Sending a Simple Message to a Service

Let’s try a simple scenario where a patient makes an inquiry specifying the doctor's specialization(category) to retrieve a list of doctors that match the specialization. The required information is available in a microservice deployed in the MSF4J profile of WSO2 EI. We will configure an API resource in WSO2 Enterprise Integrator (WSO2 EI) that will receive the client request, instead of the client sending messages directly to the back-end service, thereby decoupling the client and the back-end service. 

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Let's get started!

This tutorial includes the following sections:

Creating the message mediation artifacts

Requests going through the Enterprise Integrator are called messages, and message mediation is a fundamental part of any ESB. In this section, we will configure message mediation for requests received by the Enterprise Integrator that need to be sent to the Healthcare back-end service. We will use Eclipse-based WSO2 EI Tooling to create the message mediation artifacts and then deploy them to WSO2 EI.

Creating the deployable artifacts project

1. In Eclipse, open the Developer Studio dashboard by clicking the Developer Studio menu and choosing Open Dashboard. 
   

   
   

2. Click ESB Solution Project and create a project named SampleServices and select the following check boxes so that the relevant projects will be created.

   • Create Registry Resources Project

   • Create Connector Exporter Project

   • Create Composite Application Project

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   Click Finish.

   You have now created the following projects as shown in the Project Explorer:

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Next, inside the SampleServices ESB Config project, we will create an endpoint that will allow the Enterprise Integrator to connect to the back-end service.

Connecting to the back-end service 

To connect to the back-end service, we must expose a URL that can be used to connect to the service. To do this, we will create an endpoint for this service.

1. Right-click SampleServices in the Project Explorer and navigate to New -> Endpoint.

2. Ensure Create a New Endpoint is selected and click Next. Give the information as in the table below:
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   Click Finish. 
   
   The QueryDoctorEP endpoint you created is saved in the endpoints folder within the ESB Config Project you created.
   
   

Now that you have created the endpoint for the back-end service, it’s time to create the REST API and the relevant API resource that will receive requests from client applications, mediate them and send them to the endpoint, and return the results to the client.

Mediating requests to the back-end service

We will use WSO2 EI Tooling to create a REST API named HealthcareAPI. We will then create a resource within this API for the GET HTTP method that is used to send requests to the Healthcare back-end service and retrieve available doctor information.

1. In the Project Explorer, right-click SampleServices and navigate to New -> REST API.

2. Ensure Create A New API Artifact is selected and click Next.

3. Fill in the information as in the table below:

   
   Click Finish. Once the API resource is created, the design view of the HealthcareAPI.xml file appears so that you can start configuring the API resource. 
   

4. Click the Resource icon on the left side of the canvas. The properties for the API resource will appear on the Properties tab at the bottom of the window. (If they do not appear, you can right-click the proxy icon and click Show Properties View.)

5. On the Properties tab provide the following in the Basic property:

   • Url Style: Click in the Value field, click the down arrow, and then select URI_TEMPLATE from the list

   • URI-Template: /querydoctor/{category}
     This defines request URL format. In this case, the full request URL format is http://<host>:<port>/querydoctor/{category} where {category} is a variable. 

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6. In the Methods section of the properties tab, set the value of Get to true. This defines that the API resource created handles only requests where the HTTP method is GET.
   
   We are now ready to configure In sequence to handle requests from the client. 

7. From the Mediators palette, click and drag a Log mediator to the In sequence (the top of the canvas).
   

8. With the Log mediator selected, access the Properties tab and fill in the information in the table below:

9. We will now add the property that will extract the stock symbol from the request and print a welcome message in the log. Click the Value field of the Properties property, and then click the browse (...) icon that appears.

10. In the Log Mediator Configuration dialog, click New, and then add a property called "message" as follows:

    • Name: message

    • Type: LITERAL
      We select LITERAL because the required the log message is a static value.

    • Value/Expression: "Welcome to HealthcareService"

    
    Click OK to save the Log mediator configuration.

    We will now configure the Send Mediator to send the request message to the HealthcareService endpoint.

11. From the Mediators palette, click and drag a Send mediator to the In sequence adjoining the Log mediator you added above. Adjoining this, click and drag the QueryDoctorEP we created above the Defined EndPoints palette.

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12. From the Mediators palette, click and drag a Send mediator to the Out Sequence (the bottom part of the canvas).