Change the Default Mediation Flow of API Requests

The API Gateway has a default mediation flow for the API invocation requests that it receives. You can extend this default mediation flow to do additional custom mediation for the messages in the API Gateway. An extension is provided as a synapse mediation sequence. You design sequences using a tool like WSO2 Developer Studio and then upload the sequence via the APIM Perspective. 

Let's see how to create a custom sequence using WSO2 Developer Studio and then deploy and use it in your APIs.

1. Log in to the API Publisher.

2. Click Add to create an API with the following information and then click Implement.

   Field		Sample value
   Name		YahooWeather
   Context		/weather
   Version		1.0
   Resources	URL pattern	current/{country}/{zipcode}
   	Request types	GET method to return the current weather conditions of a zip code that belongs to a particular country

   

3. The Implement tab opens. Select Managed API, provide the information given in the table below and click Manage.

   Field	Sample value
   Endpoint type	HTTP endpoint
   Production endpoint	You can find the Yahoo weather API's endpoint from https://developer.yahoo.com/weather/. Copy the part before the '?' sign to get this URL: https://query.yahooapis.com/v1/public/yql To verify the URL, click the Test button next to it.
   Sandbox endpoint	https://query.yahooapis.com/v1/public/yql To verify the URL, click the Test button next to it.

   

4. Click Manage to go to the Manage tab, provide the following information and click Save & Publish once you are done.

   Field	Sample value
   Tier Availability	Gold
   Keep the default values for the other attributes

   

5. Download WSO2 Developer Studio (version 3.8.0 is used here) from http://wso2.com/products/developer-studio/ and open it by double clicking the Eclipse.app file inside the downloaded folder. 

6. Click Window > Open Perspective > Other to open the Eclipse perspective selection window. Alternatively, click the Open Perspective icon shown below at the top right corner.
   
7. On the dialog box that appears, click WSO2 APIManager and click OK.
   
8. On the APIM perspective, click the Login icon as shown below.
    
9. On the dialog box that appears, enter the URL, username and password of the Publisher server.
   
10. On the tree view that appears, expand the folder structure of the existing API.

11. Right-click on the in sequence folder and click Create to create a new in sequence.
    
    This is because you want the custom sequence to be invoked in the In direction or the request path. If you want it to be involved in the Out or Fault paths, select the respective folder under customsequences.

12. Name the sequence YahooWeatherSequence.

13. Your sequence now appears on the Developer Studio console. From under the Mediators section, drag and drop a Property mediator to your sequence and give the following values to the property mediator.

    Property Name	New Property
    New Property Name	YQL
    Value Type	Expression
    Value Expression	For the XPath expression, we take a part of the query in the Yahoo API's endpoint (https://developer.yahoo.com/weather/) and concatenate the zip code and country to it using the synapse get-property XPath expression: concat('?q=select%20*%20from%20weather.forecast%20where%20woeid%20in%20(select%20woeid%20from%20geo.places(1)%20where%20text%3D%22',syn:get-property('uri.var.zipcode'),',',syn:get-property('uri.var.country'),'%22)&format=json')

    

14. Similarly, add another property mediator with the following values. This is an HTTP transport property that appends its value to the address endpoint URL. Once you are done, save the sequence.

    Property Name	New Property
    New Property Name	REST_URL_POSTFIX
    Value Type	Expression
    Value Expression	Set the value of the property mediator that you created earlier as get-property('YQL')
    Property Scope	Axis2

    

15. Save the sequence.
16. Right-click on the sequence and click Commit File to push the changes to the Publisher server.
17. Log back in to the API Publisher, select the API that you created earlier and click the Edit link right next to its name to go to the edit wizard. 
    

18. Navigate to the API's Implement tab, select the Enable Message Mediation check box and select the sequence that you created for the In flow. Next, click Manage and Save & Publish the API again.

    Tip: It might take a few minutes for the sequence to be uploaded into the API Publisher. If it isn't there, please check again later.

    

19. Open the API Store, subscribe to the API that you just published and generate the access tokens in order to invoke the API.
    
20. Click the API Console tab of the API. It opens the integrated API Console using which you can invoke the API.
    

21. Give the following values for the parameters and invoke the API. You can also give any other value of your choice.

    country	usa
    zipcode	95004

    

22. Note the response that you get as a JSON object from Yahoo.
    

In this tutorial, you created a sequence to change the default mediation flow of API requests, deployed it in the API Gateway and invoked an API using the custom mediation flow.

<?xml version="1.0" encoding="UTF­8"?> 
<sequence xmlns="http://ws.apache.org/ns/synapse" 
	name="<insequence_name>" 
	onError="fault" > 
	<log level="full"> 
		<property name="MessageFlow" 
		value="­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­--------In sequence of the message flow­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­-------"/> 
	</log> 
</sequence> 

<sequence xmlns="http://ws.apache.org/ns/synapse" name="fault"  onError="faultSeq" > 
	<log level="custom">
		<property name="STATUS:" value="­­­­­­­­­­­­­­­­fault invoked­­­­­­­­­­­­­­­­­­­­­­­" /> 
		<property name="STATUS" value="Executing default 'fault' sequence" />
		<property name="ERROR_CODE" expression="get­property('ERROR_CODE')" /> 
		<property name="ERROR_MESSAGE" expression="get­property('ERROR_MESSAGE')" />
	</log>
	<switch source="//fault/type........">
		<case regex="invalid......">................</case>
		<default>...............</default>
	</switch>
	<class name="org.wso2.carbon.apimgt.usage.publisher.APIMgtFaultHandler" /> 
	<property name="RESPONSE" value="true" />
	<header name="To" action="remove" />
	<property name="NO_ENTITY_BODY" scope="axis2" action="remove" />
	........................................................................................ 
	<property name="X­JWT­Assertion" scope="transport" action="remove" /> <sequence key="_cors_request_handler_" />
	<send /> 
</sequence>

<?xml version="1.0" encoding="UTF­8"?>
<sequence xmlns="http://ws.apache.org/ns/synapse"  name="faultSeq" >
<class name="org.wso2.carbon.samples.FaultErrorMediator"></class>
<property description="" name="ContentType" scope="axis2" type="STRING" value="application/xml"/>
	<sequence key="fault"/> 
</sequence>