Web Service Description Requirements

Editors Copy 25 February 2002

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Jeffrey C. Schlimmer, Microsoft


This document describes the Web Service Description Working Group's requirements for the Web Service Description specification.

Status of this Document

This document is an editors' copy that has no official standing.

This section describes the status of this document at the time of its publication. Other documents may supersede this document. The latest status of this document series is maintained at the W3C.

This is the first W3C Working Draft of the Web Service Description requirements document. It is a chartered deliverable of the Web Service Description Working Group (WG), which is part of the Web Service Activity. The Working has agreed to publish this document, although this document does not necessarily represent consensus within the Working Group about Web Service Description requirements.

Comments on this document should be sent to ws-desc-comments@w3.org (public archive [2]). It is inappropriate to send discussion emails to this address.

Discussion of this document takes place on the public ws-desc@w3.org mailing list [3] per the email communication rules in the Web Service Description Working Group Charter [4].

This is a public W3C Working Draft. It is a draft document and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use W3C Working Drafts as reference material or to cite them as other than "work in progress". A list of all W3C technical reports can be found at http://www.w3.org/TR/.

Table of Contents

1 Notations
2 Relationship to WG Charter
3 Requirements
    3.1 General
    3.2 Simplicity
    3.3 Interface Description
    3.4 Description of Interactions with a Service
    3.5 Messages and Types
    3.6 Service Types
    3.7 Bindings
    3.8 Reusability
    3.9 Extensibility
    3.10 Versioning
    3.11 Security
    3.12 Mapping to the Semantic Web
4 Requirements from other W3C WGs
    4.1 XML Protocol
    4.2 XForms
    4.3 RDF
    4.4 P3P
5 References
6 Acknowledgements
7 Change Log

1 Notations

The following terminology and typographical conventions have been used in this document.

Each requirement has a three digit number with a prefix indicating the status as follows:

  1. A "DRnnn" (Draft Requirement) notation indicates a requirement that the WG is actively considering (has not reached rough consensus within the WG).

  2. An "Rnnn" (Requirement) notation indicates a requirement that the WG is not actively considering at present (has reached rough consensus within the WG).

  3. A "NRnnn" (Non Requirement) notation indicates a requirement that the WG has rejected.

The numbers used to identify requirements are arbitrary and do not imply any ordering or significance.

The document includes several verbatim quotes from the Web Service Description WG Charter [4] which provide context for the requirements. The quoted text is emphasized and prefixed with "Charter".

Each requirement also has one of the following three prefixes to indicate its priority:

These keywords are to be interpreted in a manner similar to that described in [1]. Changes from [1] are indicated above with emphasis.

Finally, each requirement has a prefix indicating its origin:

2 Relationship to WG Charter

The Web Service Description WG Charter [4] has two sections describing what is in-scope and what is out-of-scope of the problem space defined for the WG. The WG considers all the requirements in section 1 of [4] to be in-scope per the Charter.

Reviewers and readers should be familiar with the Web Service Description WG Charter [4] because it provides the critical context for the requirements and any discussion of them.

3 Requirements

3.1 General


[MUST NOT] Charter: The language developed by the WG must not preclude any programming model, nor assume any transport or protocol for communication between peers. (Last revised 21 Feb 2002.)


[MUST] JS: Describe constructs using the XML Infoset model. Similar to the SOAP 1.2 specifications, the WG specifications will use the XML Infoset model. (Last revised 21 Feb 2002.)


[MUST] WG: WSDL 1.2 processors must support XML Schema (http://www.w3.org/2001/XMLSchema*). (Last discussed 21 Feb 2002.)


[MUST] WG: The WG specifications must support other type systems (besides XML Schema http://www.w3.org/2001/XMLSchema*) via extensibility. (Last discussed 21 Feb 2002.)


[MUST] WG: The schema and examples for the WG specifications must be written in XML Schema (http://www.w3.org/2001/XMLSchema*). (Last discussed 21 Feb 2002.)


[SHOULD] JS: Use available XML technologies wherever possible.


[SHOULD] KL: Correct errors/inconsistencies in WSDL1.1


[SHOULD] KL: Provide better specification for document name and linking. WSDL1.1 section 2.1.1 is over simple. More detailed specification should be provided to define how the import mechanism works, especially how it's related to the import and include mechanism defined in the XML Schema specification.


[SHOULD] KL: Provide detailed examples including on-the-wire messages. (Was Best Practices and Conformance Test. Although a few examples are given in WSDL 1.1 specification, the examples are not sufficiently explained in the text, and no corresponding wired message examples are available for different binding definitions. To help clearer interpretation of the specification, more consistent and detailed examples should be provided. In addition, a technical report associated with the WSDL specification should be dedicated to provide: Use cases which illustrate typical usage scenarios of WSDL, Best practices, Conformance test suite.)


[SHOULD] KL: Enable easy Interaction with Upper layers in the web services stack. Additional technologies will be required in the future to complete the web services architecture. As one of the fundamental layers of the web services stack, though WSDL should not depend on any other layers, one of the design goals of WSDL should be easy interaction with upper layers, such as services composition layers.


[SHOULD] KL: Editorial Improvements. Consistent terminology should be used across all the sections of the specification. As one example, the usage of the terms "port" and "endpoint" is confusing to many readers. Though it states that "port" is "a single endpoint defined as a combination of a binding and a network address", the specification uses "port" and "endpoint" interchangeably in many places. Some diagrams that explain the structure of core WSDL elements and their relationship will help reduce misinterpretations.


[SHOULD] YF: The WSD specifications should be clear and easy to understand. This clarity implies that considerable editorial effort will be required in the structuring of the narrative through both outline/overview and normative reference material.


[SHOULD] YF: The WSD specification should make reasonable efforts to support applications that operate on resource constrained devices. Even though any practical device is resource constrained in any number of dimensions including but not limited to bandwidth, computational power and storage, the term "resource constrained device" often refers to hand-portable devices. This document does not attempt to define the term "resource constrained" nor what the constraints are for the available resources.


KL: Support up-to-date XML Schema. In all WSDL 1.1 examples, the October 2000 version of the XML schema is used: http://www.w3.org/2000/10/XMLSchema. We understand that the 10/2000 schema was the most up-to-dated schema available at the time WSDL1.1 was released. However, in future versions of WSDL specification, the W3C recommendation version of the XML schema should be used. The recommendation was released in May 2001: http://www.w3.org/2001/XMLSchema. (Last discussed 21 Feb 2002. Replaced with DR098, R099, and R100.)

3.2 Simplicity


[MUST] Charter: The WG specifications must be simple to understand and implement correctly. Simplicity is a key element in making distributed systems easy to understand, implement, maintain, and evolve. Although simplicity can only be measured in relative terms, the WG must ensure that the complexity of any solution produced is comparable to that of other current and widespread Web solutions.


[SHOULD] Charter: Another important aspect of simplicity is ease of deployment. The Working Group will look at various ways of deploying the Web services descriptions in a manner that is compatible with the existing Web infrastructure.


[SHOULD] JS: Specification shall be as lightweight as possible, keeping parts that are mandatory to a minimum.


[SHOULD] JS: Optional parts of the specification should be orthogonal to each other allowing non-conflicting configurations to be implemented.


[SHOULD] YF: Facilitate the creation of simple applications (fast and easy writing for simple apps).


[SHOULD] YF: Be possible to compare easily two WSDL web services


[SHOULD] YF: Since WSD is intended to be a foundation service description language, its definition should remain simple and stable over time. Explicit use of modularity and layering in the resulting design will help assure longevity. Such a framework will allow subsequent extension of the design while leaving the foundation of the design intact.


[SHOULD] YF: The WSD specification must clearly identify conformance requirements in a way that enables the conformance of an implementation of the specification to be tested (see also the W3C Conformance requirements (W3C members only)).


Charter: Focus must be put on simplicity, modularity and decentralization. (Last discussed 21 Feb 2002. Replaced with DR013, DR102, DR027.)


JS: Be simple to understand and implement correctly; comparable to other widespread Web solutions. (Last discussed 21 Feb 2002. Replaced with DR013.)

3.3 Interface Description


[MUST] Charter: The description language must describe the messages accepted and generated by the Web service. (Last revised 21 Feb 2002.)


[MUST] Charter: The description language designed will be used both by applications in order to automatically communicate between each other as well as by programmers developing Web services themselves. The language should therefore provide, in addition to the raw XML definition of the interface, human-readable comment capabilities to allow both applications and developers to make use of them.


[MUST] Charter: The language must also describe the error messages generated, if any.


[MUST] YF: Support abstract interfaces.


[MUST] YF: Support interfaces derived from abstract interfaces.


[MUST] KL: The final WSDL specification should be divided into two parts: the first part only focuses on the core interface definition language, and the second part addresses the binding extensions. This requirement concurs with the Charter's requirement for two separate deliverables.


[SHOULD] Charter: Developers are likely to want to extend the functionality of an existing Web service. The Working Group will look into extending interface descriptions in a decentralized fashion, i.e. without priori agreement with the original interface designers.


[SHOULD] WS: In a lot of cases, it is important for the server to expose some service-wide properties/attributes. These properties/attributes have the service-level scope and could be used to describe either some Qos parameters or some application specific characteristics. As an example, a service may want to expose an attribute which describes the version number of the service. Hence, a web service description language should be able to model service level attributes/properties.


[SHOULD] KL: Distinction between Interface definition and Implementation definition. A description of a web service can be logically divided into three parts: Data type definition, Service Interface definition and Service Implementation definition. The data type definition can be viewed as part of the Service Interface Definition. Analogous to defining an abstract interface in a programming language and having many concrete implementations, a service interface definition can be instantiated and referenced by multiple service implementers. WSDL 1.1 specification implies such a division by providing the mechanism for dividing a service definition into multiple WSDL documents. WSDL1.1 Section 2.1.2, Authoring Style, shows an example of separating a complete service definition into three documents: data type definition, abstract definitions and specific service bindings. However, this distinction is not clear and reference to each unit is very difficult. To facilitate easier allocation of responsibilities among different organizations (such as standard bodies and service providers) or among different teams within an organization (such as teams related to the different stages of a service's lifecycle: design time/development time, configuration time and run time), a better distinction between Interface definition and Implementation definition should be made in the specification. Elements such as message, portType, operation are abstract interface definitions, and are usually defined at design time. Elements such as binding and services usually get their value at configuration/deployment/run time. Mixing all these elements together is at least confusing to many people.


Charter: The data exchanged is usually typed and structured. This increases interoperability by having applications agreeing on semantics and also provides some level of error detection. It is expected that developers will want to use different mechanisms for describing data types and structures, depending on the purpose of the Web service. The Working Group should allow different mechanisms, and must define one based on XML Schema. (Last discussed 21 Feb 2002. Redundant with R021, R090, R100.)

3.4 Description of Interactions with a Service


[MUST] Charter: The Working Group will define a mechanism which will allow a Web service to describe the following set of operations: one-way messages (to and from the service described), request-response and solicit-response, as described in WSDL 1.1's port types.


[MUST] JJM: Must describe SOAP 1.2 MEP (Message Exchange Pattern) (charter says: "must [...] describe [...] one-way messages, [...] request-response")


[MUST] JS: Must be able to describe simple one-way messages, i.e., either incoming or outgoing (event) messages.


[MUST] JS: Must be able to describe simple request-response-fault message exchange.


[MUST] JS: Be able to describe sets of messages that form a logical group (i.e., a port type).


[MUST] JS: Be able to derive a port type from another by extension of the logical group of messages.


[MUST] JS: Be able to extend protocol descriptions using mechanisms not explicitly identified in the spec.


[MUST NOT] JS: (Not a requirement to describe arbitrary message exchanges.)


[SHOULD] IS: Able to define events. WSDL should be very specific about events, defining a special type of a message or even a separate definition entity. Currently it is missing in WSDL 1.1.


[SHOULD] IS: Define operations that can be carried out synchronously (i.e. have a short expected reply cycle) and those messages that require asynchronous notification (i.e. have long expected reply cycle). Ability to differentiate such operations is a requirement of the service definition rather than application architecture. For the async operations, providing a reference to an event definition or status inquiry operation should be required.


[SHOULD] PF: Context. Web Services requests often have an associated context -- such as the identity of the caller. This context has a logical component - typically a set of name-value pairs - and a binding into the particular format or transport. So for example, a userid and password may be bound into the HTTP transport using Basic Auth, or may be encoded in the SOAP header, and the Service implementation may be expecting that. Furthermore, there are some context items that the service may require, and there are others that are not required but may be understood. It is also possible that there are context items that are not required or understood, but the service may have a policy of passing on non-understood context items to other service it calls, or returning them to the client -- which may use them for correlation.


[SHOULD] PF: WSDL is typically used to capture the server requirements on the client. For example, the server will expect to see certain SOAP headers. When WSDL is used in higher protocols, such as an orchestration language, each side of the exchange may wish to publish their requirements, and the "client" may have a requirement on the "server". For example, the client may require the server to set a particular header on the response. In WSDL today, there is an option to try to map this into the "Out-In" or "Out" interactions, by treating them as the "conjugates" of the corresponding "In-Out" or "In-only" operations. However, this is unsatisfactory, as these interactions are not well defined, and there is no way to specify that an Out-In is actually the conjugate of an In-Out, or simply another operation that has the same messages in the opposite order. It would be more satisfactory if the concept of "conjugates" was exposed directly so that the "client" side of an interaction could publish their requirements. This could be used by proposal such as flow or orchestration languages.


[SHOULD] IS: Extensible metadefinitions. Be able to include *typed* metadata attributes for any definition element: message, part, port, operation, binding, service. The attributes may also be hierarchical (i.e. defined in another namespace).

3.5 Messages and Types


[MUST] JS: Must be able to describe messages independent of specific wire format.


[MUST] JS: Must be able to describe messages using XML Schema simple and complex types.


[MUST] JS: Be able to extend message descriptions using mechanisms not explicitly identified in the spec.


[MUST] JS: Be able to describe messages that include arrays and nested arrays.


[MUST] JS: Be able to describe messages of other protocols besides SOAP 1.2.


[MUST] YF: Clearly separate the description of the operations (messages?) from the message exchange pattern and protocol binding.


[MUST NOT] JS: (No requirement to describe semantic content of messages.)


[SHOULD] JS: Be able to describe messages using other info sets.


[SHOULD] PP: Be able to describe messages that include references (URIs) to strongly-typed referents.


[SHOULD] IS: Be able to describe references to other services (remote) or other interfaces (ports, local to this WSDL doc) that can be used as parts in message definitions. Currently (as of WSDL 1.1) message parts refer to data types (described in one or the other schema). The part must also be able to refer to a remote service (WSDL URL/Service/Port) or a local service/port qualified names. This has to be made clear as part of the standard for WS clients and service providers.


[SHOULD] YF: Support grouping functionalities (operations) that share the same message-exchange pattern and transport binding


[SHOULD] JR: Be able to classify/categorize [individual] operations. With the usage of XML schema in the ELEMENT attribute of the PART element (current WSDL spec) it is possible to use a type system as a kind of taxonomy for a semantically enriched description of parameters. To automatically search a suitable service respectively operation from a set of service descriptions it is not enough only to consider the parameters but also a kind of operation "type" (something like a taxonomy on operations). So I would suggest a kind of ELEMENT or TYPE attribute for operations.

3.6 Service Types


[MUST] KB: Describe Web services operations in an abstract format using the XML type system.


[MUST] JS: Be able to describe a logical group of port type instances (i.e., a service type).


[MUST] JS: Be able to derive one service type from another by extension of the logical group of port instances.


[MUST] KB: Group logically related operations together into abstract service types.


[MUST] JS: Be able to extend service descriptions using mechanisms not explicitly identified in the spec.


[SHOULD] JS: Be able to name an instance of a port type.

3.7 Bindings


[MUST] Charter: The information exchanged to and from a Web service can be carried in a large number of different ways. The action of carrying some XML-based communication in an underlying protocol is called, in the XML Protocol [SOAP 1.2] jargon, a binding. The description language defined should therefore describe how to reach the Web Service in a form which is orthogonal to its message exchange patterns and its messages.


[MUST] KB: Apply specific wire-format serializations (bindings) for service types.


[MUST] JS: Be able to describe endpoint location using URIs.


[MUST] JS: Be able to describe address for specific port instances within a service.


[MUST] JJM: Describe SOAP 1.2 messages, including header, body, encoding, fault, RPC, actor (charter says: "must describe messages available via interface", "must describe error messages" and "make sure SOAP 1.2 extensibility mechanism can be expressed").


[MUST] JS: Be able to describe SOAP 1.2 messages.


[MUST] JS: Provide a normative description for SOAP 1.2 messages.


[MUST] JS: Be able to describe SOAP 1.2 Header elements and Body elements.


[MUST] JS: Be able to describe SOAP 1.2 Faults.


[MUST] JJM: Describe SOAP 1.2 header and body's content type (charter says: "must define [a mechanism for describing data types and structures] based on XML Schema" and "take into account ending work going on in XML Protocol").


[MUST] JS: Provide a normative description of the binding for SOAP 1.2.


[MUST] Charter: The Working Group will also ensure that other SOAP bindings can be described.


[MUST] KL: Better Specification for Binding Extensions. In addition to the core service definition framework, WSDL1.1 introduces specific binding extensions for SOAP 1.1, HTTP GET/POST and MIME, and nothing precludes the use of other binding extensions. To keep the core service definition framework simple, a separate and more detailed specification or technical report should be dedicated for various bindings.


[MUST] KB: Apply in an orthogonal manner specific transport(s) for a binding.


[MUST] JS: Be able to describe bindings to transports other than HTTP.


[MUST] JJM: Ensure that SOAP 1.2 bindings to SMTP or BEEP (for example) can be described (charter says: "ensure that other SOAP bindings can be described").


[MUST] JS: Be able to describe the wire format of messages, whether XML, ASCII, binary, or some combination.


[MUST] MW: Must be able to describe messages that include binary data, where the binary data is transmitted efficiently.


[MUST] JS: Be able to extend transport binding descriptions using mechanisms not explicitly identified in the spec.


[MUST] JS: Be able to separate design-time from run-time information.


[MUST/SHOULD] Charter: It is expected that in the near-term future, Web services will be accessed largely through SOAP Version 1.2 (the XML-based protocol produced by the XML Protocol Working Group) carried over HTTP/1.1, or by means of simple HTTP/1.1 GET and POST requests. The Working Group will describe the following bindings:

  1. [MUST] SOAP Version 1.2; SOAP can be used over a variety of underlying protocols; the Working Group will provide a concrete SOAP Version 1.2 over HTTP binding.

  2. [SHOULD] HTTP/1.1 GET and POST requests.


[SHOULD] JJM: Describe SOAP 1.2 RPC parameters types (ibid.).


[SHOULD] JJM: Support SOAP 1.2 intermediaries (charter says: "make sure SOAP 1.2 extensibility mechanism can be expressed")


[SHOULD] PP: Support all HTTP methods (verbs), including WebDAV and allow the use of non-standard HTTP methods.


[SHOULD] PF: A clearer separation of transport and binding in the spec. The current spec builds bindings that are tied to the transport, and therefore a hard link between the port and binding information. There are obviously cases where formatting information is specific to the transport, so it would be good to see the ability to define multiple bindings and to define bindings that are independent of transport, and also perhaps independent of the exact port type. So an example would be --- define a port type, then a binding for "soap-encoded rpc style", then a binding for SOAP/HTTP, then a port for HTTP, which points to both bindings.


[SHOULD] FC: WSDL 1.1 defines services and operations and their bindings to various protocols. However, the details of how an operation is identified (either generally or specifically in particular bindings) is, shall we say, rather vague. As a result, some implementations use the namespace & element of the first child of Body (in SOAP RPC), others use SOAPAction header (in SOAP over Http), others use only the namespace, others the element name, others attempt to match the message type, etc. As a result, interoperability suffers.

It seems like a normative model (at least) for operation determination is necessary for interoperability between clients and servers from different vendors. This may be a requirement to define such a requirement for all defined bindings, as opposed to something that can be completely specified in the description. But I believe that such a requirement exists.


[SHOULD] PM: Ability to bind network address to the operation at runtime. For example, it is possible to have a service that supports operations like "Register" and "Notify" where a user will provide an email address that a Web Service can send notifications to when the user registers with the service. So the binding network address for the "Notify" operation needs to be dynamically populated at runtime. To achieve this it becomes necessary to allow a placeholder for dynamic bindings within the description of the Web Service.


[SHOULD] JJ: Based on the XML Protocol Usage Scenario (2.14 S21 Incremental parsing/processing of SOAP messages) and other requirements (a SOAP processor returning a large amount of data as attachment or message) there is a need for a SOAP processor and the SOAP client proxies to be constructed with the notion of data streaming in mind so that applications can scale well. (Especially in the case of dynamic proxy and stub creation scenarios.) This requirement for the SOAP processors imposed a requirement on the WSDL to be descriptive enough (like MIME binding or some kind of extension) to describe so that the Service Provider will do incremental parsing and processing of data (input) and the client can process the return message or attachment the same way. Without this description most of the toolkits will find it difficult to use this SOAP processor advantages for scalability and/or fail in interoperability.

3.8 Reusability


[MUST] JS: Be able to partition a description across multiple files.


[MUST] JS: Be able to use a description fragment in more than one description.


[MUST] YF: Support reusability of WSDL documents or parts of documents.


[SHOULD] IS: Be able to accommodate namespace clusters with data types (schemas) and interface definitions (message/port/binding). I.e. service may have several namespaces with types and several other namespaces with message/port definitions. That is pretty important for expressing proper OO model of a service. Very few framework implementations pay attention to this (in many cases namespaces are flattened out which results in name conflicts). I guess it is so because namespaces of various type definitions and message/port/binding definitions have never been emphasized as a requirement really.

3.9 Extensibility


[MUST] Charter: The language proposed must support the kind of extensibility actually seen on the Web: disparity of document formats and protocols used to communicate, mixing of XML vocabularies using XML namespaces, development of solutions in a distributed environment without a central authority, etc. In particular, it must support distributed extensibility.


[MUST] JJM: Must support an open content model (charter says: "must support distributed extensibility" and "will look into extending interface descriptions in a decentralized fashion").


[MUST] Charter: The Working Group will make sure that SOAP Version 1.2's extensibility mechanism can be expressed.


[SHOULD] JS: Define a set of constructions to indicate which extensions are optional versus mandatory.

3.10 Versioning


[MUST] PF: Have a simple versioning tag to identify versions of WSDL documents or services.


[SHOULD] FC: It would be good to allow for versioning of something smaller than a WSDL document.  I suspect that tools vendors will "compose" these documents, and they may sometimes contain information about a number of unrelated services (or, more correctly, services that are related in ways other than application semantics (tool vendor, server location,  etc)).  It would be good if web services themselves were versioned, the web services being the semantic "unit" being defined.

3.11 Security


[MUST NOT] JS: Compliance must not preclude building implementations that are resistant to attacks.


[SHOULD] DM: Should define best practice for signing descriptions using XML signatures, including canonicalization, transforms, XPath selectors, etc. I would probably be willing to leave this function out of the "core" functionality, because some client configuration WSDL driven tools might not care about issues of trusting WSDL input.

3.12 Mapping to the Semantic Web


[MUST] Charter: The Working Group will provide a mapping to RDF so that the information described can be easily merged with that of other applications. This mapping will be developed with the help of the RDF Interest Group.

4 Requirements from other W3C WGs

These are requirements submitted by other W3C Working Groups and Activities.


[MUST] Charter: The Working Group will also take into account the encoding work going on in the XML Protocol Working Group.


[MUST] JS: Coordinate with W3C XML Activity and XML Coordination Group.

4.1 XML Protocol

4.2 XForms

4.3 RDF

4.4 P3P

5 References

Key words for use in RFCs to Indicate Requirement Levels (See http://www.ietf.org/rfc/rfc2119.txt.)
Web Service Description Comments Archive (See http://lists.w3.org/Archives/Public/www-ws-desc-comments/.)
Web Service Discussion Archive (See http://lists.w3.org/Archives/Public/www-ws-desc/.)
Web Service Description Charter (See http://www.w3.org/2002/01/ws-desc-charter.)

6 Acknowledgements

7 Change Log

Date Editor Change
25 Feb 2002 JS Per 21 Feb teleconference, added NR prefix for non requirements, accepted 5 requirements and rejected 4. Added DR102 through DR108 for newly submitted requirements.
20 Feb 2002 JS Added DR081 through DR097. Assigned initial priorities ala [1].
13 Feb 2002 JM Created