Quality of Service Models for Web Services Eric Montrym 2/18/05

Agenda Web services Baseline Web service model Quality of Service model Expanded Quality of Service model

Web Services Self-contained, self-describing applications –Enable developers to build applications that provide services to other applications across the Web Examples –Simple: stock quotes, weather forecasts –Complex: business processes Can couple multiple services to build more complex ones

Web Services XML Industry standard, platform independent syntax for describing and structuring data Messages are self-describing –Contain data –Also contain the definition of that data (metadata)

Web Services Goal: Allow any authorized user to access any available service Standardized on 3 sets of functions –Standard transport and message protocols Allow communications between Web services and service requesters Use HTTP, requests look like ordinary Web traffic

Web Services –Directory services –Standard service-description language Use XML for messages between requestors and providers XML not only describes application data but also the SOAP (Simple Object Access Protocol) message envelopes that carry data between requestors and services

Web Services SOAP request

Web Services SOAP response

Web Services Follows the Distributed Computing model –Applications deployed as services that anyone can use –Using components from different sources has drawbacks Reliability, confidentiality, quality Web Services deal with these same disadvantages

Web Services Original Web model –Human access to server based content –Current majority of transactions Web services –Automated program-to-program communication –‘Future of the Web’

Web Services Becoming increasingly popular New opportunities for: –Increased revenue –Better/tighter relationships with customers & business partners E-business –Platform interoperability –Component re-use

Web Services Web Services are fairly common – though adoption has been slow Why? –Lack of security and transaction support –But, there are emerging standards in those areas What about service usability and utility?

Quality of Service Refers to the non-functional aspects of a Web service –Example: performance, reliability, availability How do we define Quality of Service (QoS)? –Example: transaction time, price –What task is at hand? If multiple services, how do we choose?

Agenda Web services Baseline Web service model Quality of Service model Expanded Quality of Service model

Baseline Web Service Model Service provider –application Service requestor –user/application Service registry –database Service registry Service provider Service requestor Service description Service Service descriptions Find Bind Publish

Agenda Web services Baseline Web service model Quality of Service model Expanded Quality of Service model

Quality of Service Model Businesses need assurances –How do I know that the service will meet my performance (speed) requirement? –Will the service be reliable for my mission- critical systems implementation? Foreseeable that multiple services may provide the needed service with different QoS – discovery process

Quality of Service Model Service registry Service provider Service requestor Service description Service Service descriptions Find Bind Publish QoS certifier Certify QoS Verify QoS

Quality of Service Model Certifier –Third-party (independent) –Verifies quality claims of suppliers Simple yes/no quality measurement Shortcomings –Do certificates expire? –Update method

Agenda Web services Baseline Web service model Quality of Service model Expanded Quality of Service model

Expanded Quality of Service Model Weaknesses in previous model to address –Not dynamic Needs of providers/consumers constantly changing Providers can’t update their QoS dynamically –How to verify the QoS with service provider Has anything changed since certificate issued Can we verify QoS again –We can do better than yes/no measurement

Expanded Quality of Service Model QoS registry Service provider Service requestor Service description Service Service descriptions Find Bind Publish Service Registry QoS calculation QoS feedback

Expanded Quality of Service Model QoS computation Example Result from QoS registry Using 3 QoS parameters, returns 3 services Avail.RTTCost

Expanded Quality of Service Model Normalize using 2 arrays –N, where 1 benefits requestor if value increases and 0 if value decreases –C, maximum normalized value

Expanded Quality of Service Model Create first normalization using the following –V i,j = q i,j / average (q i’s ) If average q i,j != 0 and average q i,j < c j and n j = 1 –V i,j = average (q i’s ) / q i,j If q i,j != 0 and n j = 0 and average q i,j < c j –V i,j = c j

Expanded Quality of Service Model Our normalized array Avail.RTTCost

Expanded Quality of Service Model Second normalization –Begin by grouping data (chosen for simplicity) –Apply this new array to first normalization Avail.TimeCost Avail. RTT Cost

Expanded Quality of Service Model Second normalization also uses 2 arrays –T, maximum normalized value –F, weight for each category

Expanded Quality of Service Model Second normalization uses the following –h i,j = g i,j / average (g i’s ) If average g i,j != 0 and average g i,j < t j –h i,j = t j If average g i,j = 0 or average g i,j >= t j

Expanded Quality of Service Model Results in final matrix QoS then calculated for each service

Expanded Quality of Service Model QoS registry – collecting info –User feedback End users required to update QoS for services consumed –Measured results –Satisfaction survey Results should be fair since based on runtime data –Manipulation prevented by using a set of keys

Expanded Quality of Service Model Improvements –Matrix calculation method allows for variation Number of quality parameters can be adjusted Relative weights –QoS updates happening with every service consumed QoS ratings are changeable Providers can see their scores as well as the competition

Conclusion Web services –What are they Baseline model –No QoS criteria Simple QoS model –Yes/No Expanded QoS model –Numerical representation

Expanded Quality of Service Model