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򐂰 Bottom-up

If some server-side Java code already exists, the WSDL specification can be

generated from it. The client-side Java proxy is still generated from this

WSDL document.

򐂰 Meet-in-the-middle

The meet-in-the-middle (MIM) development style is a combination of the two

previous ones. There are two variants:

– MIM variant 1

Some server-side Java code is already there. Its interface, however, is not

fully suitable to be exposed as a Web service. For example, the method

signatures might contain unsupported data types. A Java wrapper is

developed and used as input to the WSDL generation tools in use.

– MIM variant 2

There is an existing WSDL specification for the problem domain; however,

its operations, parameters, and data types do not fully match with the

envisioned solution architecture. The WSDL is adopted before server-side

Java is generated from it.

In the near future, we expect most real-world projects to follow the

meet-in-the-middle approach, with a strong emphasis on its bottom-up elements.

This is MIM variant 1, starting from and modifying existing server-side Java and

generating WSDL from it.

Level of integration between requester and provider

In a homogeneous environment, client and server (requester and provider) use

the same implementation technology, possibly from the same vendor. They

might even run in the same network.

In such an environment, runtime optimizations such as performance and security

improvements are possible. We expect such additional vendor-specific features

to become available as the Web services technology evolves.

We do not recommend enabling such features, however, because some of the

main advantages of the Web service technology such as openness, language

independence, and flexibility can no longer be exploited. Rather, you should

design your solution to loosely couple requester and provider, allowing

heterogeneous systems to communicate with each other.

7.3.3 The key challenges in Web services

Web services can potentially revolutionize application integration by providing a

layer of abstraction between the technology that requests a service and the

Chapter 7. Application and system design guidelines 179

technology that provides the service. In order to achieve this, though, there are

still technical challenges that have to be addressed. This section briefly

describes a few key issues, such as the Extended Web Services Architecture,

security, interoperability, quality of service, and distributed transactions.

Extended Web Services Architecture

In November 2002, W3C released a draft Web Service Architecture specification

that identifies the functional components, the relationships among those

components, and establishes a set of constraints to guide the desired properties

of the overall architecture.

The proposed architecture consists of a basic architecture that defines the

interactions between service requesters and service providers, as discussed in

7.3.1, “Web services architecture” on page 164.

The proposed architecture also defines an Extended Web Services Architecture

that incorporates additional features and functionality. These additional features

include:

򐂰 Asynchronous messaging

򐂰 Attachments typically used to include binary data in SOAP messages

򐂰 Caching

򐂰 Message Exchange Pattern (MEP)

򐂰 Correlation

򐂰 Long running transactions

򐂰 Reliable messages

򐂰 Message authentication

򐂰 Message confidentiality

򐂰 Message integrity

򐂰 Message routing

򐂰 Management messages

򐂰 Session

See the W3C Web Services Architecture specification for more detail:

http://www.w3.org/TR/2002/WD-ws-arch-20021114/

Security

Security concerns are the main limitation of current Web services initiatives. The

Internet and many of its prevalent technologies were not designed with security

in mind. Web services security must also be compatible with the foundational

technologies (SOAP, WSDL, XML Digital Signature, XML Encryption, and

SSL/TLS).

We further discuss the latest development in Web services security in 10.8.1,

“Security considerations for Web services” on page 354.