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 all sequences using a tool like WSO2 Developer Studio and then store the sequence in the Gateway's registry. 

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. Provide the information given in the table below and click Manage.

   Field	Sample value
   Implementation method	Backend
   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

   

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 3.7.1 is used here) from http://wso2.com/products/developer-studio/ and open it by double clicking the Eclipse.app file inside the downloaded folder. 

   Tip: To start Eclipse on a Mac for the first time, open a terminal and execute the following commands:

   cd <DevStudio_Home>/Eclipse.app/Contents/MacOS/
chmod +x eclipse
./eclipse

   Thereafter, you can start Eclipse by double-clicking the Eclipse icon in <DevStudio_Home>.

6. Click the Developer Studio menu and choose Open Dashboard. When the dashboard opens, click the ESB Config Project link.
   
7. Create a new ESB project by the name YahooConfigProject.
   
8. Click the Sequence link on the Developer Studio Dashboard and create a new sequence by the name YahooWeatherSequence and the YahooConfigProject project.
   

9. 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	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')

   

10. 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	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

    

11. Log in to the API Gateway's management console (https://localhost:9443/carbon) as admin. 
12. After logging in, click the Browse menu under the Resources menu. 
    

13. When the API Gateway's registry opens, navigate to the registry path /_system/governance/apimgt/customsequences/in. 
    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 location under customsequences.
    

    Tip: If you prefer not to use the registry to upload the sequence or want to engage a sequence to all APIs in the APIM at once, you can do so by saving the mediation sequence XML file in the file system. See Adding Mediation Extensions for details.

    

14. Click Add Resource and upload the XML file of the sequence that you created earlier.
    

15. 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. 
    

16. Navigate to the API's Manage tab, click the check box next to sequences and select the sequence that you created for the In flow. Next, 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.

    

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

19. 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

    

20. 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.