Research On Key Issues Of Service Scheduling In Real-Time Soa

With the developments of information technology and network technology, our society entered the era of global Internet. These make new requirements for conventional real-time system application with interconnection, interoperability and scalability. At the same time, for meeting large-scale distributed system application construction, service oriented architecture (SOA) has been got much extensive attention gradually both in academia and industry. It has become the guiding ideology to build new generation of distributed system applications. Therefore, Building real-time system applications with the loose-coupled and interoperability of SOA become the future development direction. Also real-time SOA has been a hot issue in academic research.Due to limited resources in real-time systems, real-time tasks are given time constraints which often be called "deadline". The scheduling problem is an important issue in the field of real-time system research. Service has been a new resource in real-time SOA. Conventional scheduling strategies for physical resources in computer (such as CPU, RAM and network) will not fit for real-time services. So we giving deep study on the service scheduling problem in real-time SOA, and proposing new suitable method for it, will get great significance in development of future real-time applications.There is a very important problem to be solved in real-time SOA scheduling research, that is "How to schedule for fulfill as many requests as possible when much more requests arrived overstep the service provider’s capacity?" This problem is particularly important for real-time SOA. Real-time is the key characteristics of real-time systems, so requests should not wait for a long time. There need a reasonable scheduling. Different service types such as atomic service or composite service will be considered respectively. We should give sufficient consideration on service runtime model, discovery runtime conflicts, and try to solve the conflicts. In this paper, we give a deep study on such scheduling issues in distributed control systems and our National863project with service computing technologies, combined with scheduling theory and Petri net model. Main works and innovations of this paper are as follows.1、 Put forward a service scheduling strategy based on historical similar requests for atomic service.Atomic service is a service does not request other services. The atomic service has limited capability, it always encapsulate operation logic of physical equipments. When requests overstep provider’s capability, how to get maximum success requests (MSR) become a key problem must to solve. To achieve the MSR goal, requests maybe fail should not get response to release resources for other requests with high success probability. If the provider gives response to a request, but it executes failure for some reason, it is a waste response for provider that no one gets profits from this execution. The provider should try to avoid this situation. By deep analysing service request’s historical similar requests, the provider should get the success rate of request’s historical similar requests, and get the success probability of target requests. Then request has highest success probability will be scheduled to execute.2> Put forward a runtime model for composite service based on petri net.Composite service will get the same problem as atomic service that maybe some sub services receive much more request than its capabilities in real-time SOA runtime. Multiple composite services may request the same atomic service to execute at the same time and they get conflicting. To solve this problem, a composite service runtime model is needed to construct. For petri net is very suitable to express these concurrent time constrained system, we give a dynamic colored timed petri net (DCTPN) to describe real-time SOA runtime base on petri net model. In DCTPN, places indicate services, transitions indicate service execution, and service execution reservation will be expressed by arc. Arcs join and removal express services composite continuously. Computation rules of DCTPN will be introduced in this paper, and some characteristic such as reachability, boundedness, firing, schedulability and conflict will be discussed too. 3、 Put forward a composite service scheduling algorithm based on DCTPN.In this chapter, we will show scheduling algorithms for composite service. These algorithms include service reservation algorithm (SRA), conflict monitor algorithm (CMA), random conflict solving algorithm (RCSA), key services searching algorithm (KSSA), etc. SRA and CMA are used to detect the conflict early. KSSA decompose the deadline of composite service into sub service deadline. RCSA try to solve conflicts by changing service’s start time, deadline randomly. Those conflicts which could not be eliminate, will be discarded with some strategies. Strategies include most conflict discard, key conflict discard and composite service finish degree discard.4、 Put forward a real-time SOA model for distributed automation system.With the characteristics of distributed control system, we proposed a layered real-time SOA model including Physical Layer, Message Layer, Service Layer and Application Layer. Service Scheduler Center (SSC) is the core of Service Layer. It includes several controller as service utilization scheduler, service configuration&manager, admission security control. These controllers work collaboratively. In message layer, we show a multiple network transform strategy, and give deep discuss in message format, message scheduling, etc. Finally, we show a prototype application base on the real-time SOA model.