SOAP Design Patterns

• Architectural design patterns capture the proven experience of software architects in the form of architectural patterns, each with distinct properties that address different architectural design problems. To a business strategist, software architecture patterns provide a high-level conceptual view of a software system independent of the specific implementation's details. To a software engineer, patterns provide a structure within which components can be designed and integrated. To those tasked with moving from requirements to design, patterns provide a starting point for developing implementation strategies.

• Figure 4.14 illustrates two of the architectural patterns in wide use today: the Layers Pattern, and the Pipe and Filter Pattern. The Layers Pattern is one of the earliest successful architectural patterns and is the basis for the design of telecommunications infrastructures such as TCP/IP. Layers describe a solution to a problem by partitioning the problem into subtasks that are assigned to individual layers. Each layer draws on services from the layer below and provides services to the layer above.

• Figure 4.14. Architectural design patterns: The Layers Pattern and the Pipe and Filter Pattern.

• A variant of the Layers pattern, the Multitier Pattern, is used to build tiered systems. For example, two-tier client-server systems partition display functionality to the client tier and business logic to the server tier. Three-tier systems add another tier for managing data.

• The Pipe and Filter Pattern is another architectural approach to systems construction that assumes a stream of data moving through a series of pipes and filters and being transformed as it travels from origin to destination. In addition to serving as transforming agents, stages in the Filter Pattern can be used to buffer data and help deal with issues that arise in working with interruptible network connections.

• The SOAP specification provides support for the filters through its ability to specify intermediary processing agents along a path from an XML document's origin to a final destination.

• Filters are also supported by the Simple API for XML (SAX), a Java application program interface for processing XML. SAX makes use of a filter class that can act as an intermediary in a complex chain of processing. The filter class is passed as the event handler to another class that generates SAX events, forwarding all or some of those events to the next handler or filter in the processing chain.

• Filters may be used to prune the document tree by not forwarding events for elements with a given name (or that satisfy some other condition), or a filter can generate new events to add parent or child elements to an existing document stream. Filters can also be used to add or remove element attributes or modify the character data.

• The Filter is related to UNIX's capability to take simple processing functions and pipe them together to perform complex tasks. The power of SOAP and SAX filters derives from the fact that simpler, easy-to-maintain filters may be chained together to produce complex XML data transformations