Adding Mediation Extensions

The API Gateway has a default mediation flow, which you can extend by adding custom mediation sequences. In API Manager there are 3 default sequences engaged as  in ,  out  and  fault . You create a custom mediation sequence either manually or using a tool such as the WSO2 API Manager Tooling Plug-in, and then engage it per API or globally to all APIs of a specific tenant. With custom mediation sequences you can modify the default mediation flow for different usabilities according to your requirement. Log the mediation flow, execute operations on Message context properties, to customize, format the requests and responses are some of them.

Default mediation flow

You cannot dynamically construct the back-end endpoint of an API using the address endpoints in the WSO2 API Manager. To achieve the requirement of a dynamic endpoint, you can use the default endpoint instead. The default endpoint sends the message to the address specified in the To header. The To header can be constructed dynamically. For example,

<sequence xmlns="http://ws.apache.org/ns/synapse" name="default-endpoint-seq">
  <property name="service_ep" expression="fn:concat('http://jsonplaceholder.typicode.com/', 'posts/')"/>
  <header name="To" expression="get-property('service_ep')"/>
</sequence>

In this example, you have constructed the service_ep property dynamically and assigned the value of this property to the To header. The default endpoint sends the message to the address specified in the To header, in this case, http://jsonplaceholder.typicode.com/posts/. For more details about working with dynamc endpoints, see Dynamic Endpoints.

<inSequence>
   <property name="ClientApiNonBlocking"
           value="true"
           scope="axis2"
           action="remove"/>
   <send>
      <endpoint name="FileEpr">
         <address uri="vfs:file:////home/shammi/file-out"/>
      </endpoint>
   </send>
</inSequence>

Creating per-API extensions

Create and upload using the WSO2 API Manager Tooling Plug-in

The recommended way to engage a mediation extension sequence per API is to create a custom sequence using the WSO2 API Manager Tooling Plug-in, upload it via its APIM Perspective and then engage it using the API Publisher. The following tutorial demonstrates how to do this: Change the Default Mediation Flow of API Requests.

Create and upload manually in the API Publisher

You can also create a mediation sequence manually and upload it from the API Publisher itself. For instance, you can copy the above default mediation flow content into an XML file. In the Implement tab of the API, select the Enable Message Mediation check box and click the Upload In Flow or Upload Out Flow field (for the example above, it needs to be uploaded to the In flow). Once the file is uploaded, save and publish the API. When you invoke the API, the request is sent to the endpoint referred to in the To header.

Editing a mediation policy

If you want to edit an already uploaded mediation policy, 

1. Select the mediation policy from the drop down list.
2. Click the download icon next to it, as shown below:
    
3. Edit the downloaded mediation XML file and re-upload it.

Create manually and save in the file system

Alternatively, you can name the mediation XML file in the pattern <API_NAME>:v<VERSION>--<DIRECTION> and save it directly in the following location:

• In the single-tenant mode, save the XML file in the <APIM_HOME>/repository/deployment/server/synapse-configs/default/sequences directory.
• In the multi-tenant mode, save the XML file in the tenant's synapse sequence folder. For example, if tenant id is 1, then save it in <API_Gateway>/repository/tenants/1/synapse-configs/default/sequences folder. 

In the naming pattern, the <DIRECTION> can be In or Out. When it is In, the extension is triggered on the in-flow (request path) and when it is Out, the extension is triggered on the out-flow (response path). To change the default fault sequence, you can either modify the default sequence or write a custom fault sequence and engage it to APIs through the API Publisher. 

An example synapse configuration of a per-API extension sequence created for the API admin--TwitterSearch version 1.0.0 is given below.

<sequence xmlns="http://ws.apache.org/ns/synapse" name="admin--TwitterSearch:v1.0.0--In">
 <log level="custom">
    <property name="TRACE" value="API Mediation Extension"/>
 </log>
</sequence>

You can copy this content into an XML file (e.g., twittersearch_ext.xml) and save it in the <API_Gateway>/repository/deployment/server/synapse-configs/default/sequences directory.

The above sequence prints a log message on the console whenever the TwitterSearch API is invoked.

Creating global extensions

You can also engage mediation extension sequences to all APIs of a specific tenant at once. To do that, simply create the XML with the naming pattern WSO2AM--Ext--<DIRECTION> and save it in the <APIM_HOME>/repository/deployment/server/synapse-configs/default/sequences directory.

An example synapse configuration of a global extension sequence is given below:

<sequence xmlns="http://ws.apache.org/ns/synapse" name="WSO2AM--Ext--In"> 
	<property name="Authentication" expression="get-property('transport', 'Authentication')"/> 
	<property name="Authorization" expression="get-property('Authentication')" scope="transport" type="STRING"/> 
	<property name="Authentication" scope="transport" action="remove" /> 
</sequence>

This custom sequence assigns the value of your basic authentication to Authorization header.

You can copy this content into an XML file (e.g., global_ext.xml) and save it in the <API_Gateway>/repository/deployment/server/synapse-configs/default/sequences directory.

When you invoke your REST API via a REST Client, configure that client to have a custom header (Authentication) for your basic authentication credential and configure the Authorization header to contain the bearer token for the API. When you  send the Authentication and Authorization headers, the Gateway drops the Authorization header, converts the Authentication to Authorization headers and sends to the backend.

    package samples.mediators; 
 
    import org.apache.synapse.MessageContext;
    import org.apache.synapse.mediators.AbstractMediator;
    import org.apache.axiom.om.OMElement;
    import org.apache.axiom.om.OMAbstractFactory;
    import org.apache.axiom.om.OMFactory;
    import org.apache.axiom.soap.SOAPFactory;
    import org.apache.commons.logging.Log;
    import org.apache.commons.logging.LogFactory;
 
    import javax.xml.namespace.QName;
 
    public class SimpleClassMediator extends AbstractMediator {

        private String variable1 = xxx;       

        private static final Log log = LogFactory.getLog(SimpleClassMediator.class);
        
        public SimpleClassMediator(){}
 
        public boolean mediate(MessageContext mc) {
            // Do somthing useful..
            // Implementation of Reading the propertly values of Message context and modifying request / logging properties
            return true;
        }
 
        public String getType() {
            return null;
        }
 
        public void setTraceState(int traceState) {
            traceState = 0;
        }
 
        public int getTraceState() {
            return 0;
        }


        public void setVariable1(String newValue) {
            variable1=newValue;
        }
 
        public String getVariable1() {
            return variable1;
        }
  
       
    }

<class name="samples.mediators.SimpleClassMediator">                
	<property name="propertyName" value="propertyValue"/>
    ....
</class>