Sending a Simple Message to a Service Using the ESB Profile

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  backend microservice deployed in the MSF4J profile of WSO2 Enterprise Integrator (WSO2 EI). We configure an API resource in the ESB profile of WSO2 EI that receives 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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This tutorial includes the following sections:

Creating the message mediation artifacts

Requests going through the ESB profile of WSO2 EI are called messages, and message mediation is a fundamental part of any ESB. In this section, we configure the message mediation process in the ESB profile that routes all received messages to the back-end service (healthcare service). We use the Eclipse-based WSO2 EI Tooling to create the message mediation artifacts and then deploy them to the ESB profile of WSO2 EI.

Creating the deployable artifacts project

Follow the steps given below to create an ESB solution project.

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

2. Click ESB Solution Project to create a new ESB project. 
   

    

3. Create a project named SampleServices.
   Be sure to 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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   create an ESB Solution Project

4. Click Finish.
   You have now created the following projects as shown in the Project Explorer:

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Next, we create an endpoint inside the SampleServices ESB Solution project that will allow the ESB 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.

3. Enter the information given below to create the new endpoint.

   Field	Value	Description

   Field	Value	Description
   Endpoint Name	QueryDoctorEP	The name of the endpoint.
   Endpoint Type	HTTP Endpoint	Indicates that we are connecting to REST back-end service.
   URI Template	http://localhost:9090/healthcare/{uri.var.category}	The template for the request URL expected by the MSF4J service deployed in the MSF4J profile. In this case, the variable 'category' that needs to be included in the request for querying doctors, is represented as {uri.var.category} in the template.
   Method	GET	Indicates that we are creating this endpoint for GET requests that are sent to the back-end service.
   Static Endpoint		Select this option because we are going to use this endpoint only in this ESB Solution project and will not re-use it in other projects. If you need to create a reusable endpoint, you create it as a Dynamic Endpoint and save the endpoint in either the Configuration or Governance Registry. For more information, see the documentation on registries.
   Save Endpoint in	SampleServices	This is the ESB Solution project we created in the last section
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   create a new endpoint

4. Click Finish. 
   The QueryDoctorEP endpoint is saved in the endpoints folder within the ESB Solution 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 use WSO2 EI Tooling to create a REST API named HealthcareAPI. Next, we 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. Enter the details given below to create a new REST API. 

   Field	Value	Description

   Field	Value	Description
   Name	HealthcareAPI	The name of the REST API in WSO2 EI.
   Context	/healthcare	Here we are anchoring the API in the /healthcare context. This will become part of the name of the generated URL used by the client when sending requests to Healthcare service. For example, setting the context to /healthcare defines that the API will only handle HTTP requests where the URL path starts with http://<host>:<port>/healthcare.
   Save location	SampleServices	This is the ESB Solution project we have already created previously.
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   create a new REST API

4. Click Finish.
   Once the API resource is created, the design view of the HealthcareAPI.xml file will appear as shown below. You can now start configuring the API resource. 

   • The top part of the canvas is the In sequence, which controls how incoming messages are mediated. 

   • The middle part of the canvas is the Out sequence, which controls how responses are handled. In this case, a Send mediator is already in place to send responses back to the requesting client.

   • The bottom part of the canvas is the Fault sequence, which allows you to configure how to handle messages when an error occurs (for more information, see Error Handling).

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   the created new REST API

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

6. On the Properties tab, provide the following as Basic properties:
   Url Style	Click the respective Value field, click the down arrow, and then select URI_TEMPLATE from the list
   URI-Template	Enter /querydoctor/{category}. This defines the request URL format. In this case, the full request URL format is http://<host>:<port>/querydoctor/{category} where {category} is a variable.
   Methods	Check if the value of Get is set to true. This defines that the API resource only handles the requests where the HTTP method is GET.
   
   We are now ready to configure the In sequence to handle requests from the client. 

7. Configure the In sequence:

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

      You can use the Log mediator to log a message when the request is received by the In sequence of the API resource. In this scenario, we will configure the Log mediator to display the following message: “Welcome to the HealthcareService”.

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

      Field	Value	Description

      Field	Value	Description
      Log Category	INFO	Indicates that the log contains an informational message.
      Log Level	Custom	When Custom is selected, only specified properties will be logged by this mediator. For more information on the available log levels, see the Log Mediator.
      Log Separator	(blank)	Since there is only one property that is being logged, we do not require a separator, so this field can be left blank.
      Properties		Follow the steps given below to 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. 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 log message is a static value.) Value/Expression: "Welcome to HealthcareService" Open properties of the log mediator
      Description	Request Log	The Description field provides the name that appears for the Log mediator icon in the design view.
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   3. Click OK to save the Log mediator configuration.

   4. Configure the Send Mediator to send the request message to the HealthcareService endpoint.

      1. From the Mediators palette, click and drag a Send mediator to the In sequence adjoining the Log mediator you added above. 

      2. From the Defined EndPoints palette, click and drag the QueryDoctorEP endpoint, which we created, right next to the empty space of the Send mediator.

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      first send mediator

8. Configure the Out sequence to ensure that we send the response from the Healthcare service endpoint, back to the client. For this, we use a Send mediator with no output endpoint defined, which defaults to sending the response back to the requesting client
   From the Mediators palette, click and drag a Send mediator to the Out Sequence (the bottom part of the canvas).

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   second send mediator

You have successfully created all the artifacts that are required to send a request through the ESB to the back-end service. We will now package these artifacts and deploy them in the ESB profile of WSO2 EI.