Process Management in Supply Chains – A New Petri-Net Based Approach
Marco von Mevius1, Richard Pibernik2,
Institute of Information Systems1, Department of Supply Chain Management2, School of Business and Economics, Goethe-University, Frankfurt, Germany
{mevius, pibernik}@wiwi.uni-frankfurt.de
Abstract
Supply chain process management (SCPM) refers to the design and control of interrelated production, logistics and information processes on an operational level. In this paper, we present an innovative approach to SCPM, based on a new type of high-level Petri-nets, so-called XML-nets. With the proposed XML-nets, the capabilities of SCPM can be significantly enhanced in comparison to existing methods. By employing XML-nets, not only single phases, i.e. modelling, analysis and control, but SCPM as a whole can be improved. XML-nets facilitate an integral approach to SCPM by using a consistent and comprehen- sible methodology throughout all relevant phases. A fur- ther application potential results from applying the widely used XML standard: A SCPM software application can be directly linked to different transactional systems, in order to exchange relevant (intra- and inter-organisational) process data. After a detailed introduction to supply chain process modelling with XML-nets, the advantages of the proposed methodology will be clearly outlined on the basis of a practical modelling example. Thereupon we demonstrate the architecture and functions of an XML-net based prototype software tool supporting SCPM.
1. Introduction
From an operations perspective, supply chains (SC’s) can be considered as a set of interrelated production, logistics and information processes, which are usually performed by different independent firms in order to produce and deliver a specified range of goods and services. These processes have to be designed and coordinated in such a way, that a given set of objectives is achieved. Most commonly, attaining a predetermined service level and minimizing total SC cost are relevant objectives, pursued by supply chain management (SCM). In this paper we refer to supply chain process management (SCPM) as the design and control of interrelated production, logistics and information processes on an operational level.
Process management in general as well as in regard to SC’s can be broadly divided into four consecutive inter- depending phases: process modelling, analysis, imple-
mentation and control. The major task of supply chain process (SCP) modelling is the representation of alternative SCP designs by problem-specific formal or semiformal process models. These models are to be derived in such a way, that an assessment of the alternative process designs in terms of total process cost and service levels is actively supported. On the basis of the cost and service level infor- mation, gained through SCP analysis, a design alternative is selected and finally implemented. SCP control is respon- sible for short term planning, execution, monitoring and regulation of the implemented real life SCP. For each of these phases, specific methods and applications have been developed and are also – to some extent – being applied in practice. Some examples are: (1) Event driven process chains (EPC), which have been subject to extensive re- search and are widely used to model business processes in general and SCP’s in particular. (2) Simulation methods and tools as well as activity based costing techniques, which are commonly applied for the analysis of SCP’s. (3) Workflow management methods, implemented in ERP, are being used for control and evaluation of SCP’s. The devel- oped methods and tools employed in the different phases of process management not only display room for improve- ment but also lack compatibility amongst each other. For example, the semi-formal EPC do not provide sufficient support for the development of simulation models and workflow management systems.
In this paper we present a new approach to SCPM, based on an innovative type of high-level Petri-nets, so- called XML-nets. In XML-nets, both supply chain data objects (e.g. customer orders and bills of material) and physical supply chain objects (e.g. materials and finished goods), are represented by XML documents. SCP’s are modelled as a manipulation of XML documents, represent- ing the underlying real supply chain objects (SCO’s).
XML-nets not only feature significant advantages in regard to SCP modelling. Their formal foundation allows straight- forward simulation and further extensive analysis. More- over, XML-nets can be directly interpreted and executed by a workflow engine. Thus supporting the development and implementation of control and monitoring tools.
We will provide a detailed description of an XML-net based approach to SCP modelling. The benefits for SCPM
will be clearly pointed out on the basis of a practical ex- ample. We will also demonstrate the architecture and functionalities of an XML-net based prototype software tool designed to support SCPM.
2. Literature Review
An extensive body of publications contributes to business process management in general as well as to SCPM in particular (see for example [2]). Different modelling tech- niques have been proposed for process modelling, most of them being based on textual programming languages or graphical notations such as dataflow diagrams, EPC, Petri-nets, or related notations [7,8,18]. They can be broadly divided into semi-formal methods, e.g. EPC, and formal methods, e.g. Petri-nets. Semi-formal methods, especially EPC, are very popular, widely used and easy to handle. They do, however, exhibit major shortcomings, especially in regard to SCP modelling. For example, EPC cannot adequately capture process dynamics, concurrency and parallelism of processes as well as asynchronous operations. These deficiencies result from their limited expression capabilities and lack of formal foundation [20]. For the latter, they do not allow for simulation and direct execution. These shortfalls can be overcome by the use of Petri-nets. On the basis of different definitions and interpretations of their static and dynamic modelling components, different types of Petri-nets have been de- rived. So-called high-level Petri-nets (e.g. predi- cate/transition nets [9] or coloured Petri-nets [10]) have proven suitable for modelling complex dynamic systems.
In the past years, several enhancements have been made in order to improve modelling capabilities of high-level Petri-nets. Oberweis developed so-called nested relation transition (nr/t) nets, which integrate the concept of com- plex structured objects into the Petri-net formalism [15].
Weitz proposed a Petri-net based methodology for repre- senting complex data objects by SGML documents, called SGML-nets [21]. Motivated by the increasing importance of the XML standard for inter-organizational document exchange, Lenz developed description and manipulation languages for XML documents and integrated these into the Petri-net formalism [11, 12]. This new type of Petri- net variant is called XML-net. So far, XML-nets have not been applied to SCPM and related fields.
However, different types of high-level Petri-nets, sto- chastic coloured Petri-nets in particular, have successfully been applied in areas related to SCP modelling and analy- sis, especially in production scheduling and control (see for example [4], [23], a survey is provided in [19]).
Zimmermann et al. [22] highlight the necessity of em- ploying a software tool for integrated modelling, analysis and control of manufacturing systems. They present an application called TimeNET, which supports modelling and analysis of manufacturing systems on the basis of a stochastic coloured Petri-net methodology. Van der Aalst and Waltmans [1] developed a toolkit that provides stan-
dard logistics and manufacturing processes, which can be combined graphically in order to build a model of logistics or manufacturing systems. Dong and Chen [6] introduce an approach for modelling and analysis of manufacturing SC’s on the basis of Computer Integrated Manufacturing Open Systems Architecture and object-oriented predi- cate/transition nets. On this methodological basis they conduct a process sequence analysis. Raghavan and Viswanadham [16] use generalized stochastic Petri-nets for modelling and analysis of interorganizational SC’s.
We propose an XML-net based approach to SCPM. As we will show, XML-nets meet the specific requirements of SCPM exceptionally well. The major advantages in regard to existing methods can be summarized as follows: (1) XML-nets are capable of capturing process dynamics, concurrency and parallelism of processes as well as asyn- chronous operations. (2) XML-nets provide superior capa- bilities for modelling complex objects with a hierarchical structure, which can typically be found in SCP’s. (3) By using the XML standard, XML-nets support the exchange of intra- and inter-organizational data. These advantages predestine XML-nets not only for modelling and analysis of SCP’s but also for an integral SCPM, including control and monitoring of SCP’s.
3. SCP Modelling with Petri-Nets 3.1. A Formal Definition of Petri-Nets
In this section we define place/transition (p/t) nets, a basic type of Petri-nets. With this formal foundation we establish the basis for introducing XML-nets in chapter 4.
A p/t net can be defined as a tuple (P,T,F,C,W,M0) (see for example [17]), where
(1) P is a finite, nonempty set of so-called places,
(2) T is a finite, nonempty set of so-called transitions (P∩ T=∅),
(3) F⊆
{
(PxT)∪(TxP)}
is the set of directed arcs (p,t) or (t,p), connecting a place p∈P with a transition t∈T or a transition t∈ with a place p∈P, T(4) C is a function, assigning each p∈P a capacitiy C(p), (5) W is a function, assigning all (p,t),(t,p)∈ a weight F
W(p,t),W(t,p), and
(6) M0 assigns each p∈P an initial marking, i.e. a non- negative finite integer of so-called tokens M0(p)≤C(p).
In a Petri-net, the dynamics of a system (in our case the SCP) are modelled by firing rules, which define the flow of the tokens. With the symbol t• we denote the set of all input places (pre-set)
{
p∈P(p,t)∈F}
of transition t∈ .T Accordingly, t denotes the set of all output places (post-• set){
p∈P(t,p)∈F}
of transition t∈T. Considering the initial marking M0(p), transition t∈T is enabled if condi- tions )M0(p)≥W(p,t (∀p∈•t) and M0(p)≤C(p)−) p , t (
W (∀p∈t•) are satisfied. Transitions are allowed to fire only, if enabled. When transition t fires, it removes
tokens from its input places p∈ and adds tokens to its •t output places p∈ t•. The number of tokens removed or added is determined by the arc weights W(p,t) (for all
t p∈•
∀ ) and W(t,p) (∀p∈t•). Starting from the initial marking M0(p), the firing of a transition t transforms M0(p) into the successor marking
°°
¯
°°®
•
∩
•
∈ +
− ∈ • •
+ ∈• •
−
=
else )
p ( M
, t t p for ) p , t ( W ) t , p ( W ) p ( M
, t t p for )
p , t ( W ) p ( M
, t t p for )
t , p ( W ) p ( M ) p ( M
0 0 0 0
1
\\ .
Likewise, successor markings M2(p), M3(p),… result from multiple firing of transitions.
3.2. A Modelling Example
We now introduce a practical example to illustrate SCP modelling with p/t-nets. This example will facilitate the upcoming analysis of SCPM on the basis of XML-nets.
A manufacturer of printers plans to introduce a new series of high-grade laser printers in Germany. All models of this series are based on a pre-assembled basic printer model. Customer specific assembly and the “flash”- process to configure the read only memory (ROM) are to be carried out at an assembly facility of a logistics service provider (LSP) in Germany. The printer manufacturer guarantees his retailers a next day service for all customer orders, arriving before 4 p.m. He requires a service level of at least 99 % on a monthly basis. For the shipping of the assembled printers from his facility to the customers, the LSP plans to utilize his own transportation capacities.
The packaged and palletized printers will be collected and shipped from the assembly facility to the LSP’s central hub at 9 p.m. every evening. An exception is made for customer orders consisting of not more than two printers.
These orders will be collected by a parcel service at 6 p.m. every evening. The outlined (sub-) processes for the customer-specific assembly of the printers can be mod- elled by a p/t-net with P=
{
p1,...,p13}
;T={
t1,...,t14}
;∞
= ) p (
C (∀p∈P); W(p,t), W(t,p) = 1 (∀ (p,t),(t,p)∈
F). The p/t-net is displayed in figure 1.
Figure 1. P/t-net for printer assembly processes The printer manufacturer instantly forwards the incoming
customer orders to the LSP via an XML-EDI interface (t1).
The received customer orders are represented by the tokens located in the place “customer orders” (p1). For every cus- tomer order a manufacturing order is generated (t2). A manufacturing order consists of a picking list, an assembly order and a flash-process instruction. Based on the picking list, the components required for customer specific assem- bly of an order are manually picked (t3), conveyed to a vacant assembly station (t4), and assembled on the basis of the corresponding assembly order (t5). Thereafter, the assembled printers are conveyed to a vacant flash-station (t6), where the printers’ ROM are configured with regard to customer’s specifications (t7). Thereupon, every printer of a customer order undergoes functional testing (t8). If every printer of an order passes functional testing, the order is packaged. Customer orders consisting of more than two printers are palletized (t9) and transferred to a certain pick- up zone (t10). Small customer orders with not more than two printers are directly transferred to a special collection zone for the parcel service (t12). The whole order is put on hold if one or more printers do not pass functional testing (t13). As there is no immediate error handling, a new manufacturing order is generated to replace the deficient printers of the incomplete order (t14).
4. Modelling SCP’s with XML-Nets
We propose XML-Nets, a novel kind of high-level Petri- nets, for modelling, analysis and control of SCP’s. XML- nets [10,12] can substantially enhance SCP modelling capabilities. In the following section we give a brief de- scription, why the proposed methodology is particularly suitable for SCPM. In section 4.2 we define SCO’s and delineate the hierarchical structure of complex SCO’s with regard to the practical example introduced in the previous section. Thereafter we provide a brief description on how SCO’s can be represented by XML documents (section 4.3). The structure and the dynamic behaviour of XML- nets will be described in section 4.4.
4.1. Applying XML-nets to SCPM
From the practical modelling example in section 3 it be- comes clear that Petri-nets are generally suited for model- ling interrelated production, logistics and information proc- esses. Especially process dynamics, concurrency and paral- lelism of processes as well as asynchronous operations, which can typically be found in SC’s can be modelled in a formal, explicit and comprehensible way. For this, they have to be considered superior to the widely used semifor- mal modelling techniques and should therefore be applied to SCPM (see section 5.1). However, they do exhibit some major shortcomings in regard to SCP modelling which necessitate the use of a more sophisticated and pertinent methodology, accounting for the specific requirements of SCPM. In order to motivate the application of XML-nets to SCPM, we will briefly outline intrinsic problems faced, when modelling and analyzing SCP’s. As we will point out
in the remainder of this section as well as in chapter 5, these problems can be overcome by the use of XML-nets.
A major issue of modelling SCP’s is an adequate rep- resentation of the numerous input and output objects relevant for the different processes performed in a SC.
Typically, these SCO’s (e.g. data objects and physical objects such as customer orders, bill of components, printers) exhibit a complex hierarchical (data) structure.
As both instances of the complex data structure of a SCO and the structure itself can be transformed by SCP’s, it is indispensable to explicitly model the complex structures and the input and output relations of relevant SCO’s. If this requirement is not fulfilled, SCPM is severely lim- ited: SCO’s cannot be directly identified within the proc- ess, relevant process data (e.g. different assembly or flash process times for different printer models as well as real- time status information) cannot be assigned to the indi- vidual objects nor can the transformation of specific ob- jects be represented and traced in an appropriate way. In p/t-nets only anonymous tokens are available for model- ling. Therefore, they are not appropriate for SCPM. As we will show below, these problems, encountered by SCP modelling, can be solved by integrating XML, which will be used for representing the complex hierarchical struc- tures of SCO’s, into the Petri-net methodology. The bene- fits for SCP analysis and control, which mainly arise from being able to model the complex structures of SCO’s will be outlined in chapter 5.
The use of the XML standard for modelling SCO’s makes the proposed methodology particularly suitable for SCPM. By using XML, we revert to a well-accepted and widely used standard, which is increasingly being em- ployed to support the exchange of complex data objects between heterogeneous SCM applications. Apart from the fact that the use of an accepted standard improves trace- ability and acceptance, XML-nets also entail considerable advantages in regard to the interaction with relevant transactional systems and SCM software applications.
Considering the example in section 3.2, it can be observed that the customer orders are transmitted via an EDI/XML interface. Therefore, the relevant data can be directly processed in a SCPM application based on XML-nets.
Further relevant data (e.g. delivery notes, status informa- tion) can easily be exchanged between XML-net based SCPM applications and different transactional systems, e.g. ERP systems. As we will point out in section 5.2 and 5.3, this can significantly enhance the scope of applica- tions employed for the analysis and control of SCP’s.
Already this short description of the benefits associated with the proposed methodology make clear, that an XML-net based approach to SCPM is particularly suitable. After an in-depth characterization of XML-net based SCP modelling, we will render these advantages more precisely with regard to our practical example.
4.2. Supply Chain Objects
Modelling complex SCO’s in an appropriate way is a pre- requisite for modelling the object flow in SCP’s. SCO’s are either an input or the result (output) of a SCP. Physical SCO’s are transformed by the processes (e.g. material, pre- assembled components) or provide transformation capaci- ties and capabilities (e.g. machines). Supply chain data objects contain information, which enables or triggers the processes (e.g. customer orders, picking lists, bills of com- ponents, manufacturing orders) or provide information on process outputs (e.g. order status reports, performance reports). Typically, SCO’s are complex: They comprise of objects, which can be considered to be on a lower level of the object hierarchy. These objects can again be complex as well as elementary objects. Elementary objects, in contrast to complex objects, do not comprise of any other objects on a lower level. They therefore constitute the lowest level in a complex objects hierarchical structure. In our modelling example, the manufacturing order is a complex object. As an instance of a pre-defined object structure, it is derived from a specific instance of a customer order as well as the bill of components and the flash-process instruction, which are defined for every printer model available. In SC’s, complex objects are commonly derived from combining internal objects (e.g. bill of components) with external objects (e.g. customer orders). As we will demonstrate in chapter 5, both the structure of SCO’s and their interrela- tions have to be modelled in order to support SCP analysis and control successfully. In the following section we de- scribe, how complex SCO’s can be modelled with XML.
4.3. Modelling Supply Chain Objects with XML An XML document type definition (DTD) allows the speci- fication of a document type with problem specific mark- ups as a class of XML documents conforming to that spe- cific DTD [12]. In complex XML applications, the textual description of XML DTDs can lead to a significant effort.
Moreover, the structure of textual DTDs is often not intui- tively traceable. To overcome these difficulties, two graphical representation formats are used for the descrip- tion of XML DTDs and their valid documents: GXSL, a graphical schema definition language and XManiLa, a related document manipulation language. A detailed de- scription of both languages can be found in [12, 13].
Figure 2. Graphical XML schema of a manufactur- ing order
GXSL is based on the Unified Modelling Language (UML) and refers to the concept of class diagrams [3]. Figure 2
FPI assembly order ANr[1]
bpm ni memory
manufacturing order MNr[1]
picking list PNr[1]
1..*
PR 1..*
1..*
element attribute
~ association 1..* ~ cardinality 1,...,∞
legend:
[1] attribut required bpm basic printer model ni ~ network interface PR ~ printer
FPI ~ flash-process-instruction ~
~
displays the XML DTD for a manufacturing order as a graphical XML schema.
Whereas GXSL is used for the description of XML DTDs, XManiLa is applied for querying and manipulating XML documents. In order to support a content-based query, constants or variables can be assigned to an ele- ment or an attribute of the document [12]. The concept is strongly related to “query-by-example” for relational databases. In regard to our example, specific queries can be modelled, e.g. a periodic determination of the number of on-time assemblies or printers which do not pass func- tional testing. Next to queries, XManiLa can also be em- ployed for manipulation, i.e. insertations, deletions and updating, of documents. These operations are either ap- plied to the whole document or to elements on lower levels of the documents hierarchical structure. In order to illustrate the concept, a simple XManiLa based query (a) and a deletion (b) is depicted in figure 3.
Figure 3. XManiLa query and deletion operations The XManiLa schema (a) describes a query for all manufacturing orders including the specific printer model
“LS920i”. The result of this query are all manufacturing orders, picking lists and assembly orders for the specific printer model. As this syntax allows for a specification of the set of XML documents conforming to a specific as- signment of constants and variables, this schema can be directly translated into a machine-readable database que- ry. By the deletion (b) the complete manufacturing order with the identifying number “#0112” is deleted from the database. A specific indentation (a black bar on the left hand side of the object) is used to mark the elements sub- ject to manipulation. The manipulation is inherited by all objects on a lower level of the hierarchy.
4.4. Modelling Supply Chain Processes with XML-Nets
In XML-nets, the graphical description of XML DTDs with GXSL and the manipulation of documents with XManiLa are integrated with the Petri-net formalism.
When using XML-nets for SCP modelling, the places can be considered as repositories for sets of predefined rele- vant SCO’s (tokens). As depicted above, the structure of any SCO can be represented by an XML DTD, which is now described by a GXSL schema. These schemas iden- tify all objects, a specific place can contain. Transforma- tions of one or more of these objects, performed by SCP’s, are represented by the transitions. In the SCP model, the transitions manipulate (create, change or de- lete) XML documents, which represent the underlying
SCO’s. A SCP is therefore modelled by a manipulation of an XML document, representing the real SCO. Associated places and transitions are connected by arcs, which display the direction of the object flow. A manipulation filter is assigned to each arc. The filter is an XML schema based on the XManiLa syntax. It specifies the XML documents, which are manipulated by the firing of a transition. It should be emphasized that – in contrast to other high-level Petri-nets – not only variables but also the hierarchical structure of objects can be manipulated. The filter can be considered as a template, which defines the set of tokens that can be manipulated by a transition. As explained in section 3.1, a transition can only fire if enabled. A transi- tion is enabled if all places in the pre-set contain at least one token, which complies with the filter, assigned to the adjacent arc. Furthermore, optional specific inscriptions can be assigned to the transitions. These so-called transi- tion inscriptions represent a set of logical expressions for certain variables of the documents, stored in the places of the pre-set. They can also include functions, which deter- mine the values of variables and the structure of the tokens in the post-set after firing the transition. If an inscription is assigned to a transition, the transition is enabled only if all places in the pre-set contain at least one token which com- plies with the filter and all logical expressions have the value “true”. We now illustrate this description of the basic concept of XML-nets on the basis of the modelling exam- ple, introduced in section 3.2. In figure 4 the processes
“generation of manufacturing orders”, “picking” and “con- vey to assembly” are modelled by an XML-net.
Relevant SCO’s for the process of generating manufac- turing orders are customer orders as well as bills of compo- nents and flash process instructions for every printer model available. They are stored in the places of the pre-set of transition t2. The customer order has a specific identifier (CNr) and contains identifiers (PrNR) for the ordered print- ers (PR). When transition t2 fires, objects in the pre-set are deleted and a new object is created: t2 deletes an instance of a customer order and also the corresponding bills of components and flash process instructions (FPI) for the ordered printers. These objects are linked by the values of the attribute PrNR. A new manufacturing order is created and inserted in p2. As described above, the manufacturing order comprises of a picking list as well as assembly orders and flash-process instructions for the ordered printers.
Transition t3 has a read only access on the picking list, included in a manufacturing order. As the read only access does not lead to a manipulation, the filter of the arc (p2,t3) is not marked by a black bar. Transition t3 represents the picking of the components and inserts the list of picked components into place p3. Transition t4 can only fire if there is a list of picked components in p3 and a manufac- turing order in p2 with an identical value of the attribute PNr. This ensures that the process “convey to assembly” is only performed after the required components have been picked.
assembly order ANr manufacturing
order MNr
picking list
PR = "LS920i"
manufacturing order MNr="#0112"
a.) b.)
Figure 4. XML-net for supply chain processes
5. Benefits of an XML-Net Based SCPM 5.1. Benefits for Supply Chain Process Modelling To highlight the SCP modelling advantages of XML-nets in comparison to existing formal and semi-formal meth- ods, we will revert to our modelling example: As de- scribed in section 4.2, the customer order as well as the manufacturing order is a set of different SCO’s. However, in most cases only subsets of SCO’s are relevant for spe- cific processes. These subsets can include objects on the same level or on different levels of the hierarchical struc- ture. For deriving a specific picking order from the cus- tomer order and the bill of components, the objects at the lowest level of the bill of components, i.e. basic printer model, network interfaces, memory, are relevant. These have to be linked to other pertinent data objects, e.g. a document containing the shelf positions. Whereas for picking, a list of required components is sufficient, for assembly, the link between components, ordered printers and also customer orders has to be available. If we as- sume that one specific customer order can be split up among different assembly and flash stations, the link between assembled/configured printers and the underly- ing customer order has to be established. For every proc- ess taking place after the assembly, the components are not relevant objects anymore. The packaging and palletiz- ing, for example, require the customer order as well as the link between finished printers and customer orders.
Up to now we have only considered the relevance of SCO’s in regard to the input, i.e. data input for planning and control of processes as well as physical inputs. As SCP modelling has to support the succeeding SCPM phases, the specific requirements of SCP analysis and control have to be considered when deriving a SCP model. For analysis and control purposes output objects,
especially data objects, are of particular interest. As de- scribed in section 3.2, the LSP has committed to a service- level of 99 %. In order to analyze the SCP with respect to this objective, it has to be possible, to measure the number of completed orders which are ready for shipment before 9 p.m. (large orders) or 6 p.m. (small orders). Thus, the com- pleted customer orders at a certain point of time are the relevant SCO’s (see section 5.3). With respect to SCP costs, utilization ratios for the assembly and flash stations may be relevant indicators for the analysis and control of the SCP. The relevant SCO’s are then not the customer orders, but the printers, assembled and configured at each assembly and flash station, including relevant information on the printer-specific process times. These examples make clear that for detailed and realistic process modelling, which meets the specific requirements of SCPM analysis and control, the structure of individual, complex SCO’s and their interrelations have to be considered in an appropriate way. The proposed methodology is particularly suitable for meeting these requirements. If these requirements are not being met, modelling, analysis and control are limited to one specific object type with a given structure, e.g. cus- tomer orders. If, for example, EPC are (consistently) used for modelling SCP’s, one has to either model the processes on the basis of printers or on the basis of customer orders as relevant SCO’s. EPC have the same expression capabil- ity as p/t-nets. Neither the structure nor the values of the variables can be manipulated. These objects then have to be considered as elementary objects with a pre-determined, flat data structure. Therefore, it is not possible to ade- quately model SCP’s, when SCO’s are complex, i.e. have different hierarchical structures – a typical case in SCP’s.
When using a Petri-net based modelling approach, not being able to consider complex object structures implies
customer orders (p1)
generate manufacturing orders (t2)
manufacturing
orders (p2) picking (t3)
bill of components
picked components (p3)
bill of omponents PrNR
bpm ni memory
FPI
manufacturing order MNr
picking list PNr
bpm ni memory
1..*
FPI
PR assembly order ANr
1..*
1..*
picking list PNr
bpm ni memory
1..*
manufacturing order MNr
picked components PNr
bpm ni memory
1..*
convey to assembley (t4)
manufacturing order MNr
picked components PNr
...
...
Picking list PNr FPI
PrNR customer order
CNr
PR PrNR
1..*
1..*
that all tokens represent SCO’s with a flat structure. This also holds for high-level Petri-nets with distinguishable tokens. The objects are distinguishable by the values of their variables. The structure, however, is flat and can only be transformed into another flat structure. Just as in EPC’s, SCP modelling is then basically restricted to one type of SCO with a pre-determined structure.
5.2. Benefits for Supply Chain Process Analysis The major task of SCP analysis is the assessment of alter- native SCP designs with respect to a given set of objec- tives. As pointed out before, Petri-nets can – due to their formal foundation – be directly executed and therefore be used for simulating the modelled process [14]. A simula- tion engine can interpret the formal syntax and semantics of the Petri-net and transform it into a machine-readable code [5]. As shown in section 4, XML-nets are consistent with Petri-net formalism and can therefore also be directly simulated. This property of Petri-nets in general and XML-nets in particular constitutes a major advantage over semi-formal methods. If a semi-formal method like EPC is used, the simulation model cannot be directly derived from the process model. The transformation of the semi-formal syntax into a machine-readable code leads to a significantly higher effort. Often, a breach of consistency between process model and simulation model can be observed.
In regard to our modelling example, a simulation analysis can, for example, be used to evaluate SCP de- signs with different assembly and flash process capacities.
For this purpose, process times (e.g. for the assembly process), represented by firing delays and stochastic proc- ess outputs (e.g. the number of deficient printers) can be assigned to the transitions.
Whereas direct simulation is an advantage, which generally applies for Petri-net based modelling ap-
proaches, XML-nets exhibit specific advantages. These are again based on the ability to model complex SCO’s. With the proposed methodology it is possible to conduct simula- tion a analysis of SCP’s on a higher and more realistic level of detail. We will demonstrate this for the handling of or- ders with deficient printers (see section 3.2). In figure 5 the relevant processes are modelled by an XML-net. As de- scribed in section 3.2, the whole order is put on hold if one or more printers do not pass functional testing. In a simula- tion run, two new objects (tokens) with different hierarchi- cal structures are generated by the firing of the transition
“negative functional testing (t14)”: (1) Incomplete orders with a set of printers tested positive and a set of printers tested negative, stored in the place “incomplete orders”. (2) Printers tested negative, stored in the place “deficient print- ers”. The deficient printers trigger the generation of a new manufacturing order to replace the deficient printers of the incomplete order, waiting in the place “incomplete orders”.
In the simulation run, the new manufacturing order is trans- ferred into the place “manufacturing order” (see figure 1) and is processed (i.e. picking, assembly, flash process) according to the description in figure 1. The synchronisa- tion of the (new, replacing) order and the waiting incom- plete order is performed when transition “complete order”
fires. For the simulation analysis, firing probabilities are assigned to the concurring transitions “positive functional testing (t9)” and “negative functional testing (t14)”. If transition “negative functional testing” fires, one or more printers of the order are marked as deficient on the basis of a pre-determined probability distribution (see chapter 6).
This example illustrates, how XML-net based modelling of the flow of complex SCO’s can enhance simulation analy- sis: It can explicitly be considered that complex SCO’s are split up into other complex objects which are synchronized after being processed asynchronously.
Figure 5. XML-net for handling deficient printers
completed orders (p7)
positive functional testing (t8)
bill of omponents PrNR
bpm ni memory
1..*
deficient printers
FPI PrNR generate
manufacturing order
...
...
negative functional testing (t13)
incomplete orders
complete order completed orders
manufacturing orders (p2) ...
boc
FPI
manufacturing order MNr approved
orders (p8)
PRneg PrNR deficient p.
customer order
customer order CNr
PR PrNR
1..*
customer order
customer order CNr
PR PrNR
1..*
customer order CNr
PR PrNR
1..*
PRneg PrNR customer order CNr
PR PrNR
1..*
1..*
customer order
PRneg PrNR
1..*
Whereas asynchronous processing can generally be mod- elled with high-level Petri-nets, splitting up and synchro- nizing complex SCO’s can neither be modelled with ex- isting Petri-net approaches nor with specific tools for discrete simulation (e.g. Arena, SIMPRO).
5.3. Benefits for Supply Chain Process Control The major tasks of SCP control are short term planning, execution and the monitoring of the real life SCP. Petri- nets in general have proven suitable for supporting proc- ess execution. They are a well accepted methodological basis for workflow management systems, which are an industry acknowledged computerized support for process execution systems. Petri-nets can be interpreted by a workflow engine, which generates and executes a work- flow model on the basis of a process model. It automati- cally triggers the processes when a certain event (e.g.
arrival of a customer order) occurs. The XML-nets in figures 4 and 5 can also be interpreted by a workflow engine. The workflow engine triggers the execution of all automated processes (e.g. generation of manufacturing orders) and supports manual processes (e.g. picking) by providing relevant information (e.g. picking lists). It con- trols the flow of SCO’s and all processes performed in order to fulfill the customer orders. As depicted in the previous sections, the proposed methodology allows for a more precise and detailed modelling of the SCP, espe- cially the complex SCO’s. This is a prerequisite for auto- matic execution as well as for providing the information required by processes, performed manually.
Next to the support of workflow management in SCP’s, an XML-net based approach displays another important benefit: Due to the detailed modelling of SCO’s it actively supports an SCP monitoring on the basis of relevant performance indicators, which can be directly integrated into the XML-net methodology. Usually, a set of interdepending indicators is used for monitoring the
performance of the SCP. The values of the indicators are measured on a periodic or continuous basis and are com- pared with target values. If a pre-determined deviation from the target value is observed, pre-defined processes are triggered automatically, e.g. the decision maker receives an alert. In our modelling example, relevant indicators are the service level, the LSP has committed to, utilization ratios for flash and assembly stations as well as the defective printer quota. These indicators are usually aggregated for certain time-intervals, e.g. a shift, one day or a month.
When using XML-nets, measuring the values of the indica- tors, comparing them with target values and triggering an alternative process can be directly integrated into the SCP model. In figure 6 XML-net based monitoring is illustrated for the service level indicator.
The service level SL is calculated by dividing the total number of customer orders completed on time, i.e. large orders (COLot) by 9 p.m. and small orders (COSot) by 6 p.m., by the total number of customer orders (CO) which arrived until 4 p.m. This calculation is performed by the transition “calculate service level” at 9.01 p.m. The total number of customer orders is determined in the counter place “count customer orders” which is linked to the place
“customer orders” (p1) (see figure 1 and 4). Likewise, the number of orders completed on time are counted by the counter places “counter completed orders (large)” and
“counter completed orders (small)”. The transitions “de- termine completed orders (large)” and “determine com- pleted orders (small)” assess the number of completed orders at 9 p.m. (large orders) and 6 p.m. (small orders). By the use of a transition inscription it is ensured, that the transitions fire, when the system time is either 6 p.m., or 9 p.m. respectively. The number of customer orders com- pleted on time are assessed by the transition “determine number of customer orders”. This transition fires, when the system time is 4 p.m.
Figure 6. XML-net based monitoring
calculate service level
SL=((COLot + COSot) /CO)*100
^ time=”2101”
CO'=CO+1
time
time=”2100”
time=”1600”
time=”1800”
completed orders (large) on time completed orders (small) on time
counter completed orders (small)
compare service level with target service
level
performance indicators
performance indicators
service level target
SL
<
“service level target“
customer orders (p1)
COSot CO'
CO
counter customer orders
time count customer SL
orders
determine customer orders
determine completed orders (small)
determine completed orders (large)
CO
time
CO
COS
time
COSot time
COLot COLot
SL total number of
customer orders
CO customer
order
COL
The transition “compare service level with target” evalu- ates the service level with a target service level, which is a token stored in the place “performance indicators”. In this place the target values of other relevant performance indicators, for example utilization ratios or the defective printer quota, can also be stored. The determination of these indicators can be implemented in a similar way as depicted in figure 6. If the determined service level is less than the target value, the fact transition “compare service level with target” is enabled. Enabling the transition automatically triggers a pre-determined process, e.g. an alert. If a traffic light function is employed, enabling the transition causes a red traffic light.
Another important feature of SCP monitoring, sup- ported by an XML-net based approach, is a continuous review of order statuses in order to determine, whether the orders in process will be finished on time. On the basis of the order status and the estimated remaining lead time, the time of completion can be assessed for orders in process.
If orders are identified, which seemingly will not be com- pleted on time, a specific process can be triggered, e.g. an alert or automatic rescheduling of the order sequence.
An additional major benefit comes along with an XML-net based SCP control: By exclusively using XML- documents, appropriate data can easily be exchanged with different transactional systems., e.g. ERP systems, and can also be made available via internet. In our example, an XML-net based control and monitoring tool could be directly linked to the transactional system of the LSP and the printer manufacturer. XML based order delivery no- tices as well as performance reports can be transmitted automatically to the printer manufacturer on a periodic basis. These documents can be directly processed in the printer manufacturer’s transactional system, e.g. for bill- ing. Similar to tracking and tracing systems, order status information can also be provided via internet or intranet for the customers or the printer manufacturer.
6. A Prototype Software Tool
In this section we outline the basic architecture and func- tionalities of a prototype software tool for an integral XML-net based SCPM, which is currently being devel- oped. The basic architecture is displayed in figure 7.
The software application provides integrated modules for XML-net based modelling (XML-Schema-Editor, XML-Net-Modeler), analysis (PerformanceSimulator) and control (PerformanceController). The XML-Schema- Editor is used for modelling (the structure of) the relevant SCO’s as graphical XML schemas (see figure 2). The module provides all graphical elements (e.g. element types, associations, cardinalities) needed for generating the graphical XML schema for any SCO. Every schema receives a distinct name and can be stored in a database.
Whereas the XML-Schema-Editor is used for data model- ling, the XML-Net-Modeler is employed for generating the SCP model.
Internet
XML- Schema Editor
XML- Net- Modeler
PerformanceSimulator PerformanceController Database
ERP Applications Data Warehouse
FPI assembly order ANr[1]
bpmnimemory
manufacturing order MNr[1]
picking list PNr[1]
1..*
PR 1..*
1..*
element attribute
~ association 1..* ~ cardinality 1,...?
legend:
[1] ~ attribute required bpm ~ bill of components ni ~ network interface PR ~ printer FPI ~ flash process instruction
accepted or ders ( p3) gen era te ma nufa cturing orders (t 4) m anuf actur ing or der s ( p4) Pi cking (t5 )
bill of com pon ents
p icked com pon ents ( p5) bill of o mpo nents PrN R bp mn i memo ry
1.. * F PI man ufact uri ng or der MNr P icki ng l ist P Nr bpmn i m emor y
1..*
FPI PR assemb ly or der ANr 1 ..*
1.. *Pi cking list PN r
b pmnimem or y 1. .*
m anuf actur ing or der MNr
pi cked com pon ents P Nr bp mn i m emo ry
1. .*
Con vey to assembl ey (t6 ) m anuf actur ing or der MNr
pick ed com ponen ts PNr .. . .. .
Pi cking list PN r FP I Pr NR cu st omer ord er CNr P R P rNR 1..*
Figure 7. Architecture of the software tool In the first phase of process modelling the XML-Net- Modeler is used for generating a basic Petri-net model of the SCP (see figure 1). The module supports this phase by providing the required elements (places, transition, arcs). In the second phase the XML-nets are generated: The basic Petri-net model for the SCP is combined with the graphical XML schemas, defined with the XML-Schema-Editor. The user assigns the relevant graphical XML schemas for the SCO’s to each place as an object specification. On the basis of the XML schema definitions for the places included in the pre-set of a transition, the user then has to specify the input objects for a transition. The corresponding manipula- tion filter is automatically assigned to the arcs connecting a transition with the places of its pre-set. In the same way, the manipulation filters are determined for the places in- cluded in the post-set. The system automatically ensures consistency by only providing feasible specification alter- natives for the manipulation filter. The user can optionally define transition inscriptions on the basis of a pre-defined set of logical operators.
The previously modelled XML-nets can be analyzed with the PerformanceSimulator as described in section 5.2.
At first, the user configures one or more simulation ex- periments. On the basis of a configuration menu the proc- ess times, i.e. firing delays, as well as probability distribu- tions for the transition outputs are defined. The system automatically generates a simulation model, performs a pre-determined number of simulation runs and stores the simulation results. On this basis, specified reports with textual and graphical evaluations can be generated.
With the module PerformanceController the operational data is linked to the process, modelled with the XML-Net- Modeler. At first, the source of the input objects, which trigger the process has to be defined by specifying a query for the first transition of the XML-net. In our modelling example the transition “arrival of customer order (t1)”
persistently reads the new customer orders from the data- base of the LSP’s transactional system. Thereafter, a data- base connection must be defined for every place of the XML-net. On the basis of the specified database connec- tions and the dynamic specifications of the transitions (fir- ing rules), defined in the XML-Net-Modeler, an auto- execute workflow schema can be generated. For monitor- ing purposes pre-configured XML-net structures for the definition of performance indicators and target values are provided. A pre-defined process can be determined, which
is triggered if the fact transition (see figure 6) is enabled.
This process can be modelled with the XML-Net-Modeler and can be linked to the fact transition. Clearly, the mod- ule ProcessController integrates process execution and monitoring as described in section 5.3.
Figure 8. XML-Net-Modeler
Due to its open architecture with configurable inter- faces, the tool can be directly linked to internal and exter- nal systems, e.g. data warehouse, ERP and other applica- tions in order to exchange relevant process data.
Up to now, the conceptual approach, proposed in this paper, has not been evaluated with respect to practical SCPM cases. With the software tool we aim at its valida- tion with regard to process management in different in- dustry-specific SC’s. The outlined architecture and the described functions constitute the basis for further devel- opment, implementation and application of the proposed methodology.
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