WSJ Interoperability

On August 12, 2003, the Web Services Interoperability Organization (WS-I), released the Final Material version of the WS-I Basic Profile 1.0 specification. This publication represents an important milestone for WS-I and the Web services community as a whole. It specifies the standards and technologies required for interoperability between Web services implementations running on different software and operating system platforms.

The Promise of Interoperability
The promise of interoperability is possibly the most important aspect of Web services technologies. That promise stems from the fact that Web services has its foundations in XML, which itself is interoperable across all platforms and programming languages. However, because Web services leverages heavily on the extensible nature of XML, the interoperability aspect of Web services is significantly challenged.

While most, if not all, vendors provide support for the established Web services standards, they are still motivated to provide added value to their customers in the form of advanced feature support for things such as security, reliability, transactions, and business process orchestration. Because many of the advanced Web services features are still in the early stages of development and adoption, developers and IT managers need more than just a checklist of (emerging) standards when making project implementation or product purchasing decisions. They need help in being able to determine when they are "coloring outside the lines" so that they can weigh the merits of incorporating these advanced features against the importance of ensuring broad interoperability of the deployed solution.

WS-I was founded with a mission to provide users of Web services technology with the guidance and tools that help them better understand where the boundary lies between the interoperable and not-necessarily-interoperable solution spaces so that they can make well-informed decisions.

About WS-I
The Web Services Interoperability Organization is an open industry effort chartered to promote Web services interoperability across platforms, applications, and programming languages. The organization brings together a diverse community of Web services leaders to respond to customer needs by providing guidance, recommended practices, and supporting resources, such as testing tools and sample applications, that enable the development of interoperable Web services.

WS-I Deliverables
The Basic Profile 1.0 is the first of a set of deliverables being produced by WS-I related to the Basic Profile. When complete, the package of deliverables produced in conjunction with all WS-I Profiles will be as follows:

• Use cases and usage scenarios: Use cases and usage scenarios capture (respectively) business and technical requirements for the use of Web services. These requirements reflect the classes of real-world requirements supporting Web services solutions, and provide a framework to demonstrate the guidelines described in WS-I Profiles.
• Profiles: A set of named Web services specifications at specific revision levels, together with a set of implementation and interoperability guidelines recommending how the specifications may be used to develop interoperable Web services.
• Sample applications: Demonstrate the implementation of applications that are built from Web services usage scenarios and use cases, and that conform to a given set of profiles. Implementations of the same sample application on multiple platforms, languages, and development tools demonstrate interoperability in action, and provide readily usable resources for the Web services practitioner.
• Testing tools: Used to monitor and analyze interactions with a Web service to determine whether or not the Web service instance or its artifacts (such as messages, WSDL, and UDDI registration components) conform to WS-I Profile guidelines.

At the time of this writing, each of the WS-I deliverables related to the Basic Profile 1.0 has been either formally approved as Final Material, or has been made public in the form of a Working Group Approval Draft.

Philosophy of the Profile
The WS-I Basic Profile was developed by the Basic Profile Working Group with a set of guiding principles that have been outlined in the Profile. These guiding principles form the "philosophy of the Profile."

Possibly the most important of these guiding principles is that there can be no guarantee of interoperability. The best that we could hope to achieve would be to improve the potential for interoperability since we were only dealing with the very basics of Web services technologies and we did not intend to address application-level semantics. Another key guiding principle is that the Profile never relaxes requirements of an underlying specification. That is to say that the Profile never changes a MUST to a SHOULD. However, the Profile often seeks to improve interoperability by reducing the optional features of an underlying specification by changing SHOULDs and SHOULD NOTs to MUSTs and MUST NOTs.

The Profile also focuses on interoperability, not functionality. While the underlying specifications may contain design flaws and inconsistencies, the Profile focuses only on those that directly affect interoperability. WS-I leaves the work of addressing any inadequacies of a specification to the standards body that is assigned stewardship of the standard.

Scope of the Profile
Each Profile has a scope that is defined by the set of referenced specifications. A Profile attempts to improve interoperability within its own scope by placing constraints on optional features of the referenced specifications, clarifications of ambiguities in the referenced specifications, and guidelines for use of the referenced specifications. A Profile does not impose constraints on that which is out of the scope of the Profile.

A key aspect of Web services is the composable nature of the specifications. WS-I Profiles are also intended to exhibit this same composable nature. They do so by defining the set of extensibility points, the extension mechanisms and parameters defined in the underlying specifications that may require out-of-band negotiation and/or agreement explicitly outside the scope of a Profile. While their use may impair interoperability, it is not subject to claims of conformance.

A Profile may place constraints on the use of extensibility points without constraining their range, so that specific uses of extensibility points may be further constrained by other Profiles to improve their interoperability when used in conjunction with the Profile.

The WS-I Basic Profile specification defines conformance of a Web service instance and its artifacts such as the messages it sends, its WSDL description and UDDI registration. The profile consists of the following set of nonproprietary Web services specifications:

• SOAP 1.1
• WSDL 1.1
• UDDI 2.0
• XML 1.0 (Second Edition)
• XML Schema Part 1: Structures
• XML Schema Part 2: Datatypes
• RFC2246: The Transport Layer Security Protocol version 1.0
• RFC2459: Internet X.509 Public Key Infrastructure Certificate and CRL Profile
• RFC2616: HyperText Transfer Protocol 1.1
• RFC2818: HTTP over TLS
• RFC2965: HTTP State Management Mechanism
• The Secure Sockets Layer Protocol version 3.0

The Profile adds constraints and clarifications to those base specifications with the intent to promote interoperability. Where the Profile is silent (i.e., imposes no clarification or constraint), the base specifications are normative. If the Profile prescribes a requirement in the form of a clarification or constraint, the Profile supersedes the underlying base specification. Some of the constraints imposed by the Profile are intended to restrict, or require, optional behavior and functionality so as to reduce the potential for interoperability problems resulting from impedance mismatch between implementations that have made different choices with regard to implementation of the optional functionality. Other Profile requirements are intended to clarify language in the base specifications that have been the source of frequent misinterpretation, resulting in interoperability problems. Where possible, the Basic Profile WG has tried to ensure that the Profile clarifications are aligned with the thinking and direction of the Working Group responsible for the stewardship of the underlying specification to which the clarification applies. For example, clarifications to the SOAP1.1 specification were often aligned with issue resolutions made by the W3C XML Protocol WG responsible for the development of the SOAP1.2 specification.

Profile Highlights
The following list highlights some of the key constraints imposed by the Profile:

• Precludes the use of SOAP encoding
• Requires the use of HTTP binding for SOAP
• Requires the use of HTTP 500 status response for SOAP Fault messages
• Requires the use of HTTP POST method
• Requires the use of WSDL1.1 to describe the interface of a Web service
• Requires the use of RPC-literal or document-literal forms of WSDL
• Precludes the use of RPC-encoded–style WSDL
• Precludes the use of solicit-response and notification style operations
• Requires the use of WSDL SOAP binding extension with HTTP as the required transport
• Requires the use of WSDL1.1 descriptions for UDDI tModel elements representing a Web service

What's Relevant to the Developer?
The WS-I Basic Profile 1.0 specification is a rather complex document. A majority of the specification is targeted at the audience of runtime platform and development tool vendors working on vendor-specific implementations of SOAP processors, WSDL parsers, code generators, and the like. You could reasonably consider the Profile to be a concerted effort by those tools and platform vendors to ensure that their respective products will either generate or host interoperable Web services instances.

However, it isn't enough that each of the major vendors adopt the Profile for their product offerings since each will likely retain support for certain features that the Profile does not sanction (such as RPC-encoded Web services) and most will offer support for features that are outside the scope of the Profile. A Web services developer or IT manager should be familiar with all of the profile specification's contents. However, certain sections of the Profile are specifically relevant to the implementation of interoperable Web services.

The following lists each substantive section of the profile specification and its relevance to a Web service practitioner.

• Section 4: Relates to SOAP and the use of HTTP binding for SOAP. As such, it is mostly of interest to those developers writing SOAP processor implementations rather than Web services developers.
• Section 5: Pertains to conformant use of WSDL, and as such should be of interest to Web services practitioners, especially those who handcraft their WSDL descriptions.
• Section 6: Pertains to Web service discovery using UDDI. This, too, should be of interest to Web services practitioners. It describes conformant approaches to registration and categorization of a Web service in a UDDI registry.
• Section 7: Relates to security of Web services using HTTP/S and should also be of interest to Web services practitioners who require security for the Web services they develop.

Many of the Profile requirements are often accompanied by examples of SOAP messages or WSDL descriptions that demonstrate both conformant and nonconformant adherence to the constraints and clarifications provided. The requirements associated with examples are likely to be of specific interest to Web services practitioners. However, the other WS-I deliverables related to the Profile may be more appropriate and relevant to the IT manager and Web service developer.

Scenarios, Sample Applications, and Testing Tools
The WS-I Sample Applications Working Group has developed deliverables based on the Basic Profile that a Web services practitioner will find useful.

• A mock supply-chain sample application that demonstrates most of the key features of the WS-I Basic Profile
• A Usage Scenarios specification that defines the most common design patterns for Web services and maps those scenarios to the Profile requirements that apply

The sample application serves a dual purpose. For vendors, it provides a means by which they can demonstrate and test their product's support for the requirements set forth by the Profile. To date, 10 vendors have produced independently developed implementations of the sample application, typically based on their respective runtime platform and/or development tooling. Each vendor has provided the source of their implementation so that Web services developers can better understand what they need to do to develop their own interoperable Web services.

The Testing Tools Working Group has delivered approval drafts of their reference testing tools for each of the major runtime platforms (Java and C#). They have also translated the constraints and requirements defined in WS-I Basic Profile 1.0 into formal test assertions that are used to configure the WS-I Testing Tools.

Web services practitioners can use the published reference testing tools to test their Web service instances, WSDL descriptions, and UDDI registrations for conformance to the Profile's requirements. IT managers can use the reports produced by the WS-I Testing Tools as a means of determining whether the Web services their developers have developed conform to the requirements of the Profile.

Future versions of the WS-I Testing Tools reports will be augmented to identify the extensibility points that are used in a Web service instance so that IT managers (and developers) can make informed decisions as to whether the solutions they develop and deploy meet the specific interoperability requirements of a given situation. If a Web service requires broad interoperability, such as might be the case with an Internet deployment of a service, they might wish to constrain the use of extensibility points to those covered by a WS-I Profile(s). Conversely, if a Web service is being deployed for use within an intranet, interoperability may not be considered as high a priority as the advanced features provided through the use of an extensibility point. IT managers can leverage the information provided by the testing tools to make an appropriate, well-informed decision based on the requirements of the given situation.

Looking Beyond WS-I Basic Profile 1.0
The WS-I Basic Profile 1.0 is, of course, just the tip of the iceberg. WS-I has already begun work on a number of follow-on profiles for Web services, including Attachments and Basic Security. Work will begin on future profiles, tackling some of the more advanced Web services features as the various specifications upon which they are based mature and stabilize and as the interoperability requirements associated with these advanced features are better understood by the community.

As WS-I releases these future profiles and their associated testing tools and sample applications deliverables, the Web services community benefits by reducing the tension induced by having to choose between the need for broad interoperability and the need for advanced functionality that is not yet broadly adopted.

References

WS-I: http://ws-i.org

WS-I Basic Profile 1.0: http://ws-i.org/Profiles/Basic/2003-08/BasicProfile-1.0a.htm

WS-I Testing Tools: http://ws-i.org/implementation.aspx

Published Dec. 31, 2003— Reads 14,961
Copyright © 2003 SYS-CON Media, Inc. — All Rights Reserved.
Syndicated stories and blog feeds, all rights reserved by the author.

About Christopher Ferris
Chris Ferris is an IBM Distinguished Engineer and CTO of Industry Standards in the Software Group Standards Strategy organization. He has been actively engaged in open standards development for XML and Web services since 1999. Ferris is former chair of the WS-I Basic Profile Working Group. He co-chairs the W3C Web Services Policy Working Group and serves as chair of the W3C XML Protocols Working Group. He represents IBM on the OASIS WS-RX Technical Committee. He is a former elected member of the OASIS Technical Advisory Board (TAB).