Do We Need Internal Behavior in Choreography Models?

Abstract

Choreographies capture the message exchanges between multiple processes. Certain choreography languages ignore the internal behavior completely, other languages o er the possibility to model internal behavior. This paper presents an example modeled in both types of languages and discusses the need to integrate internal behavior in choreographies. A choreography de nes the message exchanges between several processes. While it is fundamental that a choreography language has to include the ability to express message exchanges, it is an open question whether a choreography language should o er constructs to model internal behavior. In this paper, we present a sample choreography and use this example to discuss, whether internal behavior should be modeled in a choreography. The example is introduced in Sect. 2. Afterwards, it is modeled in Let's Dance (Sect. 3) and BPMN (Sect. 4). An overview on the support of modeling internal behavior of the most prominent choreography modeling languages is given in Sect. 5. Finally, Sect. 6 provides a conclusion and points out future work.

Introduction

Fig. 1 presents the drop-dead order scenario, where a customer requests a distributor to ship products until a given drop-dead date. The distributor neither produces the products by himself nor owns a delivery. Therefore, the distributor asks a supplier whether he can produce the requested products in time and the distributor asks a carrier whether he can carry the produced products by time. If the supplier and the carrier agree, then the customer's order is accepted and rejected otherwise. The example itself was rst presented in [ 1 ] and is used in [ 2 ] to develop transaction requirements for services. We use this example in this paper to illustrate the di erence between di erent choreography languages. drop-dead

order Customer Let's Dance [ 3 ] is a choreography language having the interaction as basic building block. Therefore, the model is called \interaction model" [ 4 ]. A choreography model of the drop-dead order scenario is presented in Fig. 2. The topmost box denotes that a customer sends an order to a distributor. Control ow is modeled using directed arcs. The rst arc points to a group, labeled with a circled A in the gure. In this group, the message exchange between the customer and the shipper and between the customer and the carrier happens in parallel. The crossed line between the acceptance and the rejection denotes that exactly one of the two message exchanges may happen. After both the supplier and the carrier have sent an answer, they have either accepted or rejected. Afterwards, group B is active. In case both the supplier and the carrier accepted the request of the distributor, they receive an acceptance of the distributor and the products are built and delivered to the customer. Otherwise, the order is rejected at the supplier and the carrier. The client's order is rejected, too.

In the presented choreography, there is no internal behavior modeled. The messages in the choreography suggest that the supplier is somehow producing products and that the carrier delivers them. However, there is no explicit description of these tasks. 4

BPMN

Model

Figure 3 presents the drop-dead order scenario using BPMN [ 5 ]. BPMN is a choreography language belonging to the category of interconnection models [ 4 ]. In interconnection models, the control ow is modeled per participant. While the interaction is one building block in interaction models, the interaction is split up in send and receive activities in the case of interconnection models. To provide a better overview on the model, one path through the model is highlighted. This path denotes the case, where both the supplier and the carrier accept the request of the distributor. First, the customer sends an order to the distributor, which asks the supplier and the carrier in parallel, whether they can produce/deliver in time. The supplier and the carrier decide and both accept the order. Since both have accepted, they are noti ed that the distributer accepted, too. Finally, the

Distributor

Order

Distributor Supplier

Request Product

Distributor Carrier

Request Delivery

Supplier Distributor

Acceptance

Supplier

Distributor

Rejection

producer produces, noti es the carrier, which picks up the products and delivers them to the customer.

Besides the split of the interactions into interconnections of send and receive activities, two internal activities of the supplier and the carrier are shown. In case of the supplier, the choreography model shows the decision activities and the produce activity. When it comes to implement a supplier based on the choreography description, the internal behavior can be used as basis for an executable BPEL process [ 6 ]. In case the choreography model is detailed enough, an IT export just has to add concrete WSDL information to the activity to turn the process into an executable BPEL process.

r e m o t s u

C Due date, items tr o u b iitr s D decide accept reject

Supplier

Carrier produce products are ready decide accept reject accept accept supplier accepted

rejection carrier accepted rejection

rejection Fig. 3. Drop-dead order scenario modeled in BPMN pickup product

y delivery

Support of Internal Actions

Language BPEL4Chor BPMN 1.2 iBPMN Let's Dance WS-CDL

Model Type interconnection interconnection interaction interaction interaction

Constructs for Internal

Behavior Available? + + { { + In this paper, we gave an overview on the support of modeling internal behavior in choreography languages. The drop-dead order scenario was introduced and modeled using Let's Dance and BPMN. Finally, we showed the support of internal actions in ve choreography languages. All languages supporting interconnection models also support the modeling of internal behavior. In the case of interaction models, two of the three languages do not support modeling internal behavior. WSCDL is the only language based on interaction models, where internal behavior can be modeled.

The examples suggest that it depends on the use-case of the choreography whether internal actions are needed. If the choreography is used to serve a basis for executable processes, the internal actions can be seen as placeholders for calls to internal services. Thus, the e ort to build executable BPEL processes seems less. We are going to describe concrete scenarios and to use them as basis for a comparison.

Acknowledgments This work is supported by the BMBF funded project Tools4BPEL (01ISE08B).

References

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