>
<p> In this example, we will use the same scenario as the previous example but using an LDP Indirect Container to show what is the use of Indirect containers and
how to use them. In the previous example, the object of the membership triple (in the case of using ldp:hasMemberRelation) or the subject (in the case of using ldp:isMemberOfRelation) was always the URL of the newly created information resource. LDP Indirect containers allow this to be any URI that is declared inside the resource to be created. This done by defining the predicate to look for in the representation of the resource to be created using the ldp:insertedContentRelation predicate of the LDP Indirect Container as explained in the following examples. </p>
<section id="navandretext">
<h2>Navigation and Retreival</h2>
<p> Similar to the previous example, first we will retrieve the representation of the LDP Indirect Container.</p>
<pre class="example" title="Product lookup request" data-include='product_lookup_req.txt' data-oninclude='fixCode'></pre>
<p>If the product resource is available, the server responds with the representation of the resource using the requested media type,
<code>text/turtle</code> or <code>application/ld+json</code> in this case.</p>
<pre title="HTTP response for product lookup" class='example' data-include='ic_product_lookup_resp.txt' data-oninclude='fixCode'></pre>
<p> Now the product container is an LDP Indirect container and we can notice one main difference. The container has an additional predicate called "ldp:insertedContentRelation". Further, we can notice that the object of the containment triples and the membership triples are not the same. While the object of the containment triple is the same (e.g. </tracker/ldp-demo/bug3>, an information resource) the object of the membership triple is now (e.g. </tracker/ldp-demo/bug3#it>, a non-information resource or real world thing). This distinction is because of the ldp:insertedContentRelation definition. How this works will be explained in the next example on creating a new resource. </p>
</section>
<!-- end navandretext -->
<section id="creationext">
<h3 id="IndirectCreate">Creation</h3>
<p>Continuing from the previous example, we can report a Bug Report against 'LDP demo' product by creating a Bug Report LDPR under the 'LDP Demo' product description LDPC.</p>
<p>The client POSTs a representation of a Bug to the Bug Tracker LDPC.</p>
<pre title="A request for creating a bug" class='example' data-include='ic_bug_create_req.txt' data-oninclude='fixCode'></pre>
<p> One thing to note is that the representation of the resource to be created contain a triple (< >, foaf:primaryTopic , <#it>). If the create is successful, the server responds with location of the newly created resource.</p>
<pre title="A response of creating new a bug" class='example' data-oninclude='fixCode'>
HTTP/1.1 201 Created
Location: /tracker/ldp-demo/67
Content-Length: 0
</pre>
<p>If the creation fails, the server will respond with an appropriate status code depending on the error. After the resource is creation, the Product A LDPC will have the following representation.</p>
<pre title="The state of the product LDPC after the bug creation" class='example' data-include='ic_bug_create_s1.txt' data-oninclude='fixCode'></pre>
<p> As you can see two new triples are added to the container. That is (</app/product1>, <ldp:contains>, </tracker/ldp-demo/bug67>) and
(</tracker/ldp-demo/#it>, <bt:hasbug>, </tracker/ldp-demo/bug67#it>). </p>
</section>
<!-- end creationext -->
<!-- section id="deleteext">
<h3 id="IndirectDelete">Deletion</h3>
<pre class="example">
DELETE /tracker/ldp-demo/bug3 HTTP/1.1
Host: example.org
</pre>
<p>If the update is successful, the server will respond with a success status and a new etag.</p>
<pre class="example">
HTTP/1.1 204 No Content
ETag: W/"123456790"
</pre>
<p> After the deletion, the representation of the container will look like the following (Show that the two triples are gone) </p>
</section -->
<!-- end deleteext -->
</section>
<section>
<h1>Security</h1>
<p>It is not the focus of the Linked Data Platform WG to provide security mechanisms for read/write Linked Data applications. Though most of the security mechanisms that are applicable to general web applications are equally applicable to Linked Data applications, there is still some space to build security mechanisms specific to Linked Data applications by leverage the Linked Data technologies and providing concrete security requirements of Linked Data applications. In this context, LDP WG has started to create a WG note on Access Control which aims to produce use cases for security scenarios of LDP applications that can be used as the input to later initiative that will be focused on developing standard security mechanisms for LDP applications.</p>
</section>
<section>
<h1 id="ldpc">LDP Implementations</h1>
A list of implementations that plan to be LDP compliant is available in the LDP Implementations wiki page. LDP Implementation report provides the coverage of the specification by each LDP implementation.
</section>
<section>
<h1 id="next">What To Read Next</h1>
The primer only provide an overview of the Linked Data Platform specifications. LDP WG has produced following documents that contribute to the Linked Data Platform specification.
<ul>
<li><a href="https://dvcs.w3.org/hg/ldpwg/raw-file/default/TR/ldp-ucr.html">Linked Data Platform Use Cases and Requirements</a> [[LDP-UCR]]</li>
<li><a href="https://dvcs.w3.org/hg/ldpwg/raw-file/default/ldp.html">Linked Data Platform 1.0 specifcation</a> [[LDP]]</li>
<li>Linked Data Platform 1.0 Primer (This document)</li>
<li><a href="https://dvcs.w3.org/hg/ldpwg/raw-file/default/ldp-bp/ldp-bp.html">LDP Best Practices and Guidelines</a> [[LDP-BP]]</li>
<li><a href="https://dvcs.w3.org/hg/ldpwg/raw-file/default/Test%20Cases/LDP%20Test%20Cases.html">Linked Data Platform 1.0 Test Cases</a>[[LDP-TESTS]]</li>
</ul>
</section>
<section class='appendix informative' id="history">
<h1>Change History</h1>
<p>The change history is up to the editors to insert a brief summary of changes, ordered by most recent changes first and with heading from which public draft it has been changed from.
</p>
<ul>
<li>2013-08-05 - Providing JSON-LD representations of the examples.</li>
<li>2013-07-03 - Moving the content from the wiki to the note.</li>
</ul>
</section>
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title: "LDP Best Practices and Guidelines",
href: "https://dvcs.w3.org/hg/ldpwg/raw-file/tip/ldp-bp/ldp-bp.html",
authors: [
"Cody Burleson",
"Nandana Mihindukulasooriya"
],
status: "WD",
deliveredBy: [
"http://www.w3.org/2012/ldp/"
],
publisher: "W3C"
},
"LDP-TESTS": {
title: "Linked Data Platform 1.0 Test Cases",
href: "https://dvcs.w3.org/hg/ldpwg/raw-file/tip/Test%20Cases/LDP%20Test%20Cases.html",
authors: [
"Raúl García-Castro"
],
status: "WD",
deliveredBy: [
"http://www.w3.org/2012/ldp/"
],
publisher: "W3C"
}
}
};
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//
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</head>
<body onLoad="primerOnLoad()">
<section id='abstract'>
This primer provides an introduction to Linked Data Platform (LDP), including examples which explain the principal concepts, including the LDP resource, the contribution of the container and the associated affordances. Two sample scenarios show how an LDP client can interact with a LDP server in the context of read-write Linked Data application i.e. how to HTTP for accessing, updating, creating and deleting resources from servers that expose their resources as Linked Data.
</section>
<section id='sotd'>
<p>
This is the first draft of LDP Primer Note of W3C LDP WG.
</p>
</section>
<section id="intro-section">
<h1 id="intro">Introduction</h1>
<p>
Linked Data[[LINKED-DATA]] is a universal approach for handling data which has the idea of data entities and links between them, where the mechanisms and principles of the Web is used to give a data layer on top of applications are delivered, through the modification, processing, visualization and sharing of information.
</p>
<p>
To construct clients and servers that read and write Linked Data resources, LDP specifies standard HTTP and RDF techniques and best practices that you should use, and anti-patterns you should avoid, The Primer aims to provide introductory examples and guidance in the use of the LDP protocol. For a systematic account the reader should consult the normative LDP reference [[LDP]]. For an overview of the use cases for LDP and the elicited requirements that guided its design, the reader should consult the LDP Use Cases and Requirements [[LDP-UCR]] and for best practises and guidelines, the reader should consult the LDP LDP Best Practices and Guidelines document [[LDP-BP]].
</p>
<b id="conventions">Conventions Used in This Document</b>
<p>The examples in this guide are given as a serialization of RDF graphs using the Turtle [[TURTLE]] and JSON-LD [[JSON-LD]] syntaxes of RDF.</p>
<div class="syntaxmenu">
<p>The buttons below can be used to show or hide the available syntaxes.</p>
<form>
<p>
<input id="hide-ts" onclick="display('turtle', 'none'); set_display_by_id('hide-ts', 'none'); set_display_by_id('show-ts', ''); return false;" type="button" value="Hide Turtle Syntax" />
<input id="show-ts" onclick="display('turtle', ''); set_display_by_id('hide-ts', ''); set_display_by_id('show-ts', 'none'); return false;" style="display:none" type="button" value="Show Turtle Syntax" />
<input id="hide-js" onclick="display('jsonld','none'); set_display_by_id('hide-js', 'none'); set_display_by_id('show-js', ''); return false;" type="button" value="Hide JSON-LD Syntax" />
<input id="show-js" onclick="display('jsonld',''); set_display_by_id('hide-js', ''); set_display_by_id('show-js', 'none'); return false;" style="display:none" type="button" value="Show JSON-lD Syntax" />
</p>
</form>
</div>
<p>Commonly used namespace prefixes omitted from the Turtle serialisations:</p>
<pre style="word-wrap: break-word; white-space: pre-wrap;">
@prefix rdf: <http://www.w3.org/1999/02/22-rdf-syntax-ns#> .
@prefix rdfs: <http://www.w3.org/2000/01/rdf-schema#> .
@prefix owl: <http://www.w3.org/2002/07/owl#> .
@prefix ldp: <http://www.w3.org/ns/ldp#> .
@prefix xsd: <http://www.w3.org/2001/XMLSchema#> .
@prefix dcterms: <http://purl.org/dc/terms/> .
@prefix foaf: <http://xmlns.com/foaf/0.1/> .
@prefix wdrs: <http://www.w3.org/2007/05/powder-s#> .
@prefix bt: <http://example.org/vocab/bugtracker#> . </pre>
<p>The JSON-LD examples refer to the following (external) context document:</p>
<pre style="word-wrap: break-word; white-space: pre-wrap;">
{
"@context":
{
"rdf": "http://www.w3.org/1999/02/22-rdf-syntax-ns#",
"rdfs": "http://www.w3.org/2000/01/rdf-schema#",
"owl": "http://www.w3.org/2002/07/owl#",
"ldp": "http://www.w3.org/ns/ldp#",
"xsd": "http://www.w3.org/2001/XMLSchema#",
"dcterms": "http://purl.org/dc/terms/",
"foaf": "http://xmlns.com/foaf/0.1/",
"wdrs": "http://www.w3.org/2007/05/powder-s#",
"bt": "http://example.org/vocab/bugtracker#"
}
}
</pre>
<p>
</p>
<h2>LDP concepts in a glance</h2>
<p>
A server hosting Linked Data Platform Resources (LDPRs) may manage two kinds of LDPRs, those resources who whose state is represented using RDF called LDP RDF Sources (LDP-RS) and those using other formats called LDP Non-RDF Source (LDP-NR) such as html files, images, other binary files, etc. Resoures respond to retreival operations using HTTP GET. Often a description conveyed in the response document will describe a specific domain entity; Status, Friendship, Product, Order, Bug, etc. On the other hand, it might contain a description of a number of different concepts. The links contained in the descriptions lead to the subsequent discovery and processing of other resources. Affordances offered by the server make discoverable the forward paths in the application. Together the resources, links and associated affordances together specify what might be termed the API.
</p>
<p>
<img src="images/resources.png" />
</p>
<p>
The LDP protocols covers read and write interactions with Resources. Writable aspects include creation of new resources (using POST or PUT), updates (using PUT or PATCH), deletion of resources and importantly creation. Resource creation is an essential feature providing structured creation of resources. Affordances published by the server show that some Resources can be used to create other Resources. This common pattern is often seen in cases where one resource is be made up of a number of others, e.g. a Document Store consists of Documents, a Bug Tracker consists of Bugs, a Photo Album consists of Photos, a Networth of a person consists of Assets and Liabilities. This special kind of Resource is a Container (LDPC), and this is able to respond to requests to create new resources. During creation the created resource is appended to its Container and a containment link between the Container and the new entry is made.
</p>
<p>
Therefore a LDPC is a specialization of a LDP-RS representing a collection of links to LDPRs or information resources [[WEBARCH]] that responds to client requests for creation, modification, and/or enumeration of its linked members and documents. The simplest container is the Basic Container (LDP-BC). It defines the basic containment described using generic vocabularly. This can be used in a generic storage service to manage a containment hierarchy of arbitrary resources.
</p>
<figure id="fig-bc">
<img src="images/bc.png" alt=".." />
<figcaption>Generic document storage using a Basic Container.</figcaption>
</figure>
<p>
Such servers do not impose any restriction on LDPRs and generally act as storage systems without any domain specific application logic and vocabularies. The first scenarion in this document concerns a document storage system based on Basic Containers.
</p>
<p>
A Direct Container allows to use some domain specific vocabulary to relate the resources of the hierarchy. The additional assertion is called the membership property.
</p>
<figure id="fig-dc1">
<img src="images/dc1.png" alt=".." />
<figcaption>Using domain vocabularly with a Direct container.</figcaption>
</figure>
<p>
Direct Containers can also use domain specific vocabulary and link from a resource other than the Container resource to the new resource.
</p>
<figure id="fig-dc_photos">
<img src="images/dc_photos.png" alt=".." />
<figcaption>Membership triples with a non-Container subject.</figcaption>
</figure>
<p>
Different facets of a Resource can be managed using multiple Containers.
</p>
<figure id="fig-dc_bugs">
<img src="images/dc_bugs.png" alt=".." />
<figcaption>Managing multiple facets of a Bug with two Direct Containers.</figcaption>
</figure>
<p>
Existing applications with their own data model and business logic can expose their data using the LDP specification. These systems impose restrictions on LDPRs since the LDP interaction should be compliant with the underlying business logic and data model. The bug tracker example presented in the latter part of the primer is an example of an application specific LDP server.
</p>
<p class="note">Formal definitions of two terms LDPR and LDPC and other concepts introduced by LDP can be found in the 'Terminology' section of the Linked Data Platform 1.0 specification [[LDP]]</p>
<p>The following provide a set of examples to show the Linked Data Platform interactions. Note, this is a primer and should not be considered as a canonical example of ideal LDP modeling.</p>
</section>
<section id="photomanager">
<h1>Online document store example (LDP Basics)</h1>
<p>
This section provides a set of examples of using an online document store application. These examples will demonstrate the behaviour of both types of LDPRs and LDP Basic Containers. Registration with the online document store application by a user results in some data storage space (a root Basic Container) where web resources that are supported by Linked Data Platform can be stored. Using this root Basic Container a user can create new documents and also child containers to further organize her documents stored in this application.
</p>
<p>
APIs of web applications are commonly documented by listing valid operations which can operate on URLs, where the URLs are described as templates. A description of a LDP based document store is contained in the following table. We note with emphasis that it is important for servers to use links as the main mechanism to reveal the location of resources. If it would be necessary to encode such templates into client applications, this would be a strong indicator that the design breaches a number of good design principles. </p>
</p>
<table class="simple">
<thead>
<th>Path</th>
<th>Method</th>
<th>Description</th>
</thead>
<tbody>
<tr>
<td rowspan="5">/{username}/</td>
<td>GET</td>
<td>Lists all the documents in the root container.</td>
</tr>
<tr>
<td>POST</td>
<td>Create a new document under the root container.</td>
</tr>
<tr>
<td>PUT</td>
<td>Update the description and/or list of files of the root container.</td>
</tr>
<tr>
<td>PATCH</td>
<td>Update the description and/or list of files of the root container.</td>
</tr>
<tr>
<td>DELETE</td>
<td>Not allowed.</td>
</tr>
<tr>
<td class="col1" rowspan="5">
<div class='code'>/{username}/{{document}/}*</div>
</td>
<td>GET</td>
<td>Retrive the document.</td>
</tr>
<tr>
<td>POST</td>
<td>Discovered from the resource affordances.</td>
</tr>
<tr>
<td>PUT</td>
<td>Update the document.</td>
</tr>
<tr>
<td>PATCH</td>
<td>Partial update to the document if PATCH is supported.</td>
</tr>
<tr>
<td>DELETE</td>
<td>Delete the project description and associated bug reports.</td>
</tr>
<tr>
<td rowspan="2">
<div class='code'>/*/*</div>
</td>
<td>OPTIONS</td>
<td>Discover the allowed operations over a resource</td>
</tr>
<tr>
<td>HEAD</td>
<td>Only retrieve metainformation about a resource</td>
</tr>
</tbody>
</table>
<p>
In this example, we will see how Alice, a user of this system, does read / write management of documents using the LDP protocol. A typical interaction with the system would start with Alice registering as a user. It is likely that registration would be a LDP based interaction, but this aspect is out of scope of this example. A consequence of the registration is allocation of space for the storage of documents, and communication of this URL to the user, e.g. a basic container at http://data.example.org/alice/. This section describes a typical flow of interactions where Alice firsts reads the root document and discovers its affordances. This is followed by subsequent examples of creation, update and delete, and finishes with how the client is able to create nested structure from the containers.
</p>
<section id="filelookup">
<h2>Looking up a basic container (GET on an LDP-BC) </h2>
<p>First Alice looks up her storage by retrieving the LDP Basic Container assigned to her to hold her documents. Alice's LDP client does this by doing a GET request on the URI, http://data.example.org/alice/. </p>
<pre title="Request - basic container retreival" class='example' data-include='getbc.txt' data-oninclude='fixCode'></pre>
<p>As her document storage was just created, it is an empty container. </p>
<pre title="Response - basic container retreival" class='example' data-include='getbc_res.txt' data-oninclude='fixCode'></pre>
<p> As shown in the example, in addition to the RDF representation of the Basic Container using the requested media type the server provides an E-tag of the resource representation and Link headers advertising that the requested resource is indeed an LDP Basic Container and it will support the LDP interaction model. </p>
<p class="note">The Linked Data Platform 1.0 specification [[LDP]] says that all LDP servers MUST support Turtle media type for LDP-RS resources and SHOULD support JSON-LD media type.</p>
</section>
<section>
<h2> Discovering the affordances (OPTIONS on an LDP-BC) </h2>
<p>
Now, Alice wants to know what she can do in her document space. She can use the OPTIONS operation to learn of the permitted operations LDP-BC that she retrieved in the previous example.
</p>
<pre title="Request - retreiving OPTIONS of a basic container" class='example'>
OPTIONS /alice/ HTTP/1.1
Host: example.org
</pre>
<pre title="Response - retreiving OPTIONS of a basic container" class='example'>
HTTP/1.1 200 OK
Allow: OPTIONS,HEAD,GET,POST,PUT,PATCH
Accept-Post: text/turtle, application/ld+json, image/bmp, image/jpeg
Accept-Patch: example/patch
Link: <http://www.w3.org/ns/ldp#Resource>; rel='type'
Link: <http://www.w3.org/ns/ldp#BasicContainer>; rel='type'
Content-Length: 0
</pre>
<p>According to the response, HTTP operations {OPTIONS,HEAD, GET,POST,PUT,PATCH} are allowed on her root container. In addition to the allowed operations, Accept-Post and Accept-Patch provides which media types are supported by the respective operations. The rel="type" Link header advertises that this resource supports LDP protocol and it is an LDP Basic Container.</p>
<p>In this case, the response tells Alice's LDP client that this is an LDP-Basic Container and the container allows her to POST things of both RDF types (text/turtle, application/ld+json) and images (image/bmp and image/jpeg).</p>
</section>
<section>
<h2> Creating a RDF resource (POST an RDF resource to an LDP-BC) </h2>
<p>
Alice can uplaod a social profile document to her store, by POSTing her FOAF personal profile document to her LDP-BC at the root of her document store. Note, the Slug header offers the server a hint about URL of the resource to be created.
</p>
<pre title="Request - creating a RDF resource" class='example' data-include='bccreate.txt' data-oninclude='fixCode'></pre>
</pre>
<pre title="Response - creating a RDF resource" class="example">
HTTP/1.1 201 Created
Location: /alice/foaf
Link: <http://www.w3.org/ns/ldp#Resource>; rel='type'
Content-Length: 0
</pre>
<p> The response to the create request provides a Link to the newly created resource using the Location header. In this case, the server has honored the hint provided by the Slug header and created the new resource in the URL http://data.example.org </p>
<p>Knowing the URL of the newly created resource, Alice can check the container again to confirm that the container correctly contains the newly created resource.</p>
<pre title="Request - basic container retreival after resource created" class="example">
GET /alice/ HTTP/1.1
Host: example.org
Accept: text/turtle
</pre>
<pre title="Response - basic container retreival after resource created" class='example' data-include='bcget_res.txt' data-oninclude='fixCode'/>
<p>
The ldp:contains containment triple discloses the newly created resource in the response.
</p>
</section>
<section>
<h2> Creating a non-RDF (binary) resource (POST an image to an LDP-BC) </h2>
<p>Next, Alice wants to upload a photo of her to the document storage. She can create an image by POSTing it in the same way she created the RDF document.</p>
<pre title="Request - creating a non-RDF resource" class="example">
POST /alice/ HTTP/1.1
Host: data.example.org
Slug: avatar
Link: <http://www.w3.org/ns/ldp#Resource>; rel="type"
Content-Type: image/png
Content- Length: nnn
### binary data ###
</pre>
<pre title="Response - creating a non-RDF resource" class="example">
HTTP/1.1 201 Created
Location: /alice/avatar
Link: <http://www.w3.org/ns/ldp#Resource>; rel="type"
Link: <http://data.example.org/alice/avatar/meta>; rel="describedby"
Content-Length: 0
</pre>
<p> </p>
<p>The outcome of creating a non-RDF is similar to creating a RDF resource. If successful, the server will return a 201 success code with a Location header that points to the created resource. Furthermore, in the case of binary resources the server may create an additional file to maintain the metadata about the binary file. In the above example, the server creates a new LDP-RS to maintain metadata about the binary resource such as creation date, owner, etc. and this metadata resource is advised using a Link header with the relation "describedby". </p>
<p> Similar to creating a RDF resource (LDP-RS), a containment triple will be added to the container when a non-RDF (LDP-NR) is created. Thus, the representation of the LDP container after creating the image looks like the following. </p>
<pre title="Container representation after the non-RDF resource creation" class='example' data-include='bc_after_bin.txt' data-oninclude='fixCode'></pre>
</section>
<section>
<h2> Update a RDF LDP resource (PUT on an LDP-RS) </h2>
<!--
I MISSED THESE BITS OUT ... (??)
<pre title="A request for retrieving a RDF resource" class="example" data-include='getfoaf.txt' data-oninclude='fixCode'/>
<pre title="The response of retrieving a RDF resource" class='example' data-include='getfoaf_res.txt' data-oninclude='fixCode'/>
-->
<p>After creating the image as shown in the previous example, Alice now wants to update her FOAF profile with a link to the image. After retrieving her FOAF profile using a HTTP GET operation, she uses HTTP PUT to update the document by amending the RDF with a link to her photo.</p>
<p> In this example, Alice's LDP client send the E-tag of the resource representation that it retrieved previously to prevent any lost update problems. </p>
<pre title="Request - updating a RDF resource" class="example" data-include='foafupdate.txt' data-oninclude='fixCode'></pre>
<pre title="Response - updating a RDF resource" class="example">
HTTP/1.1 204 No Content
Link: <http://www.w3.org/ns/ldp#Resource>; rel="type"
ETag: W/"123454322"
</pre>
<p>If the operation is successful, the document will be updated with new information.</p>
<p class="note"> Alice can also use PATCH operation to update the resource.</p>
</section>
<section>
<h2> Deleting a resource (DELETE on an LDPR) </h2>
<p>If Alice decides to delete the image, she can do that with a delete operation.</p>
<pre title="Request - deleting a RDF resource" class="example">
DELETE /alice/avatar HTTP/1.1
Host: data.example.org
If-Match: W/"123454322"
</pre>
<pre title="Response - deleting a RDF resource" class="example">
HTTP/1.1 204 No Content
Link: <http://www.w3.org/ns/ldp#Resource>; rel="type"
ETag: W/"123454322"
</pre>
<p> Once the document is deleted, the containment triple will be removed from the container. For example, after the deleting the resource the container representation will be as the following. </p>
<pre title="Container representation after resource deletion" class="example" data-include='bc_after_del.txt' data-oninclude='fixCode'></pre>
<p> For any subsequent request on the resource after the deletion, the server will respond with the appropriate HTTP response code. </p>
<pre title="Request - after deletion" class="example">
GET /alice/avatar HTTP/1.1
Host: example.org
Accept: image/png
</pre>
<pre title="Response - after deletion" class="example">
HTTP/1.1 410 Gone
</pre>
</section>
<section>
<h3 id="meta-structure">Structural Manipulation (Child Containers)</h3>
<p>In order for the client to introduce hierachy to the management of documents, the document store allows creation of documents which are containers. That enables Alice can to create a container hierarchy to organise her documents. This can be done by POSTing (a child) container representation to a (parent) container. This enables Alice to create a child container which she intends to use for image storage.
</p>
<pre title="State of Alice's document store before creating the photo (child) container" class='example' data-include='create_cr_s1.txt' data-oninclude='fixCode'></pre>
<p>To create a new container for managing photos, Alice POSTs a representation of a container (LDP-BC) to the root container. Alice express her intention that the newly created resource should be an LDP Basic Container by including a Link header in the request with the relationship 'type'. </p>
<pre title="Request - creating a new container" class='example' data-include='create_cr_req.txt' data-oninclude='fixCode'></pre>
<p>If the create is successful, the server responds with location of the newly created container for the photos.</p>
<pre title="Response - creating the new container" class="example">
HTTP/1.1 201 Created
Location: /alice/photos/
Content-Length: 0
</pre>
<p>After creation of this new container, the parent container will look like</p>
<pre title="State of Alice's document store after creating the photo (child) container" class='example' data-include='create_cr_s2.txt' data-oninclude='fixCode'></pre>
<p>and the photo container will look like the following.</p>
<pre title="State of Alice's newly created photo container" class='example' data-include='create_cr_s3.txt' data-oninclude='fixCode'></pre>
</section>
</section>
<section id="bugtracker">
<h1>Bug Tracker Example (LDP Direct containers)</h1>
<p>The previous section provided practical examples of basic LDP interactions using LDP Basic Containers. One of the limitations of LDP Basic Containers is that fixed LDP vocabulary is used to assert the relations between a container and its contained resource. However, some scenarios require domain specific vocabulary to be used to list the members of a container. For example, an application which already had its own vocabulary would like continue using the same vocabulary when using LDP protocol. LDP Direct containers allow the domain-specific vocabulary to be used when listing members in a container. This is done using the ldp:hasMemberRelation or ldp:isMemberOfRelation predicate of the Direct Containers which will be explained in the following examples. </p>
<p>In addition, in some scenarios it is necessary to add relationships between the newly created resource and some other resource (which is not necessarily the container). This allows to define relationship between any other information resource or non-information resource (real world thing). This will be done using ldp:membershipResource predicate of the Direct Container which will be explained in the following example. </p>
<p class="note">For more information on information resources (documents) vs real world entities (things) separation please refer to <a href="http://www.w3.org/TR/webarch/#id-resources">Web Arch (Section 2.2. URI/Resource Relationships) </a>, , <a href="http://www.w3.org/TR/cooluris/#semweb">Cool URIs (Section 3. URIs for Real-World Objects)</a>, <a href="http://www.w3.org/TR/urls-in-data/#landing-pages">URLs in Data (Section 3. Landing Pages and Records)</a>.</p>
<p>
The examples in this section will revolve around a very simple Bug Tracker application. Bug Tracker application records the bugs of several products allowing reporting, updating and deleting bugs and products. In contrast to the online document store example, the bug tracker wants to use a simple domain vocabulary, e.g. has_bug, to express membership relationships in the containers. LDP provides the additional interaction capability in the protocol to add the domain specific triples based on the properties defined in the LDP Direct Container.
</p>
<p>A RESTful API for a simple Bug Tracker system might be described as follows.</p>
<table class="simple">
<thead>
<th>Path</th>
<th>Method</th>
<th>Description</th>
</thead>
<tbody>
<!--tr>
<td rowspan="5">/tracker/</td>
<td>GET</td>
<td>Lists all the product descriptions.</td>
</tr>
<tr>
<td>POST</td>
<td>Create a new product description.</td>
</tr>
<tr>
<td>PUT</td>
<td>Update the app description and/or list of product descriptions</td>
</tr>
<tr>
<td>PATCH</td>
<td>Update the app description and/or list of product descriptions</td>
</tr>
<tr>
<td>DELETE</td>
<td>Not allowed.</td>
</tr-->
<tr>
<td class="col1" rowspan="5">
<div class='code'>/tracker/{product-id}/</div>
</td>
<td>GET</td>
<td>Lists the bug reports associated with a product.</td>
</tr>
<tr>
<td>POST</td>
<td>Create a new bug report associated with a product.</td>
</tr>
<tr>
<td>PUT</td>
<td>Update the project description.</td>
</tr>
<tr>
<td>PATCH</td>
<td>Not supported.</td>
</tr>
<tr>
<td>DELETE</td>
<td>Delete the project description and associated bug reports.</td>
</tr>
<tr>
<td rowspan="5">
<div class='code'>/tracker/{product-id}/{bug-id}</div>
</td>
<td>GET</td>
<td>Gets the bug report.</td>
</tr>
<tr>
<td>POST</td>
<td>Not supported.</td>
</tr>
<tr>
<td>PUT</td>
<td>Update the bug report.</td>
</tr>
<tr>
<td>PATCH</td>
<td>Not supported.</td>
</tr>
<tr>
<td>DELETE</td>
<td>Delete the bug report.</td>
</tr>
<tr>
<td rowspan="2">
<div class='code'>/*/*</div>
</td>
<td>OPTIONS</td>
<td>Discover the allowed operations over a resource</td>
</tr>
<tr>
<td>HEAD</td>
<td>Only retrieve meta information about a resource</td>
</tr>
</tbody>
</table>
<p> In the examples in this section, we will only focus on the container representation, creation and deletion of resources because that is where the Basic Containers, Direct Containers, and Indirect Containers have their differences. </p>
<section id="navandret">
<h2>Navigation and Retreival (GET on an LDP-DC)</h2>
<p>One of the main use cases of the example bug tracker is to list of the bugs of a given product. Assuming that a user got a URL of a product by out of band means, she can look it up to get more information including the bugs associated with it. To get the description of the product, a user (or her LDP client) can do a GET request on the URI of the known product resource. LDPR servers must provide text/turtle representations of the requested LDPRs and may provide RDF format representations such as JSON-LD or RDF/XML using standard HTTP content negotiation.</p>
<pre class="example" title="Request - Product Lookup" data-include='product_lookup_req.txt' data-oninclude='fixCode'></pre>
<p>If the product resource is available, the server responds with the RDF representation of the Direct Container that corresponds to the given product using the requested media type,
<code>text/turtle</code> or <code>application/ld+json</code> in this case.</p>
<pre title="Response - Product Lookup" class='example' data-include='product_lookup_resp.txt' data-oninclude='fixCode'></pre>
<p>The representation of the product contains both information about the product such as the title and the information about members of the product LDPC, i.e. the bugs associated with the product. </p>
<p>As you can see from the Link Header that is returned and the RDF representation of the container, this example uses an LDP Direct Container allowing the application to use domain specific vocabulary in the container and allowing it to create relationships to resources other than the container itself upon resource creation. For example, it manages the member relationship, (<?productURI>, bt:hasBug, <?bugURI>), using the application-specific vocabulary term, bt:hasBug. This is done by defining the ldp:hasMemberRelation predicate of the Direct Container to bt:hasBug (<?directContainerURI>, ldp:hasMemberRelation, bt:hasbug). </p>
<p class="note"> In this example, we have shown the Direct Containers with the membership triple pattern ( membership-constant-URI , membership-predicate , member-derived-URI ) using ldp:hasMemberRelation where the constant membership is resource is in the subject of the triple and the newly created resources will be added as the object of the triple. It is also possible for the Direct Container to have the membership triple pattern ( member-derived-URI , membership-predicate , membership-constant-URI ) using ldp:isMemberOfRelation predicate where the constant member resource will be the object of the triple and the newly created resource will be added as the subject of the triple. </p>
<p> In addition, in this example the bugs are associated with the product (a non-information resource with a # URI) and not with the Direct Container itself. This is done by defining the ldp:membershipResource predicate of the LDP Direct Container to the product non-information resource URI (<?directContainerURI>, ldp:membershipResource, </tracker/product1/#it>). By doing so one can define the subject of the membership triple any resource of interest.
</p>
<p>The next example illustrates the behaviour of LDP Direct containers when new resources are created and how these predicates of the Direct Containers affect the interaction model of LDP.</p>
<!-- <p>Looking up a bug is similar to looking up a product. Based on links in the representation of the Product, the client uses GET to navigate to a known Bug resource.</p>
<pre class="example" title="Bug lookup request"
data-include='bug_look_up_req.txt' data-oninclude='fixCode'></pre>
<p>The server responds with the representation of the bug.</p>
<pre title="Bug lookup response"
class='example' data-include='bug_look_up_resp.txt'
data-oninclude='fixCode'></pre -->
</section>
<section>
<h3 id="BugCreate">Creation (POST a resource to an LDP-DC)</h3>
<p>Continuing from the previous example, we can report a Bug against "LDP Demo" product by creating a Bug report (an LDPR representing the bug) under the "LDP Demo" product description LDPC by posting RDF representation of the Bug report to the LDPC associated with the product description. </p>
<pre title="A request for creating a bug" class='example' data-include='bug_create_req.txt' data-oninclude='fixCode'></pre>
<p>If the creation is successful, the server responds with location of the newly created resource.</p>
<pre title="A response of creating new a bug" class='example' data-oninclude='fixCode'>
HTTP/1.1 201 Created
Location: /tracker/ldp-demo/67
Content-Length: 0
</pre>
<p>If the creation fails, the server will respond with an appropriate status code depending on the error. If successful, the LDP Demo product description LDPC will have the following representation after the creation of new resource.</p>
<pre title="The state of the product LDPC after the bug creation" class='example' data-include='bug_create_s1.txt' data-oninclude='fixCode'></pre>
<p> As you can see two new triples are added to the container. That is (</tracker/ldp-demo/>, <ldp:contains>, </tracker/ldp-demo/bug67>) and
(</tracker/ldp-demo/#it>, <bt:hasbug>, </tracker/ldp-demo/bug67>). The former is added in any type of container and the latter is defined by the direct
container properties. </p>
<p>And the created Bug resource will have the following representation. Note that server has added a server managed property, creation date (dcterms:created), and a default value for the state (bt:isInState) to the Bug in addition to what was being POSTed.</p>
<pre title="The state of the bug LDPR" class='example' data-include='bug_create_s2.txt' data-oninclude='fixCode'></pre>
</section>
<section>
<h3 id="BugDelete">Deletion (DELETE on an LDPR associated with an LDP-DC)</h3>
This example illustrates the behaviour of Direct Container when a resource is deleted.
<pre class="example">
DELETE /tracker/ldp-demo/bug3 HTTP/1.1
Host: example.org
If-Match: W/"123454322"
</pre>
<p>If the delete is successful, the server will respond with a success status code.</p>
<pre class="example">
HTTP/1.1 204 No Content
ETag: W/"123454322"
</pre>
<p> After the deletion, the representation of the container will look like the following</p>
<pre title="The state of the product LDPC after the bug deletion" class='example' data-include='bug_delete_s1.txt' data-oninclude='fixCode'></pre>
<p> As seen from the LDP Direct Container representation above both containment triple (</tracker/ldp-demo/>, ldp:contains, </tracker/ldp-demo/bug3>) and the membership triple (</tracker/ldp-demo/#it>, bt:hasBug, </tracker/ldp-demo/bug3>) are removed from the container representation. </p>
</section>
</section>
<section id="bugtrackerextd">
<h1>Extended Bug Tracker Example (LDP Indirect containers)</h1>
<p> In this example, we will use the same scenario as the previous example but using an LDP Indirect Container to show what is the use of Indirect containers and
how to use them. In the previous example, the object of the membership triple (in the case of using ldp:hasMemberRelation) or the subject (in the case of using ldp:isMemberOfRelation) was always the URL of the newly created information resource. LDP Indirect containers allow this to be any URI that is declared inside the resource to be created. This done by defining the predicate to look for in the representation of the resource to be created using the ldp:insertedContentRelation predicate of the LDP Indirect Container as explained in the following examples. </p>
<section id="navandretext">
<h2>Navigation and Retreival (GET on an LDP-IC) </h2>
<p> Similar to the previous example, first we will retrieve the representation of the LDP Indirect Container.</p>
<pre class="example" title="Product lookup request" data-include='product_lookup_req.txt' data-oninclude='fixCode'></pre>
<p>If the product resource is available, the server responds with the representation of the resource using the requested media type,
<code>text/turtle</code> or <code>application/ld+json</code> in this case.</p>
<pre title="HTTP response for product lookup" class='example' data-include='ic_product_lookup_resp.txt' data-oninclude='fixCode'></pre>
<p> Now the product container is an LDP Indirect container and we can notice one main difference. The container has an additional predicate called "ldp:insertedContentRelation". Further, we can notice that the object of the containment triples and the membership triples are not the same. While the object of the containment triple is the same (e.g. </tracker/ldp-demo/bug3>, an information resource) the object of the membership triple is now (e.g. </tracker/ldp-demo/bug3#it>, a non-information resource or real world thing). This distinction is because of the ldp:insertedContentRelation definition. How this works will be explained in the next example on creating a new resource. </p>
</section>
<!-- end navandretext -->
<section id="creationext">
<h3 id="IndirectCreate">Creation (POST a resource to an LDP-IC) </h3>
<p>Continuing from the previous example, we can report a Bug Report against 'LDP demo' product by creating a Bug Report LDPR under the 'LDP Demo' product description LDPC.</p>
<p>The client POSTs a representation of a Bug to the Bug Tracker LDPC.</p>
<pre title="A request for creating a bug" class='example' data-include='ic_bug_create_req.txt' data-oninclude='fixCode'></pre>
<p> One thing to note is that the representation of the resource to be created contain a triple (< >, foaf:primaryTopic , <#it>). If the create is successful, the server responds with location of the newly created resource.</p>
<pre title="A response of creating new a bug" class='example' data-oninclude='fixCode'>
HTTP/1.1 201 Created
Location: /tracker/ldp-demo/67
Content-Length: 0
</pre>
<p>If the creation fails, the server will respond with an appropriate status code depending on the error. After the resource is creation, the Product A LDPC will have the following representation.</p>
<pre title="The state of the product LDPC after the bug creation" class='example' data-include='ic_bug_create_s1.txt' data-oninclude='fixCode'></pre>
<p> As you can see two new triples are added to the container. That is (</app/product1>, <ldp:contains>, </tracker/ldp-demo/bug67>) and
(</tracker/ldp-demo/#it>, <bt:hasbug>, </tracker/ldp-demo/bug67#it>). </p>
</section>
<!-- end creationext -->
<!-- section id="deleteext">
<h3 id="IndirectDelete">Deletion</h3>
<pre class="example">
DELETE /tracker/ldp-demo/bug3 HTTP/1.1
Host: example.org
</pre>
<p>If the update is successful, the server will respond with a success status and a new etag.</p>
<pre class="example">
HTTP/1.1 204 No Content
ETag: W/"123456790"
</pre>
<p> After the deletion, the representation of the container will look like the following (Show that the two triples are gone) </p>
</section -->
<!-- end deleteext -->
</section>
<section>
<h1>Security</h1>
<p>It is not the focus of the Linked Data Platform WG to provide security mechanisms for read/write Linked Data applications. Though most of the security mechanisms that are applicable to general web applications are equally applicable to Linked Data applications, there is still some space to build security mechanisms specific to Linked Data applications by leverage the Linked Data technologies and providing concrete security requirements of Linked Data applications. In this context, LDP WG has started to create a WG note on Access Control which aims to produce use cases for security scenarios of LDP applications that can be used as the input to later initiative that will be focused on developing standard security mechanisms for LDP applications.</p>
</section>
<section>
<h1 id="ldpc">LDP Implementations</h1>
A list of implementations that plan to be LDP compliant is available in the LDP Implementations wiki page. LDP Implementation report provides the coverage of the specification by each LDP implementation.
</section>
<section>
<h1 id="next">What To Read Next</h1>
The primer only provide an overview of the Linked Data Platform specifications. LDP WG has produced following documents that contribute to the Linked Data Platform specification.
<ul>
<li><a href="https://dvcs.w3.org/hg/ldpwg/raw-file/default/TR/ldp-ucr.html">Linked Data Platform Use Cases and Requirements</a> [[LDP-UCR]]</li>
<li><a href="https://dvcs.w3.org/hg/ldpwg/raw-file/default/ldp.html">Linked Data Platform 1.0 specifcation</a> [[LDP]]</li>
<li>Linked Data Platform 1.0 Primer (This document)</li>
<li><a href="https://dvcs.w3.org/hg/ldpwg/raw-file/default/ldp-bp/ldp-bp.html">LDP Best Practices and Guidelines</a> [[LDP-BP]]</li>
<li><a href="https://dvcs.w3.org/hg/ldpwg/raw-file/default/Test%20Cases/LDP%20Test%20Cases.html">Linked Data Platform 1.0 Test Cases</a>[[LDP-TESTS]]</li>
</ul>
</section>
<section class='appendix informative' id="history">
<h1>Change History</h1>
<p>The change history is up to the editors to insert a brief summary of changes, ordered by most recent changes first and with heading from which public draft it has been changed from.
</p>
<ul>
<li>2013-08-05 - Providing JSON-LD representations of the examples.</li>
<li>2013-07-03 - Moving the content from the wiki to the note.</li>
</ul>
</section>
</body>
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