Chapter 2

Service-Oriented Architecture and Web Services

Abstract

Service-oriented architecture (SOA), a hot topic in software design and development, is independent of any vendor, product, or technology. Web services, which make functional building blocks accessible over standard Internet protocols, independent of platforms and programing languages, can implement a SOA. This chapter will first provide an overview of web services, SOA, service component architecture, and service data objects. Then some open-source platforms for SOA will be introduced.

Keywords

Servie component architecture; Service data objects; Service-oriented architecture; Web service

Chapter Outline

2.1 Web Services 18

2.1.1 Overview of Web Services 18

2.1.2 Basic Standards of Web Services 20

2.1.2.1 Simple Object Access Protocol 20

2.1.2.2 Web Service Description Language 22

2.1.2.3 Universal Description, Discovery and Integration 24

2.1.3 Web Services Security 26

2.1.3.1 WS-Security 27

2.1.3.2 WS-Policy 28

2.1.3.3 WS-Trust 29

2.1.4 Web Services Transaction 30

2.1.4.1 WS-Coordination 30

2.1.4.2 WS-AtomicTransaction 30

2.1.4.3 WS-BusinessActivity 31

2.1.5 Semantic Web Services 31

2.1.5.1 OWL-S 31

2.1.5.2 Web Service Modeling Ontology 31

2.1.5.3 Semantic Web Service Ontology 32

2.1.5.4 WSDL-S 32

2.2 Service-Oriented Architecture 32

2.2.1 Overview of SOA 32

2.2.2 Model of SOA 33

2.2.2.1 Service 34

2.2.2.2 Visibility 34

2.2.2.3 Service description 34

2.2.2.4 Interaction 34

2.2.2.5 Real-world effects 34

2.2.2.6 Contract and policy 34

2.2.2.7 Execution context 35

2.3 Service Component Architecture 35

2.3.1 Concepts of SCA 35

2.3.2 Model of SCA 35

2.3.2.1 Property 36

2.3.2.2 Reference 36

2.3.2.3 Service 37

2.3.2.4 Wire 37

2.3.3 Strategy Framework 38

2.4 Service Data Objects 38

2.4.1 Concepts of Service Data Object 38

2.4.2 Framework of Service Data Object 39

2.5 Open-Source Platforms for SOA 40

2.5.1 Apache Tuscany 40

2.5.2 Eclipse SOA Tools Project 40

2.6 Summary 41

References 41

2.1. Web Services

2.1.1. Overview of Web Services

A web service is a software system designed to support interoperable machine-to-machine interaction over a network. It has an interface described in a machine-processable format (specifically WSDL). Other systems interact with the web service in a manner prescribed by its description using SOAP messages, typically conveyed using HTTP with an XML serialization in conjunction with other web-related standards. [1]

A web service is a concrete manifestation and function carrier of the concept of service computing. It is a kind of application, based on a web environment, that is adaptive, self-describing, and modular and has good interoperability. However, different organizations for the web service concept and its connotation may create different understanding and awareness. IBM believes that a web service should use XML to describe a group of messaging through XML for its operation, but these operations can be accessed via the network and still meet the target task. The description of the service provides all the details of what is necessary to interact with the service, including message formats, transport protocols, and location. Microsoft thinks that a web service is an application logic unit providing data and services to other applications. The application accesses the web service by the ubiquitous web service protocols and data formats, such as HTTP, XML, and Simple Object Access Protocol (SOAP), and it does not need to care about how to implement each web service. SUN thinks that a web service is a software component that has the features of being discoverable, reusable, and re-combinable, solving the problems or requirements of users. W3C thinks that a web service is a system supporting interoperability interaction through the network. It uses the Web Services Description Language (WSDL) to describe the service interface and uses the SOAP message based on HTTP protocol to implement the implementation between the service and the service and customer communication. Each view above emphasizes some parts of a web service, and there are no conflicts. By combining the above views, we think that the biggest characteristics of a web service lies in its interoperability and reusability. The interoperability makes the web service become a kind of attractive adhesive, used for seamless integration of heterogeneous applications and systems; and the feature of reuse makes the web service a good carrier to convert a software to service. Compared with other software entities, a web service has the following features: Can be described: A web service can be described by a service description language. Can be released: A web service can be registered in the registering center and released. Can be found: The user can send a search request to a registering center to find the service and the access information. Can be bound: The web service description information can be bound with a runable service instance or service proxy. Can be called: A web service can be called by remote code with the description information. Can be composited: The web services can be composited together to a large granularity service. In recent years, web service technology went through a long development period and formed the web service protocol stack as shown in Figure 2.1, with service communication, service description, service quality, and service processes. We can divide the development process into two parts: the web service foundation protocol developing period and the web service high-level protocol developing period. The first period completed the foundation protocols, such as SOAP [2], Web Service Description Language (WSDL) [3], and Universal Description, Discovery and Integration (UDDI) [4]. In this period, the majority of the research in technology was about service developing, testing, calling, and other basic problems. The second period completed the high-level protocols, such as Business Process Execution Language (BPEL) [5], Web Service Choreography Language (WS-CDL) [6], and Web Service Choreography Interface (WSCI) [7]. The technologies were about service interaction and cooperation. Now we are in the second period, and the service composition, service process management, service interaction, and service adoption are the hot topics. The concept of web service totally satisfies the definitions in service-oriented computing (SOC) and service-oriented architecture (SOA). Web service technology is thought to be the best for supporting SOC and SOA.
Figure 2.1 Web service protocol stack.

2.1.2. Basic Standards of Web Services

With the development of standards and protocols, web service computing has stepped into the mature period. The basic standards include SOAP, WSDL, and UDDI.

2.1.2.1. Simple Object Access Protocol

SOAP is a light protocol used in the distribution environment to exchange information, which was published by W3C in 1999. SOAP 1.0 is based totally on the HTTP protocol, while SOAP 1.1 published in May 2000 supports several different transport protocols. The latest version is SOAP 1.2, which was published in July 2001 by W3C. SOAP is a no-state single-direction message exchange protocol, containing the following four aspects: 1. Defining a format used for single-direction message exchange, it describes how to organize the information into an XML document. 2. Describing how to transfer the SOAP message, it uses many different carriers, such as the web (using HTTP protocol) and e-mail (using simple mail transfer protocol). 3. Defining a group of rules compiled within the process of an operating SOAP message and the classification of related entities; it can assign a particular receiver and can handle strategies when the messages cannot be parsed. 4. Defining a group contract about Remote Procedure Call Protocol (RPC) calling and SOAP message exchanging, it solves the problem about how to encapsulate the RPC calling into the SOAP message, re-encapsulate the SOAP message to RPC, calling back to the server and returning the final result in the same strategy to the client. The main design purpose is simple and extendable. To implement these two destinations, the core message framework of SOAP does not include some of the normal features found in other distribute systems, e.g., reliability, security, and Message Exchange Patterns (MEPs). A SOAP message consists of a SOAP envelope, encoding rules, RPC representation, and a SOAP binding. 1. SOAP envelope: SOAP protocol is based on message exchange. Each message can be thought of as an envelope with some data...