Typical Architecture for Web Services Management with Differential QoS Support
The state-of-practice regarding QoS differentiation is to provide different service interfaces for different service levels or to specify an additional parameter in the operation signature. Differentiation in terms of performance and throughput is usually achieved by using hardware-based techniques to either allocate dedicated resources for different QoS levels (i.e., physical partitioning) or by the over-provisioning of resources to absorb traffic fluctuations. The major drawbacks of these mechanisms are added complexity, higher cost, and inefficient resource utilization.

Supporting differential QoS for Web services should apply differentiation techniques at the SOAP layer rather than at lower protocol layers, such as HTTP or TCP. This allows content-based request classification while leveraging the metadata carried in the SOAP requests. Furthermore, differential QoS support should be transparent without requiring any changes to the application server, the service client, or the service implementation. Hence, unmodified commodity Web services platforms can be used without costly changes while avoiding specific hardware or OS prerequisites.

This section discusses a typical architecture that involves differential QoS support added to WSM platforms (see Figure 2). The following subsections describe key components of such an architecture.

Request Classifier
A key requirement for supporting differentiated QoS is the ability to classify the incoming requests according to a classification schema. For example, a classifier component can use the requester attributes metadata, which is generated by the authentication handler, to map the request to a service class. SAML could play a key role in service classification because of its wide acceptance as a standard in identity management to encode the user's access rights, authentication information, and other user attributes such as the user's role and service level (e.g., gold, silver...).

Admission Control
Admission control is used to control throughput and to prevent the system from being overwhelmed in the presence of overload by keeping the maximum amount of load within the system capacity. Admission control requires two prerequisites: determining the load that a particular service call will generate on a system, and knowing the capacity of the service implementation. Admission control is also responsible for identifying appropriate scheduling algorithms to enable differentiation in service such as weighted round-robin scheduling.

Request Dispatcher
There are various ways to implement the request dispatcher, particularly the queue manager component:

• The look-ahead parameter approach uses a single queue where requests are queued according to their arrival order. The scheduling algorithm can use a look-ahead parameter (k) that specifies the number of positions that will be searched from the head of the queue looking for requests of a given priority. If no request of the desired priority is found, the algorithm is repeated for the next lower level and so on. The higher the value of k, the better is the service given to higher priority requests. If k = 1, requests will be serviced according to their arrival order.
• Multi-queue dispatcher with a queue manager implements a set of logical FIFO queues, one for each service class. The scheduler then dispatches the pending requests according to the configured scheduling scheme.

References

• Erradi, A., Varadharajan, N., and Dr Padmanabhuni, S., "WS-DiffServ: A Middleware for Differentiated Service Delivery," submitted to the International Conference on Service-Oriented Computing (ICSOC 2005).
• Tosic, V. 2004, "Service Offerings for XML Web Services and Their Management Applications." PhD dissertation, Carleton University, Ottawa, Canada. http://elab.njit.edu/vladimir/TosicThesis-Final.pdf
• IBM Web Service Level Agreements (WSLA): www.research.ibm.com/wsla
• Web Services Distributed Management (WSDM): www.oasis-open.org/committees/tc_home.php?wg_abbrev=wsdm