Advanced Web Services: Secure, Reliable, Transacted Web Services: Architecture and Composition (Web Services Technical Articles)

Advanced Web Services: Secure, Reliable, Transacted Web Services: Architecture and Composition (Web Services Technical Articles): "Composability enables incremental consumption or progressive discovery of new concepts, tools and services. "

WS-Addressing, WS-Security and WS-ReliableMessaging elements are independent and these elements can be used independently without altering the processing of other elements.

WS-Policy supports a grammar for aggregating policy statements and allows the construction of more flexible and complete sets of policy. WS-PolicyAttachment specifies how to associate a policy set with XML messages and WSDL elements (operations and portTypes). WS-PolicyAssertions provides a foundational set of common policy statements that can be used to achieve interoperability.

Together WS-Policy and WS-PolicyAttachment provide the framework. Individual specifications define their domain specific policy statements

WS-Trust defines an extensible model for setting up and verifying trust relationships.

use WS-Security with public keys to start a "conversation" or "session," and use WS-SecureConversation to agree on session specific keys for signing and encrypting information.

WS-MetadataExchange specification enables a service to provide metadata to others through a Web services interface. Given only a reference to a Web service, a potential user can access a set of WSDL/SOAP operations to retrieve the metadata that describes the service. Clients can use WS-MetadataExchange at design time, when building their clients, or at runtime.

WS-Federation allows a set of organizations to establish a single, virtual security domain.

WS-ReliableMessaging defines mechanisms that enable Web services to ensure delivery of messages over unreliable communication networks.

WS-Addressing provides an interoperable, transport independent approach to identifying message senders and receivers. WS-Addressing also provides a finer grain approach to identifying specific elements within a service that send or should receive a message.

WS-Coordination is a general mechanism for starting and agreeing on the outcome of multiparty, multi-message Web service tasks.

WS-AtomicTransaction defines a specific set of protocols that plug into the WS-Coordination model to implement traditional two-phase atomic transaction protocols. It

WS-BusinessActivity defines a specific set of protocols that plug into the WS-Coordination model to implement long-running, compensation-based transaction protocols. While BPEL4WS defines a transaction model for business processes, it is WS-BusinessActivity that specifies the corresponding protocol rendering. This, again, is an example for the composability of the Web services specifications.

The Business Process Execution Language for Web Services (BPEL4WS) specification supports service composition. It enables developers to define the structure and behavior of a set of Web services that jointly implement a shared business solution. Each element of the set of services defines its interface using WSDL and WS-Policy. The composed solution is itself a Web service, which supports HTTP/SOAP messages and defines its interface using WSDL and WS-Policy.

Composition has three aspects: structure, information and behavior. BPEL4WS introduces three constructs supporting each composition aspect.