How to Model Multi-Party Service in SoaML?

What is this Tutorial About?

This tutorial explains what a multi-party service is in terms of SoaML and how to specify such a service with various SoaML diagrams in Visual Paradigm.

The example used in this tutorial is about paying tax through an online bank account. You will draw different SoaML diagrams to specify the tax payment service.

Preparation

To complete this tutorial, make sure you have Visual Paradigm downloaded and installed. Click here to download Visual Paradigm if you do not have it.

You are expected to have a basic understanding of SOA and SoaML, and how to develop an SOA with SoaML using Visual Paradigm. If you are unclear about any of these topics, please read the How to Draw SoaML Diagrams? tutorial first.

Part I - Define Interfaces in a Service Interface Diagram

In a multi-party service contract, all participants provide their own interface and use the interfaces of each party they call. Let's draw a service interface diagram for the three interfaces: Tax Payer, Tax Receiver, and Bank.

1. In a new project, create a service interface diagram by selecting Diagram > New from the toolbar. In the New Diagram window, enter Service Interface Diagram in the search field and click Next. Then, fill in the Diagram Name and Description (if any), and click OK to confirm diagram creation.
2. We are going to draw the three interfaces. Expand the Service Interface tool in the diagram toolbar and select Interface.
   
3. Let's create an interface for the tax payer. Click on the diagram to create an interface and name it `Tax Payer`.
   
4. Create two more interfaces: Tax Receiver and Bank.
   
   That's all for now. In each interface, there should be operations (or signals) to be invoked by others, but we are not going to specify them yet. When we define the choreography of the service in a sequence diagram, those operations will be generated automatically. This will be done in the coming sections.

Part II - Drawing a Service Contract Diagram

Multi-party service contracts are those that involve two or more participants. Let's draw a service contract diagram for the tax payment (multi-party) service.

1. To create a service contract diagram, select Diagram > New from the toolbar. In the New Diagram window, enter Service Contract Diagram in the search field and click Next. Then, fill in the Diagram Name and Description (if any), and click OK to confirm.
2. Select Service Contract from the diagram toolbar and click on the diagram to create a service contract. Name it Tax Payment Service.
   
3. Visualize the roles of participants in the tax payment service. Click on the New Role resource to create a role in the Tax Payment Service contract.
   
4. Name the role payer.
   
5. Let's set the type for the role. Right-click on the role and select Select Type... from the popup menu.
6. In the Select Type window, select Tax Payer and click OK.
   
7. Visualize the role of the bank. Use the Connector -> Role resource to create a new role from the payer role.
   
8. Name the role `payment gateway`.
9. Let's set the type for the role. Right-click on the role and select Select Type... from the popup menu.
10. In the Select Type window, select Bank and click OK.
    
11. From the payment gateway role, create the role receiver. Select Tax Receiver as its type.
    
12. The tax receiver may interact with the tax payer to let them know about the payment status. So, connect the payer and receiver roles. Finally, your service contract diagram should look like this:
    

Part III - Specifying Multi-Party Choreography with a UML Sequence Diagram

In a multi-party service contract, the communication between parties and the choreography can be shown in an interaction diagram. An interaction diagram, like a UML sequence diagram, shows who calls whom and when the calls are made. Let's draw a sequence diagram to specify the choreography of the tax payment service.

1. Click on the Tax Payment Service service contract.
2. Click on the tiny resource icon at the bottom-right of the shape and select New Diagram... from the popup menu.
   
3. The New Diagram window will open. In the New Diagram window, enter sequence diagram in the search field and click Next. Then, fill in the Diagram Name and Description (if any), and click OK to confirm.
4. Create three lifelines in the diagram. Name them payer, payment gateway, and receiver.
   
5. Set the classifier for the lifelines. Right-click on the payer lifeline and select Select Class > Select Class... from the popup menu. In the Select Class window, select Tax Payer and click OK.
   
6. Set Bank and Tax Receiver to be the classifiers of the payment gateway and receiver lifelines, respectively.
   
7. It's time to model the interaction between the lifelines. The interaction starts with a payment request made by the payer on a bank account. So, create a message named `pay tax` between the payer and payment gateway lifelines.
   
8. For 'pay tax' to be an operation in the Bank interface, we have to create an operation from the sequence message. Right-click on the message and select Select Operation > Create Operation "pay tax" from the popup menu.
   
9. Having received the payment request, the bank will request the tax receiver to process the payment. Create a message named `process tax payment` from payment gateway to receiver. Again, create an operation from the message.
   
10. If the payment is made correctly, the tax receiver will send a confirmation message to both the bank and the tax payer. Otherwise, the tax receiver will send a reject message to the bank, and the bank will forward the message to the tax payer. To represent this conditional flow, draw an alternative combined fragment that covers the lifelines.
    
11. Right-click on the alt tag at the top-left of the combined fragment and select Operand > Manage Operands... from the popup menu.
12. Enter the constraints for both operands in the Manage Constraints window. For the first operand, enter accept payment as the constraint. For the second, enter reject payment. Click OK to confirm the changes.
    
13. Create the messages between the lifelines. Remember to create operations for all the sequence messages you created. When finished, your sequence diagram should look like this:
    
14. When you drew the UML sequence diagram, you created operations for the three lifelines. If you check the service interface diagram now, you can see the interfaces have operations listed.
    

Part IV - Drawing a Service Participant Diagram

In a multi-party service, each participant provides its own interface and uses the interfaces of the other parties. This information can be represented with a service participant diagram. Let's draw a service participant diagram.

1. To create a service participant diagram, select Diagram > New from the toolbar. In the New Diagram window, enter Service Participant Diagram in the search field, click Next. Then, fill in the Diagram Name and Description (if any), and click OK to confirm.
2. There are three participants in the tax payment service: the bank account owner, the online bank, and the tax department. Draw them in the diagram.
   
3. The bank account owner is a consumer of the tax payment service. Create a <<Request>> port in the Bank Account Owner participant. You can create a <<Request>> port via the resource-centric interface of a participant.
   
4. Let's set the type for the port. Right-click on the port and select Select Type... from the popup menu.
5. In the Select Type window, select Tax Payer and click OK.
   
6. The bank account owner provides its interface, which is the Tax Payer interface, and according to the interaction modeled in the UML sequence diagram, we know that it uses the Bank interface. Draw the provided interface from the <<Request>> port first. Name the interface Tax Payer.
   
7. Draw the required interface from the <<Request>> port. Name it Bank.
   
8. The Online Bank participant is a provider of the tax payment service. Create a <<Service>> port in it. Then, select Bank as the port type.
   
9. Online Bank provides the Bank interface and uses both the Tax Payer and Tax Receiver interfaces. Draw the provided and required interfaces. For this special case, you have to draw two required interfaces for the Tax Payer and Tax Receiver interfaces.
   
10. The Tax Department participant is also a provider of the tax payment service. Create a <<Service>> port in it. Then, select Tax Receiver as the port type.
    
11. Tax Department provides the Tax Receiver interface and uses both the Bank and Tax Payer interfaces. Draw the provided and required interfaces.
    
    When finished, your diagram should look like this: