Sequence pattern (Seq. Pattern 1 Sequence A Sequence pattern describes the structure where a component service starts after the completion of another component service in the same process. When a service violates the SLA, there are two ways to adapt it. The service consumer can either renegotiate with the current service provider – the one that violated the SLA – to see if it can still provide the service with satisfactory quality to meet the service consumers’ requirements for the composite service, or find a new service provider to replace the current service provider. Before performing the adaptation process, the VOC of the adaptation solution needs to be computed. Since the actual values of the adapted QoS are not known until after negotiating with the service providers, we average all the possible combinations of the values of adapted QoS using current belief distributions which can be obtained from the pre-defined SLAs, previous interactions with the service providers or ServiceTrust – a service trust system based on SLA profiling as detailed in Chapter 6. Suppose there are two component services, A and B, that are compliant with the Sequence pattern, when an SLA violation occurs in service A, the SLA adaptation mechanisms must be triggered. If there are several candidate service providers for service A – very common in the open SOC environment, for each candidate service provider (including the current service provider), an estimated value is computed. Formally, ∫Ω A' u( e, p )⋅ Pr( EA' = e,PA' = p )dΩA' 6 (5.1) where V( A| A') is the estimated value from adapting service A with service provider A' , u( e, p ) is a utility function which computes the utility of a service based on the service consumer’s preferences for execution time, e, and price, p, Pr( E = e,P = p ) denotes the belief distribution of the combination (e, p) and Ω A' = <(e1, p1), (e2, p2), …, (en, pn)>A’ represents the possible combinations of the values of execution time and price from the candidate service providers for service A. Here we take the current service provider as a candidate service provider if it is still potentially capable of providing the service with satisfactory SLA. Then the service consumer computes the probability that service B needs to be rearranged due to selecting adapting service A with service provider A' . The probability, denoted as P( A| A' → B ) , is affected by three main aspects:
