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• 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 for collection and analysis of statistics. This handler only comes to effect if API usage tracking is enabled. See Publishing API Runtime Statistics 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 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 To configure the API Gateway to execute extension handlers first by uncommenting , uncomment the <ExtensionHandlerPosition> section in the <APIM_HOME>/repository/conf/api-manager.xml file and providing 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.

Writing a custom handler

Let's see how you can write a custom handler and apply it to the API Manager. In this example, we extend the authentication handler. 

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