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<body class="article">
<div id="header">
<h1>Application-Level Profile Semantics</h1>
<span id="author">Revision: 2012-10-29 (mca)</span><br />
</div>
<div id="content">
<div class="sect1">
<h2 id="_summary">Summary</h2>
<div class="sectionbody">
<div class="paragraph"><p>This document describes a media type designed to give Web application
authors and implementors a way to define, document, and apply
application-level semantic descriptions to elements within
application-agnostic media-type representations. This document also
sets a standard for creating and registering ALPS document identifers
and provides suggested conventions for applying ALPS descriptors to
existing media types (such as HTML, Collection+JSON, and HAL).</p></div>
<div class="paragraph"><p>This document registers two media-type identifiers with the IANA:
<tt>application/vnd.alps+xml</tt> and <tt>application/vnd.alps+json</tt>.</p></div>
</div>
</div>
<div class="sect1">
<h2 id="_motivation">Motivation</h2>
<div class="sectionbody">
<div class="paragraph"><p>When implementing a hypermedia client/server application using a
general media type (i.e. HTML, Atom, Collection+JSON, etc.), client
and server instances need to share application-specific information
such as data element names, link relation values, and state transfer
parameters. This information is directly related to the application
being implemented (i.e. accounting, contact management, etc.) rather
than the media type used in the representations.</p></div>
<div class="paragraph"><p>Instead of creating (and registering) and entirely new media type
(i.e. <tt>application/accounting</tt>), this standard allows representation
authors to apply application-level semantic descriptions to elements
defined by the generic media type. This increases the specificity of
the representation without reducing the generality of the media type.</p></div>
</div>
</div>
<div class="sect1">
<h2 id="_identifying_an_alps_document">Identifying an ALPS Document</h2>
<div class="sectionbody">
<div class="paragraph"><p>ALPS vocabularies are identified by a unique URI. This URI can be in the form
of a resolvable URL (i.e. <tt>http://example.org/alps/accounting</tt>) or it can be
in the form of a URN (i.e. TK).</p></div>
<div class="paragraph"><p>If the identifier is a URL, it SHOULD point to a valid ALPS document. the URL
MAY point to a human-readable document that describes the ALPS specification
for the identified use case. If the identifier is a URN, it SHOULD NOT be
treated as a resolvable identifer and instead SHOULD be treated as a unique
name for the identified use case.</p></div>
<div class="paragraph"><p>[I almost feel like we should have two URLs, an ALPS permalink and a
human-readable doc. Or should we suggest the use of content
negotiation? -leonardr]</p></div>
<div class="paragraph"><p>All ALPS URIs MUST be unique and all ALPS documents intended for
public consumption SHOULD be registered in the ALPS Registry located
at [TK].</p></div>
</div>
</div>
<div class="sect1">
<h2 id="_alps_documents">ALPS Documents</h2>
<div class="sectionbody">
<div class="paragraph"><p>ALPS Documents contain a machine-readable collection of semantic
descriptors and their human-readable details. An ALPS document can be
represented in either XML or JSON format. This section identifies the
general elements and properties of an ALPS document, their meaning,
and their use. Additional subsections provide specific details on how
to construct a valid ALPS document in XML and in JSON format.</p></div>
<div class="sect2">
<h3 id="_compliance">Compliance</h3>
<div class="paragraph"><p>An implementation is not compliant if it fails to satisfy one or more of the
MUST or REQUIRED level requirements. An implementation that satisfies all
the MUST or REQUIRED level and all the SHOULD level requirements is said
to be "unconditionally compliant"; one that satisfies all the MUST level
requirements but not all the SHOULD level requirements is said to be
"conditionally compliant."</p></div>
<div class="admonitionblock">
<table><tr>
<td class="icon">
<div class="title">Note</div>
</td>
<td class="content">
<div class="paragraph"><p>The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD",
"SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to
be interpreted as described in RFC 2119.</p></div>
</td>
</tr></table>
</div>
</div>
<div class="sect2">
<h3 id="_alps_document_properties">ALPS Document Properties</h3>
<div class="paragraph"><p>The ALPS media type defines a small set of properties. The properties
appear in both the XML and JSON formats, but are expressed in
different ways. Below is a list of the properties that can appear in
an ALPS document.</p></div>
<div class="sect3">
<h4 id="_tt_alps_tt"><tt>alps</tt></h4>
<div class="paragraph"><p>Indicates the root of the ALPS document. This MUST be the first property
in the document. This property SHOULD have one or more <tt>element</tt> child
properties.</p></div>
<div class="paragraph"><p>[Would it be okay to remove the <em>must be the first property in the
document</em> requirement? This would allow an ALPS doc to be embedded in
other docs. It would also make it possible to define ALPS as a
microformat. -leonardr]</p></div>
</div>
<div class="sect3">
<h4 id="_tt_appears_tt"><tt>appears</tt></h4>
<div class="paragraph"><p>Indicates the appearance rules to apply to the related <tt>element</tt>.
Valid values for this element are: <em>MUST</em>, <em>SHOULD</em>, and <em>MAY</em>. These
values are to be treated as defined in RFC 2119. The <tt>appears</tt>
property SHOULD appear as a property of the <tt>element</tt> and/or <tt>child</tt>
elements. If this property does not appear, the value SHOULD be
treated as <em>MAY</em>.</p></div>
<div class="paragraph"><p>[You’ve removed MUST NOT and the default MUST NOT for top-level
elements, which makes it difficult to define documents like
hCalendar. I’ve written an alternate section below. -leonardr]</p></div>
<div class="paragraph"><p>Indicates the appearance rules for the related <tt>element</tt> in a
representation. All valid values for this property are terms defined in RFC
2119, and have the meaning given in that standard.</p></div>
<div class="paragraph"><p>The <tt>appears</tt> property MAY appear as a property of the <tt>element</tt>
block. This controls whether the element may appear at the "top
level"; that is, not enclosed in any other element defined by this
ALPS document. The formal meaning of "top level" varies by the media
type; see "Applying ALPS to Existing Media Types" for
definitions. [that’s TK -leonardr]</p></div>
<div class="paragraph"><p>Valid values for <tt>appears</tt> in an <tt>element</tt> block are: <em>MUST</em>,
<em>SHOULD</em>, <em>MAY</em>, and <em>MUST NOT</em>. If this property does not appear, the
value SHOULD be treated as <em>MUST NOT</em>.</p></div>
<div class="paragraph"><p>The <tt>appears</tt> property MAY appear as a property of a <tt>child</tt>
block. This controls the rules by which the referenced element may
appear as a child of the enclosing <tt>element</tt>. The formal meaning of
"enclosing <tt>element</tt>" varies by the media type; see "Applying ALPS to
Existing Media Types" for definitions. [that’s TK -leonardr]</p></div>
<div class="paragraph"><p>Valid values for <tt>appears</tt> in an <tt>child</tt> block are: <em>MUST</em>,
<em>SHOULD</em>, and <em>MAY</em>. If this property does not appear, the value
SHOULD be treated as <em>MAY</em>.</p></div>
</div>
<div class="sect3">
<h4 id="_tt_cardinality_tt"><tt>cardinality</tt></h4>
<div class="paragraph"><p>Indicates the number of times the related <tt>element</tt> can appear. Valid
values are <em>single</em> and <em>many</em>.</p></div>
<div class="paragraph"><p>The <tt>cardinality</tt> property MAY appear as a property of an <tt>element</tt>
block. This controls how many times that element may be used at the
"top level" within a single representation.</p></div>
<div class="paragraph"><p>The <tt>cardinality</tt> property MAY appear as a property of a <tt>child</tt>
block. This controls how many times the child element may occur within
its enclosing element.</p></div>
<div class="paragraph"><p>If this property does not appear, the value SHOULD be treated as
<em>many</em>.</p></div>
</div>
<div class="sect3">
<h4 id="_tt_child_tt"><tt>child</tt></h4>
<div class="paragraph"><p>Indicates a child element of the related <tt>element</tt> This MAY appear in
the ALPS document as a child of an <tt>element</tt> block. The <tt>child</tt>
property MAY have an <tt>href</tt> property pointing to a definition of an
<tt>element</tt> block (see <tt>href</tt> for details).</p></div>
<div class="paragraph"><p>If no <tt>href</tt> appears, then the <tt>child</tt> property takes on the same
features as the <tt>element</tt> property. [Which ones? It would also be good
to mention here which properties the <tt>child</tt> tag supports, as you do for
<tt>element</tt>. -leonardr]</p></div>
</div>
<div class="sect3">
<h4 id="_tt_description_tt"><tt>description</tt></h4>
<div class="paragraph"><p>A text field that contains free-form, usually human-readable, text.</p></div>
<div class="paragraph"><p>A <tt>description</tt> block SHOULD appear as a child of <tt>element</tt>. When
present, it describes the meaning and use of the related <tt>element</tt>.</p></div>
<div class="paragraph"><p>A <tt>description</tt> block MAY appear as a child of <tt>child</tt>. When present,
it describes the meaning of the relationship between the <tt>element</tt> and
its <tt>child</tt>.</p></div>
<div class="paragraph"><p>A <tt>description</tt> block MAY appear as a child of <tt>alps</tt>. When present,
it describes the purpose of the ALPS document as a whole.</p></div>
<div class="paragraph"><p>[When writing the example docs I kept wanting to use HTML
markup. Obviously you can do this in ALPS+XML by using namespaces. Can
we explicitly say that the contents of <tt>description</tt> may be HTML
markup, a la the contents of an Atom story? -leonardr]</p></div>
</div>
<div class="sect3">
<h4 id="_tt_element_tt"><tt>element</tt></h4>
<div class="paragraph"><p>Identifies a semantic descriptor. One or more <tt>element</tt> blocks SHOULD
appear as children of the <tt>alps</tt> property.</p></div>
<div class="paragraph"><p>This property MUST have the following properties:</p></div>
<div class="ulist"><ul>
<li>
<p>
<tt>id</tt>
</p>
</li>
<li>
<p>
<tt>type</tt>
</p>
</li>
</ul></div>
<div class="paragraph"><p>It SHOULD have the following properties:</p></div>
<div class="ulist"><ul>
<li>
<p>
<tt>appears</tt>
</p>
</li>
</ul></div>
<div class="paragraph"><p>It MAY have the following properties:</p></div>
<div class="ulist"><ul>
<li>
<p>
<tt>cardinality</tt>
</p>
</li>
<li>
<p>
<tt>child</tt>
</p>
</li>
<li>
<p>
<tt>description</tt>
</p>
</li>
<li>
<p>
<tt>name</tt>
</p>
</li>
</ul></div>
</div>
<div class="sect3">
<h4 id="_tt_name_tt"><tt>name</tt></h4>
<div class="paragraph"><p>Indicates the name of the <tt>element</tt> as found in generic
representations. It MAY appear as a property of the <tt>element</tt>.</p></div>
<div class="paragraph"><p>This is used when the <tt>element</tt><em>s name is used as an <tt>id</tt> value
elsewhere in the ALPS document. For instance, if a single ALPS
document defines an element descriptor called 'customer</em> and a
safe-idempotent control called <em>customer</em>, they can’t both have
<tt>id="customer"</tt> in the ALPS document. One of them needs to have some
other <tt>id</tt>, and to set <tt>name="customer"</tt>.</p></div>
</div>
<div class="sect3">
<h4 id="_tt_href_tt"><tt>href</tt></h4>
<div class="paragraph"><p>Points to an <tt>element</tt> either within the existing ALPS document or in
another ALPS document.</p></div>
<div class="paragraph"><p>The <tt>href</tt> attribute MUST appear as part of the <tt>child</tt> element. The
value MUST be a valid URL, with a fragment identifier, resolving to an
<tt>element</tt> within an ALPS document.</p></div>
<div class="paragraph"><p>[Should <alps> itself support <tt>href</tt> so it can link to itself or a
full human-readable doc? Or should we allow a general <link> tag so
you can do rel="help"?]</p></div>
<div class="paragraph"><p>\[ Earlier you said that <tt>child</tt> didn’t need href. -leonardr \]</p></div>
<div class="paragraph"><p>[I have a half-baked idea about using <tt>href</tt> within <tt>element</tt> as well.
This would let you copy the <tt>fn</tt> descriptor defined in hCard into your
custom ALPS document (for performance reasons) but still make it clear
that your <tt>fn</tt> descriptor has the same meaning as the one defined in
hCard. -leonardr]</p></div>
</div>
<div class="sect3">
<h4 id="_tt_id_tt"><tt>id</tt></h4>
<div class="paragraph"><p>A document-wide unique identifier for the related element. This MUST
appear as a property of each <tt>element</tt>.</p></div>
<div class="paragraph"><p>The value of this property will appear within generic representations
as the name of a semantic descriptor or hypermedia control (see
Applying ALPS to Existing Media Types for details). The exception is
when an element defines <tt>name</tt> as well as <tt>id</tt>. In that case, the
value of <tt>name</tt> contains the name of the semantic descriptor or
hypermedia control, and the value of <tt>id</tt> is just a unique ID.</p></div>
<div class="paragraph"><p>When applied to an ALPS document, a URI fragment identifier points to
the element with the appropriate <tt>id</tt>. For example, The fragment
identifier <tt>customer</tt> in the URI <tt>http://example.com/#customer</tt> refers
to an ALPS element with <tt>id</tt> set to <tt>customer</tt>.</p></div>
<div class="paragraph"><p>A relative URL with a fragment identifier (e.g. "#customer") refers to
an element within the ALPS document containing the reference.</p></div>
</div>
<div class="sect3">
<h4 id="_tt_type_tt"><tt>type</tt></h4>
<div class="paragraph"><p>Indicates the type of representation control to which the <tt>element</tt> is
applied. This MUST appear for each <tt>element</tt>. Four values are defined:</p></div>
<div class="paragraph"><p>\[ Would it be OK to say that the default is <em>descriptor</em>? -leonardr \]</p></div>
<div class="ulist"><ul>
<li>
<p>
<em>descriptor</em> refers to a state element (e.g. <tt>HTML.SPAN</tt>, <tt>HTML.INPUT</tt>, etc.).
</p>
</li>
<li>
<p>
<em>safe-idempotent</em> refers to a hypermedia control that triggers a safe,
idempotent state transition (i.e. <tt>HTTP.GET</tt>, <tt>HTTP.HEAD</tt>, etc.).
</p>
</li>
<li>
<p>
<em>unsafe-idempotent</em> refers to a hypermedia control that triggers an
unsafe, idempotent state transition (i.e. <tt>HTTP.PUT</tt>, <tt>HTTP.DELETE</tt> etc.).
</p>
</li>
<li>
<p>
<em>unsafe-nonidempotent</em> refers to a hypermedia control that triggers an
unsafe, non-idempotent state transition (i.e. <tt>HTTP.POST</tt>).
</p>
</li>
</ul></div>
<div class="paragraph"><p>\[ How about just idempotent and nonidempotent? -leonardr \]</p></div>
</div>
<div class="sect3">
<h4 id="_tt_version_tt"><tt>version</tt></h4>
<div class="paragraph"><p>Indicates the version number of the ALPS document. This SHOULD appear as a
property of the <tt>alps</tt> property. Currently the only valid value is <em>1.0</em>.
If no value appears, then it SHOULD be set to <em>1.0</em>.</p></div>
</div>
</div>
<div class="sect2">
<h3 id="_xml_representation">XML Representation</h3>
<div class="paragraph"><p>The following example shows how ALPS Elements are expressed in the XML
format. This example ALPS+XML document describes a portion of the
hCalendar microformat (<a href="http://microformats.org/wiki/hcalendar">http://microformats.org/wiki/hcalendar</a>).</p></div>
<div class="paragraph"><p>[I put this in because I think an example that ties into something
preexisting is better than an example we made up for RWA.]</p></div>
<div class="listingblock">
<div class="content">
<pre><tt><alps version="1.0">
<description>
hCalendar is a simple, open format for publishing events on the web,
using a 1:1 representation of iCalendar (RFC2445) VEVENT properties
and values in HTML.
</description>
<element appearance="MAY">
<id>vcalendar</id>
<description>
A container for one or more events (vevent).
</description>
<child href="#vevent" appearance="MAY">
</element>
<element appearance="MAY">
<id>vevent</id>
<description>
A container for one event. See section 4.6.1 of RFC 2445.
</description>
<child href="#dtstart" appearance="MUST">
<child href="#summary" appearance="MUST">
<child href="#location" appearance="MAY">
</element>
<element appearance="MUST NOT">
<id>dtstart</id>
<description>
Date/time of the start of the event. See section 4.8.2.4 of RFC 2445.
</description>
</element>
<element appearance="MUST NOT">
<id>summary</id>
<description>
Short synopsis, title, or name of the event. See section 4.8.1.12
of RFC 2445.
</description>
</element>
<element appearance="MUST NOT">
<id>location</id>
<description>
Tells where the event is to be held. May be represented by nested
hCard record, adr record, geo record, or combination thereof. See
sections 4.8.1.7 and 4.8.1.6 of RFC 2445.
</description>
</element>
</alps></tt></pre>
</div></div>
<div class="paragraph"><p>This ALPS+XML document describes the <tt>rel-nofollow</tt> microformat
(<a href="http://microformats.org/wiki/rel-nofollow">http://microformats.org/wiki/rel-nofollow</a>):</p></div>
<div class="paragraph"><p>[And I put this other example in because hCalendar doesn’t define any link
relations. -leonardr]</p></div>
<div class="listingblock">
<div class="content">
<pre><tt><alps version="1.0">
<element id="nofollow" type="safe-idempotent">
<description>
Indicates that the referred resource was not necessarily linked to
by the author of the page, and thus said reference should not afford
the referred resource any additional weight or ranking by user
agents.
</description>
</element>
</alps></tt></pre>
</div></div>
<div class="paragraph"><p>[leaving in this example because a) i don’t want to cut it
unilaterally, and b) I didn’t change the JSON example
below. -leonardr]</p></div>
<div class="listingblock">
<div class="content">
<pre><tt><alps version="1.0">
<element id="hmaze" type="descriptor" appears="MUST">
<description>Indicates the root of an hMaze document.</description>
<child href="#collection"/>
<child href="#item"/>
<child href="#error"/>
<child href="#cell"/>
<child href="#inventory-link"/>
</element>
<element id="collection" type="descriptor">
<description>A collection of mazes.</description>
<child href="#cell" cardinality="many" appears="MAY">
<description>The opening cell of a maze</description>
</child>
</element>
<element id="item" type="descriptor">
<description>A single maze.</description>
<child href="#start"/ appears="SHOULD">
</element>
<element id="start" type="safe-idempotent">
<description>The beginning cell of a maze.</description>
</element>
<element id="error" type="descriptor">
<description>An error message.</description>
</element>
<element id="cell" type="descriptor">
<description>A cell in a maze.</description>
<child href="#title" appears="SHOULD">
<description>The name of the cell, e.g. "Throne Room."</description>
</child>
<child href="#north" appears="MAY"/>
<child href="#south" appears="MAY"/>
<child href="#east" appears="MAY"/>
<child href="#west" appears="MAY"/>
<child href="#exit" appears="MAY"/>
<child href="#current" appears="MUST"/>
</element>
<element id="north" type="safe-idempotent">