Guanjun Liu

Two Simple Deadlock Prevention Policies for

This paper proposes the concept of Key-resource/operation-place Pairs (KP) of S3PR (systems of simple sequential processes with resources). Based on KP, two policies are presented to prevent deadlocks in flexible manufacturing systems (FMS) that can be modeled by S3PR. The idea is to control some key resource places only to guarantee that all strict minimal siphons (SMS) never become empty, thereby making the controlled system live. It enables one to design two easy-to-implement control policies. The first one can guarantee that the controlled system is live, and the second one can also make the controlled system live if there is no SMS containing any control place in the controlled system. At last, a well-known FMS example is used to illustrate the proposed concept and policies.

{\rm S}^{3}{\rm PR}

Owing to their rapid development, services are increasing rapidly in quantity. The consequence is that there are so many services that share the same or similar functions. Therefore, it is important to select a credible and optimal service. Reputation as one of the important parameters of services plays a significant role in the decision support for service selection. This paper proposes a novel two-phase method to calculate service reputation. The first phase uses a dynamic weight formula to calculate reputation such that it can reflect the latest tendency of a service. The second one uses an olfactory response formula to mitigate the negative effect of unfair ratings. Some experiments are conducted and the results validate the effectiveness of the proposed method.

Based on Key-Resource/Operation-Place Pairs

Such concurrent systems as Web services and workflow systems can be viewed as a composition of a set of subsystems. Subsystems interact with each other through a set of message channels in order to perform a task. This work defines a class of Petri nets called interactive Petri nets (IPNs) to model these systems. IPNs can be used to analyze their behavior, find potential problems, and then improve their designs. Compatibility is an important concept for a composed system and reflects the possibility of correct/proper interaction among its subsystems. In order to characterize different cooperative abilities in practice, compatibility and weak compatibility are defined for IPNs. Some relationships among (weak) compatibility, liveness, reversibility, and boundedness are revealed. Based on them, this work proves that the (weak) compatibility problem is co-NP-hard. A taxonomy is also presented for IPNs in order to explore whether some subclasses can be analyzed efficiently. Based on it, we can identify several IPN subclasses, some of which can be analyzed in polynomial time.

A Novel Method for Calculating Service Reputation

This paper presents a class of Petri nets, process nets with channels (PNCs) that can model some types of concurrent systems in two aspects: process and interaction. Its significance lies in offering efficient analysis and verification methods for these systems. PNCs belong to the class of extended free choice nets. This paper establishes the conditions to examine their liveness, reversibility, and reachability based on their structural characteristics. Siphons, traps, and a state equation are used to describe these conditions such that analysis techniques based on reachability graphs and siphon enumeration are avoided. A polynomial-time algorithm is presented for the liveness analysis, and an effective method is also given to decide the reachability. A real-world example is used to illustrate the application of PNCs.

Interactive Petri Nets

Workflow nets (WF-nets) as a class of Petri nets are widely used to model and analyze workflow systems. Soundness is an important property of WF-nets, which guarantees that the systems are deadlock- and livelock-free and each task has a chance to be performed. Van der Aalst has proven that the soundness problem is decidable for WF-nets and we have also shown that it is PSPACE-complete for bounded ones. Since the definition of soundness is based on reachability and Van der Aalst has proven that a sound WF-net must be bounded, the soundness detection can be carried out via the reachability graph analysis. However, the state explosion problem is a big obstacle to this technique. The unfolding technique of Petri nets can effectively avoid/alleviate this problem. This paper proposes an algorithm to generate a finite prefix of the unfolding of a WF-net, called basic unfolding. Furthermore, a necessary and sufficient condition is proposed to decide soundness based on basic unfolding. In addition, some examples illustrate that the unfolding technique can save the storage space effectively.

Process Nets With Channels

Workflow nets (WF-nets) are widely used to model and verify the business process management systems and composite web services. The (weak) soundness of WF-nets is an important criterion for the correctness of these systems. This paper focuses on the complexity of solving the (weak) soundness problem. Aalst et al. have proven that the (weak) soundness problem is decidable. Our previous work has proven that the soundness problem for bounded WF-nets is PSPACE-complete. This paper shows that the weak soundness problem for bounded WF-nets is also PSPACE-complete. Aalst et al. has proven that the soundness problem is polynomially solvable for free-choice WF-nets (FCWF-nets). This paper discovers that the weak soundness problem is equivalent to the soundness problem for FCWF-nets. Therefore, the weak soundness problem for FCWF-nets is also polynomially solvable. Unfortunately, many composite web services are not modeled by FCWF-nets. Lots of them can be modeled by asymmetric-choice WF-nets (ACWF-nets). This paper proves that the soundness problem is co-NP-hard for ACWF-nets even when they are three-bounded. Additionally, this paper proves that the k-soundness problem is equivalent to the weak soundness problem for WF-nets, which implies that the k-soundness problem for bounded WF-nets is also PSPACE-complete.

A Branching-Process-Based Method to Check Soundness of Workflow Systems

Inter-organizational workflow nets (IWF-nets) can well model the interactions among multiple processesby sending/receiving messages. Compatibility and weak compatibility are crucial properties for IWF-nets.The latter guarantees that a system is deadlock-free and livelock-free while the former also guarantees that ithas no dead tasks. Our previous work proved that the (weak) compatibility problem is PSPACE-complete forsafe IWF-nets. This paper defines a class of IWF-nets in which some simple circuits are allowed. Necessaryand sufficient conditions are presented to decide compatibility and weak compatibility for this class, and theyare dependent on the net structures only. Algorithms are developed based on these conditions. In addition, weshow that the traditional net structures like siphon cannot be easily used to decide the (weak) compatibility ofIWF-nets.创新点跨组织工作流网广泛用于模拟与分析多个业务流程间的交互, 这些交互通过消息传递来实现。弱兼容性保证业务流程交互时既无死锁也无活锁, 而兼容性同时还保证业务流程的每个事件都有可能发生。然而, 我们前期工作证明: 对有界跨组织工作流网来说, 判定其 (弱) 兼容性是PSPACE完全的。因此, 为一些子类寻找有效判定条件是非常有意义的。本文定义了一类跨组织工作流网, 提出了新的网结构概念, 并基于这些网结构概念给出了判定 (弱) 兼容性的充要条件。

Some Complexity Results for the Soundness Problem of Workflow Nets

Classical workflow nets (WF-nets) are an important class of Petri nets that are widely used to model and analyze workflow systems. Soundness is a crucial property that guarantees these systems are deadlock-free and bounded. Aalst et al. proved that the soundness problem is decidable, and proposed (but not proved) that the soundness problem is EXPSPACE-hard. In this paper, we show that the satisfiability problem of Boolean expression is polynomial time reducible to the liveness problem of bounded WF-nets, and soundness and liveness are equivalent for bounded WF-nets. As a result, the soundness problem of bounded WF-nets is co-NP-hard. Workflow nets with reset arcs (reWF-nets) are an extension to WF-nets, which enhance the expressiveness of WF-nets. Aalst et al. proved that the soundness problem of reWF-nets is undecidable. In this paper, we show that for bounded reWF-nets, the soundness problem is decidable and equivalent to the liveness problem. Furthermore, a bounded reWF-net can be constructed in polynomial time for every linear bounded automaton (LBA) with an input string, and we prove that the LBA accepts the input string if and only if the constructed reWF-net is live. As a result, the soundness problem of bounded reWF-nets is PSPACE-hard.

Net-structure-based conditions to decide compatibility and weak compatibility for a class of inter-organizational workflow nets

Classical workflow nets WF-nets for short are an important subclass of Petri nets that are widely used to model and analyze workflow systems. Soundness is a crucial property of workflow systems and guarantees that these systems are deadlock-free and bounded. Aalst et al. proved that the soundness problem is decidable for WF-nets and can be polynomially solvable for free-choice WF-nets. This paper proves that the soundness problem is PSPACE-hard for WF-nets. Furthermore, it is proven that the soundness problem is PSPACE-complete for bounded WF-nets. Based on the above conclusion, it is derived that the soundness problem is also PSPACE-complete for bounded WF-nets with reset or inhibitor arcs ReWF-nets and InWF-nets for short, resp.. ReWF-and InWF-nets are two extensions to WF-nets and their soundness problems were proven by Aalst et al. to be undecidable. Additionally, we prove that the soundness problem is co-NP-hard for asymmetric-choice WF-nets that are a larger class and can model more cases of interaction and resource allocation than free-choice ones.

Complexity of the soundness problem of bounded workflow nets

This paper gives a necessary and sufficient condition for the liveness of normal nets, i.e. a normal net with a given initial marking is live if and only if it is structurally repetitive and each minimal siphon is marked in any reachable marking. Furthermore, it is proved that a normal net is structurally live if and only if it is structurally repetitive. Finally, we prove that a weakly persistent net, which is a special normal net, is live for a given initial marking if and only if it is structurally repetitive and each minimal siphon is marked in the initial marking. That is to say, the liveness of weakly persistent nets can be decided by the net structure and the initial marking only.