Jun. 19, 2002 12:00 AM
In the past two years, we have witnessed an explosion of Web services and XML communication technologies. While WSDL , SOAP, and UDDI have become the accepted bases of Web services, there are even more standards in the making.
This article is the first of a two-part series that examines the Web services technological space in order to provide an overview of some of the major Web services standards now in progress in various organizations and consortiums across the country.
General Classifications of Web Services
The description stack deals with a wide range of technologies that describe Web services in order to facilitate their common use for business process modeling and workflow choreography in B2B collaborations. The discovery stack deals with technologies that allow for directory, discovery, and inspection services. The wire stack consists of technologies that provide the steam for the runtime engines of Web services.
Figure 2 breaks these stacks into their subcomponents. Many of the available Web services technologies can be mapped to these stacks, although not all stacks have a corresponding specification or technology.
Functional Classifications of Web Services Technology
The list below shows the various functional areas of the Web services technology space. This list is not comprehensive, but it does cover most of the available technologies.
- Service description
- Communication protocols
- Transport protocols
- Directory services
- Process modeling and orchestration
In the remainder of this article, I'll address each of the functional categories of Web services technology and discuss the standards that apply.
WSDL takes a two-step approach to describing Web services. The first step is to provide an abstract definition of services and the data format; the second is to bind this abstract definition to concrete protocols. This two-step process permits reuse; it's possible to have many similar Web services based on one abstract definition with each implemented using different protocols.
WSDL is independent of any network and communication protocols, although it does define default binding to HTTP, SOAP, and MIME. Similarly, WSDL isn't tied to any type of system, although it does use XML Schema. WSDL is designed to be extensible to work with different types of systems and other network and communication protocols. Listing 1 demonstrates the simple WSDL grammar used to describe services. Note: This example isn't complete and won't parse; more namespaces need to be defined.
In Listing 1 the message element, along with the part element, defines the data in abstract terms. The operation element defines the action supported by the service. WSDL defines four basic operations: one-way, request-response, solicit-response, and notification. The portType element acts as a container for a set of abstract operations. In this example, we define a portType element, "StockQuotePortType", with a single operation, "GetLastTradePrice", which takes an input message, "GetLastTradePriceRequest", and gives an output message, "GetLastTradePriceResponse".
These abstract definitions are then bound to concrete protocols using the binding element. The port element captures the communication endpoint details, and the service element contains a list of related ports. The types element (not shown) acts as a data container holding various data type definitions. In the example, the operations in "StockQuotePortType" are bound to SOAP and HTTP.
WSDL enjoys the support of many tools. Some help generate WSDL from existing Java and C++ classes, and others generate Java and C++ classes from WSDL documents.
XML-RPC is an XML-based RPC protocol based on HTTP POST with a simple data model that came from Userland software in 1998. Compared to SOAP it is simple; in addition to RPC, SOAP provides much richer processing semantics, an enhanced data model, and support for messaging. SOAP has garnered a great deal of attention and a huge user base.
The ebXML messaging specification, built on top of SOAP, is one part of a set of ebXML specifications. WDDX, an effort from Allaire, is focused on providing a simple, lightweight data exchange mechanism for Web programming languages such as ColdFusion, ASP, Perl, and PHP. Though RPC semantics can be layered on top of WDDX, it isn't as widely adopted as SOAP for RPC purposes. Jabber is an open-source protocol that enables exchange of structured information in a near-real-time manner between two or more end points. Jabber is used in the instant messaging areas.
Let's look at SOAP in detail, since it is the protocol of choice for most Web services.
SOAP, the Protocol of Choice
SOAP is a lightweight XML-based communication protocol for the exchange of information in a decentralized, distributed environment. SOAP is neutral with regard to language, platform, and programming model, allowing both the sender and the receiver to operate in their environment of choice. SOAP documents can be exchanged over many transport protocols.
The SOAP specification can be broadly classified into four main parts:
<env:Envelope xmlns:env=In this example, the SOAP message is identified by the namespace-qualified root element "Envelope". The Envelope namespace determines the version of the SOAP specification to which a SOAP message conforms. The header element is optional; it is typically used to carry out-of-bounds information, such as transaction or security information. The header can contain any number of namespace-qualified XML elements, called entries or blocks. The above example contains one header entry named "app:trans actionId". The body element contains the essence of the message intended for the endpoint. Unlike the header element, the body element must be contained in every SOAP message; the body element can contain one or more namespace-qualified XML elements, called entries or blocks. The above example contains one application-defined body entry named "app:getStockQuote". SOAP defines one body block, called Fault, to represent errors.
As part of its encoding rules, SOAP defines a simple data model consisting of simple types, compound types similar to structs in programming languages, an array type, and an ID/HREF type that represents references. The encoding rules define a particular serialization rule for this data model. SOAP data model and encoding rules are optional. SOAP defines an "encodingStyle" attribute under the "env" namespace, which can be used to specify a particular encoding rule in effect for a specific element or group of elements.
Like encoding rules, the RPC conventions defined by SOAP are optional. In SOAP, both the request and the response of an RPC call are modeled as structs; they can also be modeled as arrays, according to recent changes in the SOAP specification. The name of the struct represents the name of the method being invoked. The parameters of a request or the results of an invocation are modeled as named accessors inside the struct. Our example message is an RPC request defined according to SOAP-RPC conventions. Though SOAP has defined a set of conventions for RPC, SOAP is not RPC-centric. It can be used for any general-purpose messaging.
SOAP can be exchanged over many transport protocols, but the SOAP 1.2 specification defines a binding to HTTP and provides an e-mail binding.
The W3C working group on SOAP is expected to publish their recommendation around August 2002. To participate or follow their progress, go to www.w3.org/2000/xp/Group.
There are at least two technologies that address this space:
SOAP with Attachments (SwA) was an effort by a group of individuals to combine the existing SOAP and MIME technologies to facilitate carrying arbitrary data in SOAP. The W3C has published SwA as a W3C note.
SwA doesn't introduce any new technology. Rather, it uses the referencing facilities in SOAP (HREF attribute) and Multipart MIME (RFC 2045) to make it possible to carry arbitrary data. The whole message is constructed as a multipart MIME message with the SOAP message as the root part. The MIME message can have any number of MIME parts, and the SOAP message can refer to any of these parts using the HREF attribute. In addition, the specification places a few more constraints (such as content-type and start parameter), and makes some recommendation on how the reference URIs in the HREF attribute can be resolved using existing RFCs.
Listing 2 shows a SOAP 1.2 message with an attached facsimile image of a signed claim form (claim061400a.tiff).
Until recently, SwA was the most popular way to carry arbitrary data in SOAP; now DIME seems to be shifting the balance. The W3C has not yet started any work on SwA.
The MB, ME, and CF fields are bitmasks indicating the "begin", "chunk", and "end" of records. The Type Name field is a 3-bit field indicating the structure of the value of the type field. DIME provides a numeric value mapping for different media and MIME types. The ID field is used to give an identifier for each DIME payload. The maximum size of the data field is limited to 4GB.
Microsoft has also published a companion Internet-Draft that shows how SOAP messages can use DIME to send arbitrary data. Using SOAP with DIME is somewhat similar to using SwA. In both cases, the SOAP message is wrapped in a compound structure with the SOAP message as the root or first message, and the referenced parts as the second. DIME specifies rules for resolving the URIs referenced through HREF attributes in SOAP messages; they are similar to SwA rules (RFC 2396 and 2557). DIME also adds on to SOAP-HTTP binding semantics by specifying the content-type as application/dime, rather than the default text/xml specified by SOAP.
It's important to note that in both SwA and DIME, the SOAP message itself travels as either a MIME or DIME message with respect to the carrier or transport protocols.
SwA Versus SOAP with DIME
The other face of UDDI is the repository itself. A repository implements the API specification with which users can publish or discover services. UDDI repositories are logically centralized and physically distributed. As of this writing, there are four node operators running UDDI registries: Microsoft, IBM, SAP, and HP.
The UDDI repository implementations are open source; users could get them and run their own in-house UDDI repositories. Though UDDI was initially touted as the technology that would open the gates for dynamic discovery of Web services and dynamic collaboration, it is more and more frequently used for Intranet and in-house repository needs.
WS-I defines a simple grammar to aggregate service description documents of various services offered at that site. The service descriptions can be in any format, such as WSDL or UDDI. There can be many service descriptions per service, and many services can be defined in a single WS-I document. WS-I also defines an extended binding grammar for both WSDL and UDDI that provides hints about what may be found in the referred service description documents.
WS-I makes some recommendations on how its documents may be made available to users, so they are easily found. WS-Inspection documents may also be placed within a content medium such as HTML.
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