Receiving Notifications from Data Services

You can create an event trigger from a query as explained below. 

Introduction

Eventing support is provided by the WS-Eventing Web services standard. When a data service request or response triggers an event, the subscribers listening to those events receive notifications. The criteria for triggering an event as well as the destination to which the event notifications should be sent are defined per data service query.  When a certain event-trigger is activated, emails will be sent to all the respective subscribers. You can set an event trigger as an input or an output event trigger in a data service query. Let's look at the usage of the two approaches.

Input event trigger

When an input event trigger is applied to a query, the event trigger is evaluated when the parameters are received by the query. An event-trigger executes an XPath expression against an XML element. This XML element is built from the input parameters that are represented by the element of the parameters wrapped by an element with the name of the query.

For example, take the following query:

<query id="incrementEmployeeSalaryQuery" useConfig="default">
   <sql>update Employees set salary=salary+? where employeeNumber=?</sql>
   <param name="increment" paramType="SCALAR" sqlType="DOUBLE" type="IN" ordinal="1" />
   <param name="employeeNumber" paramType="SCALAR" sqlType="INTEGER" type="IN" ordinal="2" />
</query>

Assuming that the values of increment and employeeNumber are value1 and value2 respectively, the XML element to be evaluated with the XPath expression is as follows. Also, note that this XML element doesn't have any namespaces associated with it.

<incrementEmployeeSalaryQuery>
   <increment>value1</increment>
   <employeeNumber>value2</employeeNumber>
</incrementEmployeeSalaryQuery>

Output event trigger

In this case, the event trigger is evaluated when a specific query is returning its result. There isn't a specific creation of an XML element that is used with the XPath expression like in the input event trigger. But the full result XML is used to evaluate it. The result will be namespace qualified. Therefore, you must write the XPath expressions accordingly.

Get started!

Follow the steps given below.

Enabling notifications for a query in a data service

Let's create a data service using the Create Data Service wizard:

1. Start or restart the ESB profile of WSO2 Enterprise Integrator.

   You need to restart the ESB profile for the changes you made to the axis2_client.xml file to reflect in the server.

2. Open the ESB profile's Management Console using https://localhost:9443/carbon, and log in using admin as the username and the password.

3. In the Data Service menu click Create.
4. Enter a name for the data service and click Next.

5. Connect to the Company database that you defined above.

   Datasource ID	Datasource
   Datasource Type	RDBMS
   Datasource Type (Default/External)	Leave Default selected.
   Database Engine	MySQL
   Driver Class	com.mysql.jdbc.Driver
   URL	jdbc:mysql://localhost:3306/Company
   User Name	Enter your MySQL server's username. Example: root
   Password	Enter your MySQL server's password.  If you have not assigned a password, keep this field empty.

6. Click Next to go to the Queries screen.
7. Click Add New Query to specify the query details:

   1. Enter UpdateAccBalance as the query ID.

   2. Enter the following SQL dialect:

      UPDATE ACCOUNT SET Balance=:Balance WHERE AccountID=:AccountID

   3. Click Generate Input Mapping to automatically generate input mappings for the AccountID and Balance fields.
      

   4. At the bottom of the page you will find the Events section :
      

   5. Click Manage Events and add a new event as shown below:

      Event Id	Enter account_balance_low_trigger. This is the ID used for identifying the event-trigger used in data services queries.
      Xpath	Enter /UpdateAccBalance/Balance<200. Represents an XPath expression that is run against the XML message presented. That is the request/response message. When this evaluation returns true, the event is triggered.
      Target Topic	Enter product_stock_low_topic. The topic to which the event notifications are published.
      Event Sink URL	Enter the email to which the event notifications need to be sent in the following format: mailto:<YOUR_EMAIL> A subscription can be any endpoint that is compliant with WS-Eventing. For example, you can use an SMTP transport to send a message to a mail inbox, where an email address is given as the subscription. Here, many subscriptions can be defined for the given topic.

      Example:
       

   6. Click Save and click Main Configuration to go back to the query page.

   7. You can now add the event as an event trigger for the query as shown below.
      Since the event sends notifications related to the data input, you need to add the event as an Input Event Trigger.
      
   8. Save the UpdateAccBalance query.

8. Create an operation for the UpdateAccBalance query as shown below.

   Operation Name	UpdateAccBalanceOp
   Query ID	UpdateBalance

   

9. Save the operation.

10. Click Finish to navigate to the Deployed Services window from where you can manage data services.

Invoking the data service 

You can try the data service you created by using the TryIt tool that is in your product by default.  

1. Go to the Deployed Services screen.
2. Click Try this Service to open the data service from the TryIt tool.
3. Click the UpdateAccBalance operation.

4. Copy the following into the TryIt tool:

   To trigger the notification, you need to have a balance that is less than 200.

   <body>
      <p:UpdateAccBalanceOp xmlns:p="http://ws.wso2.org/dataservice">
         <!--Exactly 1 occurrence-->
         <xs:Balance xmlns:xs="http://ws.wso2.org/dataservice">189</xs:Balance>
         <!--Exactly 1 occurrence-->
         <xs:AccountID xmlns:xs="http://ws.wso2.org/dataservice">1</xs:AccountID>
      </p:UpdateAccBalanceOp>
   </body>

5. Since the balance is less than 200, you receive an email.
   In the event notification messages, additional information about the event is added to the SOAP Envelope/Body element. The following code demonstrates this.

   
   

   <data-services-event>
       <service-name>$SERVICE_NAME</service-name>
       <query-id>$QUERY_ID</query-id>
       <time>$TIME</time>
       <content>
         $CONTENT
       </content>
   </data-services-event>

   
   

   • $SERVICE_NAME : Name of the service from which the event originated
   • $QUERY_ID: The id of the query that triggered the event
   • $TIME: The date/time when the event occurred
   • $CONTENT: The XML element generated in case of an input event trigger. It is used when executing the XPath expression. It contains the input parameters wrapped with the query id value. In the case of an output event trigger, it contains the full result XML