Service Modeling Language, Version 1.1 http://www.w3.org/TR/2007/WD-sml-20070926/ W3C Working Draft 26 September 2007 http://www.w3.org/TR/2007/WD-sml-20070926/ XML http://www.w3.org/TR/2007/WD-sml-20070806/ http://www.w3.org/TR/sml/ James Lynn HP Bhalchandra Pandit Microsoft Corporation Valentina Popescu IBM Corporation Virginia Smith HP This specification defines the Service Modeling Language, Version 1.1 (SML) used to model complex services and systems, including their structure, constraints, policies, and best practices. SML uses XML Schema and is based on a profile of Schematron. This section describes the status of this document at the time of its publication. Other documents may supersede this document. A list of current W3C publications and the latest revision of this technical report can be found in the W3C technical reports index at http://www.w3.org/TR/. This is the second Public Working Draft of the Service Modeling Language, Version 1.1 specification for review by W3C members and other interested parties. It has been developed by the Service Modeling Language (SML) Working Group, which is a part of the Extensible Markup Language (XML) Activity. The features and algorithms described in the normative portion of the document are specified in enough detail adequate for early implementation experiments. Publication as a Working Draft does not imply endorsement by the W3C Membership. This is a draft document and may be updated, replaced or obsoleted by other documents at any time. It is inappropriate to cite this document as other than work in progress. Comments on this document are invited and are to be sent to the public-sml@w3.org mailing list (public archive). This document was produced by a group operating under the 5 February 2004 W3C Patent Policy. W3C maintains a public list of any patent disclosures made in connection with the deliverables of the group; that page also includes instructions for disclosing a patent. An individual who has actual knowledge of a patent which the individual believes contains Essential Claim(s) must disclose the information in accordance with section 6 of the W3C Patent Policy. English Last Modified: $Date: 2007/09/25 17:21:48 $ Introduction (Non-Normative) The Service Modeling Language (SML) provides a rich set of constructs for creating models of complex services and systems. Depending on the application domain, these models may include information such as configuration, deployment, monitoring, policy, health, capacity planning, target operating range, service level agreements, and so on. Models provide value in several important ways. Models focus on capturing all invariant aspects of a service/system that must be maintained for the service/system to function properly. Models represent a powerful mechanism for validating changes before applying the changes to a service/system. Also, when changes happen in a running service/system, they can be validated against the intended state described in the model. The actual service/system and its model together enable a self-healing service/system ― the ultimate objective. Models of a service/system must necessarily stay decoupled from the live service/system to create the control loop. Models are units of communication and collaboration between designers, implementers, operators, and users; and can easily be shared, tracked, and revision controlled. This is important because complex services are often built and maintained by a variety of people playing different roles. Models drive modularity, re-use, and standardization. Most real-world complex services and systems are composed of sufficiently complex parts.  Re-use and standardization of services/systems and their parts is a key factor in reducing overall production and operation cost and in increasing reliability. Models enable increased automation of management tasks. Automation facilities exposed by the majority of services/systems today could be driven by software ― not people ― both for reliable initial realization of a service/system as well as for ongoing lifecycle management. A model in SML is realized as a set of interrelated XML documents. The XML documents contain information about the parts of a service, as well as the constraints that each part must satisfy for the service to function properly. Constraints are captured in two ways: Schemas ― these are constraints on the structure and content of the documents in a model. SML uses XML Schema [, ] as the schema language. In addition SML defines a set of extensions to XML Schema to support inter-document references. Rules ― are Boolean expressions that constrain the structure and content of documents in a model. SML uses a profile of Schematron [, , ] and XPath [] for rules. One of the important operations on the model is to establish its validity. This involves checking whether all data in a model satisfies the schemas and rules declared. This specification focuses primarily on defining the extensions to XML Schema for inter-document references, the profile of Schematron used by SML, as well as the process of model validation. It is assumed that the reader is familiar with XML Schema and Schematron. Notations and Terminology Notational Conventions The keywords "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 []. Terminology Document A well-formed XML document, as defined in []. Model A set of inter-related documents that describe a service or system.  Each model consists of two disjoint subsets of documents –definition documents and instance documents. Rule A rule is a boolean expression that constrains the structure and content of a set of documents in a model. Model Definition The subset of documents in a model that describes the schemas and rules that govern the structure and content of the model's documents. This specification defines two types of model definition document - Schema documents that conform to XML Schema and rule documents that conform to SML's profile of Schematron – but permits implementations to define other types of model definition documents. Such other types of model definition documents do not play any role in SML model validation. Model Instance A model instance is the subset of documents in a model that describe the structure and content of the modeled entities. Model Validation The model validation is the process of verifying that all documents in a model are valid with respect to the model's definition documents. Model Validator A model validator is an embodiment capable of performing model validation XML Namespaces lists XML namespaces that are used in this specification. The choice of any namespace prefix is arbitrary and not semantically significant. XML Namespaces used in this specification. Prefix XML Namespace Specification(s) sml http://www.w3.org/2007/09/sml This specification smlerr http://www.w3.org/2007/09/sml-err This specification smlfn http://www.w3.org/2007/09/sml-function This specification wsa http://www.w3.org/2005/08/addressing [] xs http://www.w3.org/2001/XMLSchema [, ] sch http://purl.oclc.org/dsdl/schematron [] xsi http://www.w3.org/2001/XMLSchema-instance XML Schema instance, as defined in [] Dependencies on Other Specifications Other specifications on which this one depends are listed in []. Conforming implementations of this specification MUST support XML 1.0 [], XML Schema 1.0 [, ], Schematron [] and XPath 1.0 []. However, conforming implementations MAY additionally support later versions of the XML, XML Schema, Schematron or XPath specifications. Schemas SML uses XML Schema [, ] to define constraints on the structure of data in a model. SML scenarios require several features that either do not exist or are not fully supported in XML Schema. These features can be classified as follows: References – XML Schema does not have any support for inter-document references, although it does support intra-document references through xs:ID, xs:IDREF, xs:key and xs:keyref. Inter-document references are fundamental to SML since a document is a unit of versioning. SML extends XML Schema to support inter-document references and a set of constraints on inter-document references. Rules – XML Schema does not support a language for defining arbitrary rules on the structure and content of XML documents. SML uses Schematron to express assertions on the structure and content of XML documents. XML Schema supports two forms of extension: "attributes in different namespace" and "application information elements"; both forms are used by SML extensions. References XML documents introduce boundaries across content that needs to be treated as a unit. XML Schema does not have any support for inter-document references. SML extends XML Schema to support inter-document references and a set of constraints on inter-document references. Support for inter-document references includes: Multiple addressing schemes for representing references. Constraints on the type of a referenced element. The ability to define key, unique, and key reference constraints across inter-document references. An SML reference is a link from one element in an SML model to another element from the same model. It can be represented by using a variety of schemes, such as and Endpoint References (EPRs) []. SML does not mandate the use of any specific scheme for representing references; implementations are free to choose suitable schemes for representing references.  References MUST be supported by model validators that conform to this specification. An SML reference MAY use one or more reference schemes. Reference elements MUST be identified by sml:ref="true" or sml:ref="1" where sml:ref is a global SML attribute whose declaration is as follows: Although either sml:ref="true" or sml:ref="1" can be used to identify an SML reference element, for the sake of brevity and consistency, the rest of this specification uses sml:ref="true" in examples and other definitions. An element that has sml:ref="true" MUST be treated as a reference element. This mechanism enables schema-less identification of reference elements, i.e., reference elements can be identified without relying on PSVI. The following example illustrates the use of sml:ref. Consider the following schema fragment:                                                                                                                                   The schema definition in the above example is SML agnostic and does not make use of any SML attributes, elements, or types. The EnrolledCourse element, however, has an open content model and this can be used to mark instances of EnrolledCourse as reference elements as shown below:   1000   John Doe             PHY101       A               http://www.university.example.org/Universities/MIT/Courses.xml#xmlns(u=http://www.university.example.org/ns)         xpointer(/u:Courses/u:Course[u:Name='PHY101'])                     http://www.university.example.org                       MAT100       B               http://www.university.example.org/Universities/MIT/Courses.xml#xmlns(u=http://www.university.example.org/ns)         xpointer(/u:Courses/u:Course[u:Name='MAT100'])                     SocialSkills       F       The first EnrolledCourse element in the above example is a reference element since it specifies sml:ref="true". Assuming that references are represented using URI and EPR schemes, it has two representations of the reference to the element for course  PHY101. The second and third EnrolledCourse elements are not reference elements; the second element specifies sml:ref="false" and the third element does not specify the sml:ref attribute. Note that the second element has a child element that contains a reference scheme referring to course MAT100, but this reference will be ignored since sml:ref="false" for the second element. An EnrolledCourse reference element can be a marked as a null reference if it defines the sml:nilref="true" attribute as shown in the following example (the first EnrolledCourse element is a null reference):   1000   John Doe             PHY101       A               MAT100       B               http://www.university.example.org/Universities/MIT/Courses.xml#xmlns(u=http://www.university.example.org/ns)         xpointer(/u:Courses/u:Course[u:Name='MAT100'])                     SocialSkills       F       Null references are different from empty references. An empty reference is a normal reference that just happen to be empty. For a null reference, the content of that reference is intentionally left empty, as an explicit declaration of intent from the document author that the reference must be empty. In the above example, first reference EnrolledCourse defines the sml:nilref="true" attribute which marks this as a null reference. By setting the reference null, the document author makes an explicit declaration that this Student doesn't have an EnrolledCourse with a name PHY101 and grade A. Reference Definitions SML Reference An element information item in an SML model instance document is an SML reference if and only if: It has an attribute information item whose [local name] is ref and whose [namespace name] is http://www.w3.org/2007/09/sml and whose [normalized value] , after whitespace normalization using collapse following schema rules, is either "true" or "1". Null Reference An element information item in an SML model instance document is an SML null reference if and only if: It is an SML reference. It has an attribute information item whose [local name] is nilref and whose [namespace name] is http://www.w3.org/2007/09/sml and whose [normalized value] after whitespace normalization using collapse following schema rules, is either "true" or "1". A null reference is an explicit declaration of intent by the document author that the reference itself does not exist, and a processing directive (NOT a hint) to processors not to search the reference for reference scheme information items. If a reference element is recognized as null, then processors MUST NOT attempt to resolve it. The question of whether a null reference is resolved or not is undefined; it is an ill-formed question. Unresolved Reference An element information item in an SML model instance document is an SML unresolved reference if and only if: It is a non-null SML reference. None of the recognized schemes resolves. Reference Semantics At Most One Target Every non-null reference MUST target (or resolve to) at most one element in a model. When a scheme or multiple schemes in a reference resolve to more than one target then the model is declared invalid. Multiple References   An element in a document MAY be targeted by multiple reference elements. These reference elements may use different schemes and/or be expressed in different ways.   Null References A model validator MUST not attempt to resolve null references. The question of whether a null reference is resolved to one or more targets is undefined; it is an ill-formed question. deref() XPath Extension Function Each model validator MUST provide an implementation of the deref() XPath extension function that is capable of resolving references expressed in the model validator's chosen scheme(s). This function takes a node-set of elements and returns a node-set consisting of element nodes corresponding to the elements referenced by the input node set.  In particular, for each node R in the input node set the output node set contains at most one element node. The output node set contains one element node for R provided that all of the following conditions are true: sml:ref ="true" for R. R contains at least one reference scheme that is understood by the implementation. The reference targets a single element in some document in the model. The output node set contains no element node corresponding to R if any of the following conditions is true: R is not an SML reference. R is a null reference. R does not contain any reference scheme that is understood by the implementation. R does not target any element in the model. Reference Schemes A reference MAY be represented by using a variety of schemes, and SML does not mandate the use of any specific schemes. A reference scheme normally uses, but is not required to use, child elements, attributes or both to capture the information necessary to identify a resource. and endpoint references (EPRs) [] are two common schemes for referencing resources. Although SML does not require the use of either scheme, it does define how a reference MUST be represented using the URI scheme and the EPR scheme. URI Scheme The URI Scheme is based on the anyURI type defined in the XML schema specification []. References that are represented using the URI scheme MUST be implemented by using the sml:uri global element as a child of reference elements, i.e., elements for which sml:ref="true".  More precisely, if a model author chooses to represent references using the URI scheme, It MUST represent each reference using an instance of the sml:uri global element declaration as a child of the reference element.   It MUST treat each instance of  the sml:uri global element declaration, whose parent element is a reference element, as a reference represented in the URI scheme. For example, if the reference element in is represented using the URI scheme, an instance of EnrolledCourse will appear as follows:   http://www.university.example.org/someUri where http://www.university.example.org/someUri is a valid URI as defined in . Suppose that a model has the following documents, and each document has an associated URI: Document URI Course PHY101 http://www.university.example.org/Universities/MIT/Courses/PHY101.xml Course MAT200 http://www.university.example.org/Universities/MIT/Courses/MAT200.xml Student 1000 http://www.university.example.org/Universities/MIT/Students/1000.xml Student 1001 http://www.university.example.org/Universities/MIT/Students/1001.xml The following is a sample instance document for Student 1000 where the references are represented in the URI scheme:   1000   John Doe             http://www.university.example.org/Universities/MIT/Courses/PHY101.xml               http://www.university.example.org/Universities/MIT/Courses/MAT200.xml       Fragment Identifier  Fragment identifiers in references that are represented using the URI scheme MUST use the following XPointer [] profile: Only two schemes – xmlns() [] and xpointer() [] – are supported. The expression specified for the xpointer scheme MUST be a restricted XPath [] expression that MUST resolve to at most one element node. In particular, this expression MUST NOT contain the union ("|") operator defined for XPath point() and range() node tests defined for the xpointer() scheme This expression can only use the functions defined in the XPath core function library []. It MUST NOT use the smlfn:deref function and/or the following functions defined for xpointer() scheme []: range-to string-range range range-inside start-point end-point here origin The following example illustrates the use of xpointer fragments. Consider the case where all courses offered by MIT are stored in a single XML document – Courses.xml – whose URI is  http://www.university.example.org/Universities/MIT/Courses.xml. In this case, the element inside Courses.xml that corresponds to the course PHY101 can be referenced as follows (assuming that Coursesis the root element in Courses.xml)   1000   John Doe                     http://www.university.example.org/Universities/MIT/Courses.xml#xmlns(u=http://www.university.example.org/ns)         xpointer(/u:Courses/u:Course[u:Name='PHY101'])             A reference element can also be used to reference an element in its own document. To see this consider the following instance document   MIT             PHY101               MAT200                   123       Jane Doe                                     #xmlns(u=http://www.university.example.org/ns)             xpointer(/u:University/u:Courses/u:Course[u:Name='MAT200']                               Here, the EnrolledCourse element for the student Jane Doe references the Course element for MAT200 in the same document. EPR Scheme References that are represented using the EPR scheme MUST be implemented by using instances of  wsa:EndpointReference global element declaration [] as  child  elements of reference elements. The following example illustrates how the EnrolledCourse reference that references course PHY101 in MIT university can be represented using the EPR scheme: [Example Under Construction]       http://www.university.example.org   Constraints on References SML supports the following attributes for expressing constraints on reference elements. Attributes Name Description sml:acyclic Used to specify whether cycles are supported for a reference. sml:targetElement Used to constrain the name of the reference's target element. sml:targetType Used to constrain the type of the reference's target element. sml:targetRequired Used to specify that a reference's target element is required to be present in the model. These attributes MUST be supported in the following specified locations only in an SML model: These attributes MUST be supported in XML Schema documents. The sml:acyclic attribute MUST be supported for any element. The sml:target* attributes MUST be supported for any element. SML defines a new property for every Complex Type Definition schema component: {acyclic} An xs:boolean value. Required. And 3 new properties for every Element Declaration and every Particle component: {target element} An Element Declaration component. Optional. {target required} An xs:boolean value. Required. {target type} A Type Definition component. Optional. sml:acyclic Model validators that conform to this specification MUST support the sml:acyclic attribute on any element in a schema document. This is a boolean attribute and its value can be either true or false. Mapping from Schema If sml:acyclic is present, then {acyclic} has the actual value of sml:acyclic; otherwise, it has the same value of {acyclic} as its {base type definition}. Rules A cyclic type can be used to derive cyclic or acyclic reference types, but all derived types of an acylic reference type are acyclic. Model validators that conform to this specification MUST enforce the following: If a complex type definition D's {base type definition} is also a complex type definition and has {acyclic} true, then D MUST have {acyclic} true. Validation If T is a complex type definition with {acyclic} true, then instances of T MUST NOT create cycles in any model. More precisely, the directed graph whose nodes are SML model instance documents and whose edges are SML references with [type definition] T (an edge is directed from the document containing the SML reference to the document containing the target element),  must be acyclic. Constraints on Targets SML supports three attributes: sml:targetElement, sml:targetRequired, and sml:targetType, for constraining the target of a reference. These three attributes are collectively called sml:target* attributes and they MUST be supported on global and local element declarations. Model validators that conform to this specification MUST enforce the following: If one/more of sml:target* attributes are specified (either explicitly or by default) for a particle P in a complex-type definition CT, then all particles in CT that have the same name as P must specify the same set of sml:target* attributes as P and these attributes must have the same values as those specified for P. In particular, all of the following must be enforced: If sml:targetElement="ns:GTE" for P then sml:targetElement="ns:GTE" for all particles in CT that have the same name as P If sml:targetRequired="true" for P then sml:targetRequired="true" for all particles in CT that have the same name as P If sml:targetRequired="false" for P then sml:targetRequired="false" for all particles in CT that have the same name as P If sml:targetType="ns:T" for P  then sml:targetType="ns:T" for all particles in CT that have the same name as P The above conditions on the use of sml:target* attributes have been defined to reduce the implementation burden  on  model validators for verifying  that the use of sml:target* attributes is consistent across derivation by restriction. These conditions enable model validators to find the restricted particle for a restricting particle using a simple name match when sml:target* attributes are specified for these particles.  In the absence of the above conditions, it is extremely difficult for SML validators to verify consistent use of sml:target* attributes across a base type and its restricted derived type. In order to verify consistent use of an sml:target* attribute on a restricted particle in the base type and its restricting particle in a restricted derived type, it is necessary to connect the particles in the derived type with  those from the restricted base type. However, this level of support is not provided by most XML Schema frameworks; thus most SML validators would otherwise need to duplicate large parts of XML Schema's compilation logic to verify consistent usage of sml:target* attributes across derivation by restriction.     sml:targetElement Model validators that conform to this specification MUST support the sml:targetElement attribute on any element declarations. The value of this attribute MUST be the qualified name of some global element declaration. Let sml:targetElement="ns:GTE" for some element declaration E. Then each element instance of E MUST reference an element that is an instance of ns:GTE or an instance of some global element declaration in the substitution group hierarchy whose head is ns:GTE. If a target element constraint is specified for a global element declaration G then it continues to apply to all global element declarations in the substitution group hierarchy whose head is G. However, a global element declaration in G's substitution group can specify a target element constraint that refines the constraint defined for G. In particular, model validators that conform to this specification MUST enforce the following: If sml:targetElement="ns:GTE" is specified for G, and SG is a global element declaration that specifies G as the value of its xs:substitutionGroup attribute, then if  sml:targetElement is specified for SG then its value MUST  be ns:GTE or the name of a global element declaration in the substitution group hierarchy whose head is ns:GTE if sml:targetElement is not specified for SG , then sml:targetElement="ns:GTE" holds for SG by default. If a target element constraint is specified for a particle P in some type B, then it continues to apply to each particle PR that is a valid restrictions of P where PR is defined in some restricted derived type of B (see Schema Component Constraint: Particle Valid (Restriction) , section 3.9.6, Constraints on Particle Schema Components, [] for XML Schema's definition of valid restrictions). However, PR can specify a target element constraint that refines the constraint defined for P.  In particular, model validators that conform to this specification MUST enforce the following: If sml:targetElement="ns:GTE" is specified for P and sml:targetElement is specified for PR , then the value of sml:targetElement for PR must be ns:GTE or the name of a global element declaration in the substitution group hierarchy whose head is ns:GTE. If sml:targetElement is not specified for PR , then sml:targetElement="ns:GTE" holds for PR by default. sml:targetRequired Model validators that conform to this specification MUST support the   sml:targetRequired attribute on any element declarations. If  sml:targetRequired ="true" for an element declaration E, then each element instance of E MUST target some element in the model, i.e., no instance of E can be null or contain an unresolved reference. Otherwise, instances of E can be null or contain unresolved references. If this attribute is not specified, then its value is assumed to be "false". Model validators that conform to this specification MUST enforce the following: If the sml:targetRequired  attribute is specified for a global element declaration G then the specified value applies by default to each global element declaration SG in the substitution group hierarchy whose head is G unless the sml:targetRequired  attribute is specified for SG . If sml:targetRequired="true" is specified for a global element declaration G then sml:targetRequired="false"  MUST NOT be specified for any element declaration in the substitution group hierarchy whose head is G. If sml:targetRequired attribute is  specified for a particle  P in some type B, then the specified value applies by default to  to each particle PR that is a valid restriction of P unless the sml:targetRequired attribute is specified for PR (see Schema Component Constraint: Particle Valid (Restriction) , section 3.9.6, Constraints on Particle Schema Components, [] for XML Schema's definition of valid restrictions). If sml:targetRequired="true" for a particle P then sml:targetRequired="false"  MUST NOT be specified for any particle PR that is a valid restriction of P.   sml:targetType The sml:targetType attribute MUST be supported on any element declarations. The value of this attribute MUST be the qualified name of some type definition. Let sml:targetType="ns:T"  for some element declaration E. Then each element instance of E MUST reference an element whose type is ns:T or a derived type of ns:T. If a target type constraint is specified for a global element declaration G then it continues to apply to all global element declarations in the substitution group hierarchy whose head is G. However, a global element declaration in G's substitution group can specify a target type constraint that refines the constraint defined for G. In particular, model validators that conform to this specification MUST enforce the following: If sml:targetType="ns:T" is specified for G, and SG is a global element declaration that specifies G as the value of its xs:substitutionGroup attribute, then if the sml:targetType attribute is specified for SG the its value    MUST be either ns:T or the name of some derived type of ns:T if sml:targetType is not specified for SG , then sml:targetType="ns:T" holds for SG by default If the target type constraint is specified for a particle P in some type B, then it continues to apply to each particle PR that is a valid restriction of P where PR is defined in some restricted derived type of B. However, PR can specify a target type constraint that refines the constraint defined for P. In particular, model validators that conform to this specification MUST enforce the following: If sml:targetType="ns:T" is specified for P and sml:targetType is specified for PR then the value of the sml:targetType for PR must be ns:T or the name of some derived type of ns:T. If sml:targetType is not specified for PR , then sml:targetType="ns:T" holds for PR by default Identity Constraints XML Schema supports the definition of key, unique, and key reference constraints through xs:key, xs:unique, and xs:keyref elements. However, the scope of these constraints is restricted to a single document. SML defines analogs for these constraints, whose scope extends   to multiple documents by allowing them  to traverse inter-document references. Model validators that conform to this specification MUST support the following elements for defining identity constraints across references: Name Description sml:key Similar to xs:key except that the selector and field XPath expression can use smlfn:deref function sml:unique Similar to xs:unique except that the selector and field XPath expression can use smlfn:deref function sml:keyref Similar to xs:keyref except that the selector and field XPath expression can use smlfn:deref function The syntax and semantics of the above elements are according to Section 3.11.2 of the XML Schema specification [XML Schema Structures], except for the following: If an SML identity constraint needs to be specified for an element declaration E, then it MUST be defined in the xs:annotation/xs:appinfo descendant element for the xs:element element for E An SML identity constraint that is specified for an element declaration  E can reuse the definition of an SML identity constraint ID' specified for some other element declaration E' by specifying the name of E' as the value of its ref attribute. In particular, If the ref attribute is specified for an SML identity constraint element that is specified for an element declaration E, then the value of ref attribute MUST NOT be name of any other SML identity constraint element specified for E. If the ref attribute is specified for an sml:key element, then the value of ref attribute MUST be name of another SML key constraint If the ref attribute is specified for an sml:unique element then the value of the ref attribute MUST be name of another SML unique constraint If the ref attribute is specified for an sml:keyref element then the value of the ref attribute MUST be name of another SML keyref constraint Either the ref attribute or the refer attribute (but not both) MUST be specified for an sml:keyref element. If the ref attribute is specified for an SML identity constraint, then the name attribute MUST NOT be specified If the ref attribute is specified for an SML identity constraint, then the selector and field child elements MUST NOT be specified If an SML identity constraint is specified for an element declaration E, then this constraint is applicable to all instances of E in a model, i.e., the identity constraint MUST be satisfied for each instance of E in a valid model The sml:selector XPath expression syntax and the XML identity constraint selector XPath syntax are identical in all respects except for 1 difference. The sml:selector XPath MAY use smlfn:deref() calls, nested to any depth, at the beginning of the expression. The XML identity constraint selector Path production is amended to support this requirement as defined below. Path ::= ('.//')? Step ( '/' Step)* | DerefExpr DerefExpr ::= (NCName ':')? 'deref(' Step (/Step)* ')'  ('/'Step)* |               (NCName ':')? 'deref(' DerefExpr ')' (/Step)* The sml:field XPath expression MUST conform to the amended BNF defined above for the selector XPath expression with the following modification. Path::= ('.//')?  ( Step '/')* ( Step | @NameTest ) |         DerefExpr ('/' @NameTest)? Each SML identity constraint that is specified for a global element declaration G MUST be treated as if it is specified by default for all global element declarations SG that are in the substitution group hierarchy whose head is G Each SML identity constraint that is specified for a particle P in a complex-type definition CT MUST be treated as if it is specified by default for all particles PR in restricted derived types of CT that are a valid restriction of P If one/more SML identity constraints are specified (either explicitly or by default) for a particle P in a complex-type definition CT, then all particles in CT that have the same name as P MUST specify the same set of identity constraints as P. This rule is defined to reduce the implementation burden for model validators. It facilitates the matching of restricting and restricted particles using their names, and avoids the replication of large parts of XML Schema's compilation logic for this purpose. University Example The following example will be used to illustrate the sml:key, sml:unique, and sml:keyref constraints across references.                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        sml:key and sml:unique XML Schema supports key and uniqueness constraints through xs:key and xs:unique, but these constraints can only be specified within a single XML document. The sml:key and sml:unique elements  support the specification of key and uniqueness constraints across documents.  We'll use the UniversityType (see ) definition to illustrate this concept. It is reasonable to expect that each student in a university must have a unique identity, and this identity must be specified. This can be expressed as follows:                                                                                                                     The sml:key and sml:unique constraints are similar but not the same. sml:key requires that the specified fields must be present in instance documents and have unique values, whereas sml:unique simply requires the specified fields to have unique values but does not require them to be present in instance documents.  Thus keys imply uniqueness, but uniqueness does not imply keys.  For example, students in a university must have a unique social security numbers, but the university may have foreign students who do not possess this number. This constraint can be specified as follows:                                                                                                                     The sml:key and sml:unique  constraint are always specified in the context of a scoping element. In the above example, the University element declaration is the context for the key and unique constraints. The following example illustrates the use of the ref attribute in an SML identity constraint:                                                         In the above example, the PrivateUniversity element declaration specifies the StudentSSNisUnique unique constraint by referencing its definition in the University element declaration. sml:keyref XML Schema supports key references through xs:keyref to ensure that one set of values is a subset of another set of values within an XML document. Such constraints are similar to foreign keys in relational databases. Key references in XML Schema are only supported within a single XML document. The sml:keyref element allows key references to be specified across XML documents, and can be used to scope references to point to elements within a valid range. The following example uses sml:keyref to capture the requirement that courses in a university can only enroll students from the same university:                                                                                                                                                                                             The above constraint specifies that for a university, the set of IDs of students enrolled in courses is a subset of the set of IDs of students in a university. In particular, the selector and field elements in StudentIDisKey key constraint identify the set of IDs of students in a university, and the selector and field elements in CourseStudents key reference constraint identify the set of IDs of students enrolled in courses.   Rules XML Schema supports a number of built-in grammar-based constraints but it does not support a language for defining arbitrary rules for constraining the structure and content of documents. Schematron [] is an ISO/IEC standard for defining assertions concerning a set of XML documents. SML uses a profile of the Schematron schema to add support for user-defined constraints. SML uses XPath, augmented with the smlfn:deref()  extension function, as its constraint language. Model validators that conform to this specification are REQUIRED to support and evaluate XPath expressions augmented with the smlfn:deref() function in the body of Schematron constraints. This section assumes that the reader is familiar with Schematron concepts; the Schematron standard is documented in [] and [, ] are good tutorials on an older version of Schematron. User-defined constraints can be specified using the sch:assert and sch:report elements from Schematron.  The following example uses sch:assert elements to specify two constraints: An IPv4 address must have four bytes An IPv6 address must have sixteen bytes                                                                               A v4 IP address must have 4 bytes.                     A v6 IP address must have 16 bytes.                                                                               A Schematron pattern embedded in the xs:annotation/xs:appinfo element for a complex type definition or an element declaration is applicable to all instances of the complex type or element. In the above example, the pattern Length is applicable for all elements whose type is IPAddress or a derived type of IPAddress. A pattern element contains one or more rules and a rule element contains one or more assert and/or report elements. Each rule specifies its context using the context attribute. This context expression is evaluated in the context of each applicable element and results in an element node set for which the assert and report test expressions defined in the rule are evaluated. The context expression is defined as an XSLT Pattern. This means that the smlfn:deref function may not be used in the location path of a context expression. In the above example, context=".". Therefore the two assert expressions are evaluated in the context of each applicable element, i.e., each element of type IPAddress. The test expression for an assert is a boolean expression, and the assert is violated (or fires) if its test expression evaluates to false. A report is violated (or fires) if its test expression evaluates to true. Thus, an assert can be converted to a report by simply negating its test expression. The following example uses report elements to represent the IP address constraints of the previous example:                                                                                             A v4 IP address must have 4 bytes.                     A v6 IP address must have 16 bytes.                                                                                   If an assert or report is violated, then the violation MUST be reported during model validation together with the specified message.  Model validation MUST evaluate each Schematron pattern for all of its applicable elements contained in the model.   The message can include substitution strings based on XPath expressions. These can be specified using the sch:value-of element. The following example uses the sch:value-of element to include the number of specified address bytes in the message:               A v4 IP address must have 4 bytes instead of the specified           bytes.       In addition to being embedded in complex type definitions, constraints can also be embedded in global element declarations. Such constraints are evaluated for each instance element corresponding to the global element declaration. Consider the following example:                                                                                                     The specified ID                   does not begin with 99.                                                   The student must be enrolled                   in at least one course.                                                         The constraints defined in StudentPattern are applicable to all element instances of the StrictUniversity global element declaration. For each StrictUniversity element, the XPath expression specified as the value of the context attribute is evaluated to return a node set, and the test expressions for the two asserts are evaluated for each node in this node set.  The context expression for the rule returns a node set consisting of all Student elements referenced by an instance of StrictUniversity, and the test expressions for the two asserts are evaluated for each element node in this node set.  Thus, these two asserts verify the following conditions for each instance of StrictUniversity The ID of each student must begin with '99' Each student must be enrolled in at least one course Model validators that conform to this specification  MUST behave as follows:   Each Schematron pattern that is embedded in the xs:annotation/xs:appinfo element for a global complex-type definition CT MUST be evaluated for all element instances of type CT in a model during the model's validation. Each Schematron pattern that is embedded in the xs:annotation/xs:appinfo element for a global element declaration G MUST be evaluated for all element instances of G in a model during the model's validation. As defined in the Schematron specification [], a pattern MUST be evaluated for an instance element by evaluating the rule elements of the pattern in the order of their definition. The context expression for a rule MUST be evaluated in the context of the instance element, and all asserts and reports contained in the first rule whose context expression evaluates to a non-empty node set MUST be evaluated for each node in this node set. Model validators that conform to this specification MUST provide a mechanism to support binding of Schematron patterns that are authored in separate documents, i.e., not embedded in schema definition, to a set of documents in a model. The mechanism for binding such Schematron patterns to a set of documents in a model is implementation-dependent and hence outside the scope of this specification.  The following example shows the constraints for StrictUniversity expressed in a separate document:                                                                                       The specified ID                   does not begin with 99.                                                   The student must be enrolled                   in at least one course.                                       The binding of the StudentPattern pattern to instances of StrictUniversity element is implementation-dependent and hence outside the scope of this specification. Localization of Error Messages smlerr:localizationid Localization of the natural-language error messages, which provide details about asserts and reports, MAY be supported by model validators that conform to this specification. Such model validators MAY support the use of smlerr:localizationid attribute on sch:report and sch:assert to specify the identity of the resource containing the localized versions of the natural-language error message. Model validators that conform to this specification but do not support smlerr:localizationid attribute MUST ignore all smlerr:localizationid attributes in a model; they MUST NOT treat the model as invalid just because it contains smlerr:localizationid attributes. The mechanisms for mapping values of smlerr:localizationid to the corresponding localization resources are implementation-dependent and hence outside the scope of this specification. Schematron Profile SML supports a conforming profile of Schematron. All elements and attributes are supported.   Limited Support If the queryBinding attribute is specified, then its value MUST be set to "xslt" Structured XML Output from Schematron Rules Schematron has rich support for natural-language error and diagnostic messages that provide details about failed assertions. As per the Schematron specification the content of the sch:assert, sch:report, and the optional sch:diagnostic elements should be natural language assertions or messages. To facilitate machine processable output from the evaluation of Schematron rules, this specification extends Schematron by adding support for structured XML output that provides details about failed assertions. This structured XML data can be consumed by an application to perform some application-specific tasks required to handle a failed assertion. This is an OPTIONAL feature and model validators that conform to this specification are not REQUIRED to support it. Model validators that conform to this specification but do not support smlerr:output element MUST ignore all smlerr:output elements in a model; they MUST NOT treat the model as invalid just because it contains smlerr:output elements. smlerr:output This element is used to specify the structured XML output for one/more failed assertions. It is supported as a child of the sch:rule element. An sch:rule element can contain multiple smlerr:output elements. The schema definition for smlerr:output is as follows:                     id = a required attribute that defines the identity of an smlerr:output element. This identity is used by an assert and/or report element to specify that the expression specified in the expression attribute of the smlerr:output element must be evaluated to generate structured XML when the assert/report fires. This identity is specified on one or more assert and/or report elements' smlerr:outputids attribute. Each time such an assert or report fires, the smlerr:output element's expression attribute is evaluated to generate structured XML.    applicationUri = an optional attribute that specifies the identity of the application for which the output is generated expression= an XPath expression that evaluates to a node set. The expression is evaluated in the context of the node selected by the context attribute in the  parent sch:rule element. This XPath expression can use the deref() extension function.  smlerr:outputids This global attribute can be used in a sch:report or sch:assert element to specify the identities of the smlerr:output elements whose expressions must be evaluated to generate XML output when the assert/report fires. smlerr:attributeNode This element is used for serialization of each attribute node in the node set resulting from the evaluation of the expression attribute on an smlerr:output element.                                   The value of the name attribute is the qualified name of the attribute whose value is being serialized. smlerr:errorDataType This element is used for enclosing the structured XML generated by an smlerr:output element.                       Semantics When a report/assert fires, then all smlerr:output elements whose ID is listed in the outputids attribute on that sch:report or sch:assert element are evaluated. For each such smlerr:output,  the expression specified in its expression attribute is evaluated, and the resulting node set is serialized into XML by concatenating each node and enclosing the serialized XML fragment in the smlerr:errorData element to create a well-formed XML document. The resulting document is returned to the application that initiated the model validation. If the node set is empty, an empty smlerr:errorData element is returned. The nodes in the node set may be serialized in any order.  Element nodes are serialized directly into their XML representation, and attribute nodes are serialized by using the smlerr:attributeNode element. All namespace bindings defined (through the sch:ns element) for the parent sch:rule, sch:pattern, or sch:schema elements remain valid and can be used in the expression specified in the expression attribute. Examples The following example illustrates the use of smlerr:output                                                                                                            A v4 IP address must have 4 bytes.                                             A v6 IP address must have 16 bytes.                                                                                                         If the report with id="v4" fires for an element ipaddress of type IPAddressType, then the output may look like       v4    
10
   
10
   
20
   
0
   
0
  The following example illustrates an smlerr:output element whose expression results in attribute nodes                                                                                 A university must have  more than 20 students                                                                                           If  the assert fires for an element of type universityType then the output may look like   MIT   false Conformance Criteria A program is a conforming SML model validator if it satisfies the following conditions: The validator MUST perform model validation as defined in this specification. Model validation is the process of examining each document in a model and verifying that this document is valid with respect to the model definition documents, i.e., each model instance document satisfies the schemas and rules defined in the applicable model definition documents. The validator MUST support XML, XML Schema and XPath but MAY also support any future versions of these specifications. The validator MUST perform Schematron rule evaluation on the #ALL phase, implying that every rule in every pattern is evaluated. A set of XML documents is a conforming SML model if it satisfies the following conditions: Each document in the model MUST be a well-formed XML document [] Each XML Schema document in the model's definition documents MUST satisfy the conditions expressed in Errors in Schema Construction and Structure (§5.1). [] Each Schematron document in the model's definition documents MUST be a valid Schematron document [] In each instance document in the model, the [validity] property of the root element and all of its attributes and descendants MUST NOT be "invalid" when schema validity is assessed by a conforming schema-aware processor with respect to the referenced XML Schema documents in the model's definition documents. [] Each document in the model MUST satisfy all applicable Schematron constraints. Each document in the model MUST satisfy all applicable sml:acyclic and sml:target* constraints. SML Extension Reference This section is a non-normative reference of the SML extensions to XML Schema and XPath. Attributes sml:acyclic Used to specify that a complex type is acyclic, i.e., its instances do not create any cycles in a model. If this attribute is set to true for a complex type D, then instances of D (including any derived types of D) can not create any cycles in a model. More precisely, the directed graph whose nodes are documents that contain the source or target elements for instances of D, and whose edges are instances of D (an edge is directed from the document containing the source element to the document containing the target element),  must be acyclic. A model is invalid if its documents result in a cyclic graph using instances of D. In the following example, Hostref is a complex type declaration whose instances can not create any cycles: ... If the sml:acyclic attribute is not specified or set to false for a complex type declaration, then instances of this type may create cycles in a model. sml:ref This global attribute is used to identify reference elements.  Any element that has sml:ref="true" will be treated as a reference element. sml:nilref This global attribute is used to identify null reference elements. This attribute MUST NOT be used on an element unless it also has sml:ref="true" specified.  Any element that has sml:nilref="true" or sml:nilref="1" will be treated as a null reference element. sml:targetElement A QName representing the name of a referenced element   sml:targetElement is supported as an attribute for any element declarations. The value of this attribute must be the name of some global element declaration. Let sml:targetElement="ns:GTE" for some element declaration E. Then each element instance of E must target an element that is an instance of ns:GTE or an instance of some global element declaration in the substitution group hierarchy whose head is ns:GTE. In the following example, the element referenced by instances of HostOS must be instances of win:Windows A model is invalid if its documents violate one/more sml:targetElement constraints. sml:targetRequired Used to specify that instances of a reference element must target elements in the model, i.e., an instance of the reference element can not be null or contain an unresolved reference which does not target any element in the model. Therefore it is an error if targetRequired="true" is specified on an element declaration where the corresponding reference element R has sml:nilref="true". In the following example, the targetRequired attribute is used to specify that application instances must have a host operating system.                                 A model is invalid if its documents violate one/more sml:targetRequired constraints. sml:targetType A QName representing the type of a referenced element     sml:targetType is supported as an attribute for any element declarations. If the value of this attribute is specified as T, then the type of the referenced element must either be T or a derived type of T. In the following example, the type of the element referenced by the OperatingSystem element must be "ibm:LinuxType" or its derived type A model is invalid if its documents violate one/more sml:targetType constraints. Elements sml:key This element is used to specify a key constraint in some scope. The semantics are essentially the same as that for xs:key but sml:key can also be used to specify key constraints on other documents, i.e., the  sml:selector child element of sml:key can contain deref functions to resolve elements in another document. sml:key is supported in the appinfo of an xs:element.   sml:keyref Applies a constraint in the context of the containing xs:element that scopes the range of a nested document reference.                                                                     > sml:keyref is supported in the appinfo of an xs:element.   sml:unique This element is used to specify a uniqueness constraint in some scope. The semantics are essentially the same as that for xs:unique but sml:unique can also be used to specify uniqueness constraints on other documents, i.e., the  sml:selector child element of sml:unique can contain deref functions to resolve elements in another document. sml:unique is supported in the appinfo of an xs:element.   sml:uri Specifies a reference in URI scheme. This element must be used to specify references that use the URI scheme. XPath functions smlfn:deref node-set deref(node-set) This function takes a node-set of elements and attempts to resolve the references contained in the elements that have sml:ref="true".    The resulting node-set is the set of elements that are obtained by successfully resolving (or de-referencing) the  reference contained in each element in the input node-set for which sml:ref="true".  For example, deref(/u:Universities/u:Students/u:Student) will resolve the reference in element Student. The target of the reference must always be an element. References Normative References Key words for use in RFCs to Indicate Requirement Levels, S. Bradner, Author. Internet Engineering Task Force, June 1999. Available at http://www.ietf.org/rfc/rfc2119.txt. Information technology ― Document Schema Definition Languages (DSDL) ― Part 3: Rule-based validation ― Schematron. International Organization for Standardization and International Electrotechnical Commission, 1 January 2006. Available at http://standards.iso.org/ittf/PubliclyAvailableStandards/c040833_ISO_IEC_19757-3_2006(E).zip Web Services Addressing 1.0 - Core, M. Gudgin, M. Hadley, and T. Rogers, Editors. World Wide Web Consortium, 9 May 2006. This version of the WS-Addressing Core Recommendation is http://www.w3.org/TR/2006/REC-ws-addr-core-20060509. The latest version of WS-Addressing Core is available at http://www.w3.org/TR/ws-addr-core. Extensible Markup Language (XML) 1.0 (Fourth Edition), T. Bray, J. Paoli, C. M. Sperberg-McQueen, and E. Maler, Editors. World Wide Web Consortium, 10 February 1998, revised 16 August 2006. This version of the XML 1.0 Recommendation is http://www.w3.org/TR/2006/REC-xml-20060816. The latest version of XML 1.0 is available at http://www.w3.org/TR/REC-xml. XML Schema Part 1: Structures Second Edition, H. Thompson, D. Beech, M. Maloney, and N. Mendelsohn, Editors. World Wide Web Consortium, 2 May 2001, revised 28 October 2004. This version of the XML Schema Part 1 Recommendation is http://www.w3.org/TR/2004/REC-xmlschema-1-20041028. The latest version of XML Schema 1.0 Part 1 is available at http://www.w3.org/TR/xmlschema-1. XML Schema Part 2: Datatypes Second Edition, P. Byron and A. Malhotra, Editors. World Wide Web Consortium, 2 May 2001, revised 28 October 2004. This version of the XML Schema Part 2 Recommendation is http://www.w3.org/TR/2004/REC-xmlschema-2-20041028. The latest version of XML Schema 1.0 Part 2 is available at http://www.w3.org/TR/xmlschema-2. XML Path Language (XPath) Version 1.0, J. Clark and S. DeRose, Editors. World Wide Web Consortium, 16 November 1999. This version of XML Path Language (XPath) Version 1.0 is http://www.w3.org/TR/1999/REC-xpath-19991116. The latest version of XML Path Language (XPath) Version 1.0 is available at http://www.w3.org/TR/xpath. XPointer Framework, P. Grosso, E. Maler, J. Marsh, and N. Walsh, Editors. World Wide Web Consortium, 25 March 2003. This version of the XPointer Framework Recommendation is http://www.w3.org/TR/2003/REC-xptr-framework-20030325/ The latest version of XPointer Framework is available at http://www.w3.org/TR/xptr-framework/. XPointer xmlns() Scheme, S. DeRose, R. Daniel Jr., E. Maler, and J. Marsh, Editors. World Wide Web Consortium, 25 March 2003. This version of the XPointer xmlns() Scheme Recommendation is http://www.w3.org/TR/2003/REC-xptr-framework-20030325/ The latest version of XPointer xmlns() Scheme is available at http://www.w3.org/TR/xptr-xmlns/. XPointer xpointer() Scheme, S. DeRose, E. Maler, and R. Daniel Jr., Editors. World Wide Web Consortium, 19 December 2002. This version of the XPointer xpointer() Scheme specification is http://www.w3.org/TR/2002/WD-xptr-xpointer-20021219/. The latest version of XPointer xpointer() Scheme is available at http://www.w3.org/TR/xptr-xpointer/. Informative References An Introduction to Schematron, Eddie Robertsson, Author. O'Reilly Media, Inc., 12 November 2003. Available at http://www.xml.com/pub/a/2003/11/12/schematron.html Improving XML Document Validation with Schematron, Dare Obasanjo, Author. Microsoft Corporation, September 2004. Available at http://msdn2.microsoft.com/en-us/library/Aa468554.aspx XML Schema Part 0: Primer Second Edition, D. Fallside and P. Walmsley, Editors. World Wide Web Consortium, 2 May 2001, revised 28 October 2004. This version of the XML Schema Part 0 Recommendation is http://www.w3.org/TR/2004/REC-xmlschema-0-20041028. The latest version of XML Schema Part 0 is available at http://www.w3.org/TR/xmlschema-0. Normative SML Schema Specifies if the element contains a reference Specifies that the reference element denotes a “null” reference. To be used only on elements for which sml:ref="true". A qualified name of an element in the referenced document. If true, requires the target element of the reference to exist in the model. A qualified name of the type of the element in the referenced document. If this attribute is set to true for a type D then instances of D should not create any cycles in a model. More precisely, the directed graph whose edges represent instances of D, and whose nodes represent documents that contain the source or target elements for instances of D, must be acyclic. References in URI scheme must be representend by this element. Normative SML Error Schema Sample Model This sample model illustrates the use of the following SML extensions: Inter-document references key and keyref constraints User-defined constraints Each application in workstation must be hosted on an operating system Application from does not have high security level. A secure workstation can only contain applications from TrustedVendor. Acknowledgements The editors acknowledge the members of the Service Modeling Language Working Group, the members of other W3C Working Groups, and industry experts in other forums who have contributed directly or indirectly to the process or content of creating this document. At the time this specification was published, the members of the Service Modeling Language Working Group were: John Arwe (IBM Corporation), Jordan Boucher (Sun Microsystems, Inc.), Pratul Dublish (Microsoft Corporation), Zulah Eckert (BEA Systems, Inc.), Sandy Gao (IBM Corporation), Philippe Le Hégaret (W3C/MIT), Paul Lipton (CA), James Lynn (HP), Kumar Pandit (Microsoft Corporation), Valentina Popescu (IBM Corporation), Virginia Smith (HP), Michael Sperberg-McQueen (W3C/MIT), Bassam Tabbara (Microsoft Corporation), Vijay Tewari (Intel Corporation), Marvin Waschke (CA), Kirk Wilson (CA).