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Adding Mediation Extensions - API Manager 1.7.0 - WSO2 Documentation
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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 create a custom mediation sequence either manually or using a tool like WSO2 Developer Studio, and then engage it per API or globally to all APIs of a specific tenant.

There are two ways to apply mediation extensions to messages:

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

Creating per-API extensions

Create and upload to the registry

Create a custom mediation sequence either manually or using a tool like WSO2 Developer Studio, and store the sequence.xml file in the governance registry. For more information, see Creating ESB Artifacts in the Developer Studio documentation. The registry collection where sequences are stored is customsequences, which is available by default in apimgt governance registry location. Given below are the registry paths:

SequenceRegistry path
in/_system/governance/apimgt/customsequences/in
out/_system/governance/apimgt/customsequences/out
fault/_system/governance/apimgt/customsequences/fault

For example, if you have an in sequence file, testInSequence, you must save it in the /_system/governance/apimgt/customsequences/in/testInSequence.xml file.

Once you create the XML file, upload it to the registry using the management console UI.

  1. Open the API-M management console (https://localhost:9443/carbon with admin/admin as the default credentials) and select Resources > Browse.
  2. Navigate to the /_system/governance/apimgt/customsequences registry location.
  3. Click Add Resource to upload the XML file.

Once an extension sequence has been uploaded to the registry, you can attach it 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 to the Manager section where you find Sequences. There, you can select In/Out/Fault sequences for the API from the drop-down lists. For example,

Create and save in the file system

Create a custom mediation sequence either manually or using a tool like WSO2 Developer Studio. For more information, see Creating ESB Artifacts in the Developer Studio documentation. 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 <API-M_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>

NOTE: The tenant username must be given as <username>-AT-<domain> in the configuration. For example, if the tenant username is testuser and the domain is wso2.com, then the name attribute in the above configuration must be testuser-AT-wso2.com--TwitterSearch:v1.0.0–In. The @ sign must be given as AT.

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 file with the naming pattern WSO2AM--Ext--<DIRECTION> and save it in the <API-M_HOME>/repository/deployment/server/synapse-configs/default/sequences directory. The <DIRECTION> can be In or Out. 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. When the direction of the sequence is In, the extension is triggered on the in-flow (request path). Similarly, 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.

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

Global Extension Sequence Example
<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 <API-M_HOME>/repository/deployment/server/synapse-configs/default/sequences directory. The above sequence prints a log message on the console on every API invocation.

Executing 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:

API Configuration
<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 the <API-M_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.

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