• Performance: Performance is the quality aspect of Web service, which is measured in terms of throughput and latency. Higher throughput and lower latency values represent good performance of a Web service. Throughput represents the number of Web service requests served at a given time period. Latency is the round-trip time between sending a request and receiving the response.
   
• Reliability: Reliability is the quality aspect of a Web service that represents the degree of being capable of maintaining the service and service quality. The number of failures per month or year represents a measure of reliability of a Web service. In another sense, reliability refers to the assured and ordered delivery for messages being sent and received by service requestors and service providers.
   
• Regulatory: Regulatory is the quality aspect of the Web service in conformance with the rules, the law, compliance with standards, and the established service level agreement. Web services use a lot of standards such as SOAP, UDDI, and WSDL. Strict adherence to correct versions of standards (for example, SOAP version 1.2) by service providers is necessary for proper invocation of Web services by service requestors. Security: Security is the quality aspect of the Web service of providing confidentiality and non-repudiation by authenticating the parties involved, encrypting messages, and providing access control. Security has added importance because Web service invocation occurs over the public Internet. The service provider can have different approaches and levels of providing security depending on the service requestor.

QoS enabled Web services

The interface definition (WSDL) specifies the syntactic signature for a service but does not specify any semantics or non-functional aspects. QoS enabled Web services require a separate QoS language for Web services to answer the following questions:

• What’s the expected latency?
• What’s the acceptable round-trip time?

A programmer needs to be able to understand the QoS characteristics of the Web services while developing applications that invoke Web services.

Ideally, a QoS enabled Web services platform should be capable of supporting a multitude of different types of applications:

1. With different QoS requirements
2. By making use of different types of communication and computing resources

When considering QoS-aware Web services, we suppose that the interface specifications are extended with statements on QoS that can be associated to the whole interface or to individual operations and attributes. In the case of a service requestor, these statements describe the required QoS associated with the service required by the client, while from a service provider’s perspective these statements describe the offered QoS associated with the service offered by the server object.

The Web service architecture design from IBM includes a separate layer called “endpoint description” to add additional semantics to service description like QoS properties.