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 Enterprise Integrator (WSO2 EI). We will configure an API resource in the ESB profile of 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. 

Let's get started!

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 will configure the message mediation process in the ESB profile that will route all received messages to the back-end service (health care service). We will use 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

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. 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

   

   Click Finish.

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

   

Next, inside the SampleServices ESB Config project, we will create an endpoint 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. Give the information as in the table below:

   Field	Value	Description
   Endpoint Name	QueryDoctorEP	The name of the endpoint defined.
   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 in this ESB Config project only 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 Config project we created in the last section

   

   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:

   Field	Value	Description
   Name	HealthcareAPI	The name of the REST API in WSO2 EI.
   Context	/healthcare	Here we are anchoring the API at "/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 Config project we have already created previously.

   
   Click Finish. Once the API resource is created, the design view of the HealthcareAPI.xml file will appears 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).

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 as Basic properties:
   • Url Style: Click the Value field, click the down arrow, and then select URI_TEMPLATE from the list
   • URI-Template: /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.
      
   

6. In the Methods section of the Property tab, set the value of Get 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. 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”.

   
   

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

   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.
   Log Seperator	(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		We will add the property in the next step, so skip this for now.
   Description	Request Log	The Description field provides the name that appears for the Log mediator icon in the design view.

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 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 endpoint we created above the Defined EndPoints palette.

    

    The In Sequence is now complete. Next, we need 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. 

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

This completes the steps for creating the artifacts required for sending 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.

Packaging the artifacts

Package the QueryDoctorEP endpoint and the HealthcareAPI resource into the Composite Application (C-App) project named SampleServicesCompositeApplication. Save all changes.

Starting the Integrator runtime and deploying the artifacts

1. In WSO2 EI Tooling, navigate to the Developer Studio Dashboard and click Server under Add Server.
   
   
   
   
2. In the Define a New Server dialog box, expand the WSO2 folder, and select the respective product and version.
   
   
   
   
3. Review the default port details for the ESB profile of WSO2 EI. Typically, you can leave these unchanged, but if you are already running another server on these ports, specify unused ports here. (See Default Ports of WSO2 Products for more information.) Click Next. 
   
   
   
   

4. To deploy the CApp project in the ESB server that we just added, select SampleServicesCompositeApplication from the list, click Add to move it into the Configured list, and then click Finish.
   
   
   
   The ESB server of WSO2 EI is now added inside Eclipse tooling.
   On the Servers tab, you can see that the server is currently stopped. Click the "start the server" icon on the Servers tab's toolbar. If prompted to save changes to any of the artifact files you created earlier, click Yes. 
   
   
   As the server starts, the Console tab will appear. You should see messages indicating that the C-App was successfully deployed. The C-App will now be available in the ESB profile's management console (Manage -> Carbon Applications -> List).

   You can also deploy the artifacts to the ESB server using a Composite Application Archive (CAR) file.

Starting the MSF4J profile

To be able to send requests to the back-end service (which is an MSF4J service deployed in MSF4J profile), you need to first start the MSF4J runtime:

1. Open a terminal and navigate to the <EI_HOME>/wso2/msf4j/bin directory.

2. Start the runtime by executing the MSF4J startup script as shown below.

   sh carbon.sh

The Healthcare service is now active and you can start sending requests to the service.

Sending requests to the ESB

Let's send a request to our REST API (HealthcareAPI), which is now deployed in the ESB profile of WSO2 EI. You will need a REST client like curl for this.

1. In your Web browser, navigate to the ESB profile's management console using the following URL:  https://localhost:9443/carbon/.
2. Log in to the management console using the following credentials:
   • Username: admin
   • Password: admin

3. In the left navigation pane, click APIs under Service Bus. Here, you can see that the REST API we created earlier (HealthcareAPI) listed. This API is now ready to receive requests and to send them to the back-end service. Here, you can also view the API Invocation URL that is used in to send the request to the service.
   

4. Open a command line terminal and enter the following request:

   curl -v http://localhost:8280/healthcare/querydoctor/surgery    

   The above request is formed as per the URI-Template defined when creating the endpoint.

   http://<host>:<port>/healthcare/{uri.var.category}

   The {uri.var.category} of the above URI is derived from the URI-Template defined when creating the API resource.

   http://<host>:<port>/querydoctor/{category}

   Other categories you can try sending in the request are:

   • cardiology

   • gynaecology

   • ent

   • paediatric

5. You will see the response message from the HealthcareService with a list of available doctors and the relevant details.

   [{"name":"thomas collins",
     "hospital":"grand oak community hospital",
     "category":"surgery",
     "availability":"9.00 a.m - 11.00 a.m",
     "fee":7000.0},
    {"name":"anne clement",
     "hospital":"clemency medical center",
     "category":"surgery",
     "availability":"8.00 a.m - 10.00 a.m",
     "fee":12000.0},
    {"name":"seth mears",
     "hospital":"pine valley community hospital",
     "category":"surgery",
     "availability":"3.00 p.m - 5.00 p.m",
     "fee":8000.0}

6. Now, check the ESB profile's server Console in Eclipse, and you will see the following message: 
INFO - LogMediator message = "Welcome to HealthcareService"

   This is the message printed by the Log mediator when the message from the client is received in the In sequence of the API resource.

You have now created and deployed an API resource in the ESB profile, which receives requests, logs a message using the Log mediator, sends the request to a back-end service using the Send mediator, and returns a response to the requesting client.