Information Technology Reference
In-Depth Information
discovery, invocation, interface, and composition. The message layer is
about the format of messages exchanged between Web services, and
between services and clients. XML [18] and JSON [20] are two major
industry standards in this layer. The discovery layer is about how
services are advertised and registered such that users can find them.
UDDI [21] serves this purpose. The invocation layer defines the
protocol specification when services are invoked. Currently, SOAP
[22] and RESTful API [23] are the two major protocols where API
stands for application programming interface. The interface layer is
about the service interface specification that describes how a service can
be called, what data structure it expects, and what it returns. WSDL [24]
is the standard in this layer and OWL-S [25] offers a semantics-
enhanced option. As we mentioned earlier, service composability is
an important principle of SOA, and therefore service composition has
been a major focus of research by both academia and industry. Among
many specifications in this layer, WS-BPEL [8] is the de facto industry
standard to compose multiple Web services into a business process and
to expose this process also as a (composite) service. OWL-S offers
semantics-enabled composite service description. WS-CDL [26], from
a higher level, defines the peer-to-peer collaborations of services by
specifying the ordered message exchanges between them. BPMN [31]
is a popular business process modeling specification, and has a tight
relation with service composition.
In Figure 1.3, there are also two cross-layer categories, that is,
programming model and management. The programming model, which
is independent of any specific programming language, defines how to
abstract functions as components and use them as building blocks to
assemble SOA solutions. Currently, Service Component Architecture
(SCA) is a popular SOA programming model; while Service Data
Object (SDO) is its data model. Service management involves many
cross-cutting and nonfunctional specifications, such as addressing,
policy, security, resource, interoperability, and transaction.
Message
XML (Extensible Markup Language) [18] is often used as the message
exchange format for interfacing Web services, though this is not
required. JSON (JavaScript Object Notation) [20] is also becoming
increasingly common. Here, we briefly introduce both languages.
