Adding a Mediation Extension

The API Gateway has a default mediation flow that is executed in each API invocation. You can do additional custom mediation for the messages in the API Gateway by extending its mediation flow. An extension is provided as a synapse mediation sequence.

You can design all sequences using a tool like WSO2 Developer Studio, and store the sequence.xml file in the governance registry. The registry collection where sequences are stored is customsequences, which is available by default in apimgt governance registry location. The registry path is /_system/governance/apimgt/customsequences. Store the InSequence files in apimgt/customsequences/in folder and the OutSequence files in apimgt/customsequences/out folder. For example, if you have an in sequence file as testInSequence, you must save it in /_system/governance/apimgt/customsequences/in/testInSequence.xml and if you have an out sequence file as testOutSequence, save it in /_system/governance/apimgt/customsequences/out/testOutSequence.xml.

There are two ways to apply mediation extensions to messages:

• Global Extensions : Apply to all APIs
• Per-API Extensions : Apply only to an intended API

The difference between a global extension and a per-API extension is simply in the name given to the sequence that you use to create it.

Creating global extensions

Given below is the naming pattern of a global extension sequence.

WSO2AM--Ext--<DIRECTION>

The <DIRECTION> can be either In or Out. When the direction of the sequence is In, the extension is triggered on the in-flow (request path). When the direction of the sequence is Out, the extension is triggered on the out-flow (response path). Shown below is an example synapse configuration of a global extension sequence.

<sequence xmlns="http://ws.apache.org/ns/synapse" name="WSO2AM--Ext--In">
    <log level="custom">
        <property name="TRACE" value="Global Mediation Extension"/>
    </log>
</sequence>

To test the code, copy it to an XML file (e.g., global_ext.xml) and save the file in the <APIM_HOME>/repository/deployment/server/synapse-configs/default/sequences directory. The above sequence prints a log message on the console on every API invocation.

Creating per-API extensions

Given below is the naming pattern of a per-API extension sequence.

<API_NAME>:v<VERSION>--<DIRECTION>

Shown below is an example synapse configuration of a per-API extension sequence. It is created for an API named admin--TwitterSearch with version 1.0.0.

<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>

To test the code in super-tenant mode, copy it to an XML file (e.g., twittersearch_ext.xml) and save the file in the <APIM_HOME>/repository/deployment/server/synapse-configs/default/sequences directory, if you are using a standalone server. In multi-tenant mode, copy the file to the tenant's synapse sequence folder. For example, if tenant id is 1, then copy it to <API_Gateway>/repository/tenants/1/synapse-configs/default/sequences folder. 

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

Selecting predefined APIs from the UI

Alternatively to the above approach, you can also attach extension sequences to an API using the API Publisher Web interface at the time the API is created. Log in to the API Publisher (https://localhost:9443/publisher) and click Add from the left panel. In the Add New API page that opens, navigate down to the Sequences section. There, you can select In/Out sequences for the API from the drop-down lists. For example,

To populate these drop-down lists, you must add mediation sequences as explained in the beginning.

Invoking the extension sequences

When an API is published, a file with its synapse configuration is created on the API Gateway. This synapse configuration has a set of handlers as shown in the following example:

<handlers>
        <handler class="org.wso2.carbon.apimgt.gateway.handlers.security.APIAuthenticationHandler"/>
        <handler class="org.wso2.carbon.apimgt.usage.publisher.APIMgtUsageHandler"/>
        <handler class="org.wso2.carbon.apimgt.usage.publisher.APIMgtGoogleAnalyticsTrackingHandler"/>
        <handler class="org.wso2.carbon.apimgt.gateway.handlers.throttling.APIThrottleHandler">
            <property name="id" value="A"/>
            <property name="policyKey" value="gov:/apimgt/applicationdata/tiers.xml"/>
        </handler>
        <handler class="org.wso2.carbon.apimgt.gateway.handlers.ext.APIManagerExtensionHandler"/>
</handlers>

The handler by the name APIManagerExtensionHandler triggers both global as well as per-API extension sequences. It reads the sequence names and determines what APIs must be invoked. By default, the extension handler is listed at last in the handler chain, and therefore is executed last. You can configure the API Gateway to execute extension handlers first. To do that, open <APIM_HOME>/repository/conf/api-manager.xml file, uncomment the <ExtensionHandlerPosition> section and provide the value top as follows:

<ExtensionHandlerPosition>top</ExtensionHandlerPosition>

This is useful when you want to execute your own extensions before our default handlers. For example, if you want to have additional security checks such as signature verification on access tokens before executing the default security handler, you can define an extension and configure the Gateway to execute extension handlers first.

For more information on Handlers, see Architecture Components.