Web Services Platform Architecture [Electronic resources] : SOAP, WSDL, WS-Policy, WS-Addressing, WS-BPEL, WS-Reliable Messaging, and More نسخه متنی

3.6. Quality of Service

Specifications in this domain are related to the quality of the experience associated with interaction with a Web service. The services in this layer specify the requirements that are associated with the overall reliability of Web services. The specific issues involving this layer include security, reliability of message delivery, and support for transactions (guaranteeing and agreeing on the outcome of a business application).

3.6.1. WS-Security

Security Chapters 12, "Security," and 13, "Advanced Security."

3.6.2. Reliable Messaging

In the Internet world, communication channels are typically unreliable. Connections break, messages fail to be delivered or are delivered more than once, and perhaps in a different sequence to that in which they were sent. Communication can become even more of an issue when the exchange of messages spans multiple transport layer connections. Although techniques for ensuring reliable delivery of messages are reasonably well understood and available in some messaging middleware products today (such as IBM WebsphereMQ), messaging reliability is still a problem. If messaging reliability is addressed by Web service developers who are incorporating techniques to deal with this directly into the services they develop, there is no guarantee that developers of different Web services will make the consistent choices about the approach to adopt. The outcome might not guarantee end-to-end reliable interoperable messaging. Even in cases in which the application developers defer dealing with the reliable messaging to messaging middleware, different middleware products from different suppliers do not necessarily offer a consistent approach to dealing with the problem. Again, this might preclude reliable message exchange between applications that are using different message-oriented middleware.

WS-ReliableMessaging addresses these issues and defines protocols that enable Web services to ensure reliable, interoperable exchange of messages with specified delivery assurances. The specification defines three basic assurances:

• In-order delivery The messages are delivered in the same order in which they were sent.

• At least once delivery Each message that is sent is delivered at least one time.

• At most once delivery No duplicate messages are delivered.

You can combine these assurances to give additional ones. For example, combining at least once and at most once gives exactly one delivery of a message. The protocol enables messaging middleware vendors to ease application development and deployment for Web services by providing services that implement these protocols, possibly layered over their existing proprietary message exchange protocols. WS-Reliable Messaging protocols allow different operating and middleware systems to reliably exchange messages, thereby bridging different infrastructures into a single, logically complete, end-to-end model for Web services reliable messaging.

WS-ReliableMessaging is Chapter 10, "Reliable Messaging."

3.6.3. Transactions

Dealing with many of today's business scenarios necessitates the development of applications that consist of multiple Web services exchanging many messages. An example might be a group of financial institutions setting up a financial offering that involves insurance policies, annuities, checking accounts, and brokerage accounts. Such applications can be complex, executing across heterogeneous, loosely coupled distributed systems that are prone to failure, and introducing significant reliability problems. For such applications, you must deal with the failure of any component Web service of the application within the context of the whole application. A coordinated orchestration of the outcome of the participating services that make up the business application is essential so that a coherent outcome of the whole business application can be agreed upon and guaranteed. Therefore, it is important that the Web services involved are able to do the following:

• Start new tasks, the execution and disposition of which are coordinated with other tasks.

• Agree on the outcome of the computation. For example, does everyone agree that the financial packages were set up?

WS-Coordination, WS-AtomicTransaction, and WS-BusinessActivity define protocols that are designed specifically to address these requirements.

WS-Coordination is a general mechanism for initiating and agreeing on the outcome of multiparty, multimessage Web service tasks. WS-Coordination has three key elements:

• A coordination context This is a message element that is associated with exchanges during the interaction of Web services. This coordination context contains the WS-Addressing endpoint reference of a coordination service, in addition to the information that identifies the specific task being coordinated.

• A coordinator service This provides a service to start a coordinated task, terminate a coordinated task, allow participants to register in a task, and produce a coordination context that is part of all messages exchanged within a group of participants.

• An interface Participating services can use the interface to inform them of an outcome that all of the participants have agreed upon.

Although WS-Coordination is a general framework and capability, WS-AtomicTransaction and WS-BusinessActivity are two particular protocols that compose with and extend the WS-Coordination protocol to define specific ways to reach overall outcome agreement. They extend this framework to allow the participants in the distributed computation to determine outcome robustly .

WS-AtomicTransaction defines Chapter 11, "Transactions."