Service orientation is the core reference model for cloud computing systems.
This approach adopts
the concept of services as the main building blocks of application and system development. Service-oriented computing (SOC) supports the development of rapid, low-cost, flexible, interoperable, and evolvable applications and systems.
A service is an abstraction representing a self-describing and platform-agnostic component that can perform any function — anything from a simple function to a complex business process.
Virtually any piece of code that performs a task can be turned into a service and expose its functionalities through a network-accessible protocol.
A service is supposed to be loosely coupled, reusable, programming language independent, and location transparent.
- Loose coupling allows services to serve different scenarios more easily and makes them reusable. Independence from a specific platform increases service accessibility. Thus, a wider range of clients, which can look up services in global registries and consume the min allocation-transparent manner, can be served.
- Services are composed and aggregated into a service-oriented architecture(SOA), which is a logical way of organizing software systems to provide end users or other entities distributed over the network with services through published and discoverable interfaces.
Service-oriented computing introduces and diffuses two important concepts, which are also fundamental to cloud computing: quality of service (QoS) and Software-as-a-Service(SaaS).
Quality of service(QoS) identifies a set of functional and non-functional attributes that can be used to evaluate the behavior of a service from different perspectives. These could be performance metrics such as response time, or security attributes, transactional integrity, reliability, scalability, and availability. QoS requirements are established between the client and the provider via an SLA that identifies the minimum values (or an acceptable range) for the QoS attributes that need to be satisfied upon the service call.
One of the most popular expressions of service orientation is represented by Web Services (WS). These introduce the concepts of SOC into the World Wide Web, by making it consumable by applications and not only humans.
Web services are software components that expose functionalities accessible using a method invocation pattern that goes over the HyperText Transfer Protocol (HTTP).
The interface of a Web service can be programmatically inferred by metadata expressed through the Web Service Description Language (WSDL); this is an XML language that defines the characteristics of the service and all the methods, together with parameters, descriptions, and return type, exposed by the service. The interaction with Web services happens through Simple Object Access Protocol (SOAP).
This is an XML language that defines how to invoke a Web service method and collect the result.
Using SOAP and WSDL over HTTP, Web services become platform independent and accessible to the World Wide Web. The standards and specifications concerning Web services are controlled by the World Wide Web Consortium (W3C).
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