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This section introduces handlers and uses an example to explain how to write a custom handler: 

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• CORSRequestHandler: Sets the CORS headers to the request and executes the CORS sequence mediation logic. This handler is thereby responsible for returning the CORS headers from the gateway or routing the requests to the backend and letting the backend send the CORS headers.
• APIAuthenticationHandler: Validates the OAuth2 bearer token used to invoke the API. It also determines whether the token is of type Production or Sandbox and sets MessageContext variables as appropriate. 
• APIThrottleHandler: Throttles requests based on the throttling policy specified by the policyKey property. Throttling is applied both at the application level as well as subscription level.
• APIMgtUsageHandler: Publishes events to BAM WSO2 Data Analytics Server (WSO2 DAS) for collection and analysis of statistics. This handler only comes to effect if API usage tracking is enabled. See the Analytics section for more information.
• APIMgtGoogleAnalyticsTrackingHandler: Publishes events to Google Analytics. This handler only comes into effect if Google analytics tracking is enabled. See Integrating with Google Analytics for more information.

• APIManagerExtensionHandler: Triggers extension sequences. By default, the extension handler is listed at last in the handler chain, and therefore is executed last. You cannot change the order in which the handlers are executed, except the extension handler. To configure the API Gateway to execute extension handler first, uncomment the <ExtensionHandlerPosition> section in the <APIM_HOME>/repository/conf/api-manager.xml file and provide the value top. This is useful when you want to execute your own extensions before our default handlers in situations like  doing additional security checks such as signature verification on access tokens before executing the default security handler.  
  See Adding Mediation Extensions.

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You can engage a custom handler to all APIs at once or only to selected APIs. To engage a custom handler to APIs, you need to add the custom handler with its logic in the <APIM_HOME>/repository/resources/api_templates/velocity_template.xml file. 

For example, the following code segment adds the custom authentication handler that you wrote earlier to the velocity_template.xml file while making sure that it skips the default APIAuthenticationHandler implementation:

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