## 2 Materials and Methods
### Service and Service Process  
A service is defined as the application of competences for the benefit of another, meaning that service is a kind of action, performance, or promise that is exchanged for value between provider and customers [15]. Service can be divided into two granularities: primitive service and complex service. A primitive service is a candidate atomic service, with a specific service capability. A complex service is a service requested by a consumer, which is a composition of a set of primitive services [16]. To support the implementation of services for customers, there must be a corresponding service system to compose various primitive services together. A service system is considered as a complex socio-technological system, and is defined as \"a value co-production configuration of people, technology, other internal and external service systems connected by value propositions, and shared information (such as language, processes, metrics, prices, policies, and laws)\" [15]. In a service system, services are interacting each other, for example, by exchanging messages. Orchestration and choreography are the main ways to realize service composition. Orchestration is concerned with the composition of services, seen from the viewpoint of single service. Choreography is concerned with the composition of services seen from a global viewpoint, focusing on the common and complementary observable behaviors [5]. Through orchestration and choreography, primitive services and complex services are integrated to create an end-to-end service process.  
Service process is an integral part of service, and it is the actual procedure, mechanism, and operation flow of service provision. The service process can be viewed as a chain or constellation of activities that allow the service to function effectively [17]. In general, the execution of service processes is constrained by a service-level agreement (SLA) between service provider and customer [18], or the quality of service (QoS) requirements of customer. The SLA specifies the metrics of the customer, used to monitor and verify the contract. QoS can be regarded as a part of SLA, mainly considering the parameters related to customer requirements, such as response time, availability, security, etc. Customers may not only care about the functionality of the service, but also want to achieve some specific service quality objectives, such as the fastest response or the most reliable service [19]. The current research on service process is mainly about the reconfiguration of service process, and the management of service process. [PERSON] [20] presented a reconfiguration region identification algorithm and middleware components to support the service reconfiguration based on end-to-end QoS constraints. [PERSON] [21] proposed an adaptive QoS-aware service process reconfiguration approach based on supplementary services and a region-based reconfiguration algorithm, to solve the problem efficiently with low reconfiguration costs. [PERSON] [18] proposed a contract template and a directed tree-graph model for SLA specification definition to manage service delivery. [PERSON] [22] argued that service process management is a way to effectively manage publication, discovery, negotiation, deployment, and implementation of services, and presented an SLA-based service process management approach for service-oriented architecture (SOA). [PERSON] [23] argued that the scope of process management consists of process design, process execution, process mining, and process intelligence, and a petri-nets based service process management method was introduced for workflow verification. Most of the literature focuses on the composition of services, not the service process itself.