4.3 Technologies Used to Create Web Service Contracts
Now that weâ€™ve described the primary parts of a Web service contract and also how they can relate to each other, letâ€™s identify the development technologies suitable for creating these parts.
Each of these technologies shares the following characteristics:
- Language â€“ Every part of a Web service contract is defined by writing code that conforms to a markup language. This is different from a traditional programming language, in that we do not compile our final results into a software program. We simply use the languages to create definitions that exist as textual documents; easy to access and read, but also reliant upon supporting platform programs for runtime processing.
- XML-Based â€“ The markup grammar used to write these languages is expressed in XML. Therefore, you end up writing a Web service contract as a set of XML documents for the purpose of exchanging messages that also exist as XML documents and themselves carry data that is further represented with XML.
- Vendor-Neutral â€“ Web service contract technologies were all developed by the same industry standards organization (the W3C) through rigorous, multi-year processes. Each was owned by a technical committee comprised of members from different vendor and practitioner organizations, and the final design of most of these language features required a consensus across this committee. Therefore, each of these technologies is also considered a legitimate industry standard.
More information regarding standards development processes, procedures, and stages is provided in Appendix B. Also note that these next sections only briefly introduce each technology; more in-depth coverage is provided in subsequent chapters.
â€œElementâ€ vs. â€œConstructâ€ vs. â€œDefinitionâ€
Before we introduce the technologies, letâ€™s first establish some additional terminology. If you are familiar with XML, you know that all of these XML-based languages are comprised of vocabularies of pre-defined elements, similar to those used in HTML (another Web language that was ratified into a standard through the W3C).
An additional term youâ€™ll see used in these next sections and throughout the remaining chapters is construct or element construct. A â€œconstructâ€ is a term used to refer to an element that contains nested child elements (in which case the element is implemented with an opening and closing tag). Think of a construct as an â€œelement container.â€
For example, an HTML table exists as a table construct comprised of various nested elements (and nested constructs).
There are no rules as to when an element is referred to as a construct. It is just helpful at times to communicate that a particular part of a contract is represented as a container of elements. The individual parts and definitions of a Web service contract that weâ€™ve been explaining all exist as element constructs that contain further details. We make a distinction between a definition and a construct (or element) when we need to discuss how a part of a contract is implemented (coded) as opposed to designed.
So a definition, a construct, and an element can all refer to the same thing, just from different perspectives.
As we get into the details of these technology languages, weâ€™ll use a style convention whereby specific language element names are displayed in a different font. For example, we might state that the port type definition is created using the portType construct which is comprised of opening and closing portType tags.
Web Services Description Language (WSDL)
WSDL is considered to be the most fundamental and important technology for Web services development because it has become the de facto language for writing Web service contracts. Using WSDL, contract structure is defined and then further detailed and extended with the other technologies covered in this section.
As you may have gathered, weâ€™ve already been discussing WSDL in this chapter. The basic Web service contract structure illustrated in previous diagrams has represented a WSDL definition, and the abstract and concrete descriptions are the fundamental parts of a WSDL structure (Figure 4.9).
Figure 4.9. The basic WSDL definition structure that we have been exploring throughout this chapter.
A WSDL document ends up acting as somewhat of a container in which elements from almost all other languages covered in this book converge. Chapters 7 and 8 introduce the WSDL language as it applies to abstract and concrete descriptions respectively. As you explore how these parts of the WSDL definition are assembled, you will be able to see just how much code in the WSDL document does not originate from the WSDL language.
Chapter 9 provides further WSDL coverage specific to the WSDL 2.0 standard, and Chapters 14 and 15 drill down into a wide range of advanced topics.
XML Schema Definition Language (XML Schema)
XML Schema provides a formal language for defining the structure and validation constraints of XML documents. Because XML is a native part of the WSDL language and because Web services operate by exchanging XML messages, XML Schema is a highly complementary technology for creating Web service contracts.
Schemas serve as the part of a contract that describes the detailed structure of messages. They define what elements and attributes may appear in the message, in what order, and what type of data they are allowed to contain.
All of the type definitions displayed in the previous figures are assumed to be created using the XML Schema language. The types section displayed earlier in the physical view of an abstract description (Figure 4.4) is technically part of the WSDL language, but its primary purpose is to house XML Schema elements.
In case youâ€™re wondering why sometimes the word â€œschemaâ€ is capitalized and other times it isnâ€™t, this is explained in the â€œXML Schemaâ€ vs. â€œXML schemaâ€ section at the beginning of Chapter 6.
XML Schema code can be embedded directly within WSDL documents or kept as separate documents and then referenced from the WSDL definitions. Each approach has its pros and cons.
When XML schemas exist as independent files, you can establish a many-to-many relationship between the schema documents and the WSDL message definitions. One schema might be used to describe several messages, particularly if they are related and share types. Similarly, each message definition can reference several different XML Schema types originating from different XML Schema documents.
Most often, you will be designing your own schema from the ground up, based on your specific requirements. At other times, you will be using an industry schema that was developed by an industry organization, a vendor, or even a private corporation. When using an industry standard XML schema, you may wish to reuse only parts of it or extend it to meet your specific needs.
There are multiple ways to structure XML documents, just like there are a number of ways to design a database. Once you have decided on an XML structure (i.e. which elements and attributes you want to use) there is also more than one way to express that structure in an XML Schema definition.
Figure 4.10 provides a preview of common XML Schema constructs that we will be covering in Chapter 6. You will have the freedom to create a variety of different types based on XML Schema features that provide a series of built-in types and allow for the assembly of composite types commonly used to express complex structures and even entire business document structures.
Figure 4.10. A logical view of a sample XML Schema definition structure comprised of different types that can then be used to define the structure of WSDL message definitions. Note the symbol on the left used to represent an XML Schema document.
Introductory XML Schema coverage is provided in the next chapter. Chapters 12 and 13 then delve into more advanced topics related to complex message design.
The WS-Policy standard specifies a policy framework specifically for Web services that enables behavior-related constraints and characteristics to be expressed as machine- readable metadata. This means that we can use this language to extend the base Web service contract provided by WSDL and XML Schema in order to add requirements or promote characteristics of the Web service that supplement the definitions in the abstract and concrete descriptions.
Figure 4.11 shows us that policy definition documents are just as modular as WSDL and XML Schema documents. As with XML Schema types, individual policy requirements (also referred to as policy assertions) can be combined or related to each other in order to form more complex composite policies (called policy expressions). You even have the ability to offer consumers a choice between two or more policies via the use of policy alternatives.
Figure 4.11. A sample WS-Policy definition with two composite policy expressions, each comprised of multiple policy assertions. Note the symbol on the left used to represent a policy document.
Another similarity that WS-Policy shares with XML Schema is how it relates to WSDL. Policies can also reside within or outside of the WSDL document. The related WS-PolicyAttachment language is used to associate (attach) policies to the various definitions in the abstract and concrete descriptions.
The WS-Policy and WS-PolicyAttachment languages are introduced in Chapter 10. More advanced topics are covered later in Chapters 16 and 17. Part of this coverage includes an exploration of how policy assertions are used by other languages to implement their features.
SOAP provides a messaging format and mechanism that supports the standardized exchange of XML messages between software programs. SOAP has become the most prevalent messaging protocol used in conjunction with WSDL.
The SOAP language is used to express the structure of XML-based envelopes that host message definitions populated with actual data. Although at runtime SOAP language elements are usually auto-generated, SOAP is very much a technology related to the design-time creation of Web service contracts.
SOAP messages and message exchanges are fundamentally defined within the message and operation definitions of the WSDL document, and various SOAP language elements are further added to the concrete description in order to bind them to SOAP as a messaging protocol.
SOAP messages are most commonly:
- transmitted via HTTP
- described using XML Schema types
- associated with WSDL definitions in the concrete description
As shown in Figure 4.12, a standard SOAP message is divided into a set of parts. These parts are individually defined by XML Schema types via WSDL message definitions.
Figure 4.12. The SOAP message structure is defined by the SOAP standard and expressed with the SOAP language. Note the use of the symbol on the right to indicate a SOAP message in this book.
There are many supplemental technologies that extend SOAP messages via the use of SOAP header blocks. Many of these technologies are part of the WS-* specifications (also known as the second-generation Web services platform).
SOAP is introduced in Chapter 11. Advanced message design considerations are covered in Chapters 18 and 19 in relation to the WS-Addressing standard which provides fundamental routing and Web service instance identification header blocks.
Technologies for the Abstract Description
Figure 4.13 shows a high level mapping between the primary abstract description definitions and the related Web service technologies that can be used to create these definitions.
Figure 4.13. A high level mapping of the abstract description parts and related industry technologies.
What this figure tells us is that the overall structure of the abstract description (including the port type, operation, and message definitions) is created using WSDL (1).
In addition to the WS-I Profile, which is light on XML Schema guidelines, the W3C XML Schema Patterns for Databinding Working Group has documented a set of best practices for designing schemas for Web services. The XML Schema patterns are divided into two categories, basic and advanced, and there is a separate specification for each:
- Basic patterns use only the basic features of XML Schema and should already be correctly implemented by all toolkits.
- Advanced patterns are those that may still cause some tool interoperability problems today, but should be considered recommended patterns for the future.
To view the actual specifications for any of the languages covered in this book, visit www.soaspecs.com.
The types part of a Web service contract is created collectively with WSDL and XML Schema (2) code in that the WSDL language provides a container construct wherein XML Schema code can be placed to express the various XML Schema types.
Finally, policy definitions can be separately created using the WS-Policy language (3). These policies can then be attached to the different WSDL definitions using the related WS-PolicyAttachment language.Technologies for Concrete DescriptionsLetâ€™s now turn our attention to the technologies used to build a concrete description for a Web service. Figure 4.14 shows us the primary concrete description parts as they relate to three of the technology languages we just introduced.
In this depiction of a concrete description, we again establish how the WSDL (1) language is responsible for creating the overall structure, including the port type, operation and message binding definitions; as well as the service, port, and address definitions.
All of the constructs used to create these definitions can be further supplemented with special SOAP language statements (2) that allow the definitions to be directly associated with the SOAP messaging protocol.
And, as with the abstract description, all parts can be further supplemented with policy statements expressed using the WS-Policy language (3) as well as the related WS-PolicyAttachment language.
The WS-I Basic Profile
The Web Services Interoperability Organization (WS-I) develops specifications dedicated to establishing baseline interoperability between Web services using the technologies we just described.
The primary WS-I specification weâ€™ll be making reference to throughout this book is the Basic Profile, a document that essentially consists of a series of requirements and recommendations as to how Web services-related technology languages should be used together. Wherever appropriate, weâ€™ll be highlighting WS-I Basic Profile guidelines to supplement and provide further insight into contract design-related content.
Figure 4.14. A look at the concrete description parts and the technology languages used to create them.
SUMMARY OF KEY POINTS
- The primary technology languages used to create the abstract description are WSDL, XML Schema, and WS-Policy.
- The primary technology languages used to create the concrete description are WSDL, SOAP, and WS-Policy.
- A WSDL document will be comprised of code originating from all of these languages.
- XML Schema and WS-Policy definitions can optionally be placed into standalone documents that are then referenced from within the WSDL definition.